May 15, 2008

Regulating wind power into a dispatchable resource

By Robert D. Castro, University of Southern California, and Fernando Pardo

Guaranteeing availability

As wind power becomes more important to more utility resource portfolios, so does the availability of wind farms. Some utilities address the issue by specifying in their contracts with wind farms a target mechanical availability for the project, such as 98% for smaller wind turbines or 95% for the megawatt-size turbines. This, along with annual generation guarantees, enables them to more effectively establish the reliability of wind power resources. The two guarantees work hand-in-glove, because it is conceivable that a wind farm could meet its annual guaranteed generation level with poorly performing wind turbines. Typically, guaranteed generation is an annual target, while mechanical availability is based on quarterly turbine performance statistics.

The main objective of requiring mechanical availability is to ensure that a wind farm is properly maintained and operated. But it also ensures that a minimum amount of energy is generated over a given period if the minimum mechanical availability number of the wind turbines is met. If this minimum production is not achieved, an energy shortfall will be declared and the wind farm owner will have to find a way to make up the difference that would have been generated had the turbines been fully available.

Calculating the guaranteed generation of a typical wind plant for a given period of time (usually one quarter of a year) illustrates these points. Guaranteed generation is calculated using the following formula:

Example calculation

Consider a 50-MW wind farm with an annual 35% capacity factor and turbines whose required mechanical availability is 97% for a given quarter. Now assume that, during a 90-day quarter, the wind farm has 30 hours of scheduled maintenance, 10 hours of power system emergency, and 40 hours when wind speeds are above or below the turbines’ operating range. Further, assume that during that quarter the wind farm generated 34,800 MWh.

For this quarter, the AH of the turbines is calculated as:

Since there are 2,160 hours in a calendar quarter, the turbines’ MAF during the same quarter would be:

The wind farm’s GG for this quarter would be the product of its EQE and MAF. In this case, the EQE would be the product of 50 MW, the number of hours in the quarter, and the projected capacity factor. Using our definitions, EQE would be 50 MW x 2,160 hours x 35%, or 37,800 MWh.

As a result, the wind farm’s GG for this quarter is 37,800 MWh x 0.9341, or 35,309 MWh. Because only 34,800 MWh are delivered to the meter, there would a shortfall of 509 MWh that the wind farm owner would have to make up.

Utilities typically buy wind energy using a power purchase agreement that may include an option to purchase the wind farm later as well, after its tax credits have expired (usually 10 years after its commissioning). In such cases, mechanical availability guarantees take on even more importance. Obviously, the utility would prefer the wind farm to have a high availability when it takes ownership. The operator, on the other hand, would be financially motivated to let turbine maintenance slip the last few years of operation. Guarantees of mechanical availability give wind farm operators an incentive to be conscientious about maintenance right up to the turnover date.

—Robert Castro (robert.castro@alumni.usc.edu) teaches graduate level power classes at the University of Southern California and negotiates wind generation contracts for a local utility. Fernando Pardo (flpardo@aol.com) is a supervisor of renewable energy development at a local utility.