Performance analysis of direct-expansion solar-assisted heat pump system

Abstract

Direct-expansion solar-assisted heat pump (DX-SAHP) system has proven to be an effective energy-saving application. At present, the majority of research on the heat transfer characteristics of the DX-SAHP system concentrates on steady-state conditions. In steady-state research, the temperature and pressure parameters of the system remain constant over time. However, the operational state of the system under actual application conditions is influenced by numerous factors. The current paper employs the moving boundary model to construct dynamic models of the condenser and collector-evaporator based on thermodynamic principles, whilst establishing steady-state models for the compressor and expansion valve to investigate the system's dynamic response characteristics. This model simulates sudden temperature changes and the compressor's speed to assess its performance in real-world scenarios. Under step increases in ambient temperature of 5 K, 10 K, and 15 K, the evaporating pressure rose significantly by 3.63 %, 6.56 %, and 10.93 %, respectively, indicating a strong sensitivity to environmental changes.The 30 % step increase in compressor speed resulted in a 1.60 % rise in condensing pressure but a 7.79 % decrease in evaporating pressure. These responses were accompanied by considerable variations in zone lengths and wall temperatures; the condenser’s two-phase zone expanded by up to 4.10 % with ambient change, while the evaporator’s two-phase zone consistently shortened by 0.097 % across compressor speed changes. These quantified insights underscore the critical influence of dynamic perturbations on system behavior, highlighting the need for advanced control strategies to optimize DX-SAHP performance under realistic operating conditions.