298 Energy andtransport for thefuture
a two- or three-bladed propeller about 50 m in diameter and a rate of
power generation in a wind speed of 12 ms−^1 (43 kmh−^1 , 27 mph or
Beaufort Force 6), of about 700 kW. On a site with an average wind
speed of about 7.5 ms−^1 (an average value for exposed places in many
western regions of Europe) it will generate an average power of about
250 kW. The generators are often sited close to each other in wind farms
that may include several dozen such devices.
From the point of view of the electricity generating companies the
difficulty with the generation of electrical power from wind is that it
is intermittent. There are substantial periods with no generation at all.
The generating companies can cope withthis in the context of a national
electricity grid that pools electrical power from different sources pro-
viding that the proportion from intermittent sources is not too large.^42
Some public concern about wind farms arises because of loss of visual
amenity. The use of more off-shore sites may therefore be more generally
acceptable than too much concentration in windy siteson-shore.
Rapid growth has occurred in many countries in the installation of
wind generators over the past decade–agrowththat continues unabated.
But most of the growth has been for electricity generation. Over 30 GW
peak operating capacity has now (2002) been built worldwide. With
this large growth, economies of scale have brought down the cost of
the electricity generated so that it is approaching the costof electricity
generated from fossil fuels (Table 11.6). Because the power generated
from the wind depends on the cube of the wind speed (a wind speed
of 12.5 ms−^1 is twice as effective as one of 10 ms−^1 ) it makes sense to
build wind farms on the windiest sites available. Some of the windiest
sites available are to be found in western Europe where some of the most
rapid growth in wind generation has occurred. In Denmark for instance,
twenty per cent of electricity is now generated by wind – increasingly
being built offshore.^43 Eventually, it is envisaged the proportion could
rise to forty to fifty per cent. Similar estimates of the eventual resource
are being made for the UK where rapid expansion, again especially of
off-shore wind energy generation, is envisaged.^44 Developing countries
are also making increased use of wind energy. For instance, it has been
estimated that India could be generating up to 10 GW of electrical power
(about a quarter of current needs) from wind by 2030.^45 With the growth
that is occurring, the proportion of global energy needs supplied from
wind energy could be substantially greater than that envisaged by the
projection in Table 11.5.
Wind energy is also particularly suitable for the generation of elec-
tricity at isolated sites to which the transmission costs of electricity from
other sources would be unacceptable. Because of the wind’s intermit-
tency, some storage of electricity or some back-up means of generation