Thermodynamic analysis of the operation of two-stage metal-hydride heat pumps

Abstract

One of the most important applications of metal hydrides is for heat pumps. A two-stage heat pump is superior to a single-stage heat pump because the former has higher efficiency and wider operational temperature range. This paper carries out a systematic study on two-stage heat pumps. Six cycles of two-stage heat pumps are analysed and compared, the selection of metal hydrides for these heat pumps is discussed, the actual COP equations are derived and the effects of some parameters on COP are examined. The study shows that the low-pressure-side two-stage heat pump is suitable for heat upgrading and the high-pressure-side two-stage heat pumps for heat amplifying and refrigerating. For high efficiency of the systems, the metal hydrides used should have ΔH H/ΔH M ≤ 1 and ΔH M/ΔH L ≤ 1 for a heat-upgrading heat pump and have ΔH H/ΔH M ≥ 1 and ΔH M/ΔH L ≤ 1 for heat-amplifying and refrigerating heat pumps. In order to obtain a high COP, it is important to develop metal hydrides with wider and flatter plateaux in their P-C-T curves, to reduce the total sensible heat capacity of the reactors and metal hydrides, and to recover some part of the sensible heat between reactors.