Heat pump technology is a new type of energy-saving refrigeration heating technology, has long been mainly used in building heating and air conditioning. Due to the heat pump heating energy saving and environmental protection in good performance, sanitary hot water supply system is also more and more use of heat pump equipment as a heat source [2]. One of the outdoor air source heat pump for the heat source, the structure is simple, does not require special room, easy to install and use, has an irreplaceable advantage in the supply of sanitary hot water, in addition to the larger air source heat pump hot water system, now has Several brands of small household air source heat pump water heaters are also on the market. However, one of the major disadvantages of air-source heat pumps is that the heating capacity and heating performance coefficient decrease as the outdoor temperature decreases. Therefore, its use is limited by the ambient temperature and is generally applicable to areas with a minimum temperature above -10 ° C [3] . The combination of heat pump technology and solar energy supply domestic hot water, many studies have been carried out at home and abroad, there are two main ways, one is directly to the air-source heat pump as a solar heating system auxiliary equipment, the other is the use of solar thermal Water is a low-temperature heat source or a solar heat pump system that uses a solar collector as an evaporator for a heat pump. The former with direct solar heating mainly air source heat pump, to solve the problem of continuity of solar heating, but still can not get rid of the ambient temperature on the heat pump heating performance; the latter entirely to solar heat pump heat source, greatly improving the solar energy Of the utilization efficiency, but still need to add other auxiliary heat sources when the solar energy resources are insufficient, and the heat pump heating capacity is limited by the solar energy heat collection, the scale is generally smaller. In large-scale solar central hot water system, air-source heat pump is undoubtedly an ideal auxiliary heating equipment. In order to improve the performance of air-source heat pump in heating operation in low temperature environment and expand its use area, combined with domestic and international solar heat pump With advanced experience in research, we have developed a solar-heat pump central hot water system that is suitable for working in low-temperature environments. The system uses a new type of low-temperature solar-assisted air-source heat pump unit and the solar collector system, solar and heat pump each other as a heat source, maximize the use of solar energy to solve rainy weather and winter temperatures are low when insufficient solar resources Hot water supply guarantee rate, so that all year round, hot water supply. 1 solar - heat pump central hot water system 1.1 Solar - heat pump central hot water system basic components Solar - heat pump central hot water system is mainly composed of solar collectors and solar assisted heating air source heat pump units, other auxiliary equipment and conventional Central hot water system is the same, including solar circulation pump, hot water heating ring pump, heat exchanger, hot water tank and controller. 1.2 Solar assisted heating air source heat pump unit 1.2.1 Solar assisted heating air source heat pump unit working principle To make air source heat pump in low temperature environment, efficient, stable and reliable operation, many domestic and foreign research institutes and manufacturing enterprises to carry out research and development and improvement There are mainly three ways to sum up. First, rely on external auxiliary heat source to improve the heat pump low temperature heating performance, such as electric heating to increase heat pump heating water temperature, the use of burner-assisted heating of outdoor heat exchangers, laying around the compressor phase change thermal storage materials to increase the low temperature conditions Heating output and so on; the second is to improve the low-temperature heating performance of the heat pump by improving the refrigerant circulation system, such as the use of two-stage compressed air source heat pump, air-source heat pump in the middle of the gas recovery circuit; third is the use of frequency conversion system , Low-temperature conditions allow the compressor to work at high speed to increase the amount of working fluid circulation, while the working fluid to the compressor spray to prevent overheating, so that the heat pump unit to normal operation. Solar assisted heat source air-source heat pump unit is based on the first way and generated, as shown in Figure 2. An additional heat exchanger is added to the evaporator of the unit. When the heat pump is operated in a low-temperature heating mode, the solar hot water above the ambient temperature flows through the auxiliary heat exchanger, exchanges heat with the outdoor air entering the evaporator to raise its temperature so that the refrigerant flows through the relatively high The environment evaporates and absorbs heat, improves the evaporation temperature, improves the working condition of the compressor.