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The Greening of IT
264 How Companies Can Make a Difference for the Environment

upper end of the range in particularly favorable sites. For example, a
1 megawatt turbine with a capacity factor of 35 percent will not produce
8,760 megawatt-hours in a year (1x24x365) but only 0.35x24x365 =
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.
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 percent capacity factor. Plants
with higher fuel cost are throttled back to follow load. Gas turbine
plants using natural gas as fuel might be very expensive to operate and
might be run only to meet peak power demand. A gas turbine plant
might have an annual capacity factor of 5 percent to 25 percent 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
ptg
wind farms allows 33 percent to 47 percent of the total energy produced
to be used as reliable, baseload electric power, as long as minimum crite-
ria are met for wind speed and turbine height.

Intermittency and Penetration Limits
Because instantaneous electrical generation and consumption must
remain in balance to maintain grid stability, this variability can present
substantial challenges to incorporating large amounts of wind power
into a grid system. Intermittency and the nondispatchable nature of
wind energy production can raise costs for regulation, incremental oper-
ating reserve, and (at high penetration levels) could require energy-
demand management, load shedding, or storage solutions. At low levels
of wind penetration, fluctuations in load and allowance for failure of
large generating units require reserve capacity that can also regulate for
variability of wind generation.
Pumped-storage hydroelectricity or other forms of grid energy storage
can store energy developed by high-wind periods and release it when
needed. Stored energy increases the economic value of wind energy
because it can be shifted to displace higher cost generation during peak
demand periods. The potential revenue from this arbitrage can offset the
cost and losses of storage; the cost of storage might add 25 percent to the