Preliminary investigation on the switching time of a photovoltaic solar-assisted heat-pump/heat-pipe hybrid system

Abstract

Integrating the passive and active two-phase heat transfer means (gravity-assisted heat pipe and heat pump) in PV/T applications can solve the freezing and corroding problems associated with the water-based PV/T collectors. However, the former fails to meet the temperature demands in unfavorable weather conditions while the latter wastes solar resources in favorable weather conditions. Integrating these two could ensure flexible operation. However, the switching time of the passive and active modes was rarely reported. In the present study, a hybrid PV/T collector/evaporator which integrated the evaporation section of the heat pipe and the refrigerant pipe of the heat pump was designed and manufactured. A hybrid system that can alternatively operate in passive and active operating mode was proposed and constructed. Furthermore, a numerical model which employs a temperature transfer matrix to couple the two operating modes is developed. Based on the validated model, the switching times of the hybrid system are revealed respectively based on the criteria of overall thermal efficiency and overall exergy efficiency. In particular, the results of the two modes operated individually are also compared and discussed. The results indicate that the hybrid system respectively needs to switch two times (from active to passive and then to active) and one time (from passive to active) with respect to overall thermal efficiency and overall exergy efficiency; the deserted mode is always inferior to that it operated individually because the initialization values deviate its normal operating pattern. Comparatively speaking, the net yields show that the hybrid system finishes a straight and narrow performance and can well balance the water temperature demand and the energy saving no matter what the switching index is; meanwhile, the switching based overall exergy efficiency is more preferred because higher net yields both in heat and electricity can be completed.