Introduction - Wind Energy Meteorology: Atmospheric Physics for Wind Power Generation - page 5

The globally available energy in the wind can be estimated from the chain of
energy conversions in the Earth's atmosphere [the numbers given here are based
on earlier seminal publications such as by Lorenz ( 1955 ) and Peixoto and Oort
( 1992 )]. The incoming solar power at the top of the atmosphere is roughly 174
300 TW (*342 W/m 2 ). 1743 TW (*3.5 W/m 2 or 55,000 EJ/year) of this power
is available in form of kinetic energy that will eventually be dissipated in the
atmosphere. About half of this dissipation takes place in the boundary layer
(871 TW or 1.75 W/m 2 ). This yields 122 TW of potential power assuming that
one fourth of the Earth's surface is accessible for wind energy generation and that
wind turbines can theoretically extract up to 56 % of this energy (Betz' limit).
Practically, maybe 50 % of this is realistic, meaning that the total potential wind
power extractability is about 61 TW (1,925 EJ/year). Other estimates which use
similar approaches come to energy amounts of the same magnitude (see e.g.,
Miller et al. ( 2011 ) who derive 18-68 TW). A more pessimistic evaluation by de
Castro et al. (2011) starts with 1200 TW for the global kinetic energy of the
Earth's atmosphere. 8.3 % of this energy is available in a 200 m deep surface layer
giving 100 TW. 20 % of the land surface is suitable for the extraction of this
surface layer energy giving 20 TW. Restricting wind parks to areas with reason-
able wind resources halves this further to 10 TW. Then de Castro et al. estimate
that only 10 % of this energy can be extracted by wind turbines. Thus, their
estimation is that just 1 TW (32 EJ/year) is the amount of energy extractable from
the wind.
While the estimate of the global kinetic energy in the atmosphere is rather
robust and yields probably more than 1,000 TW, the two critical assumptions in
these calculations are the share of this energy that is dissipated at the surface (here
varying between 8 and 50 %) and the share which can be extracted from this near-
surface kinetic energy due to technical aspects of the turbines (here varying
between 10 and 50 %). Probably a single-digit number given in TW is a realistic
estimate for the wind energy available from the Earth's atmosphere.
These numbers have to be compared to the total energy demand of mankind
which presently is roughly 15 TW (443 EJ/year) and which is expected to rise to
about 30 TW (947 EJ/year) by the middle of the century and 45 TW (1420 EJ/
year) by the end of the century (CCSP 2007 ). This comparison makes clear that
wind energy can only be part of the solution for a supply of mankind with
renewable energies. Other forms of renewable energies have to be exploited in
parallel. Furthermore, it can be expected that energy extractions of even 10 % of
the available wind energy will already have considerable effects on the Earth's
climate (see Sect. 7.4 ) .
1.5 Present Status of Wind Energy Generation
The worldwide wind energy conversion capacity reached 215 GW by the end of
June 2011, out of which 18.4 GW were added in the first 6 months of 2011