Choice of Refrigerant - Adsorbent Pairs

The best sub-atmospheric refrigerant is water. Unfortunately it is not strongly adsorbed by carbons, but refrigerators and heat pumps based on water - zeolite pairs have been built and tested in research laboratories. Methanol is adsorbed well by carbons and a solar refrigerator based on a carbon - methanol pair was marketed by Brissoneau et Lotz Marine in France. Methanol is environmentally friendly, but decomposes at temperatures around 150°C and so cannot be used for very high temperature cycles. 

Refrigerant Formula Name Normal Boiling Point (°C) Mol Wt. Latent heat L (kJ/kg) Liquid density P (kg/m3) Latent heat per unit vol. pL xlO 3 (kJ/m3) 

Other chemicals not shown are superior to R22, but it is included as the best of the 

Other refrigerants, including CFCs Rll and R12, HCFC R22, HFC R32 and hydrocarbons such as butane have been evaluated in detail by Critoph [3,4] but are significantly worse in performance than methanol or ammonia. In 1996, these two refrigerants are the only ones used in the major laboratories working on carbon adsorption cycles. 

Having chosen a suitable refrigerant, the best adsorbent must be found. Zeolites, silica gels and chemical adsorbents have been used as well as carbons, but this chapter will concentrate on the carbon adsorbents. An indication as to the range of COP s that can be expected and the influence of the type of carbon used can be obtained by modelling the performance of carbons with a range of adsorption parameters. For this purpose it is preferable to use the Dubinin-Raduschkevich 

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The application of activated carbons in adsorption heat pumps and refrigerators is discussed in Chapter 10. Such arrangements offer the potential for increased efficiency because they utilize a primary fuel source for heat, rather than use electricity, which must first be generated and transmitted to a device to provide mechanical energy. The basic adsorption cycle is analyzed and reviewed, and the choice of refrigerant-adsorbent pairs discussed. Potential improvements in cost effectiveness are detailed, including the use of improved adsorbent carbons, advanced cycles, and improved heat transfer in the granular adsorbent carbon beds. 

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