As early as the third century BC, people were aware of the power of the focused sunlight. The Greek mathematician Archimedes (287-212 BC) is famously said to have defended Syracuse (Sicily) from attack using giant mirrors that directed intense beams of sunlight on to the enemy ships, causing them to burst into flames. Today, solar power is used for more peaceful purposes, harnessed by a range of technologies to generate clean, renewable energy.
The potential of solar energy
A huge amount of sunlight strikes the Earth and is absorbed or reflected back into space during the day. The average intensity of this solar energy is roughly equivalent to 1,100 watts – the power of a light bulb per square meter (100 square foot). Most of the energy arrives in the form of heat rather than visible light. According to some estimates, the amount of solar energy arriving at the Earth’s surface is enough to supply current power demands 20,000 times over. However, this vast resource is unevenly distributed and variable.
Regions close to the Equator receive much more sunlight than those at higher latitudes, and passing clouds can absorb or scatter most of the energy before it even reaches the ground. For this reason, many applications of solar energy are only practicable in areas where bright sunlight is the norm.
Solar powered devices
Sun oven cooking appliance. Produced by Sun Oven
Probably the simplest solar powered device is the solar oven. This is simple a box with a glass lid that allows sunlight to enter, a black metal interior that absorbs nearly all the solar energy striking it, and an insulating lining to prevent heat loss. In bright sunlight, the oven can reach very high temperatures (well above the boiling point of water), and cane be used to cook food. It is particularly useful and effective in hot countries where wood or other fuel for cooking fires may be scarce.
Electricity from the Sun
There are currently two main ways of generating electricity from radiant solar energy. The most widespread is the solar or photovoltaic cell, which is a piece of semiconductor that converts light energy into electric energy. Photocells were first developed to power satellites and space probes, but are now found on everything from calculators to experimental cars.
A Solar Furnace in France. Photo courtesy of Joseph Giral
However, the best prospect for large-scale power generation comes from solar furnaces. These use huge arrays of tilting mirrors called heliostats, which turn to follow the Sun round the sky, and focus its rays on to a collector, typically located high above the ground in a tower. Water pumped through the collector boils rapidly, and the force of the expanding steam can be used to turn a turbine. This turbine drives a generator that produces electricity in the same way as many other types of power station.
So far, solar power stations make a relatively small contribution to the total world generation of electricity. Solar power will never completely replace more traditional sources of energy, because sunlight is absent for large parts of the day. However, combined with other forms of alternative energy production, solar energy will play an important role.
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