Generator Ratings & Capacity Factors: Why You Should Avoid Them

By Paul Gipe

Generator Size and Swept Area

Newcomers to wind energy and even some old-timers who should know better often equate the size of a wind turbine to it’s generator rating. A Vestas V80 for example is rated at 1.8 MW; a V82 is rated at 1.65 MW. While this may be a useful shorthand among those unfamiliar with wind energy, the use of generator size leads to a host of problems when evaluating the technology.

Rotor diameter and swept area are better measures of a wind turbine’s capability than its generator rating because it is the area swept by the rotor and not the generator’s size that captures the wind and converts it to a useful form. (See page 192 of Wind Energy for the Rest of Us for a more thorough explanation of the role of swept area.)

Swept area is the preferred measure of wind turbine size. Since most wind turbines use rotors that describe a spinning disc intercepting the wind, swept area is simply the area of the circle. Thus, the shorthand for the swept area of a conventional wind turbine is rotor diameter. The Vestas V80 uses a rotor 80 meters in diameter; the V82, a rotor 82 meters in diameter. The Enercon E48 uses a rotor 48 meters in diameter and so on.

The Vestas V82 is bigger, that is it intercepts more area of the wind stream, than the V80 even though the V82 has a smaller generator. In most areas of the world with moderate winds, the V82 will generate more electricity than the V80 simply because the V82 intercepts more wind.

Generator Size and Rated Power

The rated power system used on wind turbines is not only confusing; it can also be misleading. First, there’s no reference speed to compare one turbine to another: rated speeds range from 10 m/s to over 15 m/s. Second, some manufacturers rate their machines at peak power output and others do not. Medium-size machines, especially those using aerodynamic stall to regulate peak power, will often exceed their rated capacity, sometimes by up to 30%. A few, shall we say, less than reputable manufacturers have taken advantage of the emphasis on generator size by adding large generators to relatively small rotors. By using this rating system, it’s possible to slap a 6-foot plank on the shaft of a 25-kW generator and call it a 25-kilowatt wind turbine.

In one particularly notorious case, Fayette Manufacturing built a 10-meter turbine and saddled it with a 95-kW generator. Most other manufacturers would have rated a turbine of this size at 25-35 kW. Years of results from some 1,000 of these machines in California proved that they performed no better and often much worse than other turbines with rotors of similar swept area only driving 25-kW generators. After years of poor performance, the Fayette machines were eventually scrapped.

Unfortunately, Fayette wasn’t the only manufacturer to play the “rating” game. And it is still played today. In jurisdictions where capacity factors are used as a measure of performance and tariffs or subsidies are a function of capacity factor (or plant factor or full-load hours as they are also known), manufacturer’s can manipulate generator size-for the same size turbine–to maximize subsidies rather than the generation of electricity.

Measures of Wind Turbine Performance

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).

Typical Specific Yields for Commercial Wind Turbines (pdf)

Typical Specific Yields for Commercial Wind Turbines (xls)

Annual generation per turbine or Annual Energy Output (AEO) is used by developers, investors, farmers, 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. In the end, annual generation is what matters to the owner or investor.

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 1.8 MW turbine in the previous example produces about 2,500 kWh/kW of capacity at a 7 m/s site. This figure of merit, like capacity factor, is influenced by the rated 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 1.8 MW turbine in the example delivers a capacity factor of nearly 30% at a 7 m/s site. Because manufacturers rate their wind turbines at different wind speeds, capacity factors are useful only when the specific capacity of the turbines in kW/m² are known.

Examples of Actual Specific Yields at Sample Sites

Specific capacities are a function of how heavily the rotor is loaded. At high wind sites, it’s possible to extract more energy from the rotor because of the strong winds. To do so the turbine is modified to extract more energy and this is reflected in a higher generator rating in kilowatts and a higher rotor loading in kW/m². Modern commercial wind turbines are available in specific capacities from 0.3 kW/m² to more than 0.4 kW/m². The Vestas V80 has a specific capacity of 0.36 kW/m², whereas the V82 has a specific capacity of 0.31 kW/m². The 3 MW Vestas V90 has a very high rotor loading of 0.47 kW/m² reflecting that it’s designed for very energetic sites.

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

Annual Specific Yield or annual generation per area swept by the rotor in kWh/m²/yr 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.

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.

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

Installed Cost

The cost of energy from a wind turbine is a function of many factors, but most importantly it is a function of annual specific yield and installed cost. The units for installed cost in $/m² of rotor swept area are uncommon in the utility industry, but are more explicit than the familiar units of $/kW because of variations in the generator rating of wind turbines. A wind turbine that costs $600/m² of rotor swept area is equivalent to a turbine costing $1,500 per kilowatt if it has a specific rated capacity of 0.4 kW/m².

As with annual specific yield, specific cost is a better measure of relative wind turbine costs than $/kW because it doesn’t rely on generator ratings. Manufacturers have on occasion resorted to inflating the generator ratings to reduce the apparent cost of a wind turbine. Some manufacturers were notorious for this. That is, by giving a wind turbine a higher rating in kW than otherwise they could either raise their prices for the same $/kW or lower their apparent prices in $/kW to appear more competitive. In either case, it was still the same wind turbine and consumers were none the wiser until after the first year’s production.