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Vapour Absorption Refrigeration System Seminar Report
Solar cooling is an attractive idea because cooling loads and availability of solar radiation are approximately in phase. As the refrigeration system operates – pump work neglected - without the need for mechanical or electrical power, it is independent of electrical grids and thus may prevent in remote rural regions the spoiling of agricultural products in storage due to the lack of refrigeration. That is why there is a high demand for application of solar cooling for decentralised cold storage of food in the countries of the sun belt of the earth.
Solar cooling uses solar thermal energy to power a refrigerator, which in order to preserve food has to maintain temperatures lower than 5°C in the storage room. Heat operated cooling systems are well known. Ammonia-water absorption refrigeration systems are normally preferred for low temperature applications. The heat input for this system is required at temperatures higher than 90°C. Therefore high performance solar collectors are needed to supply a sufficient solar energy input.
Two main types of refrigeration systems are Mechanical vapour compression and absorption refrigeration system. The mechanical vapour compression system is outstanding due to its higher coefficient of performance, flexibility and compactness in manufacturing and operation. However, the fact that it is generally powered by electricity results in the emission of a large amount of CO2, which, in turn, causes the greenhouse effect. In addition, CFCs used as the working medium seriously affect the ozone layer around the globe. The absorption system, powered by either electric or wasted heat, is mainly used in large air conditioning and refrigeration systems. Owing to the environmental problem caused by CFCs and the huge energy consumption of conventional cooling system, this novel solar powered absorption refrigeration system has been developed. Novel solar powered absorption refrigeration system has many advantages in refrigeration or heat pumping application such as: Materials are environmentally friendly, chemically stable and the system can be powered by either solar energy or wasted heat.
HISTORY OF SOLAR VAPOUR ABSORPTION REFRIGERATION
Attempts have been made to run vapour absorption systems by solar energy with concentrating and flat plate solar collectors. Several small solar absorption refrigeration systems have been made around 1950s in several countries. Professor G.O.G. Lf of America is one of the pioneers in the area of solar refrigeration using flat plate collectors. A solar refrigeration system that could produce 250 kg of ice per day was installed in Tashkent, USSR in 1953. This system used a parabolic mirror of 10 sq m area for concentrating the solar radiation. F. Trombe installed an absorption machine with a cylindro-parabolic mirror of 20 sq m at Montlouis, France, which produced 100 kg of ice per day.
Serious consideration to solar refrigeration systems was given since 1965. Due to the scarcity of fossil fuel based energy sources. LiBr-water based systems have been developed for air conditioning purposes. The first solar air conditioning system was installed in an experimental solar house in University of Queensland, Australia in 1966. After this several systems based on solar energy were built in many parts of the world including India. In 1976, there were about 500 solar absorption systems in USA alone. Almost all these were based on LiBr-water as these systems do not require very high heating temperatures. These systems were mainly used for space air conditioning.
Intermittent absorption systems based on solar energy have also been built and operated successfully. In these systems, the cooling effect is obtained during the night time, while the system gets "charged" during the day using solar energy. Though the efficiency of these systems is rather poor requiring solar collector area, they may find applications in remote and rural areas where space is not a constraint. In addition, these systems are environment friendly as they use eco-friendly refrigerants and run on clean and renewable solar energy.
Solar adsorption refrigeration system with ammoniacates, sodium thiocyanate, activated charcoal, zeolite as adsorbents and ammonia, alcohols or fluorocarbons as refrigerants have also been in use since 1950s. These systems also do not require a compressor. The refrigerant vapour is driven by the adsorption potential of the adsorbent stored in an adsorbent bed. This bed is connected to an evaporator/condenser, which consists of the pure refrigerant. In intermittent adsorption systems, during the night the refrigerant evaporates and is adsorbed in activated charcoal or zeolite providing cooling effect. During daytime the adsorbent bed absorbs solar radiation and drives off the refrigerant stored in the bed. This refrigerant vapour condenses in the condenser and stored in a reservoir for night time use. Thus this simple system consists of an adsorbent bed and a heat exchanger, which acts as a condenser during the night time and, as an evaporator during the night. Pairs of such reactors can be used for obtaining a continuous cooling.
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