The Sun sends an almost unimaginable amount of energy towards Planet
Earth – around 10 17 W (one hundred thousand million million watts). In
electrical supply terms this is equivalent to the output of about one hundred
million modern fossil fuel or nuclear power stations. To state it another
way, the Sun provides in about an hour the present energy requirements of
the entire human population for a whole year. It seems that all we need do
to convert society ‘ from carbon to solar ’ is to tap into a tiny proportion of
this vast potential.
However some caution is needed. The majority of solar radiation falls on
the world ’ s oceans. Some is interrupted by clouds and a lot more arrives
at inconvenient times or places. Yet, even when all this is taken into
account, it is clear that the Sun is an amazing benefactor. The opportunities
for harnessing its energy, whether represented directly by sunlight or indirectly
by wind, wave, hydropower or biomass, seem limited only by our
imagination, technological skill and political determination.
The Sun ’ s power density (i.e. the power per unit area normal to its rays)
just above the Earth ’ s atmosphere is known as the solar constant and equals
1366 W/m 2 . This is reduced by around 30% as it passes through the atmos-