This 30,000 word glossary was written by Paul Gipe and Bill Canter in the late-1990s. I’ve added the glossary to my web site for both its historical content—many of the terms were in use during the 1980s and 1990s—and as a reference for the thousands of newcomers to the wind industry since it was first published.
The glossary was published by Forlaget Vistoft, who also published Windpower Monthly, WindStats, and Naturlig Energi. Bill Canter was the editor of WindStats. I contributed to both WindStats and Windpower Monthly.
This version is from my text file. I have no idea if this version reflects what was actually published. The document went through several revisions on both sides of the Atlantic.
I’ve made some minor edits in the text where it was obvious the term should be considered in the past tense and not present tense and where I knew I’d made an error 27 years ago. For example, I’ve corrected the rated power of the Smith-Putnam turbine to 1 MW from the original 1.25 MW.
If you find any other errors, please contact me with the corrections.
Here’s how I described the book in a promotional release.
“From HAWT to VAWT and from Anemometer to Zephyr, the glossary offers a sometimes irreverent description of the words that make up the modern wind industry and translates wind energy speak for both the uninitiated and the professional. Unlike most technical dictionaries that simply define terms, the glossary places the meaning of the term within its wind energy context. For example, consider the term AWT for Advanced Wind Turbine that is bandied about in the United States. What does it really mean? Is it merely a marketing sleight of hand? And on occasion the glossary deflates the bombastic as in
Wind turbine generator: Jargon to describe a wind turbine driving a generator. Often used by attorneys and brokers in California to impart a sense of technical sophistication.”
Glossary of Wind Energy Terms
December 31, 1996
By Paul Gipe and Bill Canter
New terms added 24 March 2024
Cupid: Shooting an arrow through the rotor with a rope attached in an attempt to bring a runaway wind turbine to a stop. Used in the Altamont, Tehachapi, and San Gorgonio passes. Historically used in Denmark on Lykkegaard wind turbines of the 1920s. (This technique is mentioned on page 420 of Wind Energy for the Rest of Us.)
Deal junky: Project developers or management keen on getting a substantial bonus for closing the “deal.” Deal junkies had a penchant for ignoring warnings from meteorologist’s about low wind speeds, or from planners about the proximity of neighbors, or environmental impacts. Creators of “magic spreadsheets.” (Mark Haller)
Dirt nap: Colloquial expression among wind techs for a wind turbine destroying itself as in “The wind turbine was taking a nap in the dirt.” Term from the early days in the Altamont Pass when the turbines sometimes pulled up their foundations and tipped over. (Mark Haller)
Flimflam: Slang for nonsense, swindle, or deception. Used in relation to outlandish concepts for harnessing the wind where the only people to make money are the promoters. See zombie wind.
Jail break: Juxtaposed to “runaway” when more than one wind turbine was in runaway. Used in reference to multiple Enertechs and ESIs in overspeed during a high wind event in the 1980s. (Mark Haller)
LOBA: Loss of blade accident, euphemism used by US Windpower (Kenetech) for a wind turbine throwing a blade.
LOMA: Loss of machine accident, euphemism used by US Windpower (Kenetech) for another wind turbine destroyed.
Magic spreadsheet: Spreadsheet values for project costs, soft costs, financing, payouts, and the all important deal junky bonus to arrive at a preconceived conclusion. “What number do you want to see?” (Mark Haller)
Trans-sonic event: When a wind turbine goes into overspeed and the blade tip exceeds the sound barrier. (Mark Haller)
Windwacko: Promoters of crazy often previously debunked ideas for harnessing the wind. See zombie wind.
Windweenie. Wide-eyed zealots who flocked to the wind industry in the 1980s with hopes of doing something good. Synonym: windjunky.
Zombie wind: Derogatory term dismissing impractical but reoccurring concepts debunked many times, e.g. “flying clotheslines,” shrouded or ducted turbines, rooftop wind, and squirrel cage VAWTs to name a few. These ideas never seem to die and keep rising from the dead, thus, zombie wind. (Robert Hamburger)
Originally published as 1997 Glossary of Wind Energy Terms (Vrinners Hoved, Denmark: Forlaget Vistoft, 1997).
The wind energy vocabulary changes continuously as the technology evolves and as new issues arise to confront designers. Thus, a glossary is not a static work, it too changes, shrinking and growing with the technology it attempts to describe.
A glossary is in part a window on a technology. The development of wind energy requires the use of many divergent professions. As such this glossary includes terms from several different fields, not just those from mechanical engineering, aeronautics, and meteorology. For example selected terms from the fields of biology, geography, aesthetics, and the electric utility industry are also included. Moreover, unlike most technical glossaries that simply define terms, this glossary places the meaning of the term within its wind energy context.
Glossary
1/7 Power law: Empirically derived rule of thumb where the wind shear exponent, α, in the Power law model is equal to 1/7, representing a stable atmosphere above open grasslands typical of America’s Great Plains. See Power law.
A-weighted scale: Weighting network for sound level measurements that selectively discriminates against low and high frequencies to roughly approximate the response of human hearing.
Abandoned wind turbines: In U.S. usage, loosely applied to all wind turbines that stand inoperative for extended periods. Few inoperative wind turbines in California are legally abandoned, though the owners may not have made any substantive efforts to operate them for many years. Some wind turbines in California have stood idle for a decade or more.
Absorption, sound: Interception and attenuation of acoustic energy by use of sound deadening materials, such as insulation and cladding, inside a wind turbine nacelle.
AC: See alternating current.
Accumulator, hydraulic: Device for the storage of hydraulic pressure typically for use in braking systems. The accumulator is a tank divided in two parts: one side contains a fluid acting against a bladder or diaphragm, the other side contains compressed air.
Acoustic noise: Unwanted audible and sub-audible sounds.
Acoustics: Study of sound.
Acuity, visual: Sharpness or keenness of visual perception.
ADEME: French agency for energy and the environment, Agence de l’Environment et de la Maîtrise de l’Energie.
Adjusted availability: Fraction of time a wind turbine is available for operation after eliminating time and outages associated with activities that are non-standard.
Advanced Wind Turbine (AWT): Term adopted by U.S. Department of Energy and the National Renewable Energy Laboratory to designate a series of wind turbines under development during the early 1990s. The designation was expropriated by one of DOE’s grant recipients to designate their product line. The company and the turbine design are both defunct.
AEI: Alternative Energy Institute, West Texas State University in Canyon, Texas. Operated wind turbine test center. Now closed.
AEO: Annual energy output. North American usage for an estimate of the total energy a wind turbine can produce per year under standard conditions.
Aeolian islands: Archipelago in the Tyrrhenian Sea north of Sicily
Aeolus: Greek god of the wind. See also Éole.
Aermotor: Trade name of the most widely recognized multiblade farm windmill built in North America. With its origin (a suburb of Chicago, Illinois) prominently displayed on its tail vane, the Aermotor became known to many outside North America as the “Chicago” mill. Note that the spelling does not include an “o.”
Aero-electric generator: Little used term for wind generator. Compound word with similar derivations as that of hydroelectric generator which is commonly accepted. Technically more correct than many terms currently in vogue as, for example, wind generator.
Aeroelasticity: The study of the effect of aerodynamic forces on elastic bodies, such as wind turbine blades.
Aerogenerator: European expression for wind generator. As in wind generator, a strict interpretation of the term would lead one to conclude that it is a machine which generates air (wind).
Aeroturbine: Similar derivation as aerogenerator.
Aesthetics: Philosophy that seeks to explain beauty. Metaphysical laws of perception. The study of the response to beauty and artistic expression. Perception of what is pleasing to the eye.
AID: Agency for International Development (U.S.)
Ailerons: Diminutive of the French aile for “little” wing. Movable control surfaces found on the outboard trailing edge of wings used for governing the roll of an airplane. Unlike wing flaps, ailerons move opposite one another. As the aileron on one wing moves up, the aileron on the other wing moves down. On airplanes, flaps are inboard of the ailerons and flaps on either side of the fuselage move in the same direction; often down, to create drag for lowering air speed. The word ailerons have also been applied to movable flaps on the outboard trailing edge of a wind turbine blade for limiting power in high winds.
Air brake: Aerodynamic device, such as pivotable tips, spoilers, or parachutes, for slowing the speed of a wind turbine rotor. Flap or spoiler. Mechanism for destroying the aerodynamic performance of a wind turbine blade to limit rotor speed and torque in high winds. See also Flap and Aileron.
Airfoil: A curved surface, such as an airplane wing, designed to create lift. Airfoils for horizontal axis or conventional wind turbines are asymmetrical with the flat side towards the wind. Vertical axis turbines use symmetrical airfoils. Some small wind turbines use airfoils with a single curved surface.
Air density: The mass of air relative to its volume. The density of air at sea level is 1.225 kg/m3. Air density decreases with increasing altitude and temperature.
Air gap: The space between the rotor and stator in a generator. (Hugh Piggott)
Altamont Pass: Low saddle separating California’s Livermore Valley from the San Joaquin Valley about 50 miles (80 kilometers) east of San Francisco. In 1994 the wind resource area ranked between third and fourth in world wind generation. During the mid-1990s, the Altamont Pass generated 0.8-1.1 TWh per year.
Alternating current (ac): An electric current that reverses its direction or polarity cyclically at 60 Hz in the Americas and 50 Hz elsewhere.
Alternator: Generator that produces alternating current.
Aluminum: Lightweight metal adapted unsuccessfully from the aircraft industry for use in wind turbine blades. The soft metal suffered fatigue failures in wind turbine applications and is no longer used in commercial wind turbine blades.
Ambient noise: Background sound level.
American farm windmill: Also called “classic” or “Chicago” windmill. Multiblade water-pumping windmill common across the breadth of the North American continent. The first wind turbine design indigenous to the New World. Multiblade wind pump commonly seen throughout North America. A technology essential for settlement on the American Great Plains, where “the cows chop the wood, and the wind pumps the water.” Known as a “classic” wind pump among those who chafe at its American origins. Still used extensively around the world.
Amp-hour: Measure of energy storage potential in batteries. One amp-hour will deliver one amp of current for one hour at the battery’s voltage. Amp-hours times battery voltage gives watt-hours of stored chemical energy.
Ampere (amp): Unit of electric current equivalent to a flow of one coulomb per second, or to the steady current produced by one volt applied across a resistance of one ohm.
Anemometer: An instrument for measuring and displaying wind speed or velocity. The term is often limited to the wind sensor, but correctly encompasses both the sensor and an indicating device.
Angle of attack: Angle between the chord line of an airfoil and the relative or apparent wind.
Annual energy output (AEO): The estimated total energy that would be produced by a wind turbine annually, assuming a Rayleigh wind speed distribution based upon an annual average wind speed and 100% availability. See AEO.
Apparent wind: Relative wind. The wind seen by a wind turbine blade as it moves through the air. The resultant of the wind vectors acting on a wind turbine blade.
Armature: The current-carrying part of a generator where current is induced. The armature may be moving or stationary. In most wind turbine applications the armature is stationary and current is drawn off the stator.
Array, wind turbine: Orderly grouping or arrangement of multiple wind turbines in relative proximity. Wind turbines are found in rectangular or geometric arrays on flat terrain and in linear arrays along dikes or breakwaters. Wind turbines can also be ordered vertically on towers of different heights in a wind wall.
Array effect: Sum of the interference of one wind turbine on those around it in a multiple turbine array.
Articulating blade: An airfoil on a straight-bladed vertical axis wind turbine that maintains its angle of attack regardless of the position on its path about the rotor axis. Any attachment of blades to their hub or central shaft through a hinge allowing out-of-plane or in plane motions. In a VAWT, an airfoil which changes its angle of attack as the rotor revolves. Blades on Darrieus rotors, in contrast, are fixed in pitch. Articulation implies the coordinated movement of a jointed assembly. For example, the blades on a Giromill or Cycloturbine are mechanically articulated to maximize lift regardless of the position the blade whether it is behind the tower, coming into the wind, or going downwind. This feature provides starting torque that is lacking in the conventional or phi-configuration Darrieus rotor.
Aspect ratio: In a vertical axis wind turbine, the ratio of the rotor’s height to its diameter. In a horizontal axis wind turbine, the ratio of the square of rotor radius divided by the projected area of one blade.
Asynchronous generator: Generator where ac frequency is not exactly proportional to the speed of the generator rotor. The induction generators used in most wind turbines are asynchronous generators because they maintain a constant frequency as rotor speed varies 1-2%. Constant frequency ac is produced by using an interconnection with a host utility to provide magnetization of the induction generator’s field.
Atlantic Wind Test Site: Canadian center for testing wind turbines on Prince Edward Island.
Attenuation, noise: To reduce the intensity of acoustic energy by use of sound deadening materials, baffling, or increase in distance between source and receptor.
Audible: Sounds that can be heard by most people with normal hearing.
Audubon: Short for National Audubon Society. North American environmental group that emphasizes appreciation and protection of birds.
Augmentor: A device that increases the air flow through a wind turbine rotor. Called concentrators on when used on the upwind side of a wind turbine and diffusers when used on the downwind side.
Autonomous system: See stand-alone power systems.
Availability: The quotient, expressed as a percent, of the total number of hours that a wind turbine is available for operation divided by the total number of hours in the period.
Average capacity: The power in kW of a wind turbine derived by multiplying the turbine’s rated power times its capacity factor during a specific period.
Average wind speed: The mean wind speed during a specific period, often one year. One of the most common measures of used to evaluate a wind resource. However, the average wind speed obscures the distribution of winds at various speeds during the period. The average power density in W/m² is a more accurate measure of the power in the wind. See Mean wind speed.
Avian mortality: Biological jargon for birds killed by wind turbines or their ancillary structures.
Avoided cost: Under federal law in the United States the incremental costs to a utility of energy and capacity, which, but for the purchase from qualifying facilities, such utility would generate for itself or purchase from another source. The sum of fixed costs (e.g., power plant, transmission lines) and variable costs (e.g., fuel) avoided by a utility when a decentralized power generator contributes energy to the utility.
AWEA: American Wind Energy Association. Wind industry trade association in the United States. No longer extant. Now part of American Clean Power Association.
AWT: See Advanced Wind Turbine.
Axial thrust: Force on a wind turbine rotor parallel to the windstream.
Azimuth: The angle about a vertical axis between a fixed heading and the direction of the object. Often applied to the orientation of a horizontal axis wind turbine. See also Yaw.
Back gearing: Term used to describe transmission system developed for American farm windmills using gears to reduce the torque needed to lift the pump rod. In contrast to “direct stroke” where the rotor lifts the pump rod for every revolution, back-geared windmills lift the pump rod once for every two-three revolutions.
Baffles, noise: Deflectors used to increase noise path and noise absorption inside a wind turbine nacelle as a means of attenuating acoustic energy.
Balance of station cost: The cost of components in a wind power plant other than the wind turbine, tower, and installation. This includes, but is not limited to conductors, transformers, substations, and interconnection.
Baseload plant: Power plant that provides a constant portion of the demand for electricity throughout the day.
Battelle Pacific Northwest Laboratory: Private contractor to the U.S. Department of Energy located in Richland, Washington specializing in wind resource assessment of the United States. Battelle’s wind resource functions have been assumed by the National Renewable Energy Laboratory’s NWTC.
Batter: Slope or inclination caused by the taper of a wind turbine tower.
Battery storage: The storage of electricity in the form of chemical energy. Batteries are often used in stand-alone wind power systems for providing electricity during periods of little or no wind.
Battery-charging wind turbines: Mostly small wind turbines that generate dc directly or rectified dc for use in stand-alone power systems that include storage batteries.
Beard, of windmill: Ornate plaque beneath main shaft on upwind side of many European smock mills displaying the name of the windmill or a proverb.
Bearingless rotor: A rotor in which the variable pitch blades are attached to the hub without bearings. A variable-pitch rotor which depends on torsion flexure of the blade spar to change blade pitch.
Beaufort number: A arbitrary scale of wind strength or force created by British naval officer Sir Francis Beaufort (1774-1857). The scale uses common examples of the wind at work, such as the fluttering of flags or the movement of trees, to describe wind strength on a scale from 0 for calm to 12 for hurricane force winds (74 mph). Also called Beaufort force.
Bedplate: Frame. A base-plate on which generator, gearing and other components of a wind turbine are fastened. Frame of a wind turbine’s nacelle which supports the rotor and other components. Also called a strongback.
Bending stress: Stress created when a load causes a cantilevered beam to flex or bend. Wind thrust on a rotor blade causes bending stresses to occur at the juncture between the blade’s root shaft and the hub.
Betz, Albert: German aerodynamicist of Gottingen, who found that an ideal wind machine could extract 16/27 (0.593) of the total power in the wind. Consequently the Betz limit is the maximum efficiency theoretically obtainable from a wind turbine.
Betz limit: The maximum efficiency theoretically obtainable from a wind turbine rotor: 16/27 of the total kinetic energy within a given capture area and a given wind speed.
Bicycle wheel turbine: Developed in the United States during the 1970s, a multiblade wind turbine that used many slender airfoils fixed between a metal ring (wheel) and a hub resembling a bicycle wheel.
Billion: 109 (US), 1012 (UK). Thousand million (UK).
Bin: Wind speed interval used for grouping wind speed data in the Method of Bins technique of wind measurement. The bins can be any width but are often 1 mph or 1 m/s, for example the 0-1 mph bin contains the occurrence of wind speeds from 0 to less than 1 mph.
Bin width: The size of a wind speed interval used in the Method of Bins (e.g., a bin having a span from 10 mph to 11 mph has a width of 1 mph).
Blade: The primary aerodynamic surface driving a wind turbine rotor. In a horizontal axis wind turbine, blades are airfoils somewhat like the airfoils on aircraft propellors. In vertical-axis machines, blades may be vanes or buckets as in Savonius rotors or airfoil sections as in Darrieus rotors. Wing in Danish usage (from Ving).
Blade activated governor: Blade pitch control mechanism used on some 1930’s era windchargers where centrifugal force acting on the blades themselves, rather than on separate weights in a traditional Watt governor, change the pitch of the blades.
Blade area: Product of blade area per blade times the number of blades in the rotor.
Blade materials: Blade materials include the surface skin or layer, the core or body of the blade, and the spar or structural element.
Blade number: The number of blades in a wind turbine rotor. Only one blade is needed. Most electricity generating wind turbines use two or three blades. Traditional European windmills used four blades. Most mechanical wind pumps use multiblade rotors because of the high torque required to lift water.
Blade pitch: Blade angle. Angle between the chord line and the direction of motion.
Blade planform: Blade shape including taper and twist. Plan view of a rotor blade as if looking down on a blade lying flat on the ground. Most blades on conventional wind turbines taper from hub to tip. Blades on vertical axis wind turbines and on some conventional machines, especially those with extruded or pulltruded blades, have a constant planform.
Blade root: Portion of a blade nearest the hub or, of a wing, nearest the fuselage. Root shaft is a shaft connecting the blade to the hub. The section of a blade closest to the hub.
Blade skin: Outer covering or surface of a blade.
Blade station: Airfoil section of a blade at a specified location along its quarter cord line.
Blade tip: The portion of the blade farthest from the hub.
Blade twist: The difference between the blade angle at the root and at the tip of a blade. Because the speed of the wind flowing over the blades varies continuously from root to tip (the tips travel much faster than the root) there is continuous variation in optimum blade angle. The blades on Denmark’s Gedser mill were twisted from 13° degrees at the hub and 0° degree at the tip.
BLM: Bureau of Land Management (U.S.). Federal agency administering public lands in the Western U.S. The BLM receives royalties from wind projects in the San Gorgonio and Tehachapi passes.
BMFT: German ministry for technology development, Bundesministerium Für Forschung und Technologie. Original sponsor of the “250 MW” subsidy program now administered by Bundesministerium für Bildung, Wissenchaft, Forschung und Technologie (BMBF).
Bonneville Power Administration: Public power marketing agency serving the Pacific Northwest of the United States.
Booster mill: Water-pumping farm windmill added in line with another to move water higher or further than that provided by a single farm windmill alone. Similar to Dutch windmill “gang” for successively lifting water out of deep polders.
Bora: Gravity squall caused by artic air descending from the mountains of the former Yugoslavia into the Adriatic Sea.
Bottom-up technology development: Description applied by Danish scholar Peter Karnøe to a style of technology development characterized by myriads of decisions and actions resulting in a gradual accretion of technology as in Danish wind development. Contrasts with what Karnøe calls “top-down” development characterized by central decision making and the search for technological “breakthroughs” as in the U.S. Department of Energy’s wind program of the 1970s.
Boundary layer: The region between a fluid (air) that is moving relative to a surface where the fluid velocity is neither that of the fluid nor the surface, but between the two.
BPA: See Bonneville Power Administration.
Brake switch: Device used to safely short circuit the conductors of a permanent-magnet alternators on small wind turbines. The load thus created can often stall the airfoils and bring the rotor to a halt. Best used when the rotor as been furled out of the wind. Not always a reliable means to stop the rotor in strong winds.
