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Details of a heat pump assisted solar water heating system, is briefly described. Refrigerants compromising of Fluorocarbons and Chlorofluorocarbons have been generally used as the working fluid. But due to its ozone depletion effect and high Global warming potential, they have been replaced by synthetic refrigerants such as R22, R134 etc. The heat pump utilizing such refrigerants has shown to attain efficiencies (COP) of the order only about 2 to 3. In this paper an attempt has been made to find out the reasons for such low efficiencies and the possibility of using CO 2 (R744) as the working fluid in heat pump assisted solar water heating systems. CO 2 is one of the few natural refrigerants which is neither flammable and nor toxic. It is commonly obtainable and does not affect the global environment like other refrigerants. A few changes to be made in the experimental set up to improve the performance of the heat pump are discussed.

An experimental study was performed to investigate the feasibility and performance of thermosyphon solar water heating (SWH) system using R744 (CO2) as the working fluid. The uniqueness of the system was the use of carbon dioxide as a working fluid; which is one of the most promising alternative natural refrigerants. For supercritical carbon dioxide, a small change in temperature or pressure can result in a large change in density, especially close to its critical point. At this pseudo critical region, density decreases rapidly with increase in temperature which aids the thermosyphon flow. An experimental setup in which 1.15 m 2 evacuated tube (U-pipe) solar collector acting as a source as well as an evaporator for the refrigerant, was designed and tested based on this principle. Experimental results have shown that it is possible to induce the natural convective flow even during solar-adverse conditions. Although during winter it was not possible to extract any useful heat gain, the system did show some promising results when operated during spring. The time-averaged collector and heat recovery efficiencies for summer were about 58% and 45%, respectively.

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