Heat pump as one of the main equipment to provide heat, with its environment-friendly and energy-saving features, has been widely used in many fields. in the text. First of all, the history of the heat pump is reviewed. The types, characteristics, use conditions and conditions of the heat pump are introduced. The problems in the application of several main heat pumps are discussed. The research progress, new technology. A heat pump and refrigerator Heat pump is a kind of refrigeration system to heat the conditioned environment by the heat released by the condenser. As far as the thermophysical process of a heat pump system is concerned, it is a special type of refrigerator used, from a working principle or from a thermodynamic point of view. Its main difference with the general refrigerator is: ①the purpose of using different. The purpose of the heat pump is heating, the focus of the study is the heat exchange of working fluid between the high-pressure side of the system through the heat exchanger and the external environment; the purpose of the refrigerator is cooling or cryogenic temperature, the focus of study is the working fluid in the system low pressure Side heat exchanger between the heat exchanger and the outside world; ② temperature range of system work is different. Heat pump is the ambient temperature as a low temperature heat source, the object to be adjusted as a high temperature heat source; the refrigerator is the ambient temperature as a high temperature heat source, the object to be adjusted as a low temperature heat source. Therefore, when the environmental conditions are equivalent, the heat pump system operating temperature is higher than the operating temperature of the refrigeration system. 2 origin and main application of heat pumps 2.1 The origin of heat pumps With the industrial revolution, the early 19th century, people on whether the heat from the lower temperature medium "pump" to the problem of higher temperature medium A strong interest. British physicist JPJoule proposed the principle of "changing the temperature of a compressible fluid by changing its pressure". In 1854, Professor W. Thomson (known as Lord Kelvin) published a paper presenting the concept of Heat Multiplier, which for the first time described the idea of ​​a heat pump. At that time, the main target of heat pump heating is civil, the total amount of heating demand is small, especially due to the heating mode and its impact on the environment there is not enough awareness. People are mainly heating coal and wood heating, thus, the development of heat pump lags far behind the long-term development of the refrigerator. In the 1930s, with the development of Freon refrigerators, heat pumps developed rapidly. Especially after World War II, the huge demand for heating supplies and the relative energy shortage brought by the rapid development of the industrial economy have promoted the development of large-scale heat supply and industrial heat pumps. The global energy crisis of 1973 further promoted the worldwide development of heat pumps. 2.2 The main application of heat pump heat pump system in accordance with the heat cycle type, usually the heat pump is divided into the following six categories: ①Vapor Compression Heat Pump Like a refrigerator, vapor compression system is also the most important application of heat pumps. According to the combination of low temperature heat source and heating medium, the vapor compression heat pump system is divided into air-air heat pump, air-water heat pump, water-water heat pump, water-air heat pump, geothermal-air heat pump And soil heat - water heat pump and several other major applications. ② gas compression heat pump and vapor compression system heat pump is different in this type of heat pump system, the work area of ​​the working medium for the overheating zone. For the gas compression heat pump system, the heat pump system and the related technologies, which mainly use carbon dioxide and humid air as the working medium, are two types of hot topics studied in related fields. Air-conditioning heat pump systems and installations that use moist air as a working medium have traditionally been used primarily for aerospace applications, for example, as air-conditioning cold and heat source equipment for aircraft cabins. For the use of wet air as the working medium of air-conditioning heat pump system in general industrial or civil construction environment regulation application possibilities, the author has essay explored. ③ steam jet heat pump steam jet heat pump works and principles and vapor compression heat pump is basically the same, only by the steam ejector instead of the compressor. This heat pump is mainly used for thermal power plant integrated heat utilization, compared with the absorption heat pump, the heat pump is less efficient, less used at present. ④ Absorption heat pump Absorption heat pump is a low-grade heat source to achieve the heat from the low-temperature heat pump to high-temperature heat pump cycle system. Common working fluid pair is "water - lithium bromide (which lithium bromide solution as a working medium with lithium bromide as a concentrated