Development Of An Activated Carbon+ HFC 134a Adsorption Refrigeration System

Abstract

The demands facing the refrigeration industry are minimal usage of conventional energy sources for compression and avoidance of ozone depleting substances. One of the approaches to combat these issues is the use of thermally driven solid sorption compression with non-ozone depleting refrigerant. In this context, the research work presented in this thesis is devoted to a comprehensive thermodynamic analysis and development of a laboratory model of an activated carbon+ HFC 134a adsorption refrigeration system. The cooling load catered to by the laboratory model is 2-5 W, mainly for thermal management of electronics. A complete thermodynamic analysis is carried out for the desorption temperatures varying from 75 to 90 oC, evaporating temperatures from -20 to 15oC and adsorption/condensing temperatures from 25 to 40 oC. A program on MatLab platform is developed for theoretical modeling. A new concept of thermal compression uptake efficiency (u) which is analogous to volumetric efficiency of a positive displacement compressor is introduced to consider the effect of void volume. The thesis also covers an investigation of two-stage and hybrid (thermal+ mechanical) cycle compression systems. It is possible to identify the conditions under which a two-stage gives a better performance than a single-stage one. It also shows that hybrid cycle system gives the best performance and saves ~40% of power compared to operation under the same conditions run with a single-stage mechanical compression refrigeration system. A heat transfer analysis of the thermal compressor is carried out to evaluate non-uniformities in bed temperature. As a part of it, the thermal conductivity of the bed under adsorbed state has been calculated. A laboratory model of activated carbon+ HFC 134a adsorption refrigeration system is fabricated to meet a 2-5 Watts cooling load based on the results from theoretical calculations. Experimental results show a fair match in the trends for the COP with analysis. The main aim of the research was to examine how effective the adsorption refrigeration system is in reducing the temperature rise of the heater used to simulate the electronic component. The heater that would have stabilized at 81, 97, 103 and 112 oC without any cooling for heat inputs of 3, 4, 4.4 and 4.9 W, respectively, would attain a cyclic steady state around 24, 26, 28, 31 oC. The influence of cycle time on the performance of the systems is also investigated. It is concluded that an activated carbon+ HFC 134a adsorption refrigeration system can be a good supplement to conventional compression refrigeration systems. In situations where heat recovery imminent this system could be a good choice. For waste heat recovery and suppression of infrared signatures of electronic components, it is ideally suited where COP becomes immaterial.