Comparative simulation and investigation of ammonia-water: absorption cycles for heat pump applications

Several recent programs in absorption research have focused on technology for domestic heating and cooling utilizing natural gas. In residential and small commercial size applications, ammonia-water cycles offer the possibility of a gas-fired heat pump for both winter heating and summer cooling, at better year-round COPs than currently available by various alternatives. Several cycles have been considered for this purpose, ranging from the simplest single effect to the GAX (Generator-Absorber heat eXchange) with its different variations. Detailed calculations of ammonia-water systems are rather difficult, particularly in complex cycles such as the GAX. This may be the reason that relatively few simulation studies have been published to date that give more than design point performance. The objective of the present study has been a comprehensive investigation of various ammonia-water cycles, with operating conditions and different design parameters varied over a wide range to compare their performance. To this end, a modular simulation program (ABSIM) was employed, which makes it possible to simulate absorption cycles in varying configurations. The cycles investigated increase in complexity from the basic single-effect cycle, through the same cycle with an added precooler, through an added solution-cooled absorber and solution-heated generator, to the GAX and branched GAX cycles with different types of branching. Each cycle was formed on the basis of the previous one by adding one or two components at each stage, resulting in increasing complexity rewarded by improved performance. This kind of investigation enabled determination at each stage of the influence of the added components on the cycle. The results of the investigation show cooling COPs ranging from about 0.5 for the simplest single-effect cycle to about 1.0 for the GAX cycle. (C) 1997 Elsevier Science Ltd and IIR.