Many food products (e.g fruits, vegetables, meats, dairy products) are stored in cooling units for periods on the order of weeks at temperatures between 0 and 4°C in order to prevent spoilage and maintain freshness and quality. Food freezing systems are required for longer term storage at -18 to -35°C. Food storage and transport take place in chambers covering a wide range of sizes from cold stores to household refrigerators. Solar cooling is of great interest especially in developing countries, where food preservation is often as difficult a problem as food production.
From an energy saving view, a solar cooling system has the capability of saving electrical energy in the range of 25-40% when compared to an equivalent cooling capacity of a conventional water-cooled refrigeration system. Therefore, the use of solar cooling systems will save energy, especially during the summer season. The contribution of these systems to the food processing sector and consequently to the economy will be high.
Solar-powered mechanical cooling, of whatever type, is presently in the developmental phase. The technology is ready, but cost factors stand in the way of vigorous marketing programs. At present, active solar cooling is not in a reasonably competitive position with respect to conventional cooling systems (energized by electricity or fossil fuel). During the last decade the situation has changed quickly because of increasing interest in renewable energy sources, especially solar energy, for reducing the use of fossil fuels and electricity.
Solar energy can be used in different systems available for cooling applications. These systems are :
* Rankine cycle-vapor compression system,
* absorption cycle system,
* adsorption system,
* jet ejector system,
* Rankine cycle-inverse Brayton cycle system, and
* nocturnal radiation system.
Among these systems, the solar-powered absorption cooling cycle is the most popular system for solar cooling applications due to the following advantages:
* quiet operation,
* high reliability,
* long service life,
* effective and economic use of low-grade energy sources (e.g. solar energy, waste energy, geothermal energy, natural gas),
* easy implementation and capacity control,
* no cycling losses during on-off operations, and
* meeting the variable cooling load easily and efficiently.