Simulation of thermal performance on unglazed solar collector for refrigerant

In order to analyze an unglazed solar collector's thermal performance,a distributed parameter and homogeneous flow mathematical model was established based on the direct expansion solar assisted heat pump water system.The overheated and two-phase regions were compartmentalized by enthalpy difference from refrigerant outlet to inlet.And the heat transfer and heat balance equations for the inner and outside of tube were set up.With dichotomy iterative calculation of tube length,the model was solved.Theoretical calculation was carried out to analyze refrigerant phase change and the influences of structural,operational and environmental parameters on solar collector efficiency.Simulation results indicate that refrigerant dryness of two-phase region changes linearly approximately;solar collector efficiency increases evidently with increases of environmental temperature and solar collector thickness.However,solar collector efficiency decreases with increases of solar radiation intensity,water temperature and the distance between collector tubes.The increase of collector tube diameter has little effect on solar collector efficiency.