Other methods of refrigeration
There are several other forms of refrigeration, the most important of which is vapour absorption, dealt with in chapter 14. Other methods, considered by Butler and Perry (1999), are as follows.
(1) Water vapour systems. The vapour pressures to achieve effective cooling are very low, making the method impractical. With a steam jet as the source of energy the COP is less than 0.3.
(2) Stirling cycle. In principle, this should have a higher COP than conventional vapour compression. However, hot and cold heat transfer areas are very small, which introduces difficulties. The method has been used in small domestic refrigerators.
(3) Acoustic cooling. Alternate compressions and expansions, causing heating and cooling, are produced in a closed tube by a sound generator. Efficiencies appear to be poor and the plant is comparatively large.
(4) Magnetic cooling. Some metals heat up when magnetised and cool when demagnetised. A prototype plant at the US Department of Energy Ames Laboratory, in Iowa, has rotated a metal disc continuously through strong and weak magnetic fields with heat transferred by water or an anti-freeze fluid to heat exchangers. Initial efficiencies are claimed to be 20 per cent better than conventional methods but with higher capital costs and a simple pay back of 5 years.
(5) Pulse tube cooling. This is similar to acoustic cooling but with a compressor instead of a sound generator, and an inert gas in the tube.
(6) Thermo-electric cooling. This makes use of the Peltier effect and is commercially available with cooling capacities up to 500 W. It has a high capital cost and very low COPs.
(7) Thermionic cooling. Cooling occurs at the negative electrode when high energy electrons flow between two electrodes in a vacuum. A better COP than vapour compression gives is possible if semiconductor materials are used. It is potentially cheap with future development.
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Fig. 9.10 Pressure-enthalpy diagram of refrigerant 134a. Reproduced with permission of the American Society of Heating, Refrigerating and Air Conditioning Engineers from the (1977) ASHRAE Handbook.
(8) Air cycle cooling. This is well established for aircraft, but with high capital cost and low efficiency. COPs are in the range 0.35 to 0.57. Air is compressed and expanded without a phase change of refrigerant.
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