Breakwater: Civil engineering structure for sheltering a harbor. Many breakwaters are suitable for supporting wind turbines. The first “offshore” wind plant was built atop the harbor breakwater at Zeebrugge, Belgium.
British thermal unit (Btu): A measurement of heat content. The amount of energy needed to raise the temperature of 1 pound of water 1 degree Fahrenheit (F).
Broadband noise: Noise across the entire frequency spectrum.
Brookhaven National Laboratory: Federal research center on Long Island, New York that specializes in the health effects from the development and use of various energy sources.
Brushing: Deformation of the branches of trees and shrubs in areas of high winds where the “branches are bent to leeward like the hair in a pelt which has been brushed one way.” Brushing is the most sensitive vegetational indicator of wind speeds and direction. Flagging and throwing indicate progressively greater wind speeds than those that cause brushing. (Putnam)
Brush windmill: In 1888 the successful manufacturer of arc lighting systems, Charles Brush, built a 56-foot (17-meter) diameter multiblade wind turbine for charging batteries at his Cleveland, Ohio estate. Brush’s 12 kW wind-powered generating system rivaled that of banking magnate J.P. Morgan in his New York mansion which operated on fossil fuel. The Brush windmill was the first American adaptation of wind energy to the generation of electricity and occurred at about the same time as Poul la Cour was beginning his experiments in Denmark.
Buffalo Ridge: Low-lying topographic feature trending southeastward from Watertown, South Dakota across southwestern Minnesota to Spirit Lake, Iowa. There is sufficient wind resources along the Minnesota portion of Buffalo Ridge to supply 60% of the state’s 1990 electricity consumption.
Bull pin: Forged riggers tool with a tapered shaft on end and a open-end wrench on the other. Tapered end is used to align bolt holes in metal tower parts.
Bull rope: Heavy rope used for temporary tower guys or raising and lowering heavy equipment from a tower.
BuRec: Bureau of Reclamation (U.S.). Federal land management agency in the Western U.S. responsible for the construction of major reservoirs in the Rocky Mountains. These reservoirs have often been eyed by federal planners for storage of wind-generated electricity. Constructors of Hamilton Standard’s il-fated WTS4, a 4 MW behemoth that stood derelict near Medicine Bow, Montana for many years.
Busbar costs: The cost of electricity at the generating plant. Busbar costs exclude the costs of transmission and distribution.
Buyback rate: The rate per kilowatt-hour a utility is willing to pay for excess energy fed into its lines by a small (relative to the grid) generator. In North America the buyback rate is usually lower than the retail rate charged by the utility for the power it sells. In Germany the buyback rate or tariff is set by the “electricity feed law” at 90% of the retail rate. In Denmark the buyback rate is set at 80% of the retail tariff.
BWEA: British Wind Energy Association. Trade association in the United Kingdom. No longer extant.
Calm: Period of no wind.
Camber: Measure of the curve in an airfoil section as the ratio of the distance from the peak of the curve to the chord line and the chord length.
Campbell plot: Graphic representation of a wind turbine’s natural frequencies as a function of rotor speed.
Cantilever: A beam, pipe, or other structure supported or anchored at only one end. An unguyed, self-standing, pipe or tubular tower is cantilevered. All modern horizontal axis wind turbines use cantilevered blades. The blades are attached to the rotor at only one point, the blade root.
Cantilevered blade: Blade fixed only at one end. Self-supporting.
Cantilevered towers: Truss or tubular towers not dependent on guy cables for remaining upright.
CanWEA: Canadian Wind Energy Association. Canadian trade group. No longer extant.
Capacitors: Device that stores an electrical charge. Capacitors or condensers are used to boost the power factor of induction generators. Capacitors for wind turbine applications are often sized to compensate for the generators volt-ampere-reactance at no-load.
Capacity, generating: Rated or peak power of a wind turbine or the sum of the rated or peak power of all wind turbines in an array in watts.
Capacity credit: The amount of power that an intermittent resource such as a wind machine can be depended upon to provide. Contrary to popular belief, a statistically determinable portion of a wind generator’s capacity can be relied upon to provide power when needed. This portion of a wind turbine’s capacity can offset an equal amount of conventional generating capacity. Wind turbines, therefore not only displace fuel, they also displace some capacity. For most temperate wind regimes, the capacity credit is similar to the capacity factor.
Capacity factor: Measure of productivity. The quotient of the actual energy generated to that possible if the generator had operated at its rated capacity (power) over the time interval of interest, most often that of one year (8,760 hours). The capacity factor of wind turbines is dependent upon reliability, performance, wind regime, and the rated power of the wind turbine.
Capital intensive: Technologies in which a large portion of total costs of production are associated with the cost of the equipment rather than with the costs of operation. Also used to differentiate from technologies which are labor-intensive or fuel intensive. Like nuclear power, wind energy is capital intensive.
Capture area: The area projected or swept by a wind turbine rotor. Also known as “reference area” or “swept area.”
Carbon dioxide: Product of fossil-fuel combustion. Non-regulated pollutant. Each kilowatt-hour of wind generation offsets the emission of approximately 1 pound (0.5 kilogram) of carbon dioxide from a natural gas-fired power plant, or 2 pounds (1 kilogram) of carbon dioxide from a coal-fired power plant.
Carbon-fiber reinforcement: Use of carbon fibers instead of glass fibers or wood to provide strength in composite materials.
Cascade effect: See slot effect.
Case-hardened gears: Surface hardening of gear sets by the shallow infusion of carbon at high temperatures and subsequent quenching. Used to extend gear life.
CAT: Centre for Alternative Technology (Wales). 60-90,000 tourists visit the exhibits of alternative energy at CAT every year.
Cathodic protection: of pipelines. During the 1930s, small windchargers were used in North America to provide a dc charge to the surface of metal pipelines. This charge offset galvanic corrosion between the metal pipe and certain soils.
CEC: California Energy Commission, Sacramento. The world’s second largest energy agency after the U.S. Department of Energy. Responsible for siting power plants.
CEGB: Central Electricity Generating Board. The former national utility serving England and Wales.
Cemaes: Village on the west coast of the Isle of Anglesey and site of the Wylfa nuclear power station. Cemaes is also the village nearest the Rhyd-y-Groes wind power plant.
Cemmaes: Village in Wales’ Dyfi Valley below Mynydd-y-Cemais plateau. See Mynydd-y-Cemais.
Centerline height: Of rotor. Distance between the ground surface and the horizontal axis of a conventional wind turbine rotor.
Central-station power plant: As opposed to dispersed sources of electricity generation, one facility produces large quantities of electricity for an entire region.
Centrally-directed research: Mechanism by which top-down technology development proceeds. Danish academic Peter Karnøe attributes the failure of wind energy research and development in the U.S. and Germany during the early 1980s because it was centrally directed.
Centre for Alternative Technology: See CAT.
Cervantes: Author of “Don Quixote,” a novel about a delusional noble who tilts (jousts) at windmills on Spain’s plains of La Mancha .
CFRE: Carbon fiber reinforced epoxy.
CFRP: Carbon fiber reinforced polyester.
Charge controller: See Regulator, voltage.
Chicago style: Multiblade mechanical wind pump used throughout the world. More than 1 million are still in use. Also known as the “classic” American water-pumping windmill, though the design has been widely copied by manufacturers in several countries. Called “Chicago” because of the dozens of factories that fabricated the familiar design in the late nineteenth century on the southern borders of Lake Michigan near Chicago. See American farm windmill.
Chinook: Warm, dry, powerful wind descending the eastern slopes of North America’s Rocky Mountains.
Chord: Distance from the leading to trailing edge of an airfoil. A specific description of blade width in blades using airfoils.
Chord line: Straight line connecting leading and trailing edge of an airfoil.
Clap-sail windmill: Translation of the Danish klapsejlsm¢lle. The type of windmill used by Poul la Cour in his experiments using the wind to generate electricity. Clap-sail is the Germanic description of the rotor control developed in England by Andrew Meikle in 1772. William Cubitt patented an improvement the Meikle’s design in 1807. In England clap-sails are known as “patent sails” as a result. Clap-sails work like Venetian blinds or jalousie shutters.
Cloth sails: Traditional European or Dutch windmills used blades made from sail cloth draped over a wooden framework. Mediterranean windmills, including those of Spain, Portugal, and Greece, use sail cloth stretched from one spar to the much like a jib-rigged sail. Princeton University experimented with a two-bladed, jib-rigged sail rotor during the 1970s.
Clusters: Of wind turbines. A group of turbines in the same vicinity.
Clutter, visual: Jumble or juxtaposition of multiple wind turbines of different types and sizes within a viewshed.
Co-ops: See Cooperatives.
COE: Cost of energy. Measure useful for comparing energy technologies of like characteristics. Commonly misused to compare the costs of fossil fuels to renewable resources because of the COE’s inability to reflect future fuel-price and environmental risk.
Coefficient of performance: Cp. The quotient of the power extracted by a wind turbine to the power in the wind. A measure of the rotor’s aerodynamic efficiency. Rotors using modern airfoils operating at their design tip speed are capable of 0.35-0.45 Cp.
Cogging: Variation in speed of a generator due to variations in magnetic flux as rotor poles pass stator poles. Cogging in permanent magnet alternators can hinder the start-up of small wind turbines at low wind speeds.
Collective pitch: Changing pitch of all blades in a rotor simultaneously by the same amount in the same direction through a common linkage.
Columbia River Gorge: Deeply incised east-west valley of the Columbia River between the states of Washington and Oregon that funnel prevailing winds through the Cascade Range. During the 1970s the U.S. DOE installed three experimental Boeing Mod-2 wind turbines on a plateau overlooking the gorge near Goldendale, Washington. Though the turbines were removed in the 1980s, the area is again the scene of wind development.
Commutator: Segmented copper cylinder in a dc generator or motor which, in combination with brushes, provides an electrical connection between the coils of the rotating armature and stationary terminals.
Compliant drive train: Torsionally flexible drive train which absorbs peak torque loads by angular displacement of the gearbox or couplings.
Composite materials: Made up of more than one material. A composite blade may be made from a metal leading edge and blade spar with a fiberglass trailing edge. See fiberglass and wood composites
Concentrator: Device that concentrates the windstream striking a wind turbine rotor. See Augmentor. Concentrators have proven too cumbersome and costly for wind turbine applications. The same benefit provided by a concentrator can be obtained less expensively by extending the rotor swept area of a conventional wind turbine.
Conductors: Metal cables used to transmit electricity.
Conduit: Metal or plastic pipe for protecting conductors from physical damage.
Configuration, wind turbine: Type and number of blades, their location with respect to the tower, and the axis of rotation of a wind turbine rotor.
Coning: Sweeping the longitudinal axis of the blades on a rotor of a conventional wind turbine downwind. Coning permits both the shedding of some bending loads and, in downwind turbines, yawing without the need for a tail vane or mechanical yaw system. Coning is most accentuated on downwind turbines, but is also apparent on some upwind turbines. Coning is shallower on upwind turbines than on downwind turbines because of the risk that the blades will strike the tower.
Coning angle: The angle between a vertical plane and the blade axis on a conventional wind turbine. This description is from the appearance of the rotor on most downwind machines which take the shape a shallow cone swept by the rotor with its apex at the hub.
Conspicuousness, visual: A measure of how prominent or easily observed a wind turbine or other object is on the landscape. Conspicuousness is not necessarily a measure of visual impact. An object that is visible on the landscape may not necessarily elicit objection.
Constant cost of energy: Cost excluding the effects of inflation. Also known as “real” cost of energy. Cost in constant currency is less than the cost in nominal currency.
Constant speed operation: Asynchronous or induction generators operate at a relatively constant speed in comparison to true variable speed generators. Asynchronous generators operate within a few percent of their nominal speed, which is governed by the frequency of the utility line.
Constant velocity test: Wind turbine test in which a steady airflow is maintained either in a wind tunnel or by moving the wind turbine relative to the ground. During the early 1980s DOE’s Rocky Flats test center measured the performance of small wind turbines by attaching them to a railroad car and driving them along a track with a diesel locomotive. An obsolete form of testing.
Consumables: Supplies that are consumed during the normal operation and maintenance of a wind turbine (e.g., hydraulic fluid, lubricants, filters, etc.).
Contactors: Electrical relays used for making “contact” between a wind generator and the utility grid or network.
Contrarotating: Wind turbine using dual rotors on the same shaft, one rotating clockwise, the other counterclockwise. The early German wind turbine NOAH used contrarotating rotors.
Control system: Wind turbine system that monitors the condition of the turbine and its environment. Depending on these conditions, the control system adjusts the operation of the wind turbine to protect it from damage or to optimize its performance.
Converter: See Inverter.
Cooperatives: Form of mutual ownership. Used frequently in Denmark, Germany, and the Netherlands for the purchase and operation of wind turbines.
Cornwall: County encompassing the windswept southwestern peninsula of England. Britain’s first wind power plant was built in Cornwall near Camelford.
Cost of Energy: The levelized cost of generating electricity during the life of a wind turbine or wind plant, including the installed cost, the cost of equity and debt, the cost of operations and maintenance, and the cost of fuel.
Cost of service: The cost to a utility for providing the generating capacity, distribution lines, and transformers. Usually expressed as a fixed charge in a utility bill regardless of the amount of power consumed. The cost of service must be recovered even if the customer uses no electricity at all, because the utility maintains the facilities whether used or not.
Counterweight: On one-bladed turbines, the mass used to counterbalance both the mass and the thrust on the single blade.
Crane pads: Graded areas near a wind turbine foundation common in California used for parking a crane in hilly terrain. Frequently access roads are used in place of separate crane pads.
Cretan sail: Identified with the island of Crete though common throughout the Mediterranean, these jib sails use wooden poles for spars from which sail cloth is strung. Pictured in the Lasitti Valley–“the valley of 10,000 windmills.”
Cross-arm of H-rotor: The structural member of an H-bladed vertical axis wind turbine that supports the blades.
Crosswind: Across the direction of the windstream.
Cube factor: The quotient of the cube root of the mean cube of the wind speeds for a speed distribution divided by the mean wind speed. Equal to the cube root of the energy pattern factor. See also Energy pattern factor. (E.W. Golding)
Cube law: The power available in the wind increases with the cube of wind speed. Doubling the wind speed increases the power in the wind eight times, 23 or 2x2x2x=8.
Cuffs: Highly tapered blade section near the hub. Results from the Glauert formula for the ideal rotor blade.
Cup anemometer: A vertical axis drag device for measuring wind speed that uses cups mounted on radial arms. The most widely used form of wind sensor.
Cupid: Shooting an arrow through the rotor with a rope attached in an attempt to bring a runaway wind turbine to a stop. Used in the Altamont, Tehachapi, and San Gorgonio passes. Historically used in Denmark on Lykkegaard wind turbines of the 1920s. (This technique is mentioned on page 420 of Wind Energy for the Rest of Us.)
Current: Flow of electricity measured in amperes.
Curtailment: In California usage, the limitation or suspension of power purchases by a utility under terms of a power purchase contract. Reduction or termination of generation from a wind turbine or a wind plant by a planned or unplanned outage, or by extraordinary operating activities such as training, tours, or excess generation.
Customer class: In the United States the division of utility customers by rate schedule, e.g., R-1 Residential.
Cut-and-fill slopes: Road cut into a hill side in steep terrain. Material from the cut slope is pushed or dumped onto the down hill side, the fill slope.
Cut-in wind speed: Wind speed at which a wind turbine begins to produce power. Not synonymous with “start-up speed,” except for wind turbines that motor the rotor to operating speed, or wind turbines that drive permanent magnet alternators. See Start-up speed. Most analysts agree that lowering the cut-in speed from the common 4.5 m/s (7-9 mph) contributes little to total energy generation.
Cut-out wind speed: Wind speed at which the wind turbine ceases generating electricity. Most medium-sized wind turbines have a cut-out speed of 30 m/s or 65 mph. Some medium-sized wind turbines have no cut-out speed. Many small wind turbines have no cut-out speed, but do furl or otherwise control the rotor instead. See Furling wind speed.
Cyclic pitch: Periodic change in blade pitch per rotor revolution of articulating, straight blades on vertical axis wind turbines.
Cycloturbine: See Giromill.
D-ring: Large metal rings in the shape of the letter “D” on work belts used for the attachment of lanyards.
Darrieus rotor: Sleek vertical axis wind turbine developed by French inventor G.J.M. Darrieus in 1929. The (phi) φ-configuration Darrieus rotor is often referred to as an “eggbeater” in North America because of its hoop shape. Darrieus wind turbines represent about 5% of the installed wind capacity in California.
Data loggers: Recording instruments that store data electronically. Loggers are used most often to store wind data at remote sites.
dB: Decibel. Logarithmic scale used to describe the difference in power or intensity between two levels in electronics and acoustics. Defined as 10 times the common logarithm of the quotient of the two levels, that is, 10 log (P1/P2). In acoustics, the faintest audible sound, the threshold of hearing, is defined as 0 dB. The threshold of pain is 120 dB.
DC: See Direct current.
Deal junky: Project developers or management keen on getting a substantial bonus for closing the “deal.” Deal junkies had a penchant for ignoring warnings from meteorologist’s about low wind speeds, or from planners about the proximity of neighbors, or environmental impacts. Creators of “magic spreadsheets.” (Mark Haller)
Decentralized systems: In wind systems, refers to establishment of autonomous units, such as households or neighborhoods, to provide electricity or heat.
Decibel. See dB.
Declared net capacity: British usage for the equivalent amount of conventional base-load capacity wind energy will offset. For example, if British nuclear plants operate at 70% capacity factor and wind turbines operate at 30% capacity factor, 1 MW DNC of wind plants is equal to 2.3 MW of installed wind capacity.
Decommissioning: Relative to wind energy, at a minimum refers to the removal of wind turbines and their towers. In some jurisdictions decommissioning also requires removal of all or a portion of the foundation and ancillary facilities.
DEFU: Research association of Danish utilities, Danske Elværker Forenings Udredningsafdeling.
Delta connection: A three-phase connection where each phase is connected between two supply conductors. (Hugh Piggott)
Delta-3 blade hinge: Blade-to-rotor hub coupling that permits movement in and out of plane and about the pitch axis. On one- and two-bladed teetered rotors, the δ3 hinge allows the blade to move out of plane as gusts strike the blade, as it does so the blade motion changes pitch slightly reducing the blade’s performance during the gust.
Demand, for power: The amount of power required to satisfy the needs of a stated sector of the economy.
Demand charge: Charge by North American utilities based on the ratio of peak demand to total power consumed. This charge is part of rate schedules for commercial and industrial customers. The demand charge compensates the utility for maintaining sufficient spinning reserve to meet a customer’s instantaneous demand.
Density altitude: Aeronautical term for the combined change in air density with altitude and temperature.
Derelict wind turbines: Generic term for inoperative, abandoned, or wind turbines that are deliberately taken out of service. Wind turbines that have been idle for an extended period regardless of wind speeds within the normal operating range.
Design Speed: Survival speed. The wind speed a wind turbine is designed to withstand without suffering catastrophic failure. Often 55 m/s (120 mph) or greater.
Design tip speed ratio: Tip speed ratio for constant speed wind turbines at rated power.
DEWI: German wind energy institute, Deutches Windenergie Institut. Research laboratory in Wilhemshaven supported by the länder of Niedersachsen (lower Saxony).
Diameter, rotor: Diameter of disk swept by a conventional horizontal axis wind turbine. The single most important descriptor of conventional wind turbine size. Shorthand for the area swept by a conventional wind turbine. In phi-configuration Darrieus rotors, the width of the rotor at its equator. In H-rotors, the length of the rotor cross arm.
Differential billing: A method by which a utility can bill customers who provide a portion of their own electrical needs, e.g., with a wind generator. Two meters are used; one measures power consumed by the customer, the other measures excess power fed back into the utility’s line. In this way, the utility can recover the cost of service and other charges by paying a lower rate for the power they buy from the customer than the rate they charge for power.
Diffuser: Downwind device that diffuses the windstream through a rotor. See Augmentor.
Dike: Linear landscape feature common on the North German Plain that border polders. Wind turbines have been installed on or alongside dikes in the Netherlands and Germany.
Diodes: An electronic check valve that allows alternating current to pass in only one direction. Diodes are used in automotive alternators to produce direct current for charging the battery. Diodes are also used for the same purpose in small wind turbines that use alternators.
Direct current (dc): Electric current that flows in one direction. See also alternating current.
Direct-drive generator: Purpose built generator that allows the low rotor speeds common in wind turbines to produce electricity without the use of a gearbox or transmission to increase rotor speed to that required to drive mass-produced generators.
