Environmental Engineering Reference
In-Depth Information
long as coal plants do not have to account economically
for their emissions. Even so, thanks to generous tax sub-
sidies wind energy is pro
table. It is not clear what role
wind would play in the United States and elsewhere
without tax breaks, but as things are, world wind power
is growing by about
% per year worldwide and faster in
the United States. There are good sites and bad sites for
wind turbines. The good ones have fairly steady wind
with at least moderate speed. The bad sites have highly
variable speeds or steady low speeds. The electric power
generated by wind grows rapidly with wind speed;
doubling the wind speed increases the output of a wind
turbine by eight times. Double the wind speed again and
the power goes up by another eight times. That cannot go
on forever, and there is a maximum speed for a wind
turbine above which it has to be shut down to prevent
damage. There is also a minimum wind speed needed to
overcome losses in the system, so there is a band of wind
speeds in which the turbine can operate effectively.
The ideal site for a wind turbine would have a steady
wind speed at the turbine
s sweet spot for energy output.
However, nature is not so kind as to provide such sites,
and a major problem for wind is its variable nature.
Sometimes it blows hard, sometimes not so hard, and
sometimes not at all. To understand wind
'
s real energy
contribution, two different numbers have to be looked at;
one is the capacity of the wind turbine and the other is its
output. The capacity is what is written on the turbine
'
s
name plate and says what the maximum output is. When
you read or hear about a wind farm with a
'
megawatt
capacity, you are reading about its maximum output in
perfect conditions. The real world is very different, and
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