Effect of energy storage materials on performance of solar heat pump system with inserted heat pipe in winter

In order to make full use of solar energy and improve the energy efficiency of solar heat pump system, an energy storage solar heat pump water heating system with inserted oscillating heat pipe is proposed, which integrates solar collector, energy storage tank and oscillating heat pipe together reasonably and effectively. The system can store solar energy with the phase change materials (PCM) filled in the collector, transfer heat efficiently by oscillating heat pipe and switch operation mode according to solar radiation, and can realize the maximum utilization of solar energy in different seasons. In summer, enough heat is transferred or stored during the day to release at night by PCM in solar collector, which is directly used to heat the circulating water through the oscillating heat pipe heat exchanger. In winter, the heat transferred or stored during the day to release at night by PCM in solar collector is low, and the heat is transferred to the heat pump evaporator by the oscillating heat pipe heat exchanger to improve the evaporation temperature of the heat pump, and thus the overall performance of the system is improved. A test rig has also been established for the performance measurement of energy storage solar heat pump water heating system with inserted oscillating heat pipe. Paraffin is chose as phase change material of the system under the consideration of capacity, phase change temperature and latent heat of phase change. Experimental study has been carried out for two years under winter conditions in Nanjing, one year for the test rig without PCMs and another year with PCMs. Under similar environmental conditions (solar radiation intensity, fluctuation and ambient temperature), the variations of the instantaneous collecting efficiency, average collecting efficiency, COP (coefficient of performance) and water temperature of the system filling or not filling PCM with the fluctuation of solar radiation are compared and studied. The comparison and experimental results show that in winter daytime under similar solar radiation intensity, fluctuation and ambient temperature, the instantaneous collecting efficiency fluctuation with PCM is 61.5% less than that of the system without PCM, which can overcome the instantaneous influence of the fluctuation of solar radiation intensity on the system. Meanwhile the average collecting efficiency with PCM is 25.5% higher than that of the system without PCM. At winter night, under similar operation conditions, COP of the system filled with PCM is over 3.0, which is nearly twice as high as that of the system without PCM, and make water temperature reach 50 ℃ in a shorter time, shortening the time by more than 20%. The results can provide theoretical basis for the popularization and application of solar energy heat pump system.