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Global renewable energy resources and use in 2050 227

cover is lost. This reduction occurs because in the Amazon the “rainfall systems are
maintained, in part, by the forest itself through contribution of water vapor to the
atmosphere through ET and through its associated influences on land–atmosphere
energy exchange” [24]. Presumably any comparable loss of tree cover from climate
change would have a similar effect.

6.4 Wind energy

6.4.1 Introduction
Wind energy is an ancient technology, with windmills several millennia old, and
wind-powered sail boats much older. Modern wind turbines were first installed in
numbers in the United States in the 1980s, but global output did not take off until
the mid-1990s [5]. With strong growth in Germany, and then Spain, global wind out-
put rose exponentially—albeit from a very small base—but since 2008, output (in
terawatt hours of electricity) has only risen linearly (see Fig. 6.1). The steady growth
in wind energy has been helped by new materials enabling much larger blade diam-
eters, and the fact that virtually all commercial wind turbines have standardized on
horizontal axis machines, although a vast number of alternative designs have been
proposed [25]. Choice is still available for blade numbers (two or three), rated output
(from fractional MW up to 6MW or more), and tower height/blade diameter ratio.
The output from wind turbines has been found to deteriorate with age of installa-
tion: Staffell and Green [26] found a 1.6% drop in annual output for turbines installed
in the United Kingdom. The result was that average load factors declined from 28.5%
when installed to 21% after 19years, which in turn would raise the cost of the elec-
tricity produced.

6.4.2 Wind energy in 2050
If the linear increases in output from 2008 to 2015 continue in coming decades, wind
could be expected to supply 3985TWh (14.3EJ) globally in 2050, compared with the
2015 output of 841TWh (3.0EJ). Although this growth represents a more than four-
fold increase, wind will still only be a marginal source in 2050, with estimates of total
primary energy use then as high as 1000EJ [12]. Even with strict land constraints on
turbine placement, this value of 14.3EJ is still well below the global technical poten-
tial of wind energy [2]. Slow growth, rather than resource limits, is what could prevent
wind energy becoming a major energy source by 2050. In any case, wind turbines are
now also being sited offshore, which increases the global wind resource base.
Although more expensive to install and maintain, this cost is offset by higher wind
speeds—and less public opposition.
Since the rebirth of wind turbines in the 1980s, blade diameters and rotor heights
have steadily risen; the greater rotor heights (up to 200m) enable both higher wind
speeds and less wind speed variation across the blades. Even higher wind speeds could
be obtained if the turbines could be placed at much greater heights, and hence greater