Heat pump technology is a new type of energy-saving refrigeration and heating technology, which has long been mainly used in the field of heating and air conditioning of buildings. Due to the good performance of heat pump heating in terms of energy saving, consumption reduction and environmental protection, sanitary hot water supply systems are increasingly using heat pump equipment as a heat source. Among them, air source heat pumps that use outdoor air as the heat source have a simple structure, do not require a dedicated machine room, are easy to install and use, and have irreplaceable advantages in sanitary hot water supply. In addition to relatively large air source heat pump hot water systems, there are now Several brands of small household air source heat pump water heaters have also been put on the market. However, a major disadvantage of air source heat pumps is that the heating capacity and heating performance coefficient decrease as the outdoor temperature decreases, so its use is limited by the ambient temperature and is generally suitable for areas with a minimum temperature above -10°C.

　　Many studies have been conducted at home and abroad on combining heat pump technology with solar energy to supply domestic hot water. There are two main ways. One is to directly use air source heat pumps as auxiliary heating equipment for solar systems, and the other is to use solar thermal energy. A solar heat pump system in which water is a low-temperature heat source or a solar collector is used as an evaporator for a heat pump. The former uses direct solar heating as the main method and is supplemented by air source heat pumps to solve the problem of continuity of solar heating, but it still cannot get rid of the influence of ambient temperature on the heating performance of the heat pump. The latter uses solar energy as the heat source of the heat pump, which greatly improves the efficiency of solar energy. Utilization efficiency, but when solar resources are insufficient, other auxiliary heat sources still need to be added, and the heating capacity of the heat pump is limited by the solar heat collection, and the scale is generally relatively small. In large-scale solar central hot water systems, air source heat pumps are undoubtedly an ideal auxiliary heating equipment. In order to improve the performance of air source heat pumps in heating operations in low-temperature environments and expand their use areas, combined with domestic and foreign solar heat pumps Based on advanced experience in research, we have developed a solar-heat pump central hot water system suitable for working in low-temperature environments. The system uses a new low-temperature solar-assisted air source heat pump unit and a solar heat collection system combined. Solar energy and heat pumps serve as auxiliary heat sources for each other to maximize the use of solar energy and solve the problem of insufficient solar energy resources in rainy weather and low ambient temperatures in winter. The hot water supply guarantee rate is to provide hot water all year round and all day long.

　　1Solar-heat pump central hot water system composition

　　1.1 Basic components of solar-heat pump central hot water system

　　The main components of the solar-heat pump central hot water system are solar collectors and solar auxiliary heating air source heat pump units. Other auxiliary equipment are the same as conventional central hot water systems, including solar circulation pumps, hot water heating ring pumps, and heat exchangers. appliances, hot water tanks and controllers, etc.

　　1.2 Solar auxiliary heating air source heat pump unit 1.2.1 Working principle of solar auxiliary heating air source heat pump unit

　　In order to make air source heat pumps operate efficiently, stably and reliably in low-temperature environments, many scientific research units and production enterprises at home and abroad have conducted research and development and improvements. There are mainly three ways to summarize. One is to rely on external auxiliary heat sources to improve the low-temperature heating performance of the heat pump. For example, electric heating is used to increase the temperature of the heat pump heating outlet water, burners are used to assist in heating outdoor heat exchangers, and phase-change heat storage materials are laid around the compressor to increase the temperature under low-temperature conditions. The heating operation output, etc.; the second is to improve the low-temperature heating performance of the heat pump by improving the refrigerant circulation system, such as using a two-stage compression air source heat pump, an air source heat pump with an intermediate air supply circuit, etc.; the third is to use a frequency conversion system , allowing the compressor to work at high speed under low temperature conditions to increase the circulation of working fluid, and at the same time spray liquid into the working chamber of the compressor to prevent it from overheating, so that the heat pump unit can operate normally.

　　The solar-assisted heating air source heat pump unit is based on the first method mentioned above, as shown in Figure 2. An auxiliary heat exchanger is added to the evaporator of the unit. When the heat pump is operating for heating in a low-temperature environment, solar hot water higher than the ambient temperature flows through the auxiliary heat exchanger and exchanges heat with the outdoor air entering the evaporator to increase its temperature, thereby causing the refrigerant to operate at a relatively high temperature. Evaporation in the environment absorbs heat, which increases the evaporation temperature and improves the working condition of the compressor.

　　2. Solar auxiliary heating air source heat pump unit

　　1.2.2 Performance characteristics of solar-assisted heating air source heat pump unit

　　Compared with ordinary air source heat pumps, solar-assisted heating air source heat pump units have the following obvious characteristics when operating under low temperature conditions:

　　(1) COP is significantly improved. Under the same ambient temperature, solar-assisted heating increases the evaporation temperature of the refrigerant system, and the heating performance coefficient of the unit is significantly improved compared to ordinary air source heat pump units.

　　(2) Prevent frosting on the evaporator and reduce defrosting time. Due to the heating effect of the auxiliary heat source, the temperature of the air entering the evaporator is increased, reducing the possibility of frosting. This prevents frosting on the surface of the evaporator, making it Maintaining high heat exchange efficiency, at the same time, the number and time of defrosting of the unit are also greatly reduced, which can save a lot of electric energy and ensure the continuous and uninterrupted operation of the heat pump unit.

