Page 306 - Alternative Energy Systems in Building Design

280 WIND ENERGY TECHNOLOGIES

The most electricity per dollar of investment is gained by using a larger generator
and accepting the fact that the capacity factor will be lower as a result. Wind turbines
are fundamentally different from fueled power plants in this respect.
If a wind turbine’s capacity factor is 33 percent, this does not mean that it is run-
ning only one-third of the time. A wind turbine at a typical location in the midwestern
United States runs about 65–80 percent of the time. Much of the time it will be gen-
erating at less than full capacity, though.

AVAILABILITY FACTOR
Availability factor, or just availability, is a measurement of the reliability of a wind tur-
bine or other power plant. It refers to the percentage of time that a plant is ready to
generate, that is, when it is not out of service or under maintenance or repairs. Modern
wind turbines have an availability of more than 98 percent, higher than most other
types of power plants. After two decades of constant engineering refinement, today’s
wind machines are highly reliable.

WIND TURBINE POWER-GENERATION CAPACITY

Utilities must maintain enough power plant capacity to meet expected customer elec-
tricity demand at all times plus an additional reserve margin. All other things being
equal, utilities generally prefer plants that can generate as needed (i.e., conventional
plants) to plants that cannot (i.e., wind plants).
However, despite the fact that the wind is variable and sometimes does not blow at
all, wind plants do increase the overall statistical probability that a utility system will
be able to meet demand requirements. A rough rule is that the capacity value of adding
a wind plant to a utility system is about the same as the wind plant’s capacity factor
multiplied by its capacity, as shown in the preceding section. Thus a 100-MW wind
plant with a capacity factor of 35 percent would be similar in capacity value to a 35-
MW conventional generator. For example, in 2001, the Colorado Public Utility
Commission found the capacity of a proposed 162-MW wind plant in eastern
Colorado (with a 30 percent capacity factor) to be approximately 48 MW.
The exact amount of capacity value that a given wind project provides depends on a
number of factors, including average wind speeds at the site and the match between wind
patterns and utility-load (demand) requirements. It also depends on how dispersed geo-
graphically wind plants on a utility system are and how well connected the utility is with
neighboring systems that also may have wind generators. The broader the wind plants are
scattered geographically, the greater is the chance that some of them will be producing
power at any given time. Figure 8.7 is a photograph of a land-based wind turbine.

WIND TURBINE ENERGY SUPPLY POTENTIAL
FOR THE UNITED STATES
Wind energy could supply about 20 percent of the nation’s electricity, according to
Battelle Pacific Northwest Laboratory, a federal research laboratory. Wind energy
resources used for generating electricity can be found in nearly every state.