Dirt nap: Colloquial expression among wind techs for a wind turbine destroying itself as in “The wind turbine was taking a nap in the dirt.” Term from the early days in the Altamont Pass when the turbines sometimes pulled up their foundations and tipped over. (Mark Haller)
Disconnect switches: Manually operated, lockable switch often located at the base of the wind turbine tower, which allows de-energizing the entire wind turbine circuit to isolate the wind turbine from the utility network and to facilitate safe maintenance of the wind turbine’s electrical components.
Discount rate: In the U.S., the interest rate charged by the Federal Reserve for loans to its member banks.
Diseconomies-of-scale: Increasing costs with increasing size. For example, above a certain size large wind turbines cost more to service than medium-sized wind turbines because of the specialized cranes necessary.
Disincentives: Something that deters an action. The initial cost of wind turbines is a disincentive to their use even though wind turbines have no fuel costs.
Disorder, visual: In reference to wind turbines, the confusing jumble of wind turbines of different types on towers of different types and differing heights in concentrated arrays.
Dispersed arrays: Arbitrarily, concentrations of wind turbines where the relative spacing across rows is greater than four diameters that between rows is greater than six diameters. In the dense arrays common in California, spacing of two diameters across the wind by six diameter downwind is not uncommon.
Distributed generation: Single or small clusters of wind turbines disseminated across the landscape in contrast to the concentration of wind turbines in large arrays or wind power plants.
Disturbance, noise: Annoyance caused by noise. A lesser impact than where noise interferes with some activity, such as sleep or communication.
Disturbance, visual: Annoyance caused by the intrusion of wind turbines on the landscape.
Diurnal variation of wind speed: Changes of wind speed between night and day.
Diversified load: A mix of different types of power-consuming devices. In residential use, various electrical appliances as opposed to space heating or water heating loads.
Diversity, of generating sources: A mix of technologies for the generation of electricity. As in the ecologists credo “in diversity, is stability,” electrical networks using a mix of generating resources are more stable than those dependent upon one plant or one fuel source.
DIY: British usage for Do it Yourself. Often applied to plans for home-built windchargers.
DOE: Department of Energy (U.S.).
Down time: Period when a wind turbine is not available for generation due to maintenance, repairs, or other causes.
Down tower: Usage by defunct Kenetech Windpower (U.S. Windpower) for repairs requiring removal of the entire nacelle from the tower.
Downwind: Lee. The side away from the wind.
Downwind, rotor configuration: A horizontal axis wind turbine in which the rotor is oriented on the lee side of the tower. Downwind rotors employ prominent coning.
Drag: Resistance of a body moving through a fluid due to friction between the surface of the body and the fluid. A force that acts to retard the movement of a wind turbine blade through the air.
Drag device: One of two major classes of wind machines: lift devices, and drag devices. Drag devices extract less energy from the wind than lift devices, and are much more material-intensive. The blades of drag devices move at a speed slower than that of the wind. See Lift devices.
Drive train: Portion of a wind turbine that transmits torque from the rotor to the generator.
Droop cable: Means for conducting electricity from the generator of a medium-sized wind turbine to ground level through the use of extra long conductors that are allowed to sag or hang down from the nacelle. The sag or droop in the cables permits the nacelle to yaw several revolutions before the conductors become twisted. This obviates the need for slip rings capable of transmitting high current.
Drop pipe: A galvanized pipe with a smooth interior in which the pump rod of a mechanical water-pumping windmill moves.
DSM: Demand side management.
DTI: Department of Trade and Industry (U.K.).
Dual-speed generator: Induction generator with dual windings. More specifically, an induction generator which excites the windings of six poles during low-power operation at which the synchronous speed is 1,000 rpm in Europe (1,200 in the Americas). For full-power operation, only four poles are energized at which the synchronous speed is 1,500 rpm in Europe (1,800 rpm in the Americas).
Dual-speed operation: By using dual generators or dual-speed generators, constant speed wind turbines can operate at two different speeds.
Dual-wound generator: See Dual-speed generator.
Ducted rotors: Wind turbine with a shroud to concentrate or diffuse the wind stream striking the rotor. See Concentrator.
Dump load: Electrical load lowest in priority in a series of loads supplied by a wind generator, a load used to absorb excess generation, a load used to maintain a desired voltage or frequency in an isolated network.
Dust, fugitive: Regulatory term in the U.S. for dust from road traffic and construction activities that crosses property boundaries. Fugitive dust can impair wind turbine operation by coating airfoil surfaces in arid regions, such as southern California, and can incur sanctions and fines against wind companies for use of unpaved roads.
DWEA: Desert Wind Energy Association. Trade group representing wind companies in the San Gorgonio Pass near Palm Springs, California.
Dwell time: The number of samples in a bin divided by the sampling rate.
Dynamic inducer: Airfoil transverse to the tip of a wind turbine blade used to induce airflow over the tip. Developed as an alternative to shrouded or ducted concentrators designed to improve energy capture. Like other forms of concentrators, it is usually less expensive and more reliable to simple extend the length of the rotor blade.
Dynamics: The branch of mechanics that examines forces and their effect on an moving object. In wind energy refers to the interaction between forces on a rotor blade or other wind turbine components and their subsequent motion.
Dynamo: British usage for dc generator.
Éole: Greek god of the wind. Also the sobriquet for the largest φ-configuration Darrieus turbine ever built.
Earthing: British for grounding. See Grounding.
Easements, noise: Right-of-way registered with local government that grants a source of noise to emit noise of a certain level, usually in excess of local ordinances, by the property owner.
Ebeltoft: Ferry terminal on the east coast of Denmark’s Jutland Peninsula where 16 turbines installed on the harbor breakwater became the world’s second “offshore” wind plant. Revenues from the municipally-owned turbines were used to pay for harbor improvements
EC: European Community (now the European Union).
Eclipse windmill: Known as the “railroad mill” during the late nineteenth century on the American Great Plains.
ECN: Energieonderzoek Centrum Nederland. The Netherlands energy research foundation. Principal government center for wind energy research in the Netherlands.
Economies-of-scale: Reduction in cost derived from increases in size, a phenomena originally associated with the increasing size of thermal power plants.
Eddies: Vortices or turbulence in a fluid acting contrary to the main current, often in the lee of an object, such as the vortices in lee of a rock in a mountain stream.
EDF: Electricité de France, French national electric utility.
Effective capacity: Dependable generating capacity calculated from the probability that generation would be available when needed.
Efficiency: Quotient of output divided by input.
Efficiency, conversion: Quotient of net output of a conversion device divided by the gross input required to produce the output.
Eggbeater: Colloquial term for φ-configuration Darrieus rotor. See Darrieus wind turbine.
Eigen frequency: Natural frequency. The frequencies at which an oscillating system will vibrate for a constant amplitude, a critical design parameter for wind turbines rotors, drive trains, and towers. The Eigen frequency of wind turbines with stiff towers is greater than the Eigen frequency of the rotor. The Eigen frequency of soft towers is below that of the Eigen frequency of the rotor. To avoid destructive oscillations on a weakly dampened structure, a wind turbine rotor must avoid operating at the Eigen frequency of the tower for any extended period. (Klaus Kaiser, TU-Berlin)
Electrification, rural: Providing a regular supply of electricity to rural residents. Previously implied the extension of central-station power to rural areas by the construction of new power lines. Today, the meaning includes the use of stand-alone or independent power systems.
Electromagnets: Magnets where the magnetism is provided by the passage of electrical current. Unlike permanent magnets, electromagnets can be controlled by controlling the current.
Electrocution, occupational hazard: Death caused by the passage of electricity through the body. An occupational hazard of working with wind-generated electricity.
Electrocution, of birds: A major cause of death among large birds of prey, especially raptors, when the bird comes into contact with a conductor and ground or two conductors of a high-voltage transmission line. Because birds of prey perch on tall objects, they frequently land on transmission poles or towers where they come in contact with the energized conductors.
Electrolysis: The use of an electric current to split water into its constituent molecules of hydrogen and oxygen. Frequently mentioned as means for using wind-generated electricity to produce hydrogen as a transportation fuel.
Electrolyte: A chemical compound that conducts the flow of electrons.
Electromagnetic interference (EMI): The disruption of telecommunications by the magnetic field induced by the passage of electrons in a conductor.
ELKRAFT: Electricity generation and transmission utility once serving the eastern half of Denmark, including the island of Zealand.
ELSAM: Electricity generation and transmission utility once serving Denmark’s Jutland peninsula.
Emissions, air pollutants: Relative to wind energy, the by-products of the mining, processing, transport, and consumption of fuels for the generation of electricity. The only emissions produced in the fuel cycle of wind-generated electricity are those from the processing of the raw materials used in the construction of the wind turbine and its ancillary structures and that of fugitive dust from traffic on service roads in arid regions. See Fugitive dust.
Emissions, noise: The creation of audible and sub-audible vibrations by a wind turbine and its components, especially that of the rotor and the drive train. See Sound Power Level.
Emissions offset: Relative to wind energy, the pollutants that would have otherwise been emitted by a fossil-fired power plant, and the mining, processing, and transport of its fuel.
Endangered Species Act: U.S. law that prohibits “taking,” that is killing, any species that is determined to be in danger of extinction. Often abbreviated as ESA.
Endesa: Empresa National de Electricidad, S.A. Spanish electric utility.
ENEL: Italian national electric utility.
Energy density: The ratio of energy per unit of the fuel’s mass. The ratio of energy stored in a battery per unit of the battery’s mass, often used to compare the abilities of batteries to store energy relative to the space they occupy.
Energy pattern factor: The quotient of the total energy available in the wind divided by the energy in the wind from the cube of the mean wind speed. (E.W. Golding)
Energy rose: A diagram which presents the direction and energy content of the wind from each point on the compass.
Energy: The amount of work done over a given period of time. Wind energy is usually expressed in mechanical terms as horsepower-hours or electrically in kilowatt-hours (kWh).
Enfield-Andreau: Experimental 24-meter diameter, 100 kW turbine designed by French inventor Andreau. Enfield Cables installed the prototype at St. Albans near London 1n 1957. The hollow rotor used centrifugal force to pull air through a turbine mounted vertically in the tower. The prototype was removed and re-installed on Grand Vent in Algeria, then a French colony, by Electricité et Gaz d’Algérie.
Environmental costs: Monetized and non-monetized costs due to the environmental impact of a technology. Environmental costs are often non-monetized. For example, the costs associated with the climatic impact of global warming from carbon dioxide emissions of fossil fuels is not reflected in the price of fossil-fuel derived energy. See also Social costs, and Externalities.
Environmental indicators: Plants or surface features that are associated with high wind speeds. Wind-flagged vegetation or wind-thrown trees are one environmental indicator of high winds, another is the shape, location, and movement of sand dunes.
EPA: Environmental Protection Agency (U.S.). Federal agency responsible for regulating air, water, and land pollution.
EPF: Energy pattern factor.
EPRI: Electric Power Research Institute.
EPRI TAG model: Shorthand for the Electric Power Research Institute’s Technical Assessment Guide for determining the comparative economics of different electricity generating technologies.
Equipment pod: Nacelle. Gondola. Enclosure at the top of the tower on a horizontal axis wind turbine housing the generator and transmission.
Equity: The non-debt portion of financing. The holders of equity in an investment assume all investment risk for the potential of collecting all the profit.
Erosion: The process by which soil is removed and transported from one location to another through the abrasive action of wind or running water. Though erosion is a natural process, accelerated erosion from human activities causes water and air pollution, denudes land of topsoil, and alters the hydrology of stream courses, leading to flooding. Erosion due to improper wind development on steep terrain is a problem in California and Spain.
Erosion control: Techniques and structures used to minimize human-induced erosion. The principal erosion control practices for wind development is to minimize soil disturbance by minimizing roads and avoiding construction on steep slopes.
Escalation rate: A number which defines the annual increase in monetary value of a specified quantity.
ETSU: Energy Technology Support Unit at the U.K.’s Harwell Laboratory. Once the principal government research center on wind energy in the U.K.
EU: European Union (formerly the European Community).
EWEA: European Wind Energy Association. Trade association for members of the European Union. No longer extant.
EWG: In U.S. parlance, Exempt Wholesale Generator under the National Energy Policy Act of 1992.
Exceedance levels: Noise. The percentage of the time that noise exceeds the given level. The level, L90, indicates the noise level which will be exceeded 90% of the time.
Excitation: The current used to generate the field in an alternator or generator.
External costs: Non-monetized costs attributed to a technology, such as the costs from extraction, processing, and combustion of a fossil-fuel, that are not reflected in its price. External costs include both social and environmental costs.
Externalities: See External costs and see also Social costs and Environmental costs.
Extrusion: Manufacturing process where metal such as aluminum or soft steel is forced (pushed) through a die into a desired shape. Extrusion produces a product of constant planform and is most frequently used for the side rails of aluminum ladders and the frames of windows. Extruded aluminum has been used for phi-configuration Darrieus wind turbine blades.
FAA: Federal Aviation Administration (U.S.).
Fabrication: Building an assembly with conventional manufacturing techniques, e.g., stamping metal into component parts and fastening them together with welds, rivets, screws or bolts.
Fail-safe braking system: When all powered systems are off, the brake is applied. Thus, if an operating wind turbine loses power, the brake is automatically applied. Power to apply the brake may be provided by springs or by a hydraulic accumulator.
Fall-restraint systems: Combination of body harness, lanyard, and attachments that limit the length of fall should a windsmith lose their footing or handhold. Fall-restraint systems commonly include a work belt or body harness with attached sleeve or clip that slides along a vertical steel cable that runs the length of the tower.
Fan: Colloquial term for rotor on American farm windmills. See also Windwheel.
Fantail: Small, multibladed rotor on late versions of the European windmill used to orientate the main rotor into the wind. The fantail’s plane of rotation is at right angles to that of the main rotor so that off axis winds will strike the fantail and reorient the main rotor squarely into the wind. Early modern European wind turbines in Denmark and Germany, such as those built by Riisager, WindMatic, and MAN, used fantails for orientation.
Fatigue, metal: The weakening and eventual failure of metal resulting from cyclic stress. All materials suffer from fatigue, though the failure of copper and aluminum is most well known. Nearly everyone is familiar with the potential for the fatigue failure of the aluminum pull-tab on a can of soft drink. The pull-tab will eventually fail at it’s joint if worked successively back and forth. The soft aluminum at the joint eventually becomes work hardened and brittle then snaps off.
Fatigue cracks: Telltale signs that the material has become work hardened and brittle where the amount of stress or the number of stress cycles exceeds the material’s limits.
Fatigue life: The number of cycles of stress reversals that a material can withstand before it begins to show signs of brittleness and fatigue failure.
Fauna: Wildlife.
FDV: Foreningen af Danske Vindmøllefabrikanter. Association of Danish wind turbine manufacturers. Now known as Vindmølleindustrien.
Feathering: Rotating a blade about it’s longitudinal axis until its chord line is parallel to the wind. One method of controlling lift and torque on a wind turbine rotor. By decreasing the area of blade surface exposed to the wind, feathering reduces thrust and stress on the rotor and tower when the rotor is at rest.
Fehmarn: Island in the Baltic Sea between Germany and the Danish island of Lolland. The island has one of the largest concentrations of wind turbines in Germany and in 1994 included more than fifty 500 kW wind turbines.
FERC: Federal Energy Regulatory Commission (U.S.). Federal agency responsible for regulating the interstate sales of electricity.
Fetch: The distance over which the wind blows with no obstructions.
Fiberglass: American English for glass reinforced polyester composites.
Field magnet: An elctro-magnet or a permanent magnet used to produce a magnetic field in an electric generator.
Field test: A performance test which is carried out under naturally occurring atmospheric conditions as opposed to tests conducted in a laboratory or wind tunnel.
Finite element: A system of analysis in structural and fluid mechanics where a mechanical system such as a wind turbine blade is divided into discrete elements connected to each other at discrete nodes or points.
Firm power: A term occasionally used to describe the contribution that a generator might make to the reliability of the overall power system.
Fixed charge rate: Multiplier used in engineering economics for determining the cost of energy. The multiplier includes the effects of inflation, the life time of the investment, and the cost of financing equity and debt.
Fixed coning: Describes a rotor with a rigid hub that does not permit out-of-plane or “flapping” deflection.
Fixed-pitch rotor: In contrast to a variable-pitch wind turbine, a rotor where the blades are fixed in pitch during operation. The blade pitch may be adjustable, but is not varied during operation.
Fixed-price contract: See also tariff.
Fjelds: Bald wind swept plateaus or hilltops found above the Arctic Circle in Sweden, Norway, and Finland.
Flagging: Wind-deformed branches of trees and shrubs found most often on the lee side of coniferous trees in areas of high winds. Branches grow downwind on the leeward side of the trunk while the trunk remains bare on the windward side “like a flagpole carrying a banner flapping in a breeze.” Flagging indicates higher winds than Brushing. (Putnam)
Flaps: Control surface usually located on the trailing edge of a wing designed to increase drag on the airfoil. Flaps are used in aircraft to reduce air speed during landing. Flaps can be used on the trailing edge of wind turbine blades to limit power in high winds by increasing drag. See air brake and ailerons.
Flapping blade hinge: Connection between a rotor blade and hub on a conventional wind turbine that allows the blade to move in and out of the plane of rotation in response to gusts.
Flapping: Blade motion in and out of the plane of rotation on a horizontal axis rotor.
Flashing: Momentary but reoccurring bright reflections from blades or nacelles. These bright flashes of light draw attention to the wind turbine and have generated complaints of visual pollution from some residents near Palm Springs, California.
Flettner rotor: The German inventor Anton Flettner crossed the Atlantic in 1925 on the Baden Baden using two upright spinning cylinders as the sole means of propulsion. These vertical axis rotors depended upon the Magnus effect for their forward thrust. Flettner also built a wind turbine that used four tapered rotating cylinders as blades. The rotor on this wind generator spanned 20 meters and stood atop a 33-meter tower. It was capable of generating 30 kilowatts in a 23-mph wind.
Flexure, blade: The downwind curve or bend in the blades of horizontal axis wind turbines caused by the wind’s thrust.
Flicker, shadow: The momentary but continuing passage of shadows across an occupied space caused by the blades of a wind turbine as the blades pass between the sun and the occupied space. Shadow flicker is more noticeable in the early morning or late evening hours or in northern latitudes where the sun is low on the horizon and shadows cast are long. Some observers find shadow flicker annoying.
Flimflam: Slang for nonsense, swindle, or deception. Used in relation to outlandish concepts for harnessing the wind where the only people to make money are the promoters. See zombie wind.
Floating-price: Pool price.
Flora: Plants
Flutter, blade: When wind turbine blades oscillate, beat, or flap rapidly with low amplitude through coupling of flapping and torsional deflections.
Flux: The lines of force due to a magnetic field.
Flyball governor: Watt governor used on 1930s-era windchargers in the U.S. to vary blade pitch with wind speed. Most commonly associated with windchargers build by the Jacobs Wind Electric Co. Often applied to any mechanical governor on the rotor of small wind turbines that uses centrifugal force acting on weights to alter blade pitch.
Flyway, migratory bird: Broad corridor through which migratory birds pass seasonally on their way to and from their breeding grounds.
Foehn or Föhn: Warm dry wind rushing down from the Alps across the lowlands of Switzerland. Similar to the Chinooks of western North America.
Footpath: British usage for hiking or walking trail.
Forced outage: Termination of wind turbine operation due to an unplanned event such as equipment failure and unplanned maintenance. Forced outages are attributed to either the wind turbine or the utility system e.g. network problems not associated with the wind turbine.
Foreground views: Close up. That part of a visual analysis of wind turbines where the wind turbines are nearest to and in front of the observer.
Free standing tower: Tower not dependent on guy cables or stays to remain upright. See Cantilevered towers.
Free wheeling rotors: Wind turbine systems that use self starting rotors and that permit the rotor to continue spinning in light winds whether or not the wind turbine is performing useful work.
Frequency, of alternating current: The number of cycles or Hertz that alternating current reverses direction per second. The frequency in North America is 60 Hertz, in Europe 50 Hertz.
Friends of the Earth: International environmental group with affiliates in most developed countries.
Frontal area: Area of a structure intercepting the wind. See Swept area.
FSO4: Final Standard Offer Number 4. Power purchase contract offered to qualifying facilities by the California Public Utility Commission as part of its Biennial Resource Plan Update (BRPU).
Fuel savings: Fuel not consumed or consumption offset by the production of wind-generated electricity. The use of wind energy provides both fuel savings and generating capacity avoided. Until the 1990s, most of the value of attributed to wind energy by electric utilities was through fuel savings.
Fuel-saver mode: Operation of a wind generator in which the power produced is used to displace conventional fuels at a utility power plant.
Full-span pitch control: In contrast to partial-span pitch control, motion of a wind turbine blade about its longitudinal axis along its entire length. In the U.S., the Mod-0A controlled torque and power in high winds by varying the pitch of the entire blade. However, the Mod-2 and Mod-5B controlled power by varying the pitch of only the outboard sections of the blades.
