Energy Production
Contrary to popular opinion, energy yields do not increase
with the cube of the wind speed, mainly because energy
is discarded once the rated wind speed is reached. To
illustrate a typical power curve and the concept of
rated output, Figure 13.3 shows a typical performance
curve for a 1.65 MW machine. Most machines start to
generate at a similar speed - around 3 to 5 m/s - and
shut down in very high winds, generally around 20 to
25 m/s.
Figure 13.3: Power curve for a 1.65 MW wind turbine
Annual energy production from the turbine whose performance
is charted in Figure 13.3 is around 1 500 MWh at a site
where the wind speed is 5 m/s, 3 700 MWh at 7 m/s and
4 800 MWh at 8 m/s. Wind speeds around 5 m/s can be
found, typically, away from the coastal zones in all
five continents, but developers generally aim to find
higher wind speeds. Levels around 7 m/s are to be found
in many coastal regions and over much of Denmark; higher
levels are to be found on many of the Greek Islands,
in the Californian passes - the scene of many early
wind developments - and on upland and coastal sites
in the Caribbean, Ireland, Sweden, the United Kingdom,
Spain, New Zealand and Antarctica.
Wind speed is the primary determinant of electricity
cost, on account of the way it influences the energy
yield so, roughly speaking, developments on sites with
wind speeds of 8 m/s will yield electricity at one third
of the cost for a 5 m/s site. Offshore wind speeds are
generally higher than those onshore. Offshore wind farms
have been completed, or are planned, in Denmark, Sweden,
Germany, the United Kingdom, Ireland and elsewhere.
Offshore wind is attractive in locations such as Denmark
and the Netherlands where pressure on land is acute
and windy hill top sites are not available. In these
areas offshore winds may be 0.5 to 1 m/s higher than
onshore, depending on the distance. The higher wind
speeds do not usually compensate for the higher construction
costs but the chief attractions of offshore are a large
resource and low environmental impact.
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