Measures of Wind Turbine Productivity

By Paul Gipe

In my 1995 book, Wind Energy Comes of Age, I discuss various measures of wind turbine productivity, including the misleading expression capacity factor. This discussion follows an explanation of how wind turbines are rated and how that influences both our conception of size and performance. Here is the excerpt on productivity.

As with wind turbine rating, there are several measures of wind turbine productivity in common use, some more meaningful than others: generation per turbine (kWh/unit), generation per unit of capacity (kWh/kW), capacity factor (%), and specific yield or generation per unit of area swept by the turbine’s rotor (kWh/m2).

Annual generation per turbine or Annual Energy Output (AEO) is used by developers, investors, and homeowners to gauge performance because it is easily understood and directly comparable to performance projections. If a homeowner is buying a single turbine, the projected generation per unit will clearly state how much energy can be expected. In the same way the homeowner can also easily monitor performance by comparing what the turbine did deliver with what was expected.

Most manufacturers provide AEO estimates with their product literature in either tabular or chart form. For example, the manufacturer of the 500 kW turbine in Table 9-8 projects an annual output of 1.24 million kilowatt-hours in Roughness Class 1, using the Danish system of wind resource classification.

Annual generation per unit of capacity in kilowatt-hour per kilowatt of rated capacity is more useful to project planners where a broad measure of productivity is more important than the number of specific machines. This measure is easily convertible to total expected generation once the total project capacity in MW is known. The 500 kW turbine in the previous example produces 2,600 kWh/kW of capacity at a 7 m/s site. This figure of merit, like capacity factor, is influenced by the rated capacity. If this turbine was rated at 750 kW, as are some turbines this size, it would only produce 1,700 kWh/kW of capacity.

Annual capacity factor is a related parameter in common use within the electric utility industry and is percentage of actual generation compared to the potential generation if the wind turbine operates at rated power for the entire year. It, too, is dependent upon the rated capacity. The 500 kW turbine in the example delivers a capacity factor of 30%. But if the machine were rated at 750 kW, the capacity factor would only be 20%, and appear less productive, even though the turbine had still generated the same amount of electricity. Capacity factors are useful only when the specific capacity of the turbines in kW/m2 are known.

The capacity factor and specific generation per rated kilowatt are useful when data on swept area is unavailable or uncertain in statistical summaries. For example See Table 9-9.

Specific Yield or annual generation per area swept by the rotor in kWh/m2/y is the ideal measure of reliability, efficiency, and a site’s wind resource. Specific yield is solely a function of wind regime and wind turbine performance, and is independent of the turbine’s rating in kilowatts.

Reliability, available wind energy, and turbine rating affect each measure in varying degrees. Turbine rating has a direct effect on generation per unit, and an inverse effect on generation per kilowatt and capacity factor. Increasing a turbine’s rated capacity may increase generation slightly by enabling the turbine to capture energy in higher winds, while at the same time lowering overall capacity factor. As discussed previously, specific rated capacity is a function of wind turbine design. Once turbine design is known, capacity factors can be related to specific yields. See Figure 9-11. Equivalent Capacity Factor.

Equivalent capacity factor. The capacity factor for various specific energy yields relative to specific rated capacity. Specific yield is a function of wind regime and wind turbine performance. Specific rated capacity is a function of wind turbine design.

The energy in the wind produces a sizable effect on all measures of productivity because of the cubic relationship between speed and power. As noted elsewhere, an increase of only 1 m/s from a 6 m/s site to a 7 m/s will boost productivity 60%. Such variations in wind speed are common from one site to another in California’s mountain passes.

Improved reliability has probably played the most important role in improving productivity since the late 1970s, when most models were originally introduced. Designers and operators have simply made wind turbines work better and more often. During the early years in Denmark, major failures occurred in half of those turbines installed at any one time. The failure rate, though, rapidly declined. Similarly, many California projects in the early 1980s were only available for operation 60% of the time. Availability, the wind industry’s measure of reliability, rapidly improved. All projects installed after 1987 in California were available for operation more than 95% of the time, and many are consistently available for operation 97-99% of the time. See Figure 9-12. Availability. Danish utilities have seen the same improvement. From 1987 through 1990, ELSAM averaged 98% availability from the 43 MW it was then operating.[1]

Percentage of time California wind power plants are available for operation.

The improvement in availability is a major technological achievement. According to Claire Lees of Field Service and Maintenance, “mo other prime mover must run as many hours without major repairs” as modern wind turbines. For comparison. consider that at an average speed of 50 mph the typical auto engine operates only 2,000 hours during a design life of about 100,000 miles. Most wind turbines exceed this in their first four months of operation. In only one year, a modern wind turbine must operate three times as long as an automobile. High availability became such an expected part of wind turbine operations by the mid 1990s that trade publications found newsworthy any hint that a company’s availability had fallen to less than 95%.

Improved reliability, improved airfoils, and adoption of taller towers have doubled specific yields in California and Denmark during the past decade. According to data from the California Energy Commission’s Performance Reporting System, BTM Consult, and Denmark’s Risφ National Laboratory, the specific yield of individual wind turbines and wind power plants in California and Denmark has steadily increased since the early 1980s. See Figure 9-13. Specific Yield in California and Denmark.

Though there has been a significant increase in specific yield of each succeeding design iteration, there has also been a steady improvement over time within each size class. For example, the average performance of early 55 kW Danish turbines increased 25% from about 400 kWh/m2/y to about 500 kWh/m2/y. Later models of this class, machines 14-16 meters (45-50 feet) in diameter, incorporated lessons learned from field experience with the earlier turbines, pushing productivity to nearly 500 kWh/m2/y. But productivity improved most dramatically as Danish manufacturers introduced newer, larger turbines, and reached an average specific yield of 850-900 kWh/m2/y for the 450 kW model.[2]

The same pattern can be seen in California where the fleet average increased 40%, from 500 kWh/m2/y in 1985 to 700 kWh/m2/y during the early 1990s, as wind companies mastered the art of maintaining wind turbines. As in Denmark, newer turbines were more productive than earlier designs. The specific yield of wind turbines installed since 1985 reached 850 kWh/m2/y during the late 1980s. Improvement in statewide capacity factor matched that of specific yield, increasing from 13% in 1985 to about 20% during the early 1990s. The capacity factor of turbines installed since 1985 averaged 23-25%.[3] Productivity will continue rising in California, though less dramatically than during the 1980s, as inoperative turbines installed during the industry’s formative years are returned to service or are replaced by more modern machines and as the early, less productive turbines become a smaller portion of the fleet as the industry expands.

The averages mask even better performance exhibited by individual designs or specific projects. Carl Weinberg, former manager of Pacific Gas & Electric’s research and development, emphasizes that projects built today will use today’s technology, and incorporate the knowledge gained from a decade of experience. U.S. Windpower, for example, operates the largest single group of wind turbines in the world. In 1992, they produced 890 kWh/m2 from nearly 4,200 wind turbines. The same year, Zond’s Sky River project produced 990 kWh/m2 from 342 turbines. And atop Whitewater Hill near Palm Springs, San Gorgonio Farms produced a remarkable 1,450 kWh/m2 from 35 machines. The top ten performing turbines in Denmark generate 1,300-1,400 kWh/m2/y of swept area.[4]

At exceptionally energetic sites, such as on the west coast of the Jutland peninsula or on Whitewater Hill, contemporary wind turbines can yield 1,000-1,250 kWh/m2/y.  San Gorgonio Farms, which operates 200 of the world’s most productive wind turbines, consistently produces 1,100-1,200 kWh/m2/y. There are also numerous sites in Northern Europe where specific yields exceed 1,000 kWh/m2/y, located in the coastal regions of Germany and the Netherlands.

Next to greater reliability, the most important contributor to higher specific yields is the use of increasingly tall towers. Tower heights have nearly doubled from 18 meters to 35 meters since the early 1980s. See Table 9-10. Doubling the tower height alone will increase the power available more than 30%. For sites yielding 600 kWh/m2/y, the taller towers will add nearly 200 kWh/m2/y, bringing total yield to 800 kWh/m2/y at a good site. This is consistent with experience in California and Denmark.

Wind Energy Comes of Age, by Paul Gipe, John Wiley & Sons, New York, 1995, 536 pages, ISBN 0-471-10924-X, pp. 161-169.

[1]. Jens Vesterdaal, “Experience with Wind Farms in Denmark,” paper presented at European Wind Energy Association special topic conference “The Potential of Wind Farms,” Herning, Denmark, September 8-11, 1992.
[2]. Finn Godtfredsen, Peter Jensen, Poul Morthorst, “Wind Energy Development in Denmark: Development in Wind Turbine Technology and Economics Since 1980,” paper presented at the annual conference of the American Wind Energy Association, Windpower 93, San Francisco, July, 1993.
[3]. California Energy Commission, “Results from the Wind Project Performance Reporting System, Annual Reports,” 1985 through 1992, Sacramento, Calif.
[4]. “Best Performing Wind Turbines,” Wind Stats, 7:1, Winter, 1994, 2-3.