Sun is the primary source of clean and abundant energy. It is estimated that 120,000 TW of radiation is deposited by the sun on the surface of the Earth. This will be more than enough for all energy requirements by human beings. The sun is the only natural power source for earth that is the reason for the circulation of global wind and ocean currents, hydrological cycle, and the biological cycles of photosynthesis and life (Lewis, et al. 2005). Energy from sun can fulfil the growing energy needs and help to reduce the increasing pollution and can be an alternative to the use of non-renewable and fossil fuels. Technological innovations can further make this source of energy more user friendly and economic. Though solar energy is present in abundance, only recently human beings have developed appropriate technology to tap this energy. The thermonuclear process occurring in the sun is responsible for creating energy. Through this process it converts about 650,000,000 tons of hydrogen to helium every second and as a result creates heat and electromagnetic radiation (Brown, 1988).
There are three basic steps involved in all routes for utilizing solar energy and they are capture, conversion, and storage (Lewis, et al. 2005). In the capture process, the collector simply collects the radiation that falls on it. In the conversion step, a fraction of the captured radiation is converted into either electricity and heat energy or heat alone. Since day and night cycle and cloud cover influences the presence of sun’s energy it is necessary to store this energy. In other words the storage unit is required because of the non-constant nature of solar energy. The storage unit helps to hold the excess energy produced during the periods of maximum sunlight, and release it when the suns radiation drops (Brown, 1988).
The methods of collecting and storing solar energy vary with the uses planned for the solar generator. There are three types of collectors and many forms of storage units. The three types of collectors are flat-plate collectors, focusing collectors, and passive collectors. The most commonly used is the flat-plate collectors which are arrays of solar panels set in a simple plane and vary in size. The output of flat-plate collectors is directly related to a few variables including size, facing, and cleanliness that determine the amount of radiation that falls on the collector. In most of the cases the collector panels have automated machinery that keeps them facing the sun.
The main difference between the flat-plate collectors and focusing collectors are basically that these have optical devices arranged to take full advantage of the radiation falling on the focus of the collector. However these are not as common as flat-plate collectors and are used in solar furnaces and few other devices. If we compare the flat-plate and focusing collectors, the focusing collectors can produce far greater amounts of energy at a single point than the flat-plane collectors. However, they lose some of the radiation that the flat-plane panels do not lose. The silicon components are used to absorb the incoming radiation loses efficiency at high temperatures, and can even be permanently damaged. The focusing collectors can create higher temperatures and need more protecting devices to protect their silicon components.
On the other hand passive collectors are totally different from the flat-plate and the focusing collectors. The passive collectors absorb radiation and convert it to heat naturally, without being designed and built to do so. For instance, the black colour absorbs maximum heat radiation from sun when compared to other colour. The passive collectors utilizes such concepts and often their natural ability to convert radiation to heat is enhanced in some way or another and a system for transferring the heat to a different location is usually added (Brown, 1988).