THERMODYNAMIC ANALYSIS OF THE OPERATION OF 2-STAGE METAL-HYDRIDE HEAT-PUMPS

One of the most important applications of metal hydrides is for heat p umps. A two-stage heat pump is superior to a single-stage heat pump be cause the for mer has higher efficiency and wider operational temperat ure range. This paper carries out a systematic study on two-stage hear pumps. Six cycles of two-stage heat primps 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 o n COP are examined. The study shows that the low-pressure-side two-sta ge 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 Delta H -H/Delta H-M less than or equal to 1 and Delta H-M/Delta H-L less than or equal to 1 for a heat-upgrading heat pump and have Delta H-H/Delta H-M greater than or equal to 1 and Delta H-M/Delta H-L less than or e qual to 1 for heat-amplifying and refrigerating heat pumps. In order t o obtain a high COP, it is important to develop metal hydrides with wi der and flatter plateaux in their P-C-T curves, to reduce the total se nsible heat capacity of the reactors and metal hydrides, and to recove r some part of the sensible hear between reactors. (C) 1996 Elsevier S cience Ltd.