Theoretical Analysis of Ammonia-Based Combined Power/Refrigeration Cycle at Low Refrigeration Temperatures

A new combined power/refrigeration cycle uses ammonia/water mixture as a working fluid to produce both power and refrigeration in the same cycle. The cycle may be designed for various combinations of power and refrigeration. In an earlier paper by the authors, the cycle was optimized for efficiency, with power as the main intended output. This study puts an emphasis on the refrigeration part of the total output especially at low refrigeration temperatures. The objective was to find out what kind of outputs could be obtained at very low temperatures for a possible application in the Mars mission. The thermal performance of this cycle at different refrigeration temperatures has been found. At each refrigeration temperature, the cycle is optimized for maximum second law efficiency using Generalized Reduced Gradient (GRG) algorithm. It is found that refrigeration temperatures as low as 205 K may be achieved using this cycle. Generally, both first and second law efficiencies decrease when refrigeration temperature drops. For a re-circulating type of solar thermal system with a source temperature of 360K, the first and second law efficiencies increase slightly as the refrigeration temperature goes down from 265K to 245K and then decrease with the refrigeration temperature, giving a maximum second law efficiency of 63.7% at 245K.