solution absorbent"), "ammonia - water (which, as working fluid ammonia, water absorbent) , "Water - calcium chloride (which calcium aluminate dilute solution as working medium, calcium chloride concentrated solution as absorbent)". ⑤ Thermoelectric heat pump Thermoelectric heat pump, also known as temperature difference electric heat pump. It is the use of Peltier effect, that is, when the DC through the two conductors composed of different circuits, it will produce a temperature difference between the two terminals of the circuit. Thermoelectric heat pump with no moving parts, reliable, long life, easy control adjustment, small vibration, low noise, small environmental pollution, but the higher the cost of thermopiles and less efficient, and therefore mainly used for some special occasions . ⑥ solar heat pump This heat pump solar collector as a heat source. Figure 3 shows one of the options for solar heat pump process. In addition to the above several heat pump, there are chemical heat pump and adsorption heat pump, vortex tube heat pump and other other mainly used in some special occasions, heat pumps. 3 The main problems and application status Vapor compression heat pump is currently the most widely used commercial heat pump. Mainly air, water or earth as a low-temperature heat source. 3.1 Air source heat pump (AirSourceHeatPump) Air as the heat source of the heat pump is called the air source heat pump or air source heat pump (AirSourceHeatPump, ASHP). It is usually made into a dual-purpose circulation system capable of cooling and heating. ASHP needs to be designed for a given climatic conditions, ensuring its capacity and efficiency over a wide range of ambient temperatures. As a result, there is a need to address such a contradiction in terms of performance by minimizing the temperature of the air source when the maximum supply is needed and at the same time, the capacity and efficiency of the unit are also minimized. In addition, the ASHP unit needs to fully consider the parameter selection of the throttle mechanism and the reasonable match between the two heat exchangers indoors and outdoors under different cycling conditions. With the objective of minimizing the total cost over the life of the unit, the authors recommend air handling parameters as a principle of match between two heat exchangers inside and outside the ASHP system. In determining the unit capacity, for the general area, because the air conditioning load is greater than the heating load, therefore, according to the air conditioning cooling load can be determined. For users in cold areas, in a certain period of time, the air conditioning load may no longer be greater than the heating load. Under these conditions, two kinds of treatment methods can be taken according to the situation: one is to determine the unit capacity based on the extreme heating load and its corresponding environmental conditions and the operating conditions of the unit; the other is to determine the unit capacity with the cooling load of the air conditioner; Heating can not meet the heating conditions, to take supplementary heating measures. The recommended condition of starting auxiliary heating in [7] is that "the operating efficiency of the heat pump system is about 1.5 to 2.0". Another problem that needs to be considered in the hot and humid areas in winter and hot in summer is the frosting on the outdoor side heat exchanger of the ASHP unit and a series of problems caused by it. Generally believed that the ambient temperature in the -5 ~ 5 ℃ range, easy to frost area, need special attention. 3.2 Water Source Heat Pump (WaterSourceHeatPump) The heat pump with water as a heat source is called a Water Source Heat Pump (WSHP). Sea water, river water, lake water and well water are usually used as low-temperature heat sources. Due to the small temperature changes of the water, the performance of the water source heat pump is usually better and more stable than that of ASHP. At present, the heat pump system using the water of the sewage pond as the low-temperature heat source has been used in practical projects and the economic performance is good. To seawater, river water or lake as a low-temperature heat pump, on the one hand, subject to natural conditions, on the other hand, the need for heat pump system, water treatment and anti-corrosion measures taken. At present, the heat pump system using well water as the low-temperature heat source is a hot spot in the research and application of water source heat pump units and systems. Well water, especially deep well water, the annual temperature is basically stable and good water quality, heat pump system is an ideal low-temperature heat source, used more in the project. However, such systems may have environmental problems such as backwater difficulties, backwater pollution and the destruction of groundwater ecological resources. From a sustainable development point of view, this is a way not to be adopted. Actually, in many countries and regions, corresponding laws prohibit the use of groundwater resources as a low-temperature heat source for heat pump systems. 3.3 soil heat pump (SoilHeatPump) soil