Furling: To roll up or take down a flag or sail. Form of overspeed protection used on traditional European windmills with both lattice or jib sails where the miller rolled up the sail. Also used in reference to a small wind turbine with a tail vane where either the tail swings toward the rotor or the rotor swings toward the tail so both are parallel. Furling prepares the wind machine for high winds or a period of inoperation.
Furling speed: The wind speed at which a wind turbine that uses furling to control rotor speed begins to furl. Many small wind turbines begin furling at 15 m/s or 30 mph.
Gear ratio: The ratio of the number of revolutions per minute between the main shaft and the output shaft of a wind turbine drive train.
Gearbox: Transmission. Mechanism that transfers the mechanical power of a wind turbine rotor to that of the load. Most commonly used to increase the speed of the main shaft to the speed required by a generator. In traditional wind pumps the gearbox is used to covert rotary motion of the windwheel to reciprocating motion of the pump rod.
Gearcase: Commonly refers to the metal case surrounding a transmission or gearbox. See Gearbox.
Gedser: Ferry terminal and port on the southernmost extremity of the Danish island of Falster 150 kilometers south of Copenhagen. Gedser is located on an exposed peninsula jutting into the Baltic Sea. In 1956 Johannes Juul installed a wind turbine at Gedser with a three-bladed, stall-regulated, upwind rotor that spanned 24 meters. For overspeed protection, Juul devised a simple system for pitching the tips of each blade. The 200 kW Gedser mill operated in regular service from 1959 through 1967. During its lifetime the Gedser mill generated 2.2 million kWh and was capable of annual yields of 800 kilowatt-hour per square meter of rotor swept area. Juul’s experimental turbine was so successful that it became the forerunner of all later Danish wind turbines. When Denmark looked again to wind energy for help in meeting the energy crisis of the 1970s, the country had a working model still standing at Gedser. The turbine has since been removed to a museum and a modern wind turbine installed atop its original concrete tower.
Geotextile mats: Woven mats of natural or synthetic fibers used as a underlayment for roads in boggy upland soils. The mats, developed by 19th century engineers laying railroads across Britain’s moorlands, distribute the loads imposed by road metal or fill material over a wide footprint. The mats can also be used to remove road material after the road is no longer needed.
GFRE: Glass fiber reinforced epoxy.
GFRP: Glass fiber reinforced polyester.
Gigawatt (GW): 1000 kilowatts, 106 watts.
Gin pole: Pole derrick. Standing derrick. Vertical pole or pipe used to raise a wind generator and commonly associated with 1930s-era windchargers. When attached to the side of a tower and extending above the tower top, a gin pole acts like a portable crane and allows the generator to be raised into position and then lowered onto the tower top. Gin poles are also used to increase hoisting leverage when raising hinged towers from the ground.
Giromill: Cycloturbine. A vertical axis H-configuration wind turbine with articulating straight blades. Rotors with articulating straight blades offer several advantages over conventional phi-configuration Darrieus rotors because they are reliably self-starting and all lift along the blade is created at a maximum radius from the rotor axis, thus maximizing rotor torque. See also Articulating blade.
Golding: E.W. Golding was the technical secretary of the Wind Power Committee of the Britain’s Electrical Research Association during the 1950s and wrote what has become a classic in wind energy literature The Generation of Electricity by Wind Power. Like Putnam before him, many of Golding’s observations are still valid today.
Gondola: Nacelle. Equipment pod.
Governor: A mechanism that automatically controls the speed of a rotor, usually by changing the pitch of the blades. A mechanism for exerting control or governing some device. In wind energy, it generally refers to a mechanism for controlling rotor speed as in the flyball governor.
Grandpa’s Knob: Long north-south ridge near Rutland, Vermont where the 1250 kW Smith-Putnam wind turbine which operated intermittently from August 1941 to its failure in March 1945.
Greenpeace: International environmental organization with affiliates in most developed countries.
Grid: Utility distribution system. Network of transmission and distribution lines carrying electricity from sources of generation to consumers.
Grid-connected wind turbines: Wind turbines that are physically linked or interconnected with a network or grid of a electricity distribution system. Such wind turbines use the network for field excitation or inverter commutation and are capable of feeding utility-compatible electricity into the grid or network.
Griggs-Putnam index: Named after biologist Robert F. Griggs of George Washington University and engineer Palmer C. Putnam for the creation of a system for using vegetational indicators of long term average wind speeds in the mid-1940s. The techniques developed by Griggs were used in the siting of the 1250 kW Smith-Putnam wind turbine on Grandpa’s Knob near Rutland Vermont in 1941.
Grounding: To connect an electrical circuit to the earth or to the ground.
Gullies: Form of erosion. Ditch or channel cut in the ground by erosion. Larger than rills, each gully may measure up to several meters across and several meters deep.
Gust: Sudden change in wind speed.
Guy anchor: Footing designed for anchoring the cables of a guyed tower.
Guy cables: Stays. Guy wire. In American usage a wire or cable used in tension to support an object such as a tower or blade. Cable support. As in a guyed tower, the cables are equally spaced about the tower so that each is under tension.
Guyed towers: Towers that use guy cables and several far flung anchors for remaining upright.
H-rotor: Form or configuration of a vertical axis wind turbine where in side or frontal views the straight vertical blades appear as the uprights of the letter H and the rotor cross arm appears as the horizontal line of the H. The blades on an H-rotor may be fixed in pitch or articulate. See also Giromill and Articulating blades.
Habitat: The environment or surroundings where an organism normally lives. Sensitive habitat implies that the environment may either be easily disrupted by construction or use or that the environment is home to a species of special concern.
Halladay, Daniel: Inventor who in 1854 built the first fully self-regulating wind pump. Until then, millers had to manually turn the spinning rotor out of the wind or reef the sails during storms. Halladay constructed a multiblade rotor made up of several movable segments. Rather than attaching these segments to the hub directly, he pivoted them about a ring. In high winds, the segments would swing open into a hollow cylinder. Early models of Halladay’s “rosette” or “umbrella” mill, used a tail vane to point the rotor, or “wheel,” windward. Later “vaneless” versions did away with the tail vane by orienting the wheel downwind of the tower.
Hand line: Rope used for raising and lowering light material and tools from a tower.
Harmonic distortion: Undesired distortion of the sinusoidal voltage and current waveform of a utility’s alternating current. Harmonics are of concern due to the damage they may cause utility and customer equipment. (Robert Putnam)
Harness, full-body: Unlike a work belt, which is solely an aid for positioning, a full-body harness is designed to maintain the wearer in an upright position after a fall and to facilitate extraction of a unconscious wearer from a confined space. Full-body harnesses include leg straps, belt, chest harness, and lanyard ring.
Hazards, occupational: Source of danger or risk to life and limb in the workplace. The principal occupational hazards of those working with wind energy are electrocution, falls, or becoming ensnared in moving machinery.
Hazards, bird: Structures that may injure or kill birds. In wind energy, exposed electrical conductors, guy cables, towers, and the moving rotor of a wind turbine are potential hazards to birds.
Head, dynamic: The vertical distance a wind pump must lift water between the level of well drawdown and the level at the well outlet plus the friction head.
Head, friction: Loss of head caused by the resistance to flow of a fluid through a pipe. The friction head of water is a function of pipe diameter, length, and the number of elbows.
Head, pumping: The vertical distance a wind pump must lift water between the level of well drawdown and the level at the top of the well.
Head: Measure of the height that a pump lifts water.
Heat pump: A reversible heating and cooling device that operates via a compressor and an evaporator causing a liquid such as Freon to circulate in a closed loop. In one half of the cycle the compressor raises the pressure of the Freon causing it to condense to liquid form and give off heat. In the other half the liquid passes to the evaporator where lower pressure makes it expand and revert to a gas, absorbing heat. In the heat-pump process either heat can be drawn off or delivered to a given space.
Heat rate: The amount of thermal input required to produce 1 kWh of electricity. The heat rate of conventional condensing cycle power plants is approximately 10,000 Btu per kWh.
Height-to-diameter ratio: For vertical axis wind turbines, the ratio of the rotor’s height to its width at the rotor’s equator. Early phi-configuration Darrieus rotors had height-to-diameter ratios near unity. Later designs are considerably more elliptical.
Hertz (hz): Unit of frequency in cycles per second.
Hinged blades: Blades on horizontal axis wind turbine in which each blade is separately hinged to the hub so as to allow some freedom of motion flapwise.
Histogram: Bar chart or graph.
Home built: American usage for a do-it-yourself (DIY) project. Some homeowners have attempted to build their own wind turbines often with little success.
Home light plant: Stand-alone or self-contained generating system originally used to provide lighting for homesteaders on the American Great Plains before the advent of the Rural Electrification Administration. Circa 1930. Wind systems were sold during this period as “light plants” using glass batteries for storage and gasoline engine-generator combinations, such as the Delco Light Plant, as a back-up power source.
Horizontal Axis Wind Turbine (HAWT): Conventional wind turbine. Wind turbine where the rotor spins about an axis near the horizontal as seen in the traditional European windmill and the American farm windmill.
Horsepower (hp): The power required in the English system to raise 550 pounds 1 ft in 1 second. One horsepower (hp) is equivalent to 746 watts.
Hub: Center of wind turbine rotor. Component at which the rotor blades are attached to the main or drive shaft of a wind turbine.
Hub height: Centerline height. Distance from the center of a conventional wind turbine rotor to ground level.
Hütter, Ulrich: German engineer whose career spanned the half century from the 1930s through the 1980s and who is credited with postulating that the most cost-effective wind turbine will use long slender blades, lightweight components, and operate at high tip speeds. Considered the father of high-speed, lightweight wind turbine design.
Hybrid power systems: A combination of a solar technologies such as a wind turbine and solar photovoltaics combined with a conventional technology such as a diesel generator used to provide power at remote sites.
IEA: International Energy Agency (Vienna)
IGBT: Integrated gate bipolar thyristors. Electronic devices used in synchronous inverters.
In-plane: Direction or motion parallel with the rotor plane.
Induction generator: An asynchronous electrical generator which must be supplied with magnetizing current. Power is only produced when the input shaft speed exceeds that necessary to generate an alternating current frequency synchronous with that of the magnetizing current.
Inertia, rotor: The resistance of a rotor at rest to being set in motion and the resistance of a rotor in motion to any change in its speed or direction.
Infra-sound: Sub-audible frequencies below 20 hertz.
Infrastructure: The basic facilities, structures, and services necessary for the functioning of a system such as electricity generation and distribution, transportation, communication, or finance.
Installed capacity: The rated power of a wind turbine. Also called nameplate capacity. The installed or rated capacity differs from average capacity as used in the Pacific Northwest of the United States and the declared net capacity as used in Britain as it is independent of actual generation or capacity factor.
Installed cost: More correctly installed price. Total price for the turnkey installation of a wind turbine including the price of the wind turbine, tower, foundation, installation, and any associated costs for interconnection.
Intercept area: Area of the wind stream swept by a wind turbine rotor. Swept area. Frontal area.
Interconnected wind turbine: A wind generator linked to the utility grid or network. Grid connected. Grid intertied.
Interest rate: A charge or cost for a loan expressed as the percent of the principal or amount loaned due during a specified period, usually per annum.
Intermediate plant: An electrical plant which is used to meet daily or seasonal variations in electrical load. The annual average use of this type of plant is less than that of base-load plants, but more than peak-load plants.
Intermittent resources: Energy resources, such as wind, hydro, and other forms of solar energy, where the fuel or prime mover is not available 100% of the time. Wind is an intermittent resource because the wind does not blow all the time and when it is blowing it may not be blowing at sufficient strength to drive a wind turbine at 100% of its rated capacity.
Intrusion aural: Unwelcome or unwanted sound. Noise.
Intrusion visual: Unwelcome or unwanted sight on a landscape.
Inversions, temperature: Meteorological condition where air temperature increases with increasing altitude or the inverse of the normal lapse rate where temperature decreases with increasing altitude. The inversion layer acts as a lid preventing the dispersal of pollutants. Wind speeds are generally greater above an inversion layer than below.
Inverter: An electrical device to convert direct current (dc) to alternating current (ac).
Investor-owned utilities: Dubbed IOU in American usage. Private, for profit utility company listed on a national stock exchange.
IRP: Integrated Resource Plan. System for determining how best to meet electrical demand over a specified planning horizon that includes improved energy efficiency and all sources of potential generation, including renewables.
IRS: Internal Revenue Service (U.S.). Equivalent to the U.K.’s Inland Revenue. Tax collecting agency of the Federal government.
ISO4: Interim Standard Offer Number 4. One of several contracts offered by the California Public Utility Commission to qualifying facilities during the early 1980s. The ISO4 offered a 30 year contract that provided a fixed schedule of capacity and energy payments during the first ten years. In the eleventh year the price for energy reverts to the then prevailing avoided cost. Often shortened to Standard Offer Number 4 though the contracts were considered provisionary.
Isolation, noise: Component or device used for reducing noise emissions that prevents the interaction between components or the transmission of vibration between components.
Isovent: Lines of equal wind speed drawn on a geographical map of an area. Analogous to isobars, lines of equal barometric pressure or elevation contours, lines of equal elevation.
Izaak Walton League: American environmental group active predominately in the Midwest.
Jacobs Wind Electric: 1930s-era manufacturer of windchargers. Jacobs built the self-styled “Cadillac” of windchargers.
Jail break: Juxtaposed to “runaway” when more than one wind turbine was in runaway. Used in reference to multiple Enertechs and ESIs in overspeed during a high wind event in the 1980s. (Mark Haller)
John Brown: British manufacturer that developed 100 kW prototype wind turbine with a 15 meter downwind rotor circa 1955.
Joule: A unit of energy in the SI system. Work done when a force of one Newton is displaced through a distance of one meter.
Juul, Johannes: Widely credited as the father of the “Danish” configuration: three-bladed, upwind, fixed-pitch, stall-regulated rotor. Juul trained in the Danish craft tradition by attending Poul La Cour’s school for windmill electricians at the Askov folkhøjskole. Juul believed in incrementally increasing the size of wartime designs with which he was familiar. He emphasized low cost, simplicity, and the use of readily available materials. In 1956 he installed the 200 kW, 24 meter diameter turbine at Gedser, which became one of the period’s most successful wind turbines.
Kaiser-Wilhelm-Koog: Site of first German wind power plant and one of Germany’s wind turbine test centers. Located in a former polder (koog) west of Marne in Schleswig-Holstein.
Kamakaze: Japanese for “sickle wind,” a wind that cuts like a knife.
Kamikaze: Divine wind. From the Japanese kami (divine) and kaze (wind). Named after a storm that destroyed a Mongol invasion fleet in 1281. Used in WWII for suicide pilots who were believed would save Japan from the invading Allies.
Khanasin: Arabic for 50, as in 50 degrees centigrade (122 degrees F), hot easterly wind from the desert in the Middle-East.
Kilowatt: Unit of power. 1000 watts.
Kilowatt-hour (kWh): Unit of energy equal to the use of 1000 watts for one hour.
Kinderdijk: Site of world’s oldest existing wind power plant where a gang of 19 windmills drained a polder southeast of Rotterdam until the 1950s.
Klapsejlmølle: Danish for traditional windmill on which the cloth sails and lattice work blades have been replaced with movable louvers, shutters, or jalousies that regulate the flow of air through the blade. In British usage “patent” sails derived from the inventions of Meikle and Cubitt. See Patent sails.
Knots: Nautical unit of speed. One knot equals one nautical mile per hour (1.15 mph, 0.515 m/s).
KWEA: Kern Wind Energy Association. Trade group representing operators of wind plants in the Tehachapi-Mojave wind resource area of Kern County, California.
LaCour, Poul: Known as a the Danish Edison, he was one of the first Europeans to experiment with using wind energy to generate electricity. LaCour built experimental wind turbines using Klapsejlmφlle at Askov folkhφjskole where he taught.
Lanchester-Betz limit: Anglophone academics argue that British engineer Frederick Lanchester should also be given credit for deriving the theoretical limits of a propeller turbine. He published his findings in 1915, several years prior to those of Albert Betz’s widely quoted paper in 1920. Both reached similar conclusions. See Betz limit. (Karl Bergey) Others have argued that the term should more correctly be even more inclusive as the Lanchester-Betz-Joukowsky limit after the Soviet aerodynamicist N. E. Joukowsky. (Giis van Kuik)
Land-sea breeze: On shore and off shore winds created by differential heating of land and large bodies of water. During daylight the land heats faster than the water causing a breeze from sea to land (sea breeze). At night the land cools faster than the water causing a breeze from land to sea.
Lanyard, safety: Nylon cord or short nylon rope often with snap hooks at either end used to attach a work belt or body harness to a fall restraint system.
Lattice towers: A free-standing tower constructed of intersecting or criss-crossing metal bars, tubes, strips, or angles that resembles the latticework of a rose arbor. See also truss towers.
Leading edge: Front of the airfoil. Region of an airfoil which first encounters the relative wind.
Learning curve: The decrease in cost of production as volume increases. One of the first demonstrations of this phenomena occurred with the production of the Ford Model T. During the 1970s the “learning curve” was often used to justify public expenditures on costly multimegawatt wind turbines. However, there were never enough large wind turbines built to gain economies from volume production and thus the value of the “learning curve” for large wind turbines was never demonstrated.
Lease: A contract permitting use of property for a specified period in exchange for payment of rent or a fee. See also Royalties.
Lee: Downwind. On the side away from the wind. Snow accumulates in the lee of mountain summits.
Leeghwater, Jan Adriaenszoon: The Dutch father of polder drainage. Literally Jan “empty” water, so named for draining the Beemster polder with 40 windmills in 1612, Purmer in 1622, Wormer in 1626, and Schermer polder near his birthplace in Noord Holland in 1635 using 50 windmills.
Levelized cost of energy: A stream of equal costs whose present value is equivalent to a stream of both higher or lower costs throughout the life-cycle of a generating plant.
Life cycle cost: The all-inclusive cost of a technology from installation through dismantlement. Renewable technologies have higher initial costs than fossil fuel power plants, but often lower life-cycle costs because of the ongoing costs for fuel during the life of a fossil fuel plant.
Lifelines: See lanyards and fall-restraint systems.
Lift, aerodynamic: The force which pulls a wind turbine blade along, as opposed to drag.
Lift device: One of two major classes of wind machines. The blades generally move in a plane perpendicular to the wind and at a very small angle to the wind. The blades of lift devices move at speeds greater than that of the wind. Blades of lift devices have airfoil shapes from which lift is created and the term derived. Lift devices can capture 50 times more power per unit of projected blade area than drag devices.
Lift-to-drag ratio: Ratio of the coefficient of lift to the coefficient of drag. Aircraft designers have long sought high ratios of lift to drag.
Lightning: An abrupt discharge of static electricity in the atmosphere. As often the tallest objects on the landscape, wind turbines are susceptible to lightning.
Line commutated: Form of electronic inversion that uses the utility’s alternating current wave form to trip electronic switches such as thyristors.
Live: In an electrical conductor or circuit, voltage is present and current can flow, producing a shock hazard. Hot.
Livestock watering: An important application for small wind turbines and small photovoltaic systems on the American Great Plains, Argentina’s Pampas, and the steppe of Central Asia. Water for livestock is not only essential for survival of livestock on these semi-arid grasslands, but also adequate supplies are critical for weight gain.
Livingston Bench: Upland on the south side of the Yellowstone River southeast of Livingston Montana long regarded as a potential site for wind development. However, by the mid-1990s only four 65-kW wind turbines were in operation.
Load: A demand for electrical power.
Load duration: The amount of time during which the demand for power exists.
Load factor: In American usage, the coefficient of average electrical load divided by peak load. In British usage, capacity factor.
Load leveling: Technique for smoothing a power profile of a customer or an entire utility over a period of time often entailing constraints on when power is used during the day.
Load-following capability: A measure of a utility system’s response to change in load. As load increases the utility system responds by generating more from existing units or by bringing more units on line. When load decreases the utility system responds by dropping units or decreasing the generation from operating units.
Loads, standby: Secondary electrical loads that are supplied only when the demand of primary or essential loads is met. See also dump loads.
LOBA: Loss of blade accident, euphemism used by US Windpower (Kenetech) for a wind turbine throwing a blade.
Logarithmic extrapolation: Method used by meteorologists to estimate changes in wind speed with height. It differs from the power law method which was derived empirically.
V= (ln (H/Z0))/(ln(Ho/Z0)) Vo
where Vo is the wind speed at the original height, V is the wind speed at the new height, Ho is the original height, H is the new height, and Z0 is the roughness length. Roughness Class 1, representing “open” land, has a roughness length of 0.03 which is equivalent to a roughness exponent, α, in the power law equation of 0.13.
Lolly shaft: Pintle. Vertical shaft about which a conventional wind turbine yaws or turns to face the wind. American term commonly used when describing the turntable or the pintle of small 1930s-era wind generators, such as the Jacobs windcharger.
LOMA: Loss of machine accident, euphemism used by US Windpower (Kenetech) for another wind turbine destroyed.
