Quantitative and Qualitative Comparison of Low-Temperature, Heat-Activated Cooling Systems

Abstract

This paper compares the quantitative and qualitative performances of three different heat-activated cooling systems, e.g. a silica-gel water adsorption system, a LiBr-H2O absorption system, and a desiccant air system. Each of these systems can be utilized at relatively low heat source temperatures, but it is unclear which of these systems is best suited to what range of heat source temperature. Our study explores answers to this question by generating quantitative results comparing their relative thermal performance, i.e. COP and refrigeration capacity, and a qualitative comparison based on the size, maturity of technology, safe operation, etc. Each of these systems is assumed to work under the following operating conditions: a condensing temperature of 29 °C, an evaporating temperature of 19 °C, a hot water temperature range of 40-120 °C, and a hot water mass flow rate of 0.4 kg/sec. Individual mathematical models are developed for each system and numerically solved using different techniques. In order to provide a fair comparison between the fundamentally different systems, a UA (overall heat transfer coefficient multiplied by the heat transfer area) value of 1.0 kW/ °C is considered for the heat exchanger that transfers heat from the supplied hot water. Furthermore, to compare systems of similar size, the mass of silica gel in adsorption and desiccant system and mass of LiBr-H2O solution in absorption system were specified such that each system provides the same amount of refrigeration (8.0 kW) at a source temperature of 90 °C. It is found that the absorption and adsorption cooling systems have a higher refrigeration capacity at heat source temperatures below 90 °C, while the desiccant air system outperforms the others at temperatures above 90 °C.