Introduction to Modern Wind Turbines - Wind Turbine Technology: Fundamental Concepts of Wind Turbine Engineering - page 86

Annual Operations and Maintenance Cost
Operating and maintenance budgets for a wind power station are sensitive to such factors
as the maturity and durability of the wind turbine equipment, the size and number of units in
the system, ease of maintenance and availability of spare parts, and weather conditions.
Availability Factor
On-line availability is closely related to operations and maintenance costs and is also
sensitive to the maturity of the wind turbine equipment. Availability factors from 0.90 to
0.97 are usually assumed when estimating the cost of energy from a wind power station after
it has been in operation for several years.
Annual Energy Output
Both the long-term energy content of the local winds and the overall eficiency of the
turbines determine the annual energy output of the wind power station in the absence of
downtime. However, as illustrated by the coeficients of energy in Figure 2-19, modern
utility-scale wind turbines are designed to extract about 33 percent of the incident wind en-
ergy, on average, regardless of their coniguration. Since the energy content of the wind is
proportional to the cube of the wind speed, it has been found that the annual average wind
speed at the mean elevation of the turbine rotors is a dominant parameter in determining the
cost of energy and the economic viability of a wind power station.
Figure 2-20 illustrates the sensitivity of the cost of energy to the annual average wind
speed, using data from Table 2-1. For this example, the mean elevation of the rotors is
Figure 2-20. Sensitivity of the cost of energy to annual average wind speed. The refer-
ence annual average wind speed is assumed to be 7.8 m/s at mid-rotor elevation.