Clipper Windpower: The Economics of Wind Energy

Editor's Note: This presentation's discussion of capacity factor and capital cost are of particular interest.

(1) Capacity Factor: Gross vs Net

It might be helpful first to define capacity factor with an example of how it is calculated. 

Definition: capacity factor is the ratio of the actual energy produced in a given period to the hypothetical maximum possible, i.e. running full time at rated power.

 Example:  a generator with a power rating of 2000 kW that runs (hypothetically) at full power for the entire year would generate 17,520,000 kW-hr  [(2000kW)*(365 days)*(24 hours)= 17,520,000 kW-hr]. However, if it generated 4,380,000 kW for the entire year it operated at a 25% capacity factor (4,380,000/17,520,000=25%)

Developers' claims regarding the amount of electricity produced annually by a wind plant are often viewed skeptically.

Typically, developers rely on the wind speed data collected at anemometers and the power curve of a whatever make/model of wind turbine they plan to install in order to predict the annual capacity factor - and thus estimate the electricity (kWh's) generated each year by their proposed facility. However, as Clipper's #7 slide shows, there are substantial deductions which need to be included in order to accurately estimate a windplant's capacity factor, with the Net Capacity Factor of a windplant being on average about 86% of the Gross Capacity Factor. Here is a hypothetical example provided by Clipper for a windplant which has a Gross Capacity Factor as determined only by wind measure data in conjunction with the power curve for the project's wind turbines:

Gross Capacity Factor (CF): 40.0%
potential CF reductions up to:
Transformer/Line Losses/Transmission -3.0
LineWake Losses -4.8
Control Alogorithm/Turbulence -1.6
Blade Contamination -1.5
Icing -1.0
Turbulence 0
Turbine Availability -3.0

Net Capacity Factor: 34.4% (assuming a Gross to Net CF Conversion average of 86.0%)

This may help explain why developers' predictions concerning the amount of electricity generated are so much greater than what is actually generated.

(2) Capital Cost

This presentation repeatedly mentions that the capital cost of a windplant is $1,500 per kW ($1.5-million per MW) of the facility's nameplate generating capacity (e.g., if 10 2-MW turbines are installed - 20 MW of combined nameplate capacity - the capital cost of installing this facility would be $30-million). This is on the high side of cost estimates we've seen thus far - but likely realistic.