Since the energy of the sun is being received as heat

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(refrigeration). Since the energy of the sun is being received as heat, the obvious choice is a system working on the absorption refrigeration cycle which requires most of its energy input as heat. Cooling is required most in summer. Hence, in this case, there is a seasonal matching between the energy needs of the space cooling system and the availability of solar radiation. A diagram of a simple solar operated absorption system is shown in Fig. 3.18. Water heated in a flat-plate collector array is passed through a heat exchanger called the generator, where it transfers heat to a solution mixture of the absorbent and refrigerant, which is rich in the refrigerant. Refrigerant vapor is boiled off at a high pressure and goes to the condenser where it is condensed into a high pressure liquid. The high pressure liquid is throttled to a low
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28 pressure and temperature in an expansion valve, and passes through the evaporator coil. Here, the refrigerant vapour absorbs heat and cooling is therefore obtained in the space surrounding this coil. The refrigerant vapour is now absorbed into a solution mixture withdrawn from the generator, which is weak in refrigerant concentration. This yields a rich solution which is pumped back to the generator, thereby completing the cycle. The rich solution flowing from the absorber to the generator is usually heated in a heat exchanger by the weak solution withdrawn from the generator. This helps to improve the performance of the cycle. Some of the common refrigerant-absorbent combinations used are ammonia-water and water-lithium bromide, the latter being used essentially for air-conditioning purposes. Typical values for the coefficient of performance (the ratio of the refrigerating effect to the heat supplied in the generator) range between 0.5 and 0.8. Unfortunately, the installation cost of a solar absorption refrigeration system is high because of the cost of the large collector array required. Thus commercialization has not taken place although a few demonstration units have been set up. 3.13.4 Power Generation The generation of electrical power is one of the most important applications of an energy source. There are several methods to generate electricity using solar energy. Solar thermal power cycles can be classified as low, medium and high temperature cycles. Low temperature cycles work at maximum temperatures of about 100°C, medium temperature cycles work at maximum temperatures up to 400°C, while high temperature cycles work at temperatures above 400°C. Low temperature systems use flat-plate collectors or solar ponds for collecting solar energy. Systems working on the solar chimney concept have also been suggested. Medium temperature systems use the line-focusing parabolic collector technology. High temperature systems use either paraboloid dish collectors or central receivers located at the top of towers.
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