Loss of load probability: The statistical likelihood that a generating unit will not be available to meet demand. Utility engineers have traditionally raised the spectre that wind generation is unreliable to meet demand because of its intermittency. Studies in the U.S. and Europe during the early 1990s showed that wind power plants exhibit a loss of load probability not unlike that of conventional sources.
Loudness, noise: A measure of the auditory response to sound, a measure of noise intensity. The human sensation of loudness depends not only on sound pressure, but also on frequency or pitch.
Low-frequency noise: Sounds below 100 hertz. Frequency range of troublesome noise most often associated with pulsating impulsive sound from downwind two-bladed wind turbines. Includes audible and sub-audible frequencies.
Lull: Calm. Period with no wind.
Lykkegaard wind turbines: Danish wind turbine patterned after Poul la Cour’s klapsejlsmφlle that was used during WWII.
Madaras rotor: J. Madaras used a slightly different approach than Flettner to harness the Magnus effect. He proposed that large cylinders rotating about a vertical axis be mounted on a circular track with an electrical generator in the wheels of each car or truck carrying a rotor. In 1933, a 90-foot-tall cylinder 28 feet in diameter was erected in Burlington, NJ.
Magic spreadsheet: Spreadsheet values for project costs, soft costs, financing, payouts, and the all important deal junky bonus to arrive at a preconceived conclusion. “What number do you want to see?” (Mark Haller)
Magnus effect: Thrust created by the wind passing over a spinning cylinder. The spinning cylinder acts as an airfoil. The effect is comparable to that of the “curve ball” in American baseball where the pitcher imparts a spin to the ball as it leaves his hand, causing a curve in the ball’s flight.
Main bearings: In a horizontal axis wind turbine, the bearings supporting the main shaft, hub, and rotor. In a vertical axis wind turbine the bearings at the top and bottom of the rotor.
Main Shaft: Spindle or axle connecting the rotor to the transmission and generator. In many conventional wind turbines the main shaft supports the rotor’s mass and load while transferring its torque. In larger wind turbines the main shaft only transmits torque, it does not support the mass and thrust of the rotor and is sometimes called a quill shaft.
Mains: British usage for central station power available from a network of transmission and distribution lines. Grid or network.
Margin of safety: In engineering, an amount beyond that needed to meet design specifications.
Marginal cost: The cost of the next unit beyond the margin or boundary of that needed.
Marine proof: Able to withstand the corrosive effects of the marine environment, that is sea spray, salt-laden moisture, etc.
Mark: British usage for iteration, version or model number as in Supermarine Spitfire Mark IV.
Market-driven development: Technology development due to market pull or a demand for a product. As opposed to centrally directed technology development through research, development, and demonstration.
Maximum design wind speed: The maximum wind speed a wind turbine in automatic, unattended operation, but not necessarily generating, has been designed to sustain without damage to structural components or loss of its ability to function.
Maximum tested wind speed: The maximum wind speed a wind turbine in automatic, unattended operation, but not necessarily generating, has sustained without damage to structural components or the loss of its ability to function.
Mean wind speed: Average wind speed. The value obtained by dividing the sum of a quantity of wind speeds by the number of wind speeds in the set. The mean wind speed obscures the time distribution of wind speeds over time, but is the simplest measure of a wind resource.
Mean-time-between failure (MTBF): A measure of reliability. Total operating hours divided by the total number of equipment failures.
Mechanical brakes: Spring-applied brakes.
Medium-sized wind turbines: Arbitrary designation for wind turbines from 10 meters in diameter to 50 meters in diameter.
Megawatt (MW): Unit of power equal to 1,000,000 watts or 1,000 kilowatts. The Boeing Mod-5b is rated at 3.5 MW.
Megawatt-hour (MWh): Unit of energy equal to 1,000 kWh. The generation or use of 1,000,000 watts in one hour.
Megawatt-sized wind turbines: Multimegawatt wind turbines.
Merida: City in Mexico on the north shore of the Yucatan peninsula known for its multitude of water-pumping windmills, many mounted on the rooftops of multistory buildings.
Metal fatigue: Property characteristic of metals where strength decreases with use, more specifically, fatigue is a function of applied stress and the number of stress cycles. For example, the failure of a copper wire twisted and untwisted until it breaks is an example of metal fatigue. If the force or stress used in bending the wire is increased, the period of time before the wire breaks is shortened. The stress a material can withstand under cyclic loading is much less than it is under static loading. Flexure in metals is not completely reversible on the microscopic level and cyclic loads cause strain-hardening and a loss of ductility. Metal, in effect, becomes brittle with use. Metal rotor blades, for example, will eventually wear out and fail due to metal fatigue.
Meters per second (m/s): Unit of wind speed in the SI system. One m/s is equal to 2.24 mph and 1.94 knots.
Method of bins: A technique for collecting or analyzing a wind speed frequency distribution by grouping wind speed data into discrete intervals (bins). For example, wind speed data collected in m/s can be stored in bins of 0-1 m/s, 1-2 m/s, 2-3 m/s and so on.
Microclimate: The wind regime or climate in the immediate neighborhood of the wind turbine.
Micrometeorology: The localized study of meteorological data, often used in planning windfarms.
Milliard: 109. Billion in American usage.
Millwright: Skilled craft of building and maintaining a traditional European windmill. The term now also applies to anyone with the skills necessary to service any heavy machinery.
Missing-tooth syndrome: The visual impact of wind turbines on the landscape is accentuated by the human tendency to first notice objects unlike the others in a group of objects. People notice a black bean in a group of white beans or a white bean in a group of black beans. So it is with wind turbines. Observers first see a non-operating wind turbine in a group of operating wind turbines.
Mistral: From Provençal for a cold, powerful, dry wind flowing down the Rhône valley from the French Alps.
Mitigation: In American usage the actions or measures taken to reduce the real or perceived environmental impacts from a project.
Mitigation, off-site: Actions or measures taken beyond the boundaries of a project site to compensate for a project’s real or perceived environmental impacts.
Modified sine wave: Marketing sleight of hand for modified square wave.
Modified square wave: Waveform produced by some solid state inverters that replicates, to varying degrees, the sinusoidal wave form produced by ac generators.
Modularity: Of or relating to discrete units which can be easily joined together to create a whole. Wind energy is modular because generating capacity can be added in units of individual wind turbines.
Moment of force: See Torque.
Mortality, bird: The number of dead birds from both natural and manmade causes. Relative to wind energy, the number of birds killed by wind turbines.
Mortality, human: Relative to wind energy, the number of people killed in accidents during the construction, operation, and removal of wind turbines.
Multiblade rotors: High-solidity rotors using numerous blades. The American farm windmill uses 10 or more blades that nearly cover the entire rotor disk. In contrast, most modern wind turbines use only two or three slender blades.
Multimegawatt wind turbines: Large wind turbines typically with rotors sweeping more than 60 meters in diameter.
Multiple rotors: Wind turbines using multiple rotors can to take two forms: dual contra-rotating rotors on the same axis, or multiple rotors on the same towers.
Municipally-owned utility: In American usage, electric utility owned by a city government and ostensibly its citizens. This form of ownership contrasts with that of Investor Owned Utilities or IOUs owned by stockholders who may reside anywhere. There are numerous small municipal electric utilities in the Midwestern United States.
Mynydd-y-Cemais: Mountaintop plateau in mid Wales near the town of Cemmaes that overlooks the Dyfi (Dovey in English) valley and site of first wind power plant in Britain to go before a public inquiry. The project was approved and post-construction surveys have found widespread public acceptance.
NACA: National Committee on Aeronautics (U.S.). Precursor to the National Aeronautics and Space Administration. No longer extant. Widely known for its series of standard airfoil configurations.
Nacelle: Machine cabin, gondola, equipment pod. The covering that houses the rotary components of a horizontal axis wind turbine. Refers more specifically to the covering of the equipment pod.
Nacelle cover: Canopy. Shroud enclosing the equipment pod. Integral component for noise control and for protection of windsmiths during inclement weather in medium-sized wind turbines.
Nameplate rating: Arbitrary rating of wind turbine in kilowatts. See Rated power.
NASA: National Aeronautics and Space Administration (U.S.).
National Research Council: NRC, Canada.
Natta: Network for Appropriate Technology and Technology Assessment (U.K.). Located at the Open University, Milton Keynes.
Natural frequency: The frequency with which a system vibrates absent any external force. See Eigen frequency.
NCAT: National Center for Appropriate Technology (U.S.). Located in Butte, Montana.
Necropsies, avian: Bird autopsies.
NEL: Formerly stood for National Engineering Laboratory (U.K.). Located southeast of Glasgow, Scotland at East Kilbride.
NESA: Formerly the Danish electric utility serving Copenhagen.
Net energy billing: One method by which a utility can compute the billing for providing electrical service to a customer who uses their own wind generator. One or two kilowatt-hour meters may be used and billing by the difference between the two meters. Excess power from a wind generator, for example, runs the un-ratcheted meter. The utility, in effect, buys back energy for the same rate that they sell energy to the consumer.
NEWIN: Dutch wind energy association.
NFFO: Non Fossil Fuel Obligation (U.K.). Early form of feed-in tariff designed to encourage use of nuclear power after privatization. To the dismay of then Prime Minister Margaret Thatcher, it was deemed to also apply to renewable energy.
NIMBY: Not In My Backyard. Slightly derogatory acronym applied to those who oppose a land use in their vicinity, despite their support of or dependence on the same land use elsewhere. Relative to wind energy, a NIMBY response is characterized by statements such as “I support wind energy, just not here.” Related to LULU or Locally Unwanted Land Use.
NIMSBY: Not in my summer backyard. Variant of NIMBY recently coined in Sweden surrounding controversy over installing wind turbines on islands frequented by summer tourists. NIMSBY implies or suggests a conflict between rural and urban inhabitants about the use of rural landscapes. While local year-round residents may support the use of wind turbines, urban dwellers who use the rural landscape only for recreation may object because of fears that the wind turbines will somehow disrupt their recreational use of the landscape. The same phenomena is also seen in Britain, Denmark, Germany and North America.
NIABY: Not In Anyone’s Backyard. Bitter depiction of groups who oppose the use of certain technologies, such as wind energy. For example, Britain’s so-called Country Guardians campaigns against wind energy, even where there is local support for the technology.
Noise: Unwanted sound
Noise ordinance: American usage for locally imposed restrictions on noise sources.
Nominal cost of energy: Includes the effects of both real cost escalation and inflation over time. Typically higher than costs expressed in real or constant dollars.
Noordoostpolder: One of the polders reclaimed from the Netherland’s former Zuiderzee by closure of the Afsluitdijk. Site of the world’s longest linear array where fifty wind turbines border the Westmeerdijk north of Urk. The six kilometer (3 mile) string of wind turbines produce 12% of the polder’s electricity.
Northwest, Pacific: American usage for the states of Washington and Oregon. Sometimes includes the states of Montana and Idaho.
NOVEM: Nederlandse Onderneming voor Energie en Milieu. Dutch ministry for energy and the environment.
NRC: National Research Council (Canada).
NRDC: Natural Resources Defense Council. U.S. environmental group.
NREL: National Renewable Energy Laboratory (U.S.). Located at Golden, Colorado. The wind division is located at the National Wind Technology Center on Rocky Flats near Boulder, Colorado.
Nuisance, noise: In American usage, sound or noise that interferes with anothers legal right by causing damage, creating an annoyance, or causing inconvenience.
O&M: Operations and maintenance.
Obstructions: Obstacles in the wind’s path that reduce its velocity and, hence, its power. Obstructions such as trees, buildings, and shelter belts retard the wind and cause damaging turbulence.
Off-peak: Refers to points on a load distribution curve or time of the day when electricity consumption is less the maximum.
Off-the-grid power systems: Stand-alone power systems. American usage for power systems independent of an electricity distribution network or grid.
Offshore: Siting or installation of wind turbines beyond the low tide range of a coastline.
Omnidirectional: The ability of accepting the wind from any direction. Unshrouded vertical axis wind turbines are inherently omnidirectional, but nearly all wind turbines are technically omnidirectional.
One off: British usage for pre-production prototype.
One-seventh power law: Wind speed profile which uses the power law with an exponent of 1/7.
Onshore: Siting or installation of wind turbines on land.
Operations and maintenance: Actions taken to operate or insure the continuing operation of wind turbines and their ancillary equipment.
Opinion, public: Explicit or implicit expression of attitudes toward the use of wind energy by inhabitants of a physiographic or political locale, region, or state.
Orientation: Yaw. Azimuth. Bearing. Position of a conventional wind turbine about a vertical axis. The rotor on a conventional upwind turbine is orientated toward the north when the winds are from the north.
ORV: American usage for Off Road Vehicles. More correctly referred to as Off Highway Vehicles. These include but are not limited to four wheel drive vehicles such as Jeeps and Land Rovers.
Out-of-plane: Direction of motion perpendicular to the rotor plane.
Overrunning clutch: A clutch that allows the driven shaft to turn freely under its own power when the speed of the driven shaft exceeds that of the driving shaft. The clutch on an automobile is a form of overrunning clutch that permits the engine to turn freely under its own power when its speed exceeds that of the starter. The overrunning clutch was used by the U.S.D.A. at its Bushland, Texas experiment station to match the mechanical characteristics of a Darrieus rotor to that of a high-volume, motor-driven irrigation pump. When sufficient wind was available the Darrieus rotor drove the well pump mechanically. When winds were insufficient, the well pump was driven electrically.
Overspeed, rotor: Refers to excessive rotor speed. Rotor speed where either fatigue will be accelerated or the rotor will fail catastrophically. See Runaway.
Overspeed control: Mechanism, device, or some combination of devices for limiting the maximum speed of the rotor to prevent the rotor from self-destruction.
Pacheco Pass: Route through the Diablo Range between California’s San Joaquin Valley near Los Banos and Monterey Bay. Small wind plants in the pass have been installed at Dinosaur Point overlooking the California Water Project’s San Luis Reservoir.
Pacific Crest Trail: See PCT.
Pacific Gas & Electric Company: Investor-owned electric utility serving northern California.
Packerhead: A brass fitting installed over the top of a well pipe to prevent overflow. When used with a water-pumping windmill, it enables pumping water uphill or into an above ground storage tank.
Panemone: Simple wind machines using drag as the driving force where the effective surface or pane moves in the direction of the wind as in the common cup anemometer. The maximum coefficient of power for simple panemones is 1/3. (Golding)
Parachutes: Method of overspeed control on some Dutch wind turbines where a parachute deploys at the tip of each blade during an overspeed event.
Parallel generation: Production of electricity in conjunction or parallel with that delivered by an electric utility network or grid. Wind power plants and individual wind turbines interconnected with the grid generate electricity in parallel with that of the local utility.
Parallel shaft transmissions: Common arrangement of gears within a gearbox where the shafts of each gear parallel that of the others.
Partial span pitch control: In contrast to full-span pitch control, where the rotation of a wind turbine blade about its longitudinal axis acts along its entire length, only the outboard sections of the blade are used. The inboard sections remain fixed in pitch. See Full-span pitch control.
Particulate pollution: Soot, smoke and respirable particles. Includes fugitive dust in the U.S.
Passerines: Songbirds. Order of small birds.
Patent sails: British usage for mechanical louvers, shutters, or jalousies that replaced traditional windmill sails. These self-regulating sails were developed by English millwright William Cubit in 1807 following on Andrew Meikle’s spring sails of 1772. Patent sails are found on later versions of windmills in Denmark, Germany, and Britain but are rarely seen on Dutch windmills. Klapsejlmφlle in Danish.
PAWEX: De Vereniging van Particuliere Windenergie Exploitanten, Dutch association of wind turbine owners or more literally the Association of Private Wind Energy Installations.
Payback: Length of time before an investment is recovered. Time it takes for a wind turbine to pay for itself. Payback fails to incorporate benefits during the entire life-cycle of an investment in renewable energy by excluding benefits after payback has occurred.
PCT: Long distance foot and equestrian path that stretches from Mexico to Canada along the Sierra Nevada Mountains of the western United States. The trail passes through wind plants in the San Gorgonio and Tehachapi Passes.
Peak load: Maximum load or force on a structure, or maximum consumption in a system of supply.
Peak power: Maximum power output of a wind turbine. Peak power is often greater than rated power especially among stall-regulated wind turbines. See Rated power.
Peak shaving: Method to reduce the need for standby generating capacity in a utility system by regulating or controlling when loads are brought on line.
Peak wind speed: Maximum instantaneous wind speed.
Peaking capacity: Generating units used to meat peak or maximum loads on a utility system. In the past peaking units have had low capital cost, but high running costs. Typically they used technologies, such as natural gas-fired turbines, that permitted them to be started and ramped up to full output quickly.
Penetration level: The relative amount of wind turbine generating capacity in an otherwise conventional electrical supply system. The quotient of wind generating capacity divided by the system’s total installed capacity.
Pennines: The north-south hilly backbone of England that includes the Yorkshire dales. Small wind power plants operate in the mid Pennines near Leeds-Bradford.
Perches, bird: Structures that enable birds to perch or roost. Lattice towers with horizontal cross girts and nacelles with external work platforms offer numerous perches and are thought to contribute to collisions between birds and wind turbines in California’s Altamont Pass.
Performance: Wind turbine productivity. Several units of measurement are in common use, including Specific yield and Capacity factor.
Performance curve: Graph of wind turbine power output relative to wind speed. See power curve.
Permanent-magnet alternators: Generators using permanent magnets to create the field. Many small wind turbines use permanent-magnet alternators.
PG&E: Pacific Gas & Electric Co.
Photovoltaic cells: Solar cells. Laminated silicon wavers that convert sunlight directly into direct current.
Piers: Columns of steel or reinforced concrete used for the foundation of wind turbine towers. The legs of lattice towers typically rest upon piers for lateral stability.
Pilot vane: Method for overspeed control on American farm windmills that used a vane in the same plane as the rotor disc. This vane extended beyond the sweep of the rotor and was intended to furl the rotor during high winds. The same effect is accomplished today on farm windmills by offsetting the rotor axis slightly from the pivot axis (yaw axis). In both versions, the tail vane used to orientate the rotor into the wind is not fixed, but instead is spring loaded. This allows the tail to furl when pressure on the rotor or pilot vane exceeds the counteracting pressure on the tail vane.
Pinned truss tower: Lattice tower which uses pinned or riveted members in a truss or triangular pattern, e.g., NASA/DOE’s tower for the MOD OA series of wind turbines. Truss towers are rigid and unyielding in contrast to tubular towers.
Pintle: Lolly shaft. Vertical shaft about which a conventional wind turbine yaws. Term used by Palmer Putnam when describing the design of the 1.25 MW Smith-Putnam turbine.
Pitch, blade: The angle between the chordline of a wind turbine blade and its direction of travel.
Pitch control: A system which varies blade pitch to control lift and hence rotor torque and power. Typically wind turbines using blade pitch control varied pitch from feather to the running position, though some designs vary pitch from the running position to stall.
Pitch-regulated rotor: Wind turbine rotor that varies blade pitch to regulate lift, and hence, torque and power.
Pitchable blade tip: Overspeed control mechanism common among early Danish wind turbines that swiveled a portion of the blade at the tip about its longitudinal axis during overspeed events. One of the characteristic features of Johannes Juul’s Gedser turbine. The change in pitch of the tip reduced lift and increased drag sufficiently to prevent the rotor from destroying itself. Pitchable blade tips differ from Tip brakes in that they form a uniform part of the blade prior to deployment, contributing to the blade’s torque.
Plan area: The area projected on a top view of an airfoil perpendicular to a section or profile of the airfoil.
Planetary transmissions: A gear box consisting of a central gear, a coaxial internal or ring gear, and one or more pinions supported on a carrier. In contrast to parallel shaft gear boxes, planetary transmission are lighter and more compact. However, in wind turbine applications they have proven noisier than parallel shaft gear boxes. Wind turbines in the 500 kW size class have attempted to marry the advantages of both technologies by using planetary gearing on the low speed side and parallel gearing on the high speed side where much of gear box noise is generated.
Plant factor: British usage. Capacity factor. The energy produced by a wind turbine divided by the energy that would have been produced had the wind turbine operated at its rated power 100 per cent of the time.
Pole shoe: Iron core of a field pole in a generator that face the armature and is surrounded by the field coils.
Pneumoconiosis: Black lung. An occupational disease of coal miners. A social cost of using coal.
Post mill: Earliest form of European windmill. The entire windmill including the rotor, gearing and mill stones rest upon and turn about a central post. Consequently, the whole machine had to be turned to face the wind. A long pole (tail pole) extended from the post mill to ground level by which the miller could turn the mill about the post.
Power: The time rate of doing work or consuming or generating energy.
Power coefficient: A measure of a wind turbine rotor’s aerodynamic performance. The quotient of the power captured by a wind turbine rotor divided by the total power in the wind stream.
