Research on the technology of the hottest solar he

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With the development of economy and the progress of science and technology, energy and environment are two prominent social problems in the world today, which makes people more aware of the importance of energy to human beings, and pay more and more attention to the use of solar energy and energy-saving heat pump technology. At present, the heat utilization of solar energy in China mainly focuses on passive solar room heating and domestic hot water provided by water heaters, while the development and utilization of active solar heating system is relatively backward. Adopting the system mode of connecting the energy-saving device heat pump with the solar heat collection equipment and heat storage mechanism can not only effectively overcome the rarity and intermittency of solar energy itself, but also achieve the purpose of saving high energy and reducing environmental pollution, which has great development and application potential. Heat pump technology is a good energy-saving air conditioning refrigeration and heating technology, which uses a small amount of high-grade electric energy as the driving energy, efficiently absorbs low-grade heat energy from the low-temperature heat source, and transmits it to the high-temperature heat source, so as to achieve the purpose of pumping heat, so as to convert the energy source with low energy quality coefficient into the energy with high energy quality coefficient (save high-grade energy), that is, the technology to improve the energy grade. With the increasing demand for access to domestic hot water, solar energy with intermittent characteristics is difficult to meet all-weather heating. To solve this problem, the combination of heat pump technology and solar energy utilization is undoubtedly a good choice

1 basic principle and type of heat pump

heat pump is a refrigerator with reverse use, which is different from the refrigerator only in the working temperature range. The working principle of the heat pump system is shown in Figure 1 [1][2]. After the evaporator absorbs heat, the high-temperature and low-pressure superheated gas of its working medium is transformed into high-temperature and high-pressure gas through adiabatic compression in the compressor, and then condensed into low-temperature and high-pressure liquid through the constant pressure of the condenser (releasing the gasification heat of the working medium, etc., and exchanging heat with the condensed water, so that the condensed water is heated into hot water for users to use). The liquid working medium is transformed into low-temperature and low-pressure liquid after adiabatic throttling by the pressure reducing valve, and enters the evaporator to absorb the heat of the heat source at constant pressure, And evaporate into superheated steam to complete a cycle process. In this cycle, the heat energy of the heat source is continuously transferred to the condensate

according to the first law of thermodynamics, there are: qg= QD + A. according to the second law of thermodynamics, the electric power a consumed by the compressor plays a compensation role, so that the refrigerant can continuously absorb heat (QD) from the low-temperature environment and release heat (QG) to the high-temperature environment, and cycle again. Therefore, the energy consumption of the compressor is an important technical and economic index. Generally, the energy efficiency of the device is measured by the coefficient of performance (COP), which is defined as:

cop=qg/a = (qd+a)/a=1+qd/a

obviously, the cop of the heat pump is always greater than 1. Therefore, heat pump is a high-efficiency energy-saving device, and it is also an important way to implement building energy conservation in the field of refrigeration and air conditioning. It plays a positive role in saving conventional energy, alleviating air pollution and greenhouse effect

all types of heat pumps have two temperature levels of evaporation and condensation. Expansion valves or capillary tubes are used to reduce the pressure and throttle the refrigerant, but there are different forms of pressure increase, mainly mechanical compression, thermal compression and steam injection compression. Among them, mechanical compression heat pump, also known as electric heat pump, has been widely used in building heating and air conditioning, occupying a leading position in the heat pump market; The heat energy compression heat pump includes absorption type and adsorption type. Among them, the water lithium bromide absorption type and ammonia water absorption type hot water units have gradually embarked on the road of commercial development, while the adsorption type heat pump is still in the research and development stage, and the problems of intermittent operation, low system performance and low cooling weight ratio must be overcome before it can be truly applied to practice. According to the different forms of heat sources, heat pumps can be divided into air source heat pumps, water source heat pumps, ground source heat pumps and solar energy heat pumps. Foreign literature generally refers to groundwater heat pump, surface water heat pump and ground source heat pump as ground source heat pump

2 principle and characteristics of solar heat pump technology

solar heat pump generally refers to the heat pump system using solar energy as the heat source of evaporator, which is different from the heat pump unit driven by solar photovoltaic or thermal power generation. It combines heat pump technology and solar energy heat utilization technology organically, which can improve the efficiency of solar collector and the performance of heat pump system at the same time. The heat absorbed by the collector is used as the low-temperature heat source of the heat pump. In rainy days, the direct expansion solar heat pump is transformed into an air source heat pump, and the non direct expansion solar heat pump is used as the auxiliary heat source of the heating system. Therefore, it can work all day and provide hot water or heat

2.1 classification of solar heat pump

according to the combination form of solar collector and heat pump evaporator, it can be divided into direct expansion soar assisted heat pump (DX-SAHP) and non direct expansion type. In the direct expansion system, the solar collector and the heat pump evaporator are combined into one, that is, the refrigerant absorbs the solar radiant energy directly in the solar collector and evaporates (as shown in Figure 2). In the non direct expansion system, the solar collector is separated from the heat pump evaporator. The solar energy is absorbed in the collector through the heat collection medium (generally water, air and antifreeze solution), and the heat is transferred to the refrigerant in the evaporator, or directly to the air or water that needs to be preheated through the heat exchanger. According to the connection form of solar collector loop and heat pump loop, the indirect expansion system can be further divided into series, parallel and double heat source systems. Series type means that the collector loop and the heat pump loop are connected in series through the evaporator, and the heat source of the evaporator is all from the heat absorbed by the solar collector loop (as shown in Figure 3); Parallel type means that the solar collector loop and the heat pump loop are independent of each other. The former is generally used to preheat the heating object of the latter, or the latter is used as the auxiliary heat source of the former (as shown in Figure 4); The double heat source type is basically the same as the series type, except that the evaporator can use two low-temperature heat sources including solar energy at the same time (as shown in Figure 5) [3]

