Subcritical recuperative air-to-water heat pump with zeotropic mixtures: Comparative and performance assessment

Abstract

For high temperature heating, traditional single-stage air-source heat pumps suffer severe challenges in low efficiency and poor operating conditions. Heat recuperation is a simple and effective method to expand the temperature lift. In this paper, two kinds of recuperative air-to-water heat pump (RAWHP) systems were investigated for heating water from 15 °C to 90 °C at an ambient temperature of 15 °C. The energy, exergy, and economy models were established to assess the performance of RAWHP systems and compare with that of transcritical CO2 heat pump (TCHP) system in detail. The results show that the three-stream RAWHP system is superior to the two-stream RAWHP system from the perspective of energy and exergetic performance. For the three-stream RAWHP system, the highest COP of 4.08 and exergy efficiency of 45.4% were obtained by propylene/n-hexane with the optimal mole fractions of 0.9/0.1. In this case, the overall temperature matching indicator is 2.87, indicating a smaller heat transfer irreversible loss. Compared with the TCHP system, RAWHP systems have more advantages in higher COP and lower payback period at the ambient temperature ranges from −25 °C to 25 °C. The three-stream RAWHP system by using propylene/n-hexane (0.9/0.1) is recommended due to its outstanding energetic and economic performance.