SOLAR ENERGY FOR POWER GENERATION
Electric power is generated by means of solar energy in two ways.
(i) by using a thermodynamic closed loop cycle and
(ii) by using photovoltaic solar cells.
In the first method, solar energy is collected as usual on a collector's surface and then transferred to an absorber. A fluid flowing through the absorber receives solar energy (heat) and transfers this to a machine. This machine will convert heat energy into mechanical energy. By coupling an electrical generator this mechanical energy is converted into electrical energy.
In the second method, electricity generation is direct with the help of solar cells. These photovoltaic solar cells convert solar radiation into electricity.
Power Generation Using Flat Plate Collector:
The cycle of solar thermal power generation is shown in figure.
The energy of the sun is collected by water flowing through an array of flat plate collectors. Booster mirrors which reflect radiation on to the flat plate collectors may also be used so as to get maximum possible temperature. The hot water at about 100°C is stored in a well insulated thermal storage tank. From here it flows through a vapour generator. A working fluid such as methylchloride flows through a vapour generator as shown. Heat transfer takes place between hot water and this organic working fluid which has low boiling point. Consequently, at an ordinary atmospheric pressure, the working fluid evaporates at about 90°C. This vapour leaves the generator to work in a prime mover (turbine). A turbine coupled to an electric generator produces electric power. The vapour exhausts into a condenser where it is- cooled to a liquid form by some cooling medium. The working find, now in liquid form is pumped to generator again. It may be noted that the power plant works on Rankine Cycle.
This system is applicable to 10 to 50 kW power plants. Efficiency of collector system would be about 25% and that of Rankine Cycle would be about 7 to 8% only because of low temperature difference between vapour leaving the generator and condensate. Overall efficiency of the plant is as low as 20%.
The other working fluids are Hexafluoro Benzene, Steam, Freon fluids such as R-11, R-113, R-114, Toluene etc.
Power Generation Using Parabolic Concentrator:
In this system, parabolic concentrator is used to collect solar radiation. Figure shows a cylindrical parabolic collector. The collector pipe, preferably with a selective absorber coating is used as an absorber. The absorber is placed along focus axis as shown.
The parabolic reflector is made of highly polished aluminium or of a silvered glass or of a thin film of aluminised plastic. Instead of a continuous form, the reflector may be constructed from a number of ion flat strip on a parabolic base.
When the reflector unit is mounted on a frame, it can be tilted according to sun orientation using chain drive.
Cycle for power generation is same as that in Figure. The only change is that flat plate collector is replaced by a parabolic concentrator shown in figure.
Photovoltaic Conversion:
In Photovoltaic conversion, the solar radiation falls on 'solar coils' which convert the sunlight directly into electricity.
Silicon cell is the most popular one that is used in this method of energy conversion. A cell is made of thin wafers about 300 μm thick each and provided with n-type and p-type doped silicon. Metal electrodes made from a Ti-Ag solder are attached to the top and bottom sides of the cell. On the top side, the electrode is in the form of a metal grid with fingers which permit the sunlight to go through. While at the bottom side, the electrode completely covers the surface as shown in figure. An antireflection coating of silicon oxide about 0.1 μm thick is applied on top surface.
When radiation falls on cell, it is absorbed and electronhole pairs are created. These are the pairs of positive and negative charges which are separated because of the p-n junction. The solar radiation is converted into electricity directly and the direct current thereby produced is collected by electrodes and flows to the external load.
The efficiency of such cells is about 10 to 12%. They have no moving parts and their maintenance is easy.
Gallium arsenide is another solar cell material. Cells made of this material give 20 to 25% efficiency and retain this efficiency even at higher temperatures than silicon cells.