Study On The Switching Logics Of Active And Passive Phase Change Heat Transfer Modes Of A Photovoltaic Solar-assisted Heat-pump/gravity-assisted Heat-pipe Hybrid System

Integration of active and passive phase change heat transfer methods(heat pumps and gravity-assisted heat pipes)with integrated solar photovoltaic/thermal(PV/T)utilization can form PV solar-assisted heat pump(PV-SAHP)systems and PV/T gravity-assisted heat-pipe(GHP-PV/T)systems,respectively,both of which can resolve the corrosion and freezing situations associated with the conventional water-based PV/T collectors.However,the former wastes solar resources in higher solar irradiation conditions while the latter fails to meet the water temperature demands in lower solar irradiation conditions.Photovoltaic solar-assisted heat-pump/gravity-assisted heat-pipe(PV-SAHP/GHP)hybrid system combines active and passive phase change heat transfer methods,which can switch between the PV-SAHP and GHP-PV/T systems to meet the household water temperature requirements while reducing electric power consumption.At present,there is an extreme lack of research into the switching operation logic for hybrid systems.Therefore,in this paper,a combination of experiments and simulations is employed to investigate the switching logic of the PV-SAHP/GHP hybrid system.First of all,a hybrid PV/T collector-evaporator was devised and manufactured.For this purpose,the evaporation sections of the GHP-PV/T and the refrigerant pipe of the PV-SAHP were alternately mounted on the baseplate.In particular,“arch-bridges” were predesigned in the refrigerant pipe to reserve spaces for the heat pipes.On this basis,a PV-SAHP/GHP hybrid system performance test platform was proposed and constructed,and long-term experimental tests were carried out to reveal and compare the dynamic and overall performances of the two separate operating modes.Moreover,the advantages and disadvantages of the different operating modes were also examined with the help of efficiency(thermal efficiency and comprehensive efficiency)linear fitting and output factors.The results show that the total thermal gain and total electricity gain of the GHP-PV/T operating mode are8.77 k Wh and 1.88 k Wh,respectively,with daily average thermal and electrical efficiencies of 30.22% and 10.83%,respectively,and daily average overall efficiency and exergy efficiency of 36.87% and 7.09%,respectively;while the above power and efficiency of the PV-SAHP operating mode are 17.83 k Wh,2.00 k Wh,44.66%,10.96%,51.39% and-6.34% respectively for the PV-SAHP operating mode;The comprehensive results show that the GHP-PV/T mode completes higher output factors,which reflect a satisfactory energy-saving effect,but the final water temperature fails to meet the daily demand.While the PV-SAHP system can achieve a higher heat gain,but the larger power consumption leads to a lower energy output factor.In particular,the exergy output factor is less than 1.0 and the energy-saving is unsatisfactory.Therefore,based on the experimental results,efficiency fits and output factors judgment,the initial recommendation is to operate the GHP-PV/T mode during intense radiation and operate the PV-SAHP mode in other cases to balance the water temperature and energy-saving effect of the hybrid system.Secondly,numerical models of the GHP-PV/T system and the PV-SAHP system in order were established,and employed a temperature transfer matrix to couple them.Finally,based on the verified numerical model,the switching strategies of the hybrid system in order are revealed based on the criteria of overall thermal efficiency(OTE)and overall exergy efficiency(OEE).In particular,the results of the two modes worked individually are also contrasted and discussed.The results display that the hybrid system respectively requires switching two times(from active to passive and then to active)and one time(from passive to active)for OTE and OEE;the hybrid system saves 54.6% and66.6% of electricity based on the OTE and OEE switching operating respectively compared to operating the PV-SAHP system.the deserted pattern is eternal inferior to that it worked individually due to the initialization values deviate its normal operating mode.Comparatively speaking,the net yield indicates that the hybrid system can excellently balance the water temperature need and the energy-saving effect no matter what the switching index is.