Environmental

Energy from the Sun

Tides are driven by gravitational energy and plate tectonics by the heat generated by the radioactive

decay of elements in the Earth’s mantle, but the energy driving the atmosphere, oceans, and living

organisms is supplied by the Sun. To a limited extent this energy can also be harnessed directly to

perform useful work for humans. Solar heat can be used directly to warm buildings and water,

desalinate water, and cook food. Sunlight can be converted into electricity. Electrical power can also

be generated from wind and sea waves and, because the atmospheric circulation responsible for

wind and wind-driven waves is driven by solar heat, these are also forms of solar energy.

The outer layer of the Sun, which is what we see and the region from which the Sun radiates, is at a

temperature of about 6000 K and it radiates energy at 73.5×106

W from every square metre of its

visible surface (the photosphere; being entirely gaseous, the Sun has no solid surface). The figure

can be calculated because the Sun behaves as a ‘black body’. This is a body that absorbs all the

energy falling on it and radiates energy at the maximum rate possible; the rate is calculated by using

Stefan’s law9

and is proportional to the absolute temperature raised to the fourth power.

The Sun radiates in all directions and the Earth, being a very small target at a distance of 150 million

km, intercepts 0.0005 per cent of the total. At the top of the Earth’s atmosphere this amounts to about

1360 W m-2, a value known as the ‘solar constant’.

Solar output is not as constant as this name suggests. Between 1981 and 1984, it decreased by 0.07

per cent (HIDORE AND OLIVER, 1993, p. 166). This is a small deviation, but a decrease of about

0.1 per cent sustained over a decade would be sufficient to produce major climatic effects and a 5 per

cent decrease might trigger a major glaciation. Cyclical variations in the Earth’s rotation and orbit

also alter the solar constant. These are believed to be the major cause of large-scale climatic change,

and variations in solar output, marked by changes in sunspot activity, are linked to less dramatic

changes, such as the Little Ice Age, a period when average temperatures were lower than at present

which lasted from about 1450 to 1880. Some scientists believe that the recent climatic warming and

rise in atmospheric carbon dioxide concentration are both wholly due to the marked increase in

energy output of the Sun since about 1966 (CALDER, 1999).

Radiant heat and light are both forms of electromagnetic radiation, varying only in their wavelengths,

and the Sun radiates across the whole electromagnetic spectrum. According to Wien’s law10, the

wavelength at which a body radiates most intensely is inversely proportional to its temperature, so

the hotter the body the shorter the wavelength at which it radiates most intensely. This is not

surprising, because electromagnetic radiation travels only at the speed of light (beyond the Earth’s

atmosphere, in space, about 300000 km s-1) and the only way its energy can increase is by reducing

the wavelength. Very short-wave (high-energy) gamma (10-4–10-8 µm) and X (10-3–10-5 µm) solar

radiation is absorbed in the upper atmosphere and none reaches the surface. Radiation with a

wavelength between 0.2 and 0.4 µm is called ‘ultraviolet’ (UV); at wavelengths below 0.29 µm,

most UV is absorbed by stratospheric oxygen (O2

) and ozone (O3

). The wavelengths between 0.4

and 0.7 µm are what we see as visible light, with violet at the short-wave end of the spectrum and

red at the long-wave end. These are the wavelengths at which the Sun radiates most intensely, with

an intensity peak at around 0.5 µm in the green part of the spectrum.