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:

GG = EQE x MAF = EQE x MAR
x AH/TH

Where: GG is guaranteed generation. EQE is the expected quarterly energy generated. MAF is the mechanical availability factor. (Wind turbine manufacturers typically guarantee the mechanical availability of their units for two to five years following installation. After this period, it is up to the wind farm owner/operator to perform the maintenance necessary to ensure their required availability.) MAR is the mechanical availability requirement (a percentage usually around 97%).

AH is actual hours

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