Environmental Engineering Reference
In-Depth Information
Figure 4. Frequency and energy probability.
Since wind speed is not constant, a wind farm's annual energy production
is never as much as the sum of the generator nameplate ratings multiplied by
the total hours in a year. The ratio of actual productivity in a year to this
theoretical maximum is called the capacity factor. Typical capacity factors are
20-40%, with values at the upper end of the range in particularly favorable
sites. For example, a 1 megawatt turbine with a capacity factor of 35% will not
produce 8,760 megawatt-hours in a year (1x24x365), but only 1x0.35x24x365
= 3,066 MWh, averaging to 0.35 MW. Online data is available for some
locations and the capacity factor can be calculated from the yearly output.[7]
Unlike fueled generating plants, the capacity factor is limited by the
inherent properties of wind. Capacity factors of other types of power plant are
based mostly on fuel cost, with a small amount of downtime for maintenance.
Nuclear plants have low incremental fuel cost, and so are run at full output and
achieve a 90% capacity factor. Plants with higher fuel cost are throttled back
to follow load. Gas turbine plants using natural gas as fuel may be very
expensive to operate and may be run only to meet peak power demand. A gas
turbine plant may have an annual capacity factor of 5-25% due to relatively
high energy production cost.
According to a 2007 Stanford University study published in the Journal of
Applied Meteorology and Climatology, interconnecting ten or more wind
farms can allow an average of 33% of the total energy produced to be used as
reliable, baseload electric power, as long as minimum criteria are met for wind
speed and turbine height.
