Thermodynamic analysis of single-stage compression air-source heat pumps with different recuperation ways for large temperature lift

Abstract

Traditional single-stage compression air-source heat pumps for large heat supply temperature lift suffer tough challenges in high pressure ratio and low efficiency. In this paper, we focus on the large temperature lift heat pump water heater systems with a temperature lift of 68 K. Three binary refrigerants R1270/R1234yf, R1270/R600 and R1270/Hexane, three kinds of heat recuperation methods without recuperation, single-phase recuperation, and two-phase recuperation are adopted. System optimization is implemented by the genetic algorithm to acquire optimal operational conditions and refrigerant compositions. Exergy analysis is used to reveal the irreversible loss distribution in systems. The sensitivity analysis of ambient temperature and heat supply temperature is conducted. The results reveal the crucial role of big temperature glide of refrigerant to decrease irreversible losses in condensation and heat recuperation. The optimal COP and volumetric heat capacity of the heat pump with two-phase heat recuperation are 6%, 70% higher than that of the traditional heat pump when the ambient temperature is 25 °C. Moreover, the heat pump with two-phase heat recuperation shows a higher COP when the heat supply temperature exceeds 72 °C and shows COP of 2.1 even the ambient temperature is -20 °C. The strong ability of the two-phase heat recuperative heat pump to enlarge the temperature lift efficiently is verified.