2.2 technical characteristics of solar heat pump

solar heat pump organically combines solar energy utilization technology with heat pump technology, and has the following technical characteristics [4]:

1) compared with the traditional solar direct heating system, the biggest advantage of solar heat pump is that it can adopt a simple structure collector, and the cost of heat collection is very low. In the direct expansion system, the working temperature of the solar collector is consistent with the evaporation temperature of the heat pump, and close to the outdoor temperature. In the non direct expansion system, the solar collector loop is often used as the low-temperature heat source of the evaporator, and the temperature of the collector medium is usually 20 ℃ ~ 30 ℃. Therefore, the heat dissipation loss of the collector is very small, and the efficiency of the collector is correspondingly improved. Studies have shown that even if ordinary flat plate collectors with simple and cheap structure are used in non cold areas, the collector efficiency is as high as 60% - 80%, and even bare plate collectors without cover plate and insulation can be used

2) due to the shortcomings of low density, intermittency and instability of solar energy, conventional solar heating systems often need to adopt large heat collection and storage devices, and equipped with corresponding auxiliary heat sources, which not only causes high initial investment of the system, but also makes it difficult to arrange large-area heat collectors. Based on the energy conservation of the heat pump and the efficiency of the collector, under the same heat load conditions, the collector area and accumulator volume required by the solar heat pump are much smaller than those of the conventional system, making the system structure more compact and the layout more flexible

3) under the condition of good solar radiation, solar heat pump can often obtain higher evaporation temperature than air source heat pump, so it has a higher heating performance coefficient (COP can reach more than 4), and the heating performance is less affected by the decline of outdoor temperature

4) because solar energy is everywhere and inexhaustible, the application range of solar heat pump is very wide, which is not limited by local water source conditions and geological conditions, and has little impact on the natural living environment

5) like other types of heat pumps, solar heat pumps also have the advantage of "one machine for multiple purposes", that is, they can be heated in winter, cooled in summer, and provide domestic hot water all year round. Due to the heat storage device in the solar heat pump system, the cold storage operation can be carried out by using the power in the valley at night in summer for the cooling during the day, which is not only cheap, but also conducive to the peak load shifting of power

6) considering the filling amount and leakage of refrigerant, direct expansion solar heat pumps are generally suitable for small heating systems, such as household water heaters and heating and air conditioning systems. It is characterized by small heat collection area, compact system, high heat collection efficiency and heat pump performance, good adaptability and high degree of automatic control. Especially when applied to the production of hot water, it has the advantages of high efficiency and energy saving, convenient installation, all-weather and so on. Its cost is equivalent to that of air source heat pump water heater, and its performance is superior

7) non direct expansion system has the advantages of diverse forms, flexible layout, wide application range and so on, which is suitable for central heating, air conditioning and hot water supply systems. Easy to integrate with the building

3 research status of solar heat pump water heater

as early as the early 1950s, joda rapid oil drainage N and therkeld, the pioneers of solar heat utilization, pointed out the advantages of solar heat pump, which can improve the efficiency of solar collector and the performance of heat pump system at the same time. Subsequently, Japan, the United States, Sweden, Australia and other developed countries have invested a lot of human and material resources in the in-depth research and development of solar heat pump, and implemented a number of solar heat pump demonstration projects in various places, such as hotels, residences, schools, hospitals, libraries and swimming pools, which have achieved certain economic benefits and good social benefits. Today, with the increasingly serious energy and environmental problems, solar heat pump has attracted more and more attention because of its remarkable energy conservation and environmental friendliness. In recent years, Turkey, Indonesia and other developing countries have also conducted a lot of research on solar heat pumps [5][6]. In terms of industrial development, solar king series solar heat pump heating equipment in the United States and quantum series solar heat pump water heaters in Australia are typical product examples

the research on solar heat pump in China started late, and the relevant literature and reports have been in more than ten years. Tianjin University, Southeast University, Qingdao Institute of architecture and engineering, Shanghai Jiaotong University, etc. have successively carried out experimental and theoretical research on solar heat pump, and achieved some results. Tianjin University has accurately reported the mechanical properties of this kind of spring on the series solar energy heat pump heating water system. The experimental research and theoretical analysis show that the system can operate reliably all year round, provide users with 50 ℃ domestic hot water, and the cop reaches 2.64 ~ 2.85 (winter) and 2.61 ~ 3.5 (summer) [7]. Qingdao Institute of architecture and engineering has carried out experimental research on the series solar heat pump heating system. The system has multi-function regulation ability. The heat pump unit works stably during heat pump heating in winter, and the average COP reaches 2.71, which has obvious energy-saving effect [8]. Shanghai Jiaotong University has conducted experimental research on the direct expansion solar heat pump water heater. The water heater can provide 150L domestic hot water at 45 ~ 50 ℃ all day, and the daily power consumption is about 1kW · H (summer) ~ 2 kW · H (winter). Its split structure is especially suitable for high-rise or multi-storey buildings. In addition, this water heater can be used in rainy days

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