Power conditioning: Treatment or conversion of electricity produced by a generator or delivered by a distribution system. See also Synchronous inverter. Typically variable speed wind turbines generate electricity with variable frequency and voltage. In most applications this electricity must be conditioned or treated before it can be used by devices requiring constant frequency and constant voltage.
Power curve: Chart of a wind turbine’s instantaneous power across a range of wind speeds. The same data is frequently presented as a table of the instantaneous power produced at wind speeds from cut-in to cut-out. Hypothetical or idealized power curves are often shown as a line on a chart with power on the vertical axis and wind speed on the horizontal axis. Actual measured power curves are shown as scatter diagrams. Power curves derived from field tests are based on the weighted average of measurements taken when the rotor is both speeding up and slowing down.
Power density: Measure of the strength of a wind resources in Watts per square meter (W/m²). The amount of power per unit area of the free windstream. The energy flux in the windstream.
Power duration curve: Graph of wind power versus time.
Power electronics: Electronic devices used in power modern conditioning equipment, including but not limited to thyristors and integrated-gate bipolar transistors.
Power factor: Relationship between true power and apparent or imaginary power. A measure of the degree with which current and voltage are out of phase with that from the utility’s central generating station and the generator is consuming reactive power. See volt-ampere reactance.
Power form: Standard description of characteristics which describe the form in which power produced by a wind turbine is made deliverable to the load, e.g. voltage, phase, and form of current. Recommended by the American Wind Energy Association for inclusion in product literature.
Power in the wind: The ability of the wind to do work per unit of time. Often given in terms relative to a unit of cross sectional area of the wind stream, for example in Watts per square meter.
P= 1/2 τV3, where τ is air density and V is the velocity or speed of the windstream.
Power law: Widely used empirical model for the increase in wind speed with increasing height above ground.
U=Ur(z/zr)α
where U is the wind speed, Ur is the wind speed at the reference height, z is the height above ground, zr is the reference height, and α is a wind shear exponent derived from surface roughness. See also Prandtl logarithmic model.
Power quality: A subjective or qualitative comparison of the characteristics of voltage, frequency, current harmonics (for alternating current) of electricity produced by a wind turbine relative to the norm or range of values acceptable to a load or receiving utility systems.
Power train: Drive train.
PPC: Greek national electric utility
Prandtl logarithmic model: Formula for determining the variation in wind speed with height above the ground surface derived from the principles of fluid mechanics.
U(z)=(U*/k)ln(z/z0)
where U(z) is the wind speed in m/s at height z in meters, U* is the friction velocity, k is the von Karman constant (about 0.4), and z0 is the roughness length in meters that characterizes the terrain. See also Roughness length.
Prevailing wind: Most frequently occurring wind. The prevailing wind may not necessarily represent the most wind energy at a site. Because of the cubic response of power to wind speed, less frequent but stronger winds may contain more wind energy. This distinction is important when siting a wind turbine near obstructions.
Price: Cost plus profit. The price of wind turbines is the value customers or buyers pay and not necessarily what it costs the manufacturer to build the wind turbine or construct and operate a wind power plant.
Projected area: Area of a structure intercepting the wind transverse to the direction of the wind. The area of the circle intercepting the wind represented by the spinning rotor of a conventional horizontal axis wind turbine. See also Swept area.
Propagation, noise: The spreading of sound waves from a source.
Propeller, propellor: Sometimes incorrectly used to describe the rotor of a wind turbine. The rotor of a wind turbine is propelled by the wind. The rotor does not propel the wind through the rotor or propel the wind turbine through the air.
PRS: The California Energy Commission’s Performance Reporting System which at one time required California’s wind plant operators to report quarterly on the performance of their wind turbines.
PSC: American usage for Public Service Commission. State regulatory agency that typically has jurisdiction over electricity and other publicly regulated monopolies. Also known as Public Utility Commissions in some states.
Public Utility Regulatory Policies Act (PURPA): Bans discrimination against small power producers in the United States and directs utility purchase of small power production at a rate commensurate with the utility’s avoided (marginal) costs.
Publicly-owned utility: In American usage, an electric utility that is ostensibly owned by the people of a nation, state, or other geographic unit.
PUC: American usage for Pubic Utility Commission. State regulatory agency with jurisdiction over electricity and other publicly regulated monopolies. Also known as Public Service Commissions in some states.
Pulltruded blades: Wind turbine blades made from the pulltrusion process.
Pulltrusion: Manufacturing process where fiberglass strands or mats are pulled through a resin filled vat and then through a heated die emerging into the desired shape, such as that of an airfoil, in nearly a finished form.
Pulse Width Modulation: Electronic technique for adjusting the current to a load by rapid switching. (Hugh Piggott)
Pumped storage: Means of storing electrical energy where excess electricity is used to pump water uphill into a reservoir where it is then released through a hydroelectric turbine to generate electricity when needed. In the past, pumped storage has been associated with the need for using excess nuclear generation at night.
PURPA: Public Utility Regulatory Policies Act.
Putnam, Palmer Cosslett: Noted American engineer and inventor who led the development of the 1 MW Smith-Putnam turbine. During WWII Putnam became famed not for his wind turbine, but for his development of the DUKW and WEASEL amphibious vehicles.
PV: Photovoltaics or solar cells.
QF: Qualifying Facility under section 210 of the Public Utility Regulatory Policies Act in the United States.
Quad: Measure of energy. Quadrillion (1015) BTUs.
Quarter chord: The point along the chord line of an airfoil one quarter of the chord length from the leading edge.
Quill shaft: A hollow mainshaft used on the Boeing Mod-2 to transmit rotor torque to the gearbox.
Quixote, Don: Protagonist in Cervantes classic tale of self-delusion. “Quixotic” has become synonymous with foolhardy acts in response to a deluded sense of chivalry. Don Quixote attacked a colony of 38 “giants” (traditional windmills) on hills overlooking the plains of La Mancha in central Spain.
Radial station: Point or position along a blade as specified by its distance from the rotor axis measured perpendicular to the rotor axis.
RAL: Rutherford Appleton Laboratory (U.K.).
Raptor-proofed perches: Attempt to prevent perching by birds of prey on lattice towers in California’s Altamont Pass by using various devices.
Raptors: Birds of prey such as hawks, eagles, falcons, and kestrels.
Rare-earth magnets: Permanent magnets fabricated from metallic elements with atomic numbers from 57 to 71, the so-called lanthanide series. Wind turbine designers sometimes use rare-earth magnets, such as those made from a neodymium-iron-boron compounds, because of their greater magnetic density than the common Ferric oxide magnets.
Rate schedule: Published charges by each utility for cost of providing service, cost of generation, cost of maintaining reserve capacity, and cost of fuel to each customer class. See tariff.
Rated capacity: The output power of a wind machine operating at the constant speed and output power corresponding to the rated wind speed.
Rated power: The power produced by a wind generator in kilowatts at its rated wind speed. Often confused with peak power, though on some small wind turbines rated power is also peak power. Rated power is a misleading descriptor of wind turbine size and harks back to the technology’s formative years as a branch of electric utility research. A more reliable indicator of wind turbine size is rotor diameter.
Rated rotor speed: The nominal rotational speed of a wind turbine rotor when it is producing its rated power.
Rated wind speed: The wind speed at which a wind turbine produces its rated power. Rated wind speeds of different wind turbines vary from 8 m/s (18 mph) to 16 m/s (36 mph). The rated wind speed can indicate the wind regime for which the wind turbine was designed. Wind turbines with lower rated wind speeds are intended for low wind speed regimes.
Ratepayers: Utility customers.
Rayleigh wind speed frequency distribution: Idealized distribution of wind speeds over time. The Rayleigh distribution is a special case of the Weibull distribution where the Weibull constant is two and the shape of the distribution depends on only mean wind speed.
Reactive power: Power necessary for the field excitation of induction generators. Reactive power can be inductive or capacitive.
Real cost of energy: Assumes no inflation, also known as “constant” cost, can include actual cost escalation over time. Typically lower than nominal cost, which includes the effects of inflation.
Rebar: Reinforcing bar or rod used in strengthening concrete.
Receptors: One that receives. Person or animal that may hear or sense a sound. Ordinances governing community noise are often described in terms of their effects on potential receptors.
Rectified alternating current: Direct current produced by feeding an alternating current through a diode, an electronic device that restricts current flow to only one direction.
Rectifier: A electronic device, such as a diode, that converts alternating current (ac) to direct current (dc) by restricting current flow to only one direction.
Rectifier bridge: A rectifier circuit using several diodes to produce a continuous dc output from an ac source. (Hugh Piggott)
Reefing: Furling. Bringing in, hauling down, or rolling up sails on a wind-driven machine that uses sails. Millers on traditional European windmills were constantly on the alert to reef or unfurl the mills’ sails as needed.
Reflection: Light, sound, or other form of energy reflected or bounced off of a wind turbine. Reflection of the harsh desert sunlight off of metallic wind turbine nacelles or the glossy gel coat of wind turbine blades near Palm Springs, California have caused complaints about “sparklies” from some neighbors. Similarly there have been scattered incidents of television and radar signals being reflected from wind turbines in the U.S. and Europe.
Reforestation: To replant a logged area with a forest cover. Wind energy’s need for unobstructed exposure to the wind has led to land use conflicts with reforestation in Denmark where wind energy and reforestation compete for the same undeveloped coastal sites. Wind turbines today (2024) are so large, they extend well above the forest cover.
Regulator, voltage: Device that controls or regulates battery voltage in a charging circuit by limiting the charge current. Charge controller.
Relative wind: Apparent wind. The wind seen by a blade as it moves through the air. The resultant of the wind and the wind created by the blade as it moves through the air.
Reliability: Dependability. The measure of reliability used in the wind industry is mechanical availability or the mean time between failure of a component sufficient to stop a wind turbine’s operation. See Availability. Often incorrectly associated with intermittency. Wind turbines, though extremely reliable, are dependent upon the wind. Wind turbines, thus, operate intermittently despite their dependable ability to generate electricity when the wind is present.
Repair: To fix or restore to sound operating condition.
Repowering: Removing existing wind turbines and replacing them with newer, more cost-effective machines. Repowering existing sites in California’s aging wind plants has produced striking improvements in productivity. At one site in the San Gorgonio Pass repowering increased installed capacity from 29 MW to 35 MW, more than doubled annual generation and specific yield. At a site in the Tehachapi pass repowering increased annual generation five times and nearly tripled specific yield.
RERL: Renewable Energy Research Laboratory located at the University of Massachusetts in Amherst.
Reserve margin: The difference between the total system dependable capacity and the actual or anticipated total system peak load for a specified period.
Residential electrical consumption: Electricity used by homeowners.
Resistance: In an electrical conductor, opposition to the flow of currrent. Measured in Ohms.
Resonance: Response to a periodic force in which the resulting amplitude of oscillation becomes large when the frequency of the driving force approaches the natural frequency of a system or component. A common problem in wind turbine design is avoiding resonance between the frequency of which a blade passes the tower and the natural frequency of the wind turbine and its tower.
Respondent: One who responds or replies to public opinion surveys.
Retrofit: Space-age jargon for repairing or improving an existing structure or machine by adding new or modernized components.
Revegetation: To insure that eroded or disturbed land grows a cover of vegetation. Post construction activity required to establish or re-establish vegetation on soils disturbed by grading for roads, ditching for buried power lines, and excavations for tower foundations.
Reynolds number: A dimensionless number describing the aerodynamic state of an operating airfoil. The number is used, along with angle of attack, to describe the limits of a particular airfoil’s lift-to-drag ratio and the condition where stall occurs. Wind turbine airfoils typically operate in the range of Reynolds numbers from 0.5 million to 5 million. (Richard L. Hales)
Rib: Structural member that gives form to an airplane wing or shape to an airfoil used in a wind turbine blade.
Rigid hub: Attachment of wind turbine blades to the rotor hub where there are no degrees of freedom. A hub where there are no hinges permitting flapping or coning.
Ring generator: Generator or alternator where the generator diameter is large relative to the length of the generator case. In contrast to conventional generators where the diameter is some fraction of the generators length, the diameter of ring generators is several times the length or depth of the generator case. Ring generators are used in low-speed applications. For wind turbines, ring generators permit the use of direct drive and the elimination of step-up transmissions. For conventional wind turbines, the diameter of the ring generator approaches 10% of the wind turbine’s rotor diameter.
Risø: Danish national laboratory near Roskilde on the island of Zealand. Famous for its early wind turbine test center.
Rocky Flats: Site of U.S. nuclear weapons plant near Boulder, Colorado and now home to the National Renewable Energy Laboratory’s Wind Division and its National Wind Technology Center.
Root, blade: Portion of a wind turbine blade where it attaches to the hub.
Root shaft: Nearly archaic term from early wind turbines for a bar or rod that connects a wind turbine blade with its hub. Most wind turbines today use some other form of blade attachment.
Rosette windmill: Umbrella mill. American water-pumping windmill where the vanes of the rotor were assembled in hinged segments. The segments flew open in high winds to regulate the speed and power of the rotor. Because they were more complicated and costly than the fixed rotors of their late nineteenth century competitors, they eventually fell out of favor. Occasionally, one can be found at wayside tourist attractions on the Great Plains.
Rotor, generator: The spinning component of an electrical generator.
Rotor, wind turbine: Fan, Wheel, Turbine. A term borrowed from the helicopter industry for a system of rotating airfoils, such as a helicopter rotor. The assembly of blades and hub on a wind turbine.
Rotor axis: Axis about which a wind turbine rotor revolves. On conventional wind turbines the rotor axis is nearly horizontal. On Darrieus wind turbines the rotor axis is vertical. The use of “rotor axis” to define the type of wind turbine in modern wind energy terminology is a departure from historical usage. Historians of technology describe the “plane” through which the rotor passes. Thus historians term horizontal axis wind turbines “vertical windmills,” and vertical axis wind turbines as “horizontal windmills.”
Rotor diameter: On conventional or horizontal axis wind turbines, the diameter of a circle swept by the rotor perpendicular to the axis of rotation. On vertical axis wind turbines, the maximum distance perpendicular to the axis of rotation. For a phi-configuration Darrieus turbine this is the equator of the ellipse swept by the rotor. For an H-configuration turbine it is approximately the length of the rotor cross arm. A term commonly used to describe the size of a wind turbine. In general, the larger the rotor diameter, the greater the amount of energy captured from the wind because the area swept by the rotor of a conventional wind turbine increases with the square of the diameter.
Rotor plane: On conventional wind turbines, the plane perpendicular to the axis of rotation.
Rotor speed: Angular velocity of a rotor. Often given in revolutions per minute (rpm).
Rotor tilt: In a conventional wind turbine the angle between the rotor’s axis of rotation and the horizontal. For example the axis of the 1.25 MW Smith-Putnam turbine was 12.5 degrees above the horizontal. The rotors on early post windmills were probably horizontal, but millers soon learned that tilting the windshaft above the horizontal moved the rotor’s weight and thrust more directly over the tower while enabling greater clearance between the blades and the tower. Both effects were advantageous. Some wind turbine manufacturers have erroneously believed or falsely claimed that their rotors were tilted above the horizontal to better catch the wind. This belief was possibly fostered by an early treatise on the aerodynamics of sails published in the mid-1920s which illustrated the angle of clothes on a clothesline in high winds.
Rotor torque: The moment produced by a rotor about its axis. In a wind turbine torque is a function of lift and the distance from the hub where the lift is produced.
Roughness, blade surface: Imperfections or debris on the surface of an airfoil that disturb or disrupt the airflow. The performance of some airfoils is sensitive to surface roughness. When used on wind turbines in arid environments the productivity of these airfoils can be cut by as much as 50% unless the blades are washed regularly.
Roughness, earth surface: The unevenness of the earth’s surface due to topography, vegetation, and the built environment. This roughness disturbs the flow of wind within the boundary layer.
Roughness length: Measure in meters used in the Prandtl logarithmic model for estimating the variation of wind speed with height. Roughness length increases with increasing surface roughness. Roughness length varies from as little as 10-5 meters for surfaces with little or no obstructions, such ice and open mud flats, to 4 meters in urban areas. See also Prandtl logarithmic model.
Royalties: A share or portion of the proceeds from the sale of wind-generated electricity paid to a landowner for the use of their land. Royalties vary from country to country and depend upon several factors, including the profitability of wind generation and competition for use of the land. Royalties in the United States range from 2 percent to more than 5 percent. See also Lease.
RSPB: Royal Society for the Protection of Birds in Britain. Equivalent to the National Audubon Society in the United States.
Run of the wind: Distance the wind travels during a given period. Measure recorded by simple anemometers called wind-run odometers. Wind speed can be determined by dividing the wind run by the time period of measurement.
Runaway: When a wind turbine rotor is out of control. See overspeed.
Rural Electrification Administration: REA. New Deal-era agency created by the Rural Electrification Act to bring central-station electricity to rural inhabitants of the United States. Expansion of the REA spelled doom for windcharger manufactures in the 1940s, though some areas did not receive power until the mid-1950s.
Safety, occupational: The risk or freedom from risk of injury or death on the job or in the workplace. The manufacturer, installation, and operation of wind turbines entails a certain degree of risk to those who work with the technology. Fourteen men have been killed working with wind energy between 1976 and 1996.
Safety, public: The public’s risk or freedom from risk of being injured or killed by the transport, installation, or operation of wind turbines. No member of the public has ever been killed by the development or use of wind energy.
Sail wing: A wind turbine blade patterned after the sails of Mediterranean windmills especially those found on Crete and in Portugal. The sailwing uses fabric sails strung between a spar and a taut cord. A sail wing uses a tubular spar for a leading edge, a taut cord for a trailing edge, and sail cloth strung between the two. The sail wing is recognized for its low cost and self-regulating features.
Sails: A section of fabric fitted to the spars and rigging of a vessel to use the power of the wind for propulsion. Because of their likely derivation from the sails of ships, the blades used to drive traditional European windmills are commonly called sails whether or not they use a fabric covering. The term is also applied to the individual blades of the American farm windmill rotor, and sometimes refers to the blades of modern wind turbines.
Sandia National Laboratory: Albuquerque, New Mexico. One of the national laboratories in the United States resulting from development of the atom bomb and contracted by the U.S. Department of Energy for research on Darrieus wind turbines.
San Gorgonio Pass: Near sea level gateway from Southern California’s coastal region to the Sonoran Desert about 90 miles (150 kilometers) east of Los Angeles. Bounded on the north by Mount San Gorgonio and on the south by Mount San Jacinto, the pass has some of the strongest winds in California. During the mid-1990s, wind turbines in the pass generated 500-600 million kWh per year.
Santa Ana: Powerful hot dry wind that blow from the high deserts of Southern California into the Los Angeles basin.
Savonius rotor: S-rotor. Simple drag device producing high starting torque developed by the Finnish inventor, Sigurd. J. Savonious. Sometimes called an S-rotor because of its appearance in plan view. Due to its ease of construction and high torque, it is well suited for water-pumping in developing countries. For a drag device (similar to a cup anemometer with some flow recirculation) it is fairly efficient (31%), but like other drag devices is inefficient in its use of materials and, consequently, has had limited application.
SCAQMD: South Coast Air Quality Management District. Regulatory agency responsible for cleaning the air of the Los Angeles basin.
Scavaging: The removal of dead and decaying animals from a locale by predators or by animals that feed on carrion. The rate at which dead birds are scavenged or removed from a site by carrion-eating animals, such as coyotes, vultures, and ravens, is an important factor in determining the total number of birds killed by wind turbines relative to those counted by an observer.
SCE: Southern California Edison. Investor-owned electric utility serving southern California.
Scheduled outage: Termination of wind turbine operation due to a planned event such as scheduled (or planned) maintenance, training, tours, etc. Scheduled outages are attributed to either the wind turbine, the utility system, or non-operational wind turbine related events.
Schneefresser: Swiss wind known as the snow-eater.
Sea breeze: Light on shore wind caused by differential heating of the land and water. The daytime wind associated with the land-see breeze effect. See Land-sea breeze.
Self excitation: When the field of a generator is excited internally as opposed to excitation provided by an external power source.
Self starting: The ability of a wind turbine rotor to begin spinning solely using the power in the wind without the aid of an external power source. The manner of operating a wind turbine with this capability. Some wind turbines use rotors that are capable of self starting, however, these turbines motor the rotor up to operating speed to prevent the rotor from operating for any length of time at the rotor’s resonant frequency.
Self furling: Form of rotor control used on conventional small wind turbines using tail vanes for orientation where the rotor automatically folds towards the tail in high winds. This can be accomplished by use of a pilot vane or by offsetting the rotor axis from the pivot or yaw axis. This moment or torque is counterbalanced by weights or springs in the American farm windmill or by careful design of the hinge between the tail vane and the nacelle in modern small wind turbines. See Furling and Vertical furling.
Self-regulating windmill: Term applied to American farm windmills that use self furling to protect the rotor in high winds.
Self-Supporting Tower: Free-standing or cantilevered tower. Tower that does not require guy wires to remain upright.
SEP: Samenwerkende Elecktriciteits-produktiebedrijven. Association of Dutch Electric Utilities
SERI: Solar Energy Research Institute (U.S.). Renamed the National Renewable Energy Laboratory in the mid-1990s.
Serviceability: Ease of maintenance or repair.
Service hoist: Small winch sometimes located in the nacelle of medium-sized wind turbines for raising and lowering objects from the nacelle.
Servomotor: Electric or hydraulic motor used to control a device. The yaw motor used on a wind turbine to orient the turbine into the wind is a servomotor.
Setback, aesthetic: American usage for a buffer zone between wind turbines and a site’s property line intended to compensate or ameliorate the visual intrusion of the wind turbines in a vista.
Setback, public safety: American usage for a buffer zone between wind turbines and a site’s property line intended to protect the public from any real or perceived hazard from the wind turbines.
Shank: Shaft.
Shear: Force acting at right angles to the longitudinal axis of a bar, beam, or rod.
Short circuit: Accidental connection of low resistance established across a circuit causing an excessive or dangerous flow of current. Short.
Shroud: A structure surrounding a wind turbine rotor to concentrate or augment the windstream in order to extract more energy from the wind. A structure to deflect the wind around an object, such as a wind turbine tower, to reduce drag or turbulence.
Shunt regulator: Electrical device that dumps or diverts current from a source to a ballast load, such as a resistance heater, for voltge control. (Hugh Piggott)
Shunt wound: Connecting the field windings of a generator in parallel with those of the armature. Field excitation is provided by current from the armature. Because it is self-exciting, shunt-wound generators are dependent upon residual magnetism in the pole shoes of the field.
Sine wave: Waveform where the amplitude varies with the sine of a linear function of time as represented by a sinusoidal curve such as the voltage and current waveforms produced by ac generators.
Single-phase: An alternting current supply with only one voltage component or waveform. (1Φ) Rural areas in North America are typically served by single-phase distribution lines. Most wind turbines designed for interconnection with the electricity network produce three-phase current.
Sirocco: Italian for a hot humid southerly wind in southern Italy, Sicily and the Mediterranean originating on the Sahara Desert. Also spelled scirocco.
Site survey: Assessing the potential for using wind energy at a specific location, including wind resources, access, and potential land use conflicts.
Slewing: Yawing or turning to the wind.
Slew ring: Yaw ring or gear on wind turbines using active yaw.
Sling: Strap of nylon webbing or steel cable with loops in both ends, or a simple loop of nylon webbing used for cradling or hoisting heavy equipment with a crane. Used frequently when assembling and erecting a wind turbine.
Slip: Difference between synchronous speed and the operating speed of an induction generator. Given as percentage of synchronous speed. The slip increases with load on an induction generator. The slip of most induction generators is less than 3% or less than 50 rpm for a 1500 rpm generator, however some induction generators can slip as much as 10%.
Slip rings: A series of metal rings used to transmit electric current or signals from a rotating shaft to a fixed shaft. Used on many small wind turbines to transfer electricity from the generator in the nacelle, which yaws in response to changes in wind direction, to fixed conductors on the tower. Medium size wind turbines use droop cables instead of slip rings for conducting electricity from the generator to ground level. See also Droop cable.
Slot effect: The benefit derived from air flow through a gap between a slat or auxiliary airfoil and a wing or second airfoil. The slot effect was first noticed on jib-rigged sail boats where the foresail directs flow over the mainsail, accentuating the mainsails performance.
Slow-speed shaft: See Main shaft. The shaft connecting a wind turbine rotor and its drive train, so called because it spins at lower speed than the output or high-speed side of a stepup gearbox needed to drive conventional generators.
Smeaton, John: (1724-1792) British civil engineer who rebuilt the Eddystone lighthouse. Recognized as one of the earliest professional engineers, Smeaton conducted the first scientific studies of wind turbine rotors and published his results “On the Construction and Effects of Windmill Sails” in 1759. His conclusions, among others, was that the maximum power of windmill sails was nearly proportional to the cube of wind speed.
Smidth, F.L.: Danish company which designed machinery for working with concrete and which used this technology to build concrete silos and chimneys during the 1930 and 1940s. One of the world’s first firms to marry the rapidly advancing field of aerodynamics to wind turbine manufacturing. During WWII the firm manufactured several 17.5 meter and 24 m “Aeromotors” that used modern airfoils upwind of a concrete tower.
Smith-Putnam wind turbine: The world’s largest wind turbine until the wind energy revival of the 1970s. Named after engineer Palmer Cosslett Putnam and the S. Morgan Smith Co., a manufacturer of hydroelectric turbines. Putnam assembled a talented team of engineers and academics to build a 1 MW wind turbine with a two-bladed downwind rotor 175 feet (53 meters) in diameter. The turbine was installed atop Grandpa’s Knob on Lincoln Ridge near Rutland, Vermont in October 1941 and operated sporadically until it threw a blade in March 1945 and was dismantled. During its short lifetime the turbine generated only 62,000 kilowatt-hours.
Smock mill: Traditional European windmill in which the cap containing the rotor, mainshaft and gear assembly turns about the vertical axis of the tower to face the wind. So named because the flared wooden towers resemble a peasant’s smock. In contrast to the smock mill, the entire tower and cap of post mills turn to face the wind.
SMUD: Sacramento Municipal Utility District. Electric utility serving Sacramento, California and former operator of the Rancho Seco nuclear power plant.
Snap hooks: A hook closed on the open side by a spring-driven plate that is used on safety lanyards and fall restraint systems.
Social costs: Non-monetary costs associated with a technology by its direct and indirect effect on human welfare. See also External costs.
Soft towers: Towers where the fundamental system frequency is less than the blade passage frequency. (Louis Divone)
Solidity: Quotient of total blade area divided by frontal or swept area of a wind turbine. In general, rotors with high solidity are less productive than those with low solidity. The three-bladed Gedser mill had a solidity of 0.09. The solidity of American-designed high-speed, two-bladed turbines, such as the Mod-0 and the ESI-54, were 0.03.
Sound levels: See Noise levels.
Sound power level: Lw. A measure of acoustic power in decibels. The source or emission strength of wind turbine noise derived by field measurements of the sound pressure level.
Lw= Lp + 10 log(4πR²)
where Lp is the measured sound pressure level, and R is the slant distance from the nacelle. If the sound pressure level was measured on a reflective panel, 6 dB must be deducted from Lp. Sound power level is used in various noise models to project noise at various distances from a wind turbine. The sound power level of most commercial wind turbines varies from 95 dB(A) to more than 100 dB(A).
Sound pressure level: Lp. A measure in decibels of pressure relative to a reference pressure of 20 micronewtons/m². Because sound pressure levels decrease with increasing distance from the source, location is always specified or implied. For most discrete sources, such as wind machines, the distance from the listener is just as important as the noise level of the source. For example, Danish wind turbine manufacturers estimate that the noise from a typical medium-sized wind turbine will drop to 45 dB(A) within 150 meters (500 feet). Not to be confused with Sound power level, though both measures use the same units: decibels.
Span: The overall length of the airfoil in the direction perpendicular to the cross section.
Spar: A structural member running the length of an airplane wing or wind turbine blade.
Sparklies: American colloquialism. See Reflections.
Specific capacity: Total energy generated in kilowatt-hour divided by the rated power of the wind turbine in kilowatts. At good sites medium-sized wind turbines can generate more than 2,000 kWh/kW of rated capacity. At extremely energetic sites some wind turbines have produced in excess of 4,000 kWh/kW per year. Unlike specific yield, specific capacity is dependent upon the arbitrary rating of the wind turbine in kilowatts.
Specific land use: Measure of the amount of land used by wind turbines relative to installed capacity, swept area, or annual generation. Open arrays, such as those found in Europe with an 8 rotor diameter by 10 rotor diameter spacing, occupy 100 m² of land per m² of swept area. Dense packed arrays, such as those found in California, may use as little as 20 m² of land area per m² of swept area for an array with a 3 by 6 rotor diameter spacing.
Specific tower head mass: A measure of a wind turbine’s material intensity. The mass of the nacelle and rotor relative to the area swept by the rotor in kg/m². Lightweight high-speed turbines can have a specific mass as low as 10 kg/m² while large wind turbines have weighed in at more than 70 kg/m². Most medium-sized European wind turbines have a specific tower head mass of 20-30 kg/m².
Specific yield: A measure of wind turbine productivity within a specific wind resource. Net kilowatt-hour generation per square meter of rotor swept area per year (kWh/m²/yr). Unlike capacity factor, specific yield is independent of arbitrary wind turbine ratings and is, thus, a more reliable indicator of performance.
Speed, generator: rpm or revolutions per minute.
Speed, rotor: rpm or revolutions per minute of wind turbine’s rotor.
Speed increaser: Transmission. Gearbox. Increases speed of low-speed or main shaft from turbine rotor to that needed to drive a generator or other device.
Speed up effect: Increase in wind speed induced by terrain features, such as the curvature of a hill.
Spinning reserves: The plant capacity, in excess of actual load, which can be called on to produce electricity at very short notice.
Spoilers, blade: A long narrow panel that when raised on the upper surface of an airfoil destroys or spoils lift and increases the drag of the airfoil. Method of overspeed control on wind turbine rotors.
Square wave: An oscillation of the voltage waveform between two extremes without any intermediary steps such as that produced by early solid-state inverters.
Squirrel cage rotors: The rotor of an induction generator where coils of copper bars or wire are arranged in slots on an iron core and are short-circuited on the ends of the rotor by rings.
Stage mill: Smock mill with a stage or platform from which the miller could work the sails.
Stall, aerodynamic: A condition of an airfoil when an excessive angle of attack causes separation of the flow over the airfoil resulting in a loss of lift and an increase in drag. This is a dangerous condition in aircraft, but it can be put to constructive use in limiting power from a wind turbine rotor.
Stall regulation: Power control using stall. The blades of a fixed-pitch wind turbine operating at a constant rotor speed can be designed to stall in high winds to limit the wind turbine’s maximum power.
Stall-controlled rotors: Wind turbine rotors that use stall to limit maximum power in high winds.
Stand-alone power system: Electric power system independent of the network or grid often used in remote locations where the cost of stringing lines from large central power plants is prohibitive.
Star or Y connection: A three-phase connection where each phase is connected between a supply conductor and a common or neutral point. (Hugh Piggott)
Start-up wind speed: Minimum wind speed at which a wind turbine rotor at rest will begin to spin. The start-up wind speed differs from the cut-in wind speed, when the wind turbine begins producing usable power, but in some small wind turbines the two are the same.
Stator, generator: That part of a generator which is stationary as opposed to the rotor which revolves.
Stays: A support or brace. Nautical expression for guy rope or cable used to support a mast or spar. Early Danish wind turbines, such as the Windmatic 14S, were patterned after the Gedser mill which used struts and stays to brace the rotor.
Stiff towers: Towers where the lowest natural frequency of the system is higher than the blade passage frequency. (Louis Divone)
Stock: The spar used to support the sail of a traditional European windmill. Most Dutch windmills used stocks from a single continuous piece of timber. German, Danish, and British windmills used one piece, two piece, and three piece stocks made from either timber or metal.
Stock-watering: Providing ample water for cattle to accelerate weight gain as well as to insure their survival on the steppes of North America during the droughts of late summer. An important application for wind pumps.
Storage, compressed air: The ability to transform kinetic energy to potential energy in the form of compressed air in a reservoir. In practice surplus electricity is used to drive a compressor. When energy is needed, compressed air is released through a turbine where it is used to burn a fuel under pressure. In this way the compressed air reservoir replaces the compressor side of the gas turbine.
Storage, pumped: The ability to transform kinetic energy to potential energy in the form of water in a reservoir. Surplus electricity is used to pump water uphill where it is stored in a reservoir until needed. When electricity is needed, water is released to drive hydroelectric turbines.
Storage batteries: The ability to transform kinetic energy to potential energy in the form of chemical reactions in batteries.
Straight-bladed VAWT: As the name implies, the airfoils are straight as opposed to the curved (troposkein) shape of the blades on a phi-configuration (Eggbeater) Darrieus. H-rotors use straight blades.
Stress-Relieved: Heat treatment for metals that allows internal stresses to dissipate.
Strip chart recorders: Obsolete form of recording device that uses long sheets or strips of paper on which data is recorded.
Struts: Rods or bars used to brace a tower or wind turbine blade. The Gedser mill used struts and stays to strengthen the rotor.
Sucker rod: North American, the wooden or metal pole connecting the windwheel to the pump cylinder in a water-pumping farm windmill.
Sulfur oxides: SOx. Pollutant emitted by the combustion of fossil fuels which contributes to acid rain.
Surge protection: Electronic device designed to dissipate rapid increases in voltage or current.
Survey, public opinion: A statistical poll of attitudes of a representative sample of the population in a given region.
Survival wind speed: Maximum sustained wind speed a wind turbine can withstand without catastrophic damage. See design wind speed.
SWECS: Archaic jargon for Small Wind Energy Conversion System or small wind turbine.
Swept area: The area of the wind stream swept by a wind turbine rotor. For a conventional wind turbine, the swept area is the area of a circle.
A=πR2
where R is the rotor’s radius. For H-configuration VAWTs the swept area is
A=HD
where H is the length or height of the blades and D is the diameter of the rotor. For phi-configuration Darrieus, swept area is approximately the area of an ellipse. See also Intercept area and Capture area.
Synchronized stop of two-bladed rotors: A technique thought to ameliorate the visual intrusion of two-bladed wind turbines where the rotors of all units in an array are parked in the same position, preferably horizontal. First proposed by landscape architects at the University of California Polytechnic in Pomona during the mid-1980s.
Synchronous generator: Generator where ac frequency is directly proportional to the speed of the generator’s rotor. Most wind turbines interconnected with the utility network use asynchronous generators because of the generator’s simplicity. Those wind turbines that use synchronous generators produce variable frequency current at variable voltages.
Synchronous inverter: Power conditioning device for synchronizing wind generator frequency and voltage with that from the grid or network and, where necessary, inverting DC to AC.
Tvind: Between 1976 and 1978 the Tvind school near Ulfborg on Jutland’s North Sea coast assembled Tvindkraft, one of the world’s largest wind turbines. Though the 54 meter wind turbine never was permitted to operate at its rated power of 2 MW, it did generate about 1 million kilowatt-hours per year to heat the school. Tvind operated some 50,000 hours before it broke a blade in 1993. The blade was replaced and the wind turbine was still in regular service as of mid-2024.
Tændpibe-Velling Mærsk: A geometric array of 100 wind turbines on Denmark’s Jutland peninsula situated southeast of Ringkøbing on the east side of Ringkøbing fjord. One of the first wind power plants built in Europe. The turbines have since been removed and the site repowered.
Tag line: A light-weight rope used to steady unwieldy loads being hoisted by a crane. Tag lines are frequently used to prevent damage of a wind turbine rotor as it is lifted to the nacelle.
Tail vane: As in the weather vane, the vertical surface that aligns itself parallel to the wind. The tail vane of a small conventional upwind turbine keeps the rotor facing into the wind.
Taper, blade: Narrowing of chord from root to tip. Taper can be linear or nonlinear. High-performance rotor blades are tapered for much the same reason as the blades are twisted: the tangential velocity of a point on the blade decreases the closer the point is to the hub. Consequently, the tip-speed ratio declines towards the hub and a greater solidity is required to maintain an optimum coefficient of performance. This greater solidity can be provided by tapering the blade to a much wider chord at the root.
Tariff: Rate schedule. A schedule of prices or fees for the use of electricity.
Tax credit: Under U.S. tax law, a credit directly reduces tax liability. In contrast a tax deduction reduces net taxable income and only indirectly reduces tax liability. One dollar of tax credits is worth one dollar in after tax value to a U.S. taxpayer.
Tax deduction: Under U.S. tax law, a deduction reduces net taxable income. This reduces taxable liability in proportion to the level of taxation. One dollar in deductions is worth $0.30 in after tax value for someone in the 30% tax bracket.
Teetered hub: Rotor hub with a pivot that permits two-bladed rotors of conventional wind turbines to move as a rigid body several degrees perpendicular to the plane of rotation (±10 degrees). So called because the motion of the rotor resembles that of a playground teeter-totter or seesaw. Teetering is a passive means for reducing the aerodynamic loading on a wind turbine rotor and the cyclic loads on the wind turbine’s drive train.
Tehachapi Pass: The only all-weather route through California’s Sierra Nevada Mountains. The pass follows the trace of the Garlock fault through the east-west trending Tehachapi Mountains. The Tehachapi wind resource area is one of the world’s largest producer of wind-generated electricity. During the mid-1990s wind turbines in the Tehachapi Pass generated 1.3-1.4 Terawatt-hours per year.
Terrain enhancement: Increase in wind speed induced by terrain features, such as the curvature of a hill. Terrain features, such as a narrow mountain pass, that increases wind speed.
Three-phase: An alternting current supply where each of three voltage components or waveforms differ in phase by 1/3 of a cycle or 120 degrees. (3Φ) All medium-size and nearly all wind turbines designed for interconnection with the grid produce three-phase current. The simplest three-phase system uses three conductors. Each conductor serves as the return path for the other two.
Throwing: Deformation of trees and shrubs from high winds where the trunk as well as the branches lean or bend to leeward. Throwing indicates higher winds than flagging. (Putnam)
Throw line: Light rope thrown over a limb, cross-arm, or tower to raise a heavier rope.
Thrust, on rotor: Drag in the streamwise direction that acts to force the blades of an upwind rotor towards the tower. Overturning force on a wind turbine and tower.
Thrust, on tower: Drag in the streamwise direction that acts to force the tower to bend downwind. Overturning force.
Thyristors: High speed electronic switches used in electronic inverters or power conditioning equipment.
Tip, blade: That portion of a wind turbine blade opposite the hub or root end.
Tip brake: Means of overspeed control incorporating a plate located at the end of a wind turbine blade such that when deployed the plate dramatically increases drag. The term is sometimes incorrectly applied to Pitchable blade tips. Unlike pitchable blade tips, tip brakes do not contribute to the blade’s torque when undeployed. Tip brakes create parasitic drag and turbulence that accentuates tip noise.
Tip loss: The loss of lift associated with air flow around the tip of a blade instead of across it. Both AeroVironment in the United States and the Technical University of Delft in the Netherlands have attempted to design blades with devices that prevent tip losses. However, commercial blade manufacturers have found that is cheaper and structurally more sound to extend the blade’s span than to add tip augmentors.
Tip speed: Speed at the tip of a rotor blade as it moves through the air, often given in feet or meters per second. The tip speed of “slow running” medium sized wind turbines ranges from 50 m/s to 60 m/s. The tip speed of “fast running” medium sized wind turbines ranges from 70 m/s to 120 m/s. Aerodynamic noise is nearly proportional to tip speed.
Tip speed ratio: The ratio of blade tip speed to wind speed. Modern wind turbines use high-speed airfoils that operate at tip-speed ratios above five. Lift devices use airfoils that operate at tip-speed ratios of more than 1. Drag devices run at tip speed ratios of less than 1.
Tip-vortices: Power robbing noise creating turbulence found at the end of wind turbine blades. The swirling spirals of air are similar to eddies downstream of a rock in a swift mountain stream.
Top-down technology development: Command and control of technology development by a central organization, such as a government agency or other large institution.
Tornado vortex: American boondoggle. Wind generation concept funded by the U.S. Department of Energy through the Solar Energy Research Institute’s Advanced and Innovative Concepts program during the 1970s. The concept conceived of a large cylindrical tower resembling a natural-draft cooling tower with slotted openings and guide vanes that would direct incoming air to create a tornado vortex. This vortex would draw in further air at the base of the tower. The rising column of air within the tower would drive a high-speed turbine. No prototype was ever built.
Toroidal accelerator rotor platform: TARP. Another candidate in U.S. Department of Energy’s Advanced and Innovative Concepts program during the 1970s. In the TARP concept, a structure or augmentor is constructed in the shape of a hollow doughnut (toroid) at which dual high-speed turbines are mounted tangentially on either side of the core. Accelerated flow around the toroid drives the turbines. No prototype was never built.
Torque: Turning or twisting force. A force that produces a rotating motion or the force created by a rotating motion. When a wrench is used to tighten a bolt, torque (force times the moment arm) is applied to the bolt. For the same amount of force, torque is increased by increasing the moment arm. Power is the product of torque and shaft speed or rpm. To transmit an equal amount of power a rotating shaft can either operate at high speeds and low torque or low speeds and high torque. As wind turbine rotors increase in size, their rotational speed decreases. Consequently, to transmit the power available, torque must increase. As torque increases so does shaft size. At higher torques, a larger diameter shaft is needed to resist the force acting to twist the shaft in two.
Torque ripple: Effect on drive train from two-bladed vertical axis wind turbines. The lift and hence torque produced by the blades of vertical wind turbine varies with the position of the blades as they move about around the tower. Lift, and hence torque, is greatest when the blades are moving across the wind and least when moving with or against the wind. Thus the torque varies from a maximum to a minimum several times per rotor revolution.
Tower mill: Similar to the smock mill except that the tower is much taller than that in a Smock mill and usually made of stone or brick instead of wood
Tower shadow: Wake created by windstream in the lee of a tower. Each blade on a downwind rotor must pass through the wake once per revolution creating the characteristic “whop-whop-whop” of downwind rotors.
Tower: A component of conventional wind turbines necessary to elevate the rotor above the ground.
Trailing edge: Opposite the leading edge of an airfoil.
Trail: American usage for footpath or bridleway.
Tramontana: Provençal for a strong cold wind flowing down the northeastern flank of the Pyrenees towards the Mediterranean Sea. Not as well known as the Mistral but equally as powerful.
Tramontano: Italian for cold north wind.
Transfer switch: Electrical switch for simultaneously disconnecting a utility customer from the utility network while connecting them to a independent power system. Frequently used in rural America to provide power for emergencies loads during outages of the utility distribution system.
Transient: A pulse or other temporary spike in voltage or current in an electrical distribution system. Transients can be caused by lightning, by the opening and closing of switch gear, or by operation of certain loads.
Translation: Change in physical position. Used to describe the motion of wind machine’s blades and rotor as in a translating airfoil. Drag translators are devices that convert the energy in the wind into the motion of a blade by means of aerodynamic drag.
Transmission: Device for transmitting the power of the rotor to that of the load, most commonly used for converting the low speed of the main shaft to the high speed required by a generator. See also Gearbox.
Transmission and distribution: The combination of power lines, transformers, switch gear and other electrical equipment used by an electric utility to transfer electric power from a source of generation to the utility’s loads.
Trans-sonic event: When a wind turbine goes into overspeed and the blade tip exceeds the sound barrier. (Mark Haller)
Troposkein: Greek for a shape that approximates that of a rope fixed at both ends and sagging due to gravity. The shape of a spinning “skip rope” in American parlance. Many phi-configuration Darrieus rotors used blades curved to approximate the troposkein shape to eliminate bending stresses.
Truss towers: Free-standing towers constructed from a rigid triangular framework of metal beams, bars, tubes, or angles. See also Lattice towers.
Tubular tower: Tower made from rolled steel plate, generally unguyed though sometimes with stays near ground level.
Turbine: Rotor. Sometimes used in reference to a rotor with a large number of blades, for example, the “bicycle wheel turbine.”
Turbulence: Sudden changes in both wind speed and direction associated with air flow about an obstruction. Turbulence is undesirable because it decreases harnessable wind power and increases wear and tear on a wind turbine.
Turbulence intensity: Quotient of instantaneous wind speed divided by the mean wind speed for a given period.
Turntable: Revolving platform as in phonograph turntable. In American usage, it refers to an assembly of a shaft or pintle and a platform that allows a conventional wind generator to yaw in response to changes in wind direction.
Twist, blade: Variation of blade angle or pitch from root to tip. The pitch is greatest at the root and smallest at the tip, where the blade is parallel, or nearly so, to its direction of travel. Johannes Juul varied the pitch of the blades on the Gedser mill from 13 degrees at the hub to 0 degrees at the tip.
Two-speed generators: Asynchronous or induction generators with dual windings. One set of windings energizes the field of six poles. The second set of windings energizes the field of four poles. Under normal wind conditions the controller energizes only four poles, enabling the generator to reach synchronous speed at 1500 rpm in Europe or 1800 rpm in North America. During low winds, the wind turbine controller energizes six poles enabling the generator to reach synchronous speed at 1000 rpm in Europe or 1500 rpm in North America. This permits operation of the wind turbine rotor at two speeds.
Two-speed operation: The functioning of a constant-speed wind turbine rotor at dual speeds by use of a two-speed induction generator. Operation at two constant speeds enlarges the envelope of wind speeds over which the constant speed wind turbine rotor is most aerodynamically efficient.
Two-stage gearbox: A series of gears in a transmission that are arranged in two steps.
UCS: Union of Concerned Scientists. Environmental group in the United States that has aggressively encouraged energy efficiency and renewable energy development.
Umbrella mill: Rosette mill. Daniel Halliday’s first version of the self-regulating, water-pumping windmill. Halliday hinged the wooden slats (commonly used for blades at the time) about a metal ring. The blades were counterbalanced so that under normal operation the blades would form the familiar disc of the farm windmill. During high winds, however, the force on the blades counteracted the weights and the wind swung the blades into a hollow cylinder. This allowed the wind to pass through the rotor unimpeded. Also dubbed the rosette, the umbrella mill was supplanted by the more popular fixed rotors that were the direct forebears of the American farm windmills seen today.
Uniformity, visual: Resemblance in size, color, shape, and configuration. Method proposed by landscape architects in California to reduce the visual intrusion of wind turbines on a landscape by minimizing contrasts between like machines. Objects need not be identical, but merely appear similar to elicit the positive human response to uniformity.
Upper Midwest: The states of Iowa, Minnesota, and Wisconsin and portions of the states of North and South Dakota. Vast reaches of open land and moderate-to-good wind resources have long enticed wind development. By the mid-1990s only Buffalo Ridge in southwestern Minnesota had seen any extensive activity.
Upwind: Windward. The side facing the wind.
Upwind, rotor configuration: A horizontal axis wind turbine in which the rotor is oriented on the windward side of the tower. Upwind turbines employ some means for orienting the rotor into the wind. Small wind turbines use a tail vane. Medium-sized wind turbines mechanically orient the rotor into the wind.
USDA: U.S. Department of Agriculture. Administers an experiment station at Bushland in the Texas Panhandle where it tests wind turbines for rural use.
Usefulness: Concept by California landscape architects that visual intrusion of wind turbines on the landscape is deemed acceptable by many observers when the observer perceives that the wind turbines are being used, that is operating, and are useful, that is producing electricity.
Utility interconnection: The point of electrical connection between a wind turbine and an electric utility’s distribution or transmission network. In contrast, a stand-alone power system is not connected to an electric utility.
Utility-compatible generation: Electrical generation of alternating current that meets an electric utility’s requirements for voltage and frequency.
Vane: Wooden or metal blade on the American farm windmill.
Vaneless mill: Umbrella or Rosette mill. Early American farm windmill that did not use a tail vane, hence vaneless, to orient the rotor with changes in wind direction. The rotor on vaneless mills operated downwind of the tower.
Variable Geometry VAWT: Non-articulating (fixed-pitch) straight bladed vertical axis wind turbine which reduces its intercept or frontal area to limit power in high winds. Invented by Peter Musgrove at Reading University, England and pursued by Sir Robert McAlpine in an unsuccessful attempt to commercialize the concept. Up to its rated speed, the rotor has an H-shape with the rotor blades in a vertical position. Above its rated speed, the geometry of the rotor varies as the blades are flung from the vertical about a hinge where they are attached to the rotor cross arms. This movement reduces the rotor intercept area, limiting the amount of power the wind turbine will capture and preventing the rotor from destroying itself.
Variable pitch: A method of controlling rotor torque and power by changing blade pitch.
Variable speed generator: Any of several kinds of generators and accompanying power conditioning equipment that enables a generator to operate at variable speeds while generating utility-compatible electricity.
Variable speed operation: The operation of a wind turbine where rotor speed is proportional to wind speed. Operating the turbine at variable speed enables the rotor to maintain an aerodynamically optimum relationship between tip speed and wind speed, and it permits the rotor to store the energy in gusty winds as inertia rather than forcing the drive train to absorb the increased torque instantaneously.
Variable-pitch rotor: Wind turbine rotor using variable pitch blades.
Variable-stroke wind pump: Mechanical wind pump that automatically varies the pump’s stroke to optimize the relationship between rotor torque and the load.
Velocity: A vector of speed and direction.
Velocity duration curve: Graph of velocity or wind speed and the amount of time or duration the wind occurs at a range of wind speeds.
Vertical axis wind turbine (VAWT): Wind turbine whose rotor spins about a vertical axis, for example, Darrieus rotors and cup anemometers. Not to be confused with the term “vertical wind turbines” used by historians and industrial archeologists that refers to the rotor plane, not the axis of rotation.
Vertical furling: Form of rotor control on conventional small wind turbines where the rotor tips vertically toward the tail vane in high winds. This movement is counterbalanced by weights, springs, or careful design of the hinge axis. See also Furling and Self furling.
Vertical wind speed profile: Graphical representation of the change in wind speed with height.
Village electrification: Supply of electricity to rural communities currently without power. With modern hybrid wind and solar power systems it is now possible to provide electricity without extending the distribution system from central stations.
Visibility: The state of being visible or of being easily observed. Visibility is distinct from visual intrusion or visual impact. An object, such as a wind turbine, that is visible is not necessarily an intrusion on a landscape. See also Conspicuousness.
Volt (V): The unit of electric potential or electromotive force equal to the difference between two points where the current flow of one amp dissipates one Watt of power.
Volt-ampere reactance (VAR): The reactive power consumed when magnetizing the field in a generator equal to the product of current, voltage, and the sine of the phase difference between current and voltage. A measure of reactive power.
Vortex generators: Auxiliary aerodynamic devices attached to the low pressure surface of airfoils which delay flow separation, increasing the airfoil’s aerodynamic performance. Small vanes normal to the low-pressure surface of the airfoil set at an angle to the incoming flow that create vortices on the wing surface. The vortices increase lift, but also increase drag.
Vortices: Eddies. A whirling mass of water or air.
Wake: Turbulence downstream of an obstruction in a moving fluid.
Water churn: A vessel in which water is agitated to generate heat.
Water-pumping windmills: Wind pumps. Most often applied to farm windmills that mechanically pump water.
Watt: Unit of power equal to one amp at one volt.
Watt governor: Flyball governor. Named for James Watt, the Scottish inventor of the condensing-cycle steam engine (1769). The revolving weights or flyballs, acted upon by centrifugal force, limit shaft speed through a mechanical linkage. Though often credited to Watt, this early feedback mechanism was used on traditional European windmills long before the age of steam.
Watt-hour (Wh): Unit of electrical energy equal to the generation or use of one watt for one hour.
Weibull wind speed distribution: A mathematical idealization of the distribution of wind speeds over time: the amount of time the wind blows at a given wind speed.
F(U1≥U)=8760exp[-(U/C)k]
where exp is an exponential function, C is the empirical Weibull scale factor in m/s, and k is the empirical Weibull shape factor. The Rayleigh distribution is a special case of the Weibull distribution where k, the shape factor, is 2.
Weldment: A multi-piece metal assembly fastened together by welds rather than bolts or other means.
Well casing: Pipe installed in the well bore to prevent the well wall from collapsing.
Wheel: Rotor on American farm windmill. Colloquial expression for rotor of multiblade wind turbine.
WIMP: Wind Industry Monitoring Program of Riverside County California.
Wincharger: Brand of windcharger or battery charging wind turbine manufactured in the United States from the 1930s into the 1970s.
Wind assist: Innovative approach to using wind energy where the wind turbine provides all the power to the load when the wind is sufficient, and where it assists a conventional power source when the winds are inadequate to deliver all the power needed. When the wind is not blowing, the conventional source picks up the load as the wind turbine stands by.
Wind atlas: Collection of wind resource maps for a region.
Wind dependent availability: The amount of time a wind turbine is available for operation relative to the amount of time wind speeds are within the turbine’s operating range.
Wind-driven generator: Wind generator. More accurate but less commonly used description of a wind-electric turbine.
Wind dynamo: Wind turbine. Wind-electric generator. From Volta Torrey’s description in Wind Catchers (1976) for a machine that converts the wind’s mechanical energy into electrical energy.
Wind Energy Conversion System (WECS): Archaic jargon for wind machines. A generic term that includes wind turbines, wind generators, and windmills that was used by the U.S. Department of Energy and its contractors during the 1970s and 1980s.
Wind engine: American farm windmill adapted for mechanical tasks other than pumping water.
Wind farm: Wind power plant. Array. An array of multiple wind machines sited within one geographic area and operated as one unit. The term was derived from the literary association between farming and harvesting wind energy: wind generation and farming depend upon seasonal cycles, the turbines are planted in rows like fields of corn (maize), and both are practices predominantly found in rural areas.
Wind furnace: Wind turbines designed strictly for heating. Advocates of the wind furnace concept believe that the same winter winds that rob a house of its warmth could be used for heating. Most attempts at manufacturing wind turbines solely for home heating have failed commercially.
Wind generator: Wind machine used to generate electricity.
Wind machine: Generic expression for any device that translates wind energy into motion, whether it be motion in one direction as in a sail boat or motion around an axis as in a wind turbine.
Wind park: See Wind power plant. Incorrect colloquial usage. In American and British English “park” connotes recreational enclaves or sylvan settings publicly protected from development. Arrays or clusters of multiple wind turbines are more correctly described as wind power plants. During the 1970s utilities in North America envisioned building “energy parks” of multiple nuclear reactors. Propagandists for the utilities deliberately chose the term “parks” to mislead public debate. Continued use of the term “wind park” by the wind industry may be construed as a similar attempt to deceive the public.
Wind plant: See Wind power plant, see also Wind turbine. In both American and European usage the term can mean individual wind turbine. In North America Marcellus Jacobs used the term to describe individual wind turbines during the 1930s and again during the 1970s.
WindplantTM: KENETECH Windpower trademark for a wind power plant built by the defunct Livermore, California manufacturer.
Wind power plant: Array multiple of wind turbines in one locale operated as one entity, much like the multiple steam turbines at a typical coal-fired power plant. Any cluster of wind turbines used for the bulk generation of electricity.
Wind power station: Electric Power Research Institute (U.S.) description of wind power plant.
Wind pump: Wind turbine used to raise water. There is the equivalent of more than 100 MW of wind pumps in use worldwide. See also American farm windmill.
Wind regime: Sum of regional wind characteristics.
Wind rose: Circular bar graph of wind speed, direction, and sometimes frequency of occurrence. It gets its name from the radiating bars that represent the principal points of the compass.
Wind run: Distance the wind travels during a given period. When used over a specific time period, an average measure of wind speed can be found. For example, if an anemometer records that 150 miles of wind passed through in a 10-hour period, the wind speed averaged 15 mph during the period.
Wind-run odometer: Simple device used for deriving average wind speed that measures and records the number of miles or kilometers that pass the anemometer.
Wind shear: Change in wind speed with height above the ground. See also wind speed gradient.
Wind shear exponent: A number derived empirically from surface roughness and used in the Power law model for estimating changes in wind speed with changes in height. See Power Law. The wind shear exponent for low-grass steppes such as the American Great Plains is approximately 1/7.
Wind speed: Wind velocity without the directional component of the vector. Colloquial for wind velocity.
Wind speed duration curve: Graphical representation of the amount of time the wind equals or exceeds a given speed with time on the horizontal axis and wind speeds on the vertical axis.
Wind speed frequency curve: Graphical presentation of the Wind speed frequency distribution.
Wind speed frequency distribution: The number or occurrences or the amount of time the wind occurs at discrete wind speeds. Presented in both tabular form and in graphical form. In graphical form wind speed is shown on the horizontal axis and the number of occurrences or time is shown on the vertical axis.
Wind speed gradient: Variation of wind speed with height above the ground. Normally wind speed increases with height. The rate of increase varies with the roughness of the terrain. The rougher the terrain, the greater the rate at which wind speed increases with height, because of greater frictional effects on air flow in the air nearest the ground.
Wind speed profile: Graphical representation of changes in wind speed with height above the ground.
Wind structure: Details of wind flow over an area. Includes variations in direction with height as well as speed, gustiness, and the non-uniform nature of the wind.
Wind turbine: As in water turbine. Often used as a generic term for any wind machine. Correctly used it refers only to a rotor made up of high-speed airfoils. Wind generator.
Wind Turbine Generator (WTG): Jargon to describe a wind turbine driving a generator. Often used by attorneys and brokers in California during the 1980s to impart a sense of technical sophistication.
Wind vane: Device for indicating wind direction. Weather vane.
Wind velocity: Wind speed in common parlance.
Wind-assisted irrigation: The application of wind energy as an aid or supplement or high-volume irrigation applications. Concept developed by the U.S. Department of Agriculture’s Bushland, Texas experiment station to help farmers on the High Plains.
Wind-diesel systems: Hybrid power systems using wind turbines in parallel with diesel generator sets for remote villages.
Wind-electric generator: Wind generator as in hydro-electric generator.
Windcharger: American usage for battery charging wind turbines from the pre-REA era (1930-1950).
Windmill: Generic term for wind machine. Specifically refers to a wind machine used for grinding grain. However, the familiar Dutch or European windmill was used not only to grind grain but also to cut timber, pump water, shred tobacco, and perform a multitude of other industrial tasks. In North America usage, the term windmill most often refers to a multiblade water-pumping wind machine: the “American farm windmill.”
Windmill, Dutch: Windmill commonly associated with the Netherlands, although found in nearly all European countries.
Windmill, water-pumping: Variously known as farm, Chicago, or Classic American farm windmill. Wind pump.
Windpower generator. Wind generator. Wind Turbine
Wind pump: Water-pumping windmill.
Windsmiths: Modern millwrights. Those who operate and maintain wind turbines. Coined by US Windpower (Kenetech) in the 1980s.
Windwacko: Promoters of crazy often previously debunked ideas for harnessing the wind. See zombie wind.
Wind wall: An array of wind turbines arrayed vertically on towers of different but uniform heights as well as arrayed horizontally across the prevailing wind. The wind wall in the Tehachapi Pass developed by Zond Systems in 1985 has been dismantled and the site repowered.
Windward: Upwind. On the side facing the wind. Snow is swept clear from the windward side of mountain summits.
Windweenie. Wide-eyed zealots who flocked to the wind industry in the 1980s with hopes of doing something good. Synonym: windjunky.
Windwheel: North American usage for the rotor on the water-pumping farm windmill. The revolving elements in a wind machine: blades, vanes, hub and shaft.
Windy city: Chicago, Illinois. Named for the blowhards (boosters) who promoted the city’s hosting of the Columbian Exposition in 1883. No connection with wind or wind energy.
Wing: Translation of Danish “Vinge” for wind turbine blade or vane.
Wood-composites: Assemblage of wood blocks or veneer and polyester or epoxy resins for use in wind turbine blades.
Work belt: Positioning device. Waist belt of wide leather or woven nylon straps used with a lanyard for freeing the hands while working in an elevated position. Often incorrectly called a safety belt.
Yaw: Rotation about a vertical axis of a horizontal axis wind turbine nacelle to maintain alignment with the wind direction.
Yaw angle: Angle about a vertical axis between the wind direction and the rotor axis of a horizontal axis wind turbine.
Yaw, active: A horizontal axis wind turbine yaw control system that uses an electrical or hydraulic servomechanism to orient the rotor with the wind in response to a signal from a wind vane.
Yaw axis: Vertical axis about which a horizontal axis wind turbine nacelle changes its orientation in response to changes in wind direction.
Yaw control: Means for orienting the rotor into or out of the wind or for limiting yaw movement. Wind turbine yaw control is subdivided into active and passive mechanisms. Passive yaw control uses forces in the wind itself to orient the nacelle with changes in wind direction, whereas active yaw uses a signal from a sensor to direct the nacelle into or out of the wind.
Yaw dampening: Means for diminishing or retarding yaw movement.
Yaw, passive: A horizontal axis wind turbine yaw control system that uses the natural aerodynamic forces of the wind to orient the nacelle and rotor in proper alignment with the wind. There are three common forms of passive yaw: coning on downwind rotors, tailvanes on small upwind rotors, and the fan-tail on either upwind or downwind turbines.
Yaw rate: The speed at which a horizontal axis wind turbine reorients the nacelle to changes in wind direction. Small wind turbines using tail vanes have high yaw rates compared to medium-sized wind turbines using active yaw control.
Yield, specific energy: Quotient of annual energy output divided by rotor swept area and given in kWh/m²/yr. The most reliable measure of overall wind turbine performance relative to a specific wind regime.
Zaan district: Site on the Zaan river in the Dutch province of Noord Holland with the largest documented concentration of windmills during the Netherlands’ golden age. Historians attribute the birth of the industrial revolution to the more than 700 windmills used along the Zaan in the 17th century.
Zaanse Schans: Open air museum of operating windmills adjoining Zaandam in the Dutch province of Noord Holland.
Zeebrugge: Belgian port that once contained 23 wind turbines installed on the harbor breakwater. Site of the world’s first “offshore” wind power plant.
Zephyr: Greek god of the west wind.
Zombie wind: Derogatory term dismissing impractical but reoccurring concepts debunked many times, e.g. “flying clotheslines,” shrouded or ducted turbines, rooftop wind, and squirrel cage VAWTs to name a few. These ideas never seem to die and keep rising from the dead, thus, zombie wind. (Robert Hamburger)
