Research Of Solar Assisted Heat Pump System Based On Phase Change Thermal Storage

Solar-assisted heat pump uses solar energy as low-level heat source,which can improve the heating performance of heat pump system.However,solar energy has the disadvantage of discontinuity and instability.The key to solve this problem is to use reliable and excellent thermal storage materials.This process reduces the mismatch in time,intensity and location between energy supply and demand and improves the energy efficiency of solar thermal energy technology.The research of regenerative solar heat pump system with thermal storage includes the following four aspects:studying on phase change materials with suitable phase change temperature,high phase change latent heat and high thermal conductivity,studying on heat storage and release characteristics of phase change heat storage device,design and optimal selection of operation mode of phase change heat storage heat pump system.Firstly,calcium chloride hexahydrate is selected as phase change material for solar low-temperature thermal storage.In order to improve supercooling and low thermal conductivity of CaCl₂·6H₂O in application,with CaCl₂·6H₂O as phase change materials?PCM?,expanded graphite?EG?as carrier and SrCl₂·6H₂O as nucleating agent,a composite PCM was prepared by physical adsorption method,and to study the thermophysical properties of the PCM.The experimental results indicated that:adding 10 wt.%of EG and 2 wt.%of SrCl₂·6H₂O into CaCl₂·6H₂O,phase change latent heat of the composite PCM could be 151.6J/g,thermal conductivity could increase to 3.328 W/?m·K?,and the degree of supercooling kept within 2°C.The thermal conductivity and supercooling of the PCM were significantly improved.The prepared composite PCM had advantages of stable thermocycling performance,short period of heat storage-release and high heat storage density.Secondly,a new type of solar phase change heat storage device is designed and built with CaCl₂·6H₂O/EG composite phase change material as heat storage material and flat heat pipe as heat transport component.The heat storage and release performance of the heat storage device and the temperature distribution of phase change materials were studied.The experimental results indicated that:The solar phase change regenerator can effectively and steadily realize the function of heat storage and release,and the heat release power is about200W when the material is in the phase change stage.In order to match the phase change regenerator with the heat pump system,the regenerator was improved.The heat release power of the improved solar phase change regenerator can reach more than 500W,and the heat storage and release efficiency are 65%/92%respectively.Finally,with solar phase change heat storage device as one of the evaporators of the heat pump system,a solar heat pump water heater system based on phase change heat storage is designed and built.According to different climatic conditions,heat pump system has three operation modes:air source heat pump,solar regenerative heat source heat pump and composite heat source heat pump.The heating performance of the system under three modes was studied.The experimental results indicated that:Compared with solar heat storage source and air source heat pump,the evaporation temperature of regenerative heat pump increased significantly,COP increased from 2.65 to 3.23,and the heating performance increased by21.9%.Composite heat source heat pump has two operation modes of switching evaporator and double evaporator.The average heating capacity of switching mode is slightly lower than that of double evaporator mode,but the heating performance is slightly improved.The heating performance of composite heat source heat pump system is related to environmental temperature and heat storage.A lot of experiments are needed to verify the system to get a better operation mode.