Investigation of ammonia/water hybrid absorption/compression heat pumps for heat supply temperatures above 100 °C

The hybrid absorption/compression heat pump (HACHP) using ammonia-water as working fluid is a promising technology for development of a high temperature industrial heat pump. This is due to two properties inherent to the use of zeotropic mixtures: non-isothermal phase change and reduced vapour pressures. Using standard refrigeration components (28 bar) HACHP up to 100 °C are commercially available. Components developed for high pressure NH3 (52 bar) and transcritical CO2 (140 bar) increase the limiting allowable pressures. It is therefore relevant to evaluate the feasible supply temperatures using these components. A technically and economically feasible solution is defined as one that satisfies constraints on the coefficient of performance (COP), low and high pressure, compressor discharge temperature and volumetric heat capacity. The ammonia mass fraction of the rich solution and the liquid circulation ratio both influence these constraints. The paper investigates feasible combinations of these parameters. A numerical HACHP model is developed in Engineering Equation Solver (EES). The constrained parameters are evaluated for a range of combinations for systems with supply temperatures of 100 °C, 125 °C, 150 °C and 175 °C. Results show that standard components are applicable up to 100 °C, equipment for high pressure NH3 up to 125 °C, and equipment for transcritical CO2 up to 175 °C.