Abstract
The reverse Carnot cycle, the high and low heat source of which are constant temperatures, decides the theoretical limit of COP of conventional heat pumps used as air conditioners etc., but the cycle which decides the theoretical limit of COP of a heat pump used as tap water heater is considered the cycle whose high heat source varies temperature with constant specific heat, because the specific heat of tap water is constant and tap water heating with high temperature rise is needed. In this paper, the theoretical COP of this cycle is clarified. And upper limit COP of heat pump cycle with various refrigerants for tap water heating is studied and finally the refrigerant which achieves the highest COP for supplying hot water is clarified.
