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.