In the context of global warming, solar energy assumes tremendous importance as an alternative source of energy. We shall review the present status of utilization of solar energy.

Many ancient cultures such as the Egyptians, the Greeks and the Romans considered the brilliant, searing hot ball in the sky as a God that gave light and warmth to earth. In those olden times, directly or indirectly, the Sun dominated human societies. Their subsistence depended on the energy given by the Sun, the life cycles of communities revolved around the seasons set by the Sun. The Sun was recognized as the life giver, the all- powerful provider. The Sun was worshipped in all the ancient civilizations of Egypt, Greece, India, etc. In the belief-systems of many communities of people such as the Zoroastrians, the Aztecs of Mexico, the Incas of Peru, Hindus, Native American tribes, etc. the Sun occupied an important position.

Magnificence of the Sun has been understood only in the modern times. Sun is the centre of our planetary system and the planet earth rotates around the Sun. Now we know the enormity of the Sun in relation to earth. The Sun is a ball of burning hydrogen gas and is about 1,500,000 times bigger than the earth. Its surface temperature is about 27,000,000 deg C. Due to this high temperature it emits radiation in all directions The Solar radiations cover a very wide range of frequencies. A very small part of the energy is emitted as visible radiation – the light, as we call it. Radiations from the Sun extend from infra-red (lower in frequencies than the lowest visible radiation frequency – red) and well into the ultra-violet (higher in frequencies than the highest visible radiation frequency – violet).

Our planet Earth rotates around the Sun and the solar radiation carrying energy falls on the Earth. About 5% of the radiation coming towards the earth is reflected away from the earth and the atmosphere absorbs about 15%. Thus the maximum solar power we can receive at the equator is about 1000 W/m2. Presence of clouds, dust, etc. reduces this further and on an average we receive only about 800 W/m2 of solar power at the equator

Solar energy gives us the light and heat, and plants convert the solar energy by photosynthesis into chemical energy for their growth. The plants and the trees provide fruits, grains and also the fuel – wood, grass, etc. The whole interdependent kingdom of living organisms – plants, animals and humans – draw their sustenance from this life giving source of energy. (Fuels such as oil, gas and coal that we use today were formed from fossilized plants and trees and in a way they represent the solar energy stored by nature). All the energy received from the Sun is not utilized in this manner and the excess energy results in increase of temperature of the earth and the oceans. This causes infra-red radiation from the earth outward into atmosphere.

Solar energy—power from the sun—is free and inexhaustible. Man’s ingenuity has always tried to find ways of using this free bounty of energy to his best advantage. Human civilization has always used the energy of the Sun as far back as they have existed on this planet. We receive enough energy from the Sun in barely twenty days of sunshine to make up for all the energy stored in Earth's fossil fuel reserves. We realize that currently we utilize barely 1% of this energy and it has a tremendous potential as an important inexhaustible alternative source of energy. .