The Sun is the star of our galaxy and its rays provide us with all the energy we will ever need. Even so, because of the distance between Earth and the Sun, coupled with the effects of clouds and our atmosphere, very little of the energy that began the journey reaches us.
By the time sunlight arrives on the earth’s surface, it is composed of 49 percent visible light, 45 percent infrared radiation, plus lesser amounts of ultraviolet and other types of radiation. Most people who are unaware of the science, assume that we harness the light element of sunlight, but that is not so. It is the radiation that can be converted into electrical energy or – which is the easier route – into heat (thermal energy).
The science of solar power is in a very early stage. If only we had better technology, we could use sunlight for all of the planet’s energy requirements. The advantages of this are easy to see: it is a never-ending supply (for the next million or more years, at least) and is totally pollution-free, unlike the rapidly disappearing supply of fossil fuels we currently rely on.
Solar energy is relatively cheap. It’s free at source and the costs of harnessing it – although high at the moment – are coming down all the time. Most people just do not realize how vast the potential is. Every single day, 200,000 (that’s two hundred thousand) times the electricity we generate on earth in a day, arrives in the shape of solar power.
Solar panels are what the scientists call “collectors”, and their job is to attract solar energy and convert it into heat. There are two main types of collectors: flat-plate collectors and concentrating collectors. Each type has to be pretty large to collect enough radiation. Even in a sunny climates like those of India, Florida and Morocco, a total of 40 square meters is of solar collector is needed to provide just a single person’s energy requirements.
Simple flat-plate collectors are made up of a blackened metal plate, covered with sheets of glass. The sunlight falls on the panel and the heat is trapped. Tubes at the rear of the device hold what are known as “carrying fluids” – usually water treated with antifreeze -and that will take the heat to an insulated boiler or other collection and storage device.
When greater efficiency and higher temperatures are needed, a concentrating, or focusing, collector is used. Sometimes they are called “solar furnaces” and they work by concentrating sunlight from a large area into a smaller space, using mirrors and/ or lenses. Temperatures of 2,000 C (3,600 F) or more have been achieved. Typically they will be used to power steam engine generators.
Solar radiation can also be converted directly into electricity by photovoltaic cells. They work by generating a small electric voltage (around 2 watts) when light strikes the junction between a metal and a semiconductor, such as silicon. To generate hundreds or thousands of kilowatts of electric power (a solar electric plant) you need to connect lots of these cells together. But because the energy efficiency of current photovoltaic cells is only about 15-20%, vast and very costly assemblies of such cells are required to produce even moderate amounts of power. That’s why we only tend to find them on watches and calculators.
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