Limiting performance of vapour compression heat pumps for heating fluids

Abstract

When a vapour compression heat pump is used to heat a fluid over a range of temperatures the refrigerant cycle may be fundamentally different from a standard Rankine cycle which operates only between isothermal reservoirs. This paper considers two strategies for heating fluids: in one approach the fluid is heated in a single pass condenser; in the other the body of fluid is kept fully mixed at all stages of the heating process. An isothermal source reservoir is assumed and the performance of nine common refrigerants is examined. In the ideal limit the mixed fluid process has a higher COP than the single pass method. But in practice there is likely to be no clear advantage in using a fully mixed sink fluid, especially where the temperature lift is greater than 50°C. The single pass heating process has an advantage under these conditions that lower peak condensing temperatures may generally be achieved than for the mixed fluid heating process.