Enthusiasts for solar power need to be reminded that, through its role in photosynthesis and in powering the atmosphere, the Sun is already our primary source of renewable energy. Or to put it another way solar photons convert naturally into chemical fuel and heat.1 Photosynthesis yields biomass which can serve as fuel and food and it creates ATP, the main source of energy for living things and thus their muscles. In the process it liberates the oxygen required by most kinds of combustion Solar heat sets up the differences in temperature and pressure that propel wind and waves; and by providing rainfall and melting snow it generates the mechanical energy that drives water mills and their hydroelectric successors.
The only significant non-solar sources of energy are the tides (the Sun, though undoubtedly much larger than the Moon, is so distant that its pull is only half as strong as the Moon's), geothermal heat - heat from the Earth's interior - and nuclear power. An estimate by the International Energy Agency put their respective contributions to global energy consumption in 2006 at a pathetic 0.0004% for tidal power, 0.06% for wind and a thousand times more but still only 0.4% for geothermal. Nuclear accounted for 6.5%.
Yet as a direct source of energy the Sun contributed only 0.04%.The figure probably underestimates current use, especially as many consumers of solar energy are found in isolated locations, such as cattle ranches and the Space Station, where power generation on a small scale makes sense; but not by much.
What further scope is there for solar power? There are two main techniques for trapping the sun's rays: by using them as a source of heat and by converting the radiation into electricity. As with the solar constant the analysis is best expressed in the unit familiar to all electricity consumers: the watt (W) and its metered equivalent the kilowatt hour (kWh). A 60 W bulb burning for 17 h uses just over 1 kWh (60 x 17 = 1,020 W). Large units are usually expressed in megawatts or MW (that is to say 1,000 kW) and the corresponding megawatt hours (MWh).
On an average day the Sun delivers the equivalent of about 1.4 kW per square metre (m2) at the top of the Earth's atmosphere. The quantity used to be called the solar constant but, as we saw in Chapter 3, it varies during the 11-year cycle and, though fairly steady during the year near the Tropics, it will fall close to zero during winter at the poles and of course everywhere at night. About 8/10 of this wattage is lost on its way to the ground because it is reflected by clouds or absorbed by gases
C. Vita-Finzi, The Sun: A User's Manual, 121
doi: 10.1007/978-1-4020-6881-2_8, © Springer Science + Business Media B.V. 2008
in the atmosphere. Even so, the total solar energy that reaches the Earth's land areas in 1 day totals 2,000 times the energy consumed by humans during a whole year. The annual energy receipts for the Sahara are 2,500 kWh per m2. In NW Europe the figure is nearer 900 kWh/m2. As Isaac Asimov puts it, solar energy is copious but dilute (Fig. 8.1).
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