How Energy Is Produced - The Renaissance of Renewable Energy - page 101

The Potential of Solar Energy
The power of the sunlight hitting the Earth's surface (known as irradiance) in midday and
under ideal conditions is about 1,000 watts per square metre. Considering that the sun
does not shine at night, that atmospheric conditions typically reduce solar irradiance by
roughly 35 per cent on clear, dry days and by about 90 per cent on overcast days, the
average solar irradiance on the ground is about 200 watts per square metre. While not all
countries are equally endowed with sunshine, a significant contribution to the energy mix
fromdirect solar energyispossible foralmost every country (see Table 4.4 ).Overoneyear,
our planet's total solar energy 'income' has been estimated at 3.9 million exajoules (billion
billion joules), more than 7 million times the primary energy supply of the human race
(530EJ).Butbeforewegroweuphoricatthepotential ofthislargelyunharnessedresource,
we need to factor in a few limitations, such as land availability, meteorological conditions,
local demands for energy services, and environmental and social constraints. Estimates of
the global technical potential of solar energy (not diversified by technology) vary greatly,
fromroughly1,500to50,000exajoulesperyear.Thisiscertainlycauseforoptimism,ifnot
euphoria, since even the lower estimate is three times our current global supply of energy
(Arvizu et al. 2011a ; IEA 2012a ) .
Figure 4.22. Solar irradiance (watts per square metre) on the Earth's surface in January
(above) and July (below). Source: Arvizu et al. ( 2011a ) .
The Potential of Solar Heating
More than 200 million households worldwide use solar hot water collectors. The vast
majority (86.4 per cent) of glazed and unglazed water and air collectors currently in
operation are installed in China (101.5 GW), Europe (32.5 GW, mostly in Germany), and
the United States and Canada (15 GW). Other countries in which solar water heating plays
an important role include Japan, Greece, Israel, Brazil, and Austria (REN21 2012 ; Weiss
and Mauthner 2011 ) .
A survey of the countries that account for more than 90 per cent of the solar thermal
market worldwide showed that the annual collector yield of all water-based solar thermal
systems in operation in 2010 was 0.58 exajoules (162 GWh). 7 Although this roughly
corresponds to 15 million tons of oil equivalent and 50 million tons of carbon dioxide