| Photovoltaic efficiency declined with the increase of the working temperature. This leads to a relatively lower efficiency when solar cells operating without a cooling service. In order to utilize this part of thermal energy, which was discharged to the environment directly, a comprehensive utilization of the solar energy is a good alternative. Researches on this field have been widely carried on inside and outside china.References revealed two main modes were adopted by various authors in photovoltaic/photothermic (PV/T) utilization of the solar energy: in the first mode water was used as the coolant of the PV panels and the heated water was used for domestic usage or as a thermal resource of a heat pump. In the second mode air was used as the coolant, and the hot air was inducted to a room for space heating. The results demonstrated the photovoltaic efficiency was improved with the cooling effect, and the overall efficiency was also lifted to a higher level.However the water or air was usually heated to a relatively high temperature on the purpose of a direct usage. This was on the opposite direction of the photovoltaic efficiency improvement. This paper proposed a novel system which was directly coupled of a PV solar collector and a heat pump. The solar collector acted as the evaporator of the heat pump. In the evaporator, refrigerant absorbed heat from the PV base plate, and discharged it to the cooling water in the condenser. In this way the PV panel's temperature dropped and the photovoltaic efficiency was improved at the same time. This novel system was named as photovoltaic solar assisted heat pump (PV-SAHP). Research has revealed it has a superior thermal and electrical performance.A distributed parameters approach has been successfully used to simulate the PV evaporator. On this base, the model of the PV-SAHP system was also built. Researches were focused on the structural parameters and environmental parameters, which influenced the performance of the heat pump. Compared to the lumped parameters approach, the distributed parameters approach has a higher accuracy. It can give the temperature distribution of the refrigerant along the tube, and calculate the thermal /electrical outputs at the same time.Based on the above theories we designed and built the experimental rigs of the PV-SAHP system. Experimental researches were taken under winter weather conditions of Hefei region. The results of the experiment and simulation agreed very well, which demonstrated the system's model has a high accuracy. The thermal efficiency, electricity efficiency and the overall efficiency of the PV evaporator were G.53～0.75, 0.124～0.135, and 0.64～0.87 respectively. Compared to a water-based or air-based PV/T collector the overall efficiency was 10% higher. The coefficient of performance (COP) reached 8.4 with an average value of 6.5, which demonstrated the PV-SAHP system has a superior performance.With the models above, this paper also give a numerical study of the PV-SAHP system working in a variable frequency condition. When PV-SAHP operates, the solar radiation intensity and ambient temperature varied in a wide range, so it is necessary to adjust the mass flow rate of the refrigerant according to the environment parameters. This can effectively avoid two-phase flow appears at the outlet of the evaporator, or control the superheated degree of the refrigerant within a suitable region. If the two-phase flow flushed into the compressor, the compressor would be seriously damaged. If the superheated degree is too high, the heat loss to the environment increased and the thermal efficiency of the evaporator dropped on the contrary. This paper adopted a two-stage modulation through the compressor frequency and the expansion valve to match the mass flow rate with the thermal load, which ensured the PV-SAHP system working in a healthy, effective state.
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