　　(3) Improve the working environment of the air-conditioning compressor and extend the service life of the unit. When the ambient temperature is low, the compression ratio of the air-conditioning compressor increases sharply, and the exhaust temperature of the compressor often exceeds the allowable working range of the compressor, resulting in compression Frequent starts and stops of the compressor will prevent it from working properly. In the long run, it will damage the overall performance of the compressor and reduce the service life of the air conditioning equipment. Using solar energy as an auxiliary heat source to increase the evaporation temperature of the system indirectly improves the working environment of the compressor. It not only solves the problem of the compressor not working properly in low-temperature environments, but also effectively extends the service life of the entire heat pump unit.

　　1.2.3 Design of solar auxiliary heat exchanger

　　The auxiliary heat exchanger is located outside the heat pump evaporator. As a component of the heat pump unit, it is designed and produced simultaneously with the heat pump unit. It uses a fin tube heat exchanger with the same appearance size and material as the evaporator. The heat exchange area of the auxiliary heat exchanger, the air temperature rise and the distance between it and the heat pump evaporator should be determined based on the auxiliary heat provided by the solar collector, solar water temperature, ambient temperature, evaporation temperature of the heat pump unit, exhaust volume and other parameters. Design Calculation.

　　1.3 Solar collector

　　Currently, the solar collectors commonly used in solar water heating projects mainly include flat-plate solar collectors, all-glass evacuated tube collectors, U-tube evacuated tube collectors, heat pipe evacuated tube collectors and DC evacuated tube collectors. Five types of heaters. For relatively large solar central hot water systems used throughout the year, the solar collectors are required to have a certain pressure bearing capacity, relatively high heat collection efficiency, relatively small pipeline resistance, strong frost resistance, and easy maintenance. Among these types of solar collectors, although all-glass evacuated tube collectors have high heat collection efficiency and a large market share, they are not suitable for use in large-area solar hot water systems because they cannot operate under pressure and are prone to freezing and cracking. Most of them are used as heat collecting components of household solar water heaters. The other four types of solar collectors are all metal heat absorbers, which can operate under pressure and are suitable for use in large-scale solar hot water projects.

　　Flat plate collectors are an early type of solar heat collecting device. They have always been the leading product in the world's solar energy market and are widely used in various low-temperature hot water heating fields. However, with the emergence of vacuum tube solar collectors, they have become more and more popular. Due to the limitations of its own structure, it no longer has an advantage in heat collection efficiency. Due to anti-freeze issues and heat collection performance, which is greatly affected by seasons and environment, it is currently mainly used in areas with higher winter temperatures in the south. It has poor operating results in cold areas in the north. , not recommended for use in large hot water projects.

　　U-shaped vacuum tube collectors, heat pipe vacuum tube collectors and DC vacuum tube collectors are products developed on the basis of all-glass vacuum tube collectors. The common features of the three are relatively high heat collection efficiency. As a heat absorber, metal can operate under pressure. However, based on a comprehensive evaluation of heat collection efficiency, leakage prevention, scale prevention, durability, safety, reliability, installation and maintenance difficulty, etc., heat pipe vacuum tube collectors are the most suitable for use in The types of solar collectors used in central hot water supply systems are U-tube vacuum tube collectors and DC vacuum tube collectors. The heat pipe vacuum tube collector uses heat pipes for heat transfer, dry connection, and no water leaks in the pipes. It has small heat capacity, fast heat transfer, freezing resistance, thermal shock resistance, strong pressure bearing, good thermal insulation, no leakage, and easy maintenance. Advantages: U-tube vacuum tube collectors and DC collectors use concentric sleeves in the vacuum tube to directly heat the working medium. In addition to having the characteristics of high operating temperature, strong pressure bearing capacity and good thermal shock resistance, their integrated The thermal efficiency is higher than other forms of collectors, and it can be installed horizontally, simplifying the installation bracket, reducing the installation site area, and preventing the collector from affecting the appearance of the building. It has strong adaptability in combining solar energy with buildings, but its installation procedure is relatively Heat pipe vacuum tube collectors are complex, have many interfaces, are prone to leakage during operation, and have relatively high system maintenance costs.

　　2Solar-heat pump central hot water system working principle

　　The combination of solar energy and solar-assisted heating air source heat pump is used as the heat source of the central hot water system. The purpose is to complement each other so that the two can complement each other and serve as backup. When the sunshine is sufficient, solar energy will be used first to heat hot water and the solar collector will be used to generate The low-temperature hot water is used as the auxiliary heat source of the solar-assisted heating air source heat pump, thereby improving the operating conditions of the heat pump and improving its heating performance. This combination allows both to work under relatively stable and efficient conditions, ensuring that the system can supply sanitary hot water all year round. The heating process of an air source heat pump is essentially the utilization of solar thermal energy contained in the air. According to the working characteristics of the heat pump, during the operation of the entire hot water system, only one of the heat supplied by the heat pump unit operating as an auxiliary heat source A small part comes from electric energy, so the solar-heat pump central hot water system greatly improves the utilization rate of solar energy and reduces the consumption of primary energy.

　　The operation of the solar-heat pump central hot water system mainly has the following four working conditions:

　　(1) Solