Simply put, Solar Energy means energy from the sun. With a surface temperature of approximately 5800K, the sun radiates energy in the form of electromagnetic waves of all wavelengths. The "White Light" that we see falls under the visible spectrum (with wavelengths from 400 to 700 nanometres).
This energy (in the form of EM Waves) is incident on the earth's atmosphere. Part of it is reflected, a fraction is absorbed by the air and the remainder goes on to the surface. It is this part of the sun's energy that we are trying to harness.
Image Courtesy : Wikipedia
The first thing that we need to understand is that all forms of energy on earth have their origins in solar energy.For example, the chemical energy stored in plants is a result of photosynthesis, which requires the presence of sun. Temperature gradients in the atmosphere induce convection currents, which result in wind. When we say "Solar Energy", what we actually mean is the energy in the form of EM waves that is incident on earth, not any of its derivatives.
Solar Energy can be harnessed in a variety of ways, to serve many different purposes. One of the most important setups in this regard is the Greenhouse, which is based on the "Greenhouse Effect" that we hear so much about. In layman's terms, these greenhouses are based on the fact that several materials (such as glass) allow EM waves with low wavelengths (or high energy) to pass through, but reflect back EM waves with large wavelengths (or low energy). This means that plants inside a greenhouse receive heat energy from the sun, but their own heat (which has a much smaller wavelength than that of the sun) is unable to escape. Such a setup allows us to maintain hot and humid condition inside a greenhouse, simulating a tropical environment in a country which does not receive ample solar radiation. This is immensely helpful if we want to grow tropical plants (such as tea and coffee) in a non - native environment.
Solar energy can also be used for heating purposes, thanks to the Greenhouse effect again. In Solar Water heaters, for example, water is made to run through metal pipes. These metal pipes are put in a box with a glass lid, and exposed to the sun. In time, the environment inside the box becomes hot, and the metal casing gets heated up. This hot metal then exchanges its heat with the flowing water, which heats up the water supply.
Electricity can also be produced from solar energy, using both direct and indirect methods. The direct methods use "Photovoltaics". These are materials which develop an electric current when exposed to solar radiation. The mechanism is simple - solar radiation, when incident on the atoms of a photo-voltaic material, knocks off a few electrons. These electrons when set in motion produce an electric current. Elements such as Silicon and Germanium are exhibit photo-voltaic behavior. The main drawback of Photo - Voltaic cells is that only a small current is produces, which means that in order to get a significant amount of electricity, a large number of such cells have to be set up. The high cost of photovoltaic materials further hampers the cause.
Indirect methods store the sun's energy in the form of enthalpy, which is released to produce energy. For example, in many solar power plants, the sun's rays are focused onto a single device or a tower. Water is passed through this tower, which is vaporized. This steam, which is at a high pressure and temperature, is used to drive turbines which produce energy.
Solar energy is one of the most important renewable source of energy, thanks to its abundance and availability in most places of the world. Along with wind energy, it is the source of energy which might as well as drive our future.
