Performance comparison of solar heat pump system with different phase change materials

Abstract: The solar-assisted heat pump system combining with solar energy collection and storage can overcome the reliability on solar energy. A novel solar heat pump water heater integrated with collector, storage and evaporation is presented. The system consists of compressor, water-cooled condenser, solar storage/evaporation/collector, thermal expansion valve and hot water tank. The collection/storage/evaporator integrates the solar collectors, storage tank and evaporator together. The evaporator of the heat pump system is arranged in the solar vacuum tube in the form of a U-shaped evaporator, phase change materials filled in each vacuum evaporation tube. The system separately chooses the paraffin and decanoate as phase change material under the consideration of their capacity, phase change temperature and latent heat of phase change. The experiments were conducted on instantaneous collection efficiency, coefficient of performance, instaneous coefficient of performance and specific volumetric dilatation of decanoate and paraffin. The comparative studies were made on instantaneous collection efficiency, coefficient of performance, instaneous coefficient of performance and time of heating cycling water of the system with decanoate and paraffin under the typical weather conditions in spring in Nanjing. The results show that the instaneous collection efficiency has a negative correlation with the solar energy. In rainy days, the instaneous collection efficiency is higher than that in sunny days because the phase change material store lower energy in the rainy days than in sunny days. The instaneous collection efficiency of the paraffins is higher than that of the decanoate because the capacity and latent heat of paraffin are both higher than that of decanoate. Under different conditions, the coefficient of performance and the instantaneous coefficient of performance of the system filling with paraffin are both higher than the system filling with decanoate, because the paraffin has higher phase change temperature and higher latent heat of phase change. Moreover, the system filling with paraffin has a better stability, its maximum amplitude change is 53.78% while decanoate maximum amplitude changes is 57.17%. However, the vacuum tubes have the danger of splitting up because the the specific volumetric dilatation of the paraffin is 17.17%, while the specific volumetric dilatation of the decanoates is 3.27%. The system arranges brushes outside the evaporation tubes in ranges to solve the problem of splitting up of vacuum tubes. Moreover, the relationship between instantaneous collection efficiency and instantaneous coefficient of performance of the system was studied, the results show that instantaneous collection efficiency changes against the solar energy, it decreases as the solar energy increases, while instantaneous coefficient of performance changes with the solar energy. Under the same solar radiation intensity, through computation the change of instantaneous collection efficiency with solar energy is higher than the change of instantaneous coefficient of performance with solar energy. The water temperature changes with different energy storage materials were also studied. The results show that the system filling with paraffin and decanoate takes 240 and 280 min respectively to heat water. Through linear fitting, the results show that the paraffin has higher temperature increasing slope and higher heating rate.