Solar energy is the most abundant & cleanest energy resource on earth. The amount of solar energy that hits the earth’s surface in an hour is almost the same as the amount required by all human activities in a year. Solar energy can be used mainly in three ways one is direct conversion of sunlight into electricity through PV cells, the two others being concentrating solar power (CSP) and solar thermal collectors for heating and cooling (SHC). India is endowed with abundant solar energy, which is capable of producing 5,000 trillion kilowatts of clean energy. Country is blessed with around 300 sunny days in a year and solar insolation of 4-7kWh per Sq. m per day. If this energy is harnessed efficiently, it can easily reduce our energy deficit scenario and that to with no carbon emission. Many States in India have already recognized and identified solar energy potential and other are lined up to meet their growing energy needs with clean and everlasting solar energy. In near future Solar energy will have a huge role to play in meeting India’s energy demand. .
b) SOLAR PV TECHNOLOGY
Solar Photovoltaic (PV) cells convert solar light directly to electricity .Photovoltaic can literally be translated as light-electricity.;
Crystalline silicon (c-Si) is the oldest technology for solar PV modules. C-Si modules are subdivided in two main categories: i) single crystalline (SC-Si) and ii) multi-crystalline (mc-Si).
A thin film is a newer technology in comparison to the crystalline silicon. They are subdivided into three main families: i) amorphous (a-Si) and micro morph silicon (a-Si/µc-Si), ii) Cadmium-Telluride (CdTe), and iii) Copper-Indium-Diselenide (CIS) and Copper-Indium- Gallium-Diselenide (CIGS).Emerging technologies encompass advanced thin films and organic cells. The latter are about to enter the market via niche applications.
Concentrator technologies (CPV)
Concentrator technologies (CPV) use an optical concentrator system which focuses solar radiation onto a small high- efficiency cell. CPV technology is currently being tested in pilot applications. Novel PV concepts aim at achieving ultra-high efficiency solar cells via advanced materials and new conversion concepts and processes. They are currently the subject of basic research.
c. Solar thermal Technology
Solar energy is used as heat source for heating purposes for direct use and to generate steam for generating electricity through turbines.
Different technologies for solar thermal power plants making use of concentrating solar energy systems are:
i) Parabolic troughs
Parabola has the property of focusing the incoming radiation as its focus. Working on this principle, linear concentrators of parabolic shape are coated with highly reflective material and can be turned in angular movements towards the sun position and concentrate the incoming solar radiation onto a long-line receiving absorber tube. A working fluid is used to transfer the absorbed solar energy, which is then piped to an exchanger or a conventional conversion system. Parabolic trough systems cannot make use of diffused radiation as they use only direct-beam sunlight and require tracking systems to keep them focused toward the sun and are best suited to areas with high direct solar radiation. Most systems are oriented either east-west or north-south with single-axis tracking during the day.
ii) Solar Tower (Central Receiving System)
Central receiver systems use heliostats to track the sun by double axes mechanisms following the azimuth and elevation angles with the purpose to reflect the sunlight from many heliostats oriented around a tower and concentrate it towards a central receiver situated atop the tower. This technology has the advantage of transferring solar energy very efficiently by optical means and of delivering highly concentrated sunlight to one central receiver unit, serving as energy input to the power conversion system. In spite of the elegant design concept and in spite of the future prospects of high concentration and high efficiencies, the central receiver technology require more development for further up scaling plant performance. Its main attraction consists in the prospect of high process temperatures generated by highly concentrated solar radiation to supply energy to the topping cycle of any power conversion system and to feed effective energy storage systems able to cover the demand of modern power conversion systems.
Different receiver heat transfer media that have been successfully used are water/steam, liquid sodium, molten salt, ambient air, oil.
Solar Tower plants have the good long-term perspective for high conversion efficiencies and for use of very efficient energy storage systems by utilization of high temperatures in order to enlarge the solar capacity or solar share.
iii) Linear Fresnel
The Linear Fresnel technology uses long, flat or slightly curved mirrors to focus sunlight onto a linear receiver located at a common focal point of the reflectors. The receiver runs parallel to and above the reflectors and collects the heat to boil water in the tubes, generating high-pressure steam to power the steam turbine (water/direct steam generation, no need for heat exchangers). The reflectors make use of the Fresnel lens effect, which allows for a concentrating mirror with a large aperture and short focal length. This reduces the plant costs since sagged-glass parabolic reflectors are typically much more expensive. Since the optical efficiency as well as the working temperatures are considerably lower than with other CSP concepts, saturated steam conditions have to be considered for this technology. Development is now heading from demonstration plants to bigger, commercialized projects. The receiver is stationary and so fluid couplings are not required (as in troughs and dishes). The mirrors also do not need to support the receiver, so they are structurally simpler. When suitable aiming strategies are used (mirrors aimed at different receivers at different times of day), this can allow a denser packing of mirrors on available land area.