Geology Reference
In-Depth Information
6.6 Energy from the sun
According to Skinner (1986), solar radiation is the largest energy input of the Earth,
accounting for 99.985% of the total. From the 173,000 TW of incoming solar radia-
tion, about 30% is directly reflected unchanged, back into space, by the clouds, sea,
land, ice and snow. Around 350 TW are used for creating winds, ocean currents,
waves, etc. Evaporation and precipitation collectively use approximately 4,000 TW
of the sun's energy, which leads to the storage of water and ice. Only 40 TW are
effectively employed in the process of photosynthesis, leading to the production of
biomass and eventually of fossil fuels.
6.6.1 Solar power
The exergy flow of solar radiation heating the land and oceans amounts to
43,200 TW (Szargut, 2003). This is about three thousand times more than the
present power needs of the entire globe: 17 TW in 2010. In fact, the energy sup-
plied by the sun in just one minute is enough to meet the global power need for
an entire year (Khan et al., 2008). Unfortunately, technology is not yet su ciently
developed to make use of this immense quantity. Solar power density at the Earth's
surface is 125-375 W/m 2 and an average photovoltaic panel, with 15% e ciency,
may deliver 15-60 W/m 2 . Even though this seems relatively low, solar cell conver-
sion e ciency has increased from some 6% in 1954 to 40% in 2006. Unsurprisingly
therefore, the size of solar power stations has exponentially increased from 500 kW
in 1977 to about 40 GW in 2010 (REN21, 2011).
According to Johansson et al. (2004) and Schock (2005), electricity generated
directly by utilising solar photons to create free electrons in a PV cell is estimated to
have a technical potential of at least 450,000 TWh/yr (around 51 TW - Table 6.8)
. The roadmap forecast 2050 (IEA, 2010b), consideres that solar PV could supply
11% of the global electricity production. Recent developments in thin film solar
cells increases the likelihood of this forecast becoming a reality (see Sec. 1.4.2).
Apart from extracting solar radiation through photovoltaic panels, solar energy
can also be utilised via solar heating collectors. Solar thermal capacity reached 185
GWth in 2010 (REN21, 2011) and is expected to increase dramatically due to new
building regulations, especially in Europe 7 .
Additionally, promising experiences with concentrated solar power plants (CSP)
show that other forms of technology could produce clean electricity. In CSPs, the
solar flux can either be concentrated by parabolic trough-shaped mirror reflectors
(30 - 100 suns concentration), central tower receivers requiring numerous heliostats
(500 - 1000 suns), or parabolic dish-shaped reflectors (1000 - 10,000 suns) to heat
a working fluid, which in turn is transferred to a thermal power conversion system.
According to Philibert (2004), 1 km 2 of land at low latitude, receiving high levels of
7 See Directive 2002/91/EC on the energy performance of buildings.
 
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