A high-temperature electrically operated heat pump water heater is evaluated in terms of the viability of employing capacity control using zeotropic refrigerant mixtures. The system coefficient of performance (COP) is improved by introducing capacity control, which offers continuous modulation by varying heat pump capacity to match the load. This is accomplished by using a zeotropic refrigerant mixture and changing the composition (x) of the circulating mixture. The zeotrope R-22/R-142b is selected due to the requirements for a high condensing temperature and a wide capacity range. A comparison is made of the energy-saving potential of the R-22/ R-142b mixture in a capacity-controlled heat pump, relative to an R-22 heat pump. Computer simulations show that the capacity-controlled heat pump, operating between compositions of 100% R-22 and 70% R-22, shows a 29.6% improvement in energy conversion when compared with a conventional R-22 heat pump water heater. An advanced heat pump water heater is proposed, which would exploit the advantages of using a zeotropic mixture.

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