The following article appeared in an 1999 edition of Renewable Energy World.
Wind energy is booming. Not since the heyday of the American farm windmill has wind energy grown at such a dramatic pace. By the new millennium, more than 40,000 medium-size wind turbines will be in operation worldwide, mostly in California, Europe, and India. These commercial wind turbines, including those found in California’s giant wind power plants, will produce 20 terawatt-hours (20,000 million kilowatt-hours) of wind-generated electricity annually.
But the success of medium-size wind turbines is only part of the story. Small wind turbines have found their role expanding as well. Whether it’s on the contemporary homestead of Ed Wulf in California’s Tehachapi Mountains, in the Chilean village of Puaucho overlooking the Pacific Ocean, or on the Scoraig peninsula of Scotland’s wind-swept west coast, small wind turbines are making an important difference. While their contributions may be small in absolute terms, small wind turbines make a big difference in the daily lives of people in remote areas around the globe. Small wind turbines may produce only a few tens of kilowatt-hours (kWh) per month, but this electricity goes much further and provides as much, if not more, value to those who depend upon it as the generation of their bigger brethren.
Today there are more than 50 manufacturers of small wind turbines worldwide, and they produce more than 100 different models. Altogether manufacturers in western countries have built about 60,000 small wind turbines during the last two decades. And tens of thousands more have been manufactured in China for use by nomads on the Mongolian steppes.
The largest number of machines built fall into in the micro wind turbine category. These are wind turbines so small you can carry them in your hands. Though micro turbines have been around for decades for use on sailboats, they have only gained prominence in the 1990s as their broader potential for off-the-grid applications on land has become more widely known.
Southwest Windpower awakened latent consumer interest in micro turbines with the introduction of its sleek Air 303. Since its debut in 1995, Southwest Windpower has shipped 18,000 of the popular and inexpensive turbines. Southwest’s Air 303 quickly gained ground on the stalwarts of micro turbines manufacturers Marlec, LVM, and Ampair. Marlec built more than 20,000 of its multiblade Rutland model during the 1980s and early 1990s, mostly for the marine market. Not far behind is LVM. They have built some 15,000 of their multiblade turbine. Ampair, another British manufacturer of micro turbines for the yachtsmen, has shipped more than 5,000 of their signature product, the Ampair 100.
The number of the more familiar household-size turbines built by companies such as Bergey Windpower and World Power Technologies in the United States and Proven in Scotland, though significant, is far smaller. Between them, two American producers have built more than 4,000 small wind turbines not unlike the windchargers once used throughout the Great Plains.
During the 1930s several Midwestern manufacturers built small battery-charging wind turbines for remote homesteads on the American steppes that stretch from Texas to Alberta. Often these windchargers were the only source of electricity for many farms and ranches in the days before central-station power was extended to the hinterlands. Thousands of wind turbines producing a few hundred watts to several kilowatts were built. Household names like Sears, Zenith, and Montgomery Wards could be found emblazoned on their tail vanes, along with less well-known brands such as Parris-Dunn.
With the advent of rural electrification, the windcharger industry collapsed in the late 1940s and early 1950s. And it wasn’t until the 1970s that the oil crises spurred renewed interest in small wind turbines. In those days the shortest route to wind energy was salvaging a 1930s-era windcharger and rebuilding it. Many did, and rebuilt Jacobs and Wincharger turbines, some now 50 years old, are still in service. It’s not uncommon even today to find advertisements for these machines in “Home Power” magazine.
During the 1980s manufacturers developed new designs for small wind turbines, incorporating lessons learned from the windcharger period. Most of the new battery-charging turbines switched from direct current (DC) generators to permanent-magnet alternators. The alternating current (AC) of these machines is rectified to DC for charging batteries or powering an inverter.
Several manufacturers in the 1980s built small wind turbines using induction generators for direct interconnection with an electric utility’s power lines. Though technically an elegant solution for integrating wind turbines with the utility grid, small interconnected wind turbines proved a commercial failure in the United States for regulatory and political reasons.
In Europe, however, small wind turbines driving induction generators, known to engineers as asynchronous generators, found a much more receptive climate, especially in Denmark, Germany, and the Netherlands. Experimenters, hobbyists, and small metal-working shops began building small wind turbines in the mid-1970s. Designed to supplement electricity from the local utility, the wind turbines gradually grew in size: from 10 kW to 15 kW, then 30 kW. By 1982 they reached the then incredible size of 50 kW. This grassroots effort spawned a billion dollar industry that today builds wind turbines from 500 kW to 1.5 megawatt.
Small Turbine Technology
Small wind turbines often confuse the uninitiated. Small wind turbines can present a bewildering variety of shapes and sizes. At one time the situation was even worse, when practically every conceivable form was on the market. Fortunately, the technology has now steadily evolved toward a common configuration and though they may look different, most are actually quite similar. The differences today are more subtle, much like the differences between photovoltaic panels: differences in how they generate electricity and in how they are controlled.
Small wind turbines can be built in many different shapes, and have been. But today nearly all small wind turbines are upwind, horizontal-axis wind machines where the rotor spins in front of the tower about a line parallel with the horizon.
Two or Three Blades
There has been a long and bitter debate about the merits of using two or three rotor blades on small wind turbines. (There was even a brief foray by a German manufacturer that used only one blade.) The only advantage of two blades over three is that two are cheaper. But it’s a case of penny wise and pound foolish. Turbines with three blades run more smoothly than two, and that usually means they will last longer. After long experience with small wind turbines, the U.S. Department of Agriculture’s Nolan Clark opts for three-blade rotors over the two-blade machines still occasionally found.
Most small wind turbines use composite materials, such as fiberglass (glass reinforced polyester), for their rotor blades. A few still use wood. Some have shifted to more exotic composites using carbon fiber instead of glass. None use aluminum because of its propensity to metal fatigue.
Because of their size, small wind turbines can’t afford the yaw motors and mechanical drives of the bigger upwind turbines. Nearly all small wind turbines use tail vanes to point the rotor into the wind. One of the only exceptions is Proven Engineering’s downwind turbines.
Wind turbines work in a far more rugged environment than photovoltaic panels that sit quietly on the roof. (One quickly appreciates this after watching a small wind turbine struggling through a gale.) There’s no foolproof way to evaluate the robustness of small wind turbine designs. You certainly can’t rely on the manufacturer’s pronouncements. No manufacturer of small wind turbines is going to tell a potential customer that their product is only suitable for light winds.
In general, heavier small wind turbines have proven more rugged and dependable than lightweight machines. Mick Sagrillo, a small wind turbine guru in the United States, is a proponent of what he calls the “heavy metal school” of small wind turbine design. Heavier, more massive turbines, he says, typically run longer.
Heavier in this sense is the weight or mass of the turbine relative to the area swept by the rotor. By this criteria, a turbine that has a relative mass of 10 kilograms (kg) per square meter may be more robust than a turbine with a specific mass of 5 kg per square meter.
All wind turbines typically have a means for controlling the rotor in high winds. Overspeed control is one of the characteristics that sets different brands of small wind turbines apart. Most micro and mini wind turbines “furl” or fold about a hinge so that the rotor swings toward the tail vane. Some furl the rotor vertically (World Power’s Whisper 500 and Southwest Windpower’s Windseeker), others furl the rotor horizontally toward the tail (Bergey Windpower, Marlec, LVM, and World Power). Several designs of household-size turbines pitch the blades (Proven and Vergnet), and one both pitches the blades and furls the rotor toward the tail vane (Wind Turbine Industries).
Most small wind turbines use permanent-magnet alternators. This is the simplest and most robust generator configuration and is nearly ideal for micro and mini wind turbines. There is more diversity in household-size turbines. Bergey Windpower sticks with its permanent-magnet alternator, but Wind Turbine Industries uses a conventional wound-field alternator, while France’s Vergnet uses an off-the-shelf induction generator.
One noticeable characteristic of some permanent-magnet alternators used by small wind turbine manufacturers, such as Bergey and World Power, is their inside-out design. The case to which the magnets are attached, sometimes called the magnet can, rotates outside the stator. In this configuration the blades can be bolted directly to the case, and they often are.
Most small wind turbines using alternators produce three-phase AC to make best use of the space inside the generator. Some battery-charging models rectify the AC to DC at the generator; others rectify it at a controller which can be some distance from the generator.
Today’s Market for Small Wind
The once promising prospect of tens of thousands of small wind turbines whirring above farms, ranches, and homes across the breadth of the United States, feeding electricity into the utility network, never materialized.
When the US Congress passed the Public Utility Regulatory Policies Act (PURPA) in 1978 small wind technology seemed poised for explosive growth. In part, the effort succeeded. More than 4,000 small wind turbines were installed in North America for this purpose. Similarly, California’s giant wind farms are also a direct result of PURPA and Congress’ lucrative tax incentives. But a host of problems beset wind turbines big and small. Some problems were technical: the turbines didn’t work as well as expected. Some problems were commercial: firms entered and left the business so fast that it was hard to tell who was a manufacturer and who wasn’t. And then the price of oil collapsed, so did interest in small wind turbines.
It was old, pedestrian uses, such as battery charging and water pumping, that saved small turbine manufacturers from extinction. Because these applications were more prosaic than feeding electricity into the utility system on a par with that from a nuclear plant, they were viewed as not quite so alluring in the American context and were overlooked for many years. But the technology survived.
The situation today is just the opposite of that in the early 1980s. Few small turbines are being interconnected with utility networks. Only committed environmentalists and the utilities themselves are interested in this technology, says Mike Bergey of Bergey Windpower.
Unlike those living off the grid, who are dependent on their power systems and thus place a high value on the power their systems produce, utility or mains customers are typically more cost-conscious. In strict economic terms, small wind turbines seldom make sense where utility power is already available, and excess generation can’t be banked with the utility through net-metering regulations.
The boom in small wind turbines, especially in North America, has been in battery-charging systems. The reasons are part political, part technological. In the United States, connecting a small wind turbine to the network simply doesn’t pay. But it can for those who live beyond the utility’s lines.
The development of cheap micro and mini wind turbines has made wind energy affordable and relatively easy to use for those living off the grid without mains power. The development of new inverters, compact fluorescent lamps, and other low-power electronic devices, as well as the widespread availability of photovoltaic panels has revolutionized living off the grid with a hybrid power system using both wind and solar energy.
The new millennium holds vast promise. The worldwide demand for small wind turbines has never been greater. Growing populations and aspiration in the third world are signaling a steadily swelling demand for small wind turbines. These machines will become part of hybrid power systems for village electrification, obviating the need for strapped central governments to extend power lines to every village. The demand for turbines to supply these applications will continue to put pressure on manufacturers of small wind turbines to improve reliability, ease of installation, and service.
Technology is inherently political, say high-tech pundits in the Silicon Valley. And efforts to tackle global environmental problems could lead to surprising political action. Germany’s highly successful electricity feed law resulted from just such a combination of political opportunism and pragmatism.
These and several other encouraging trends offer the tantalizing prospect that small wind turbines could eventually fulfill the vision wind energy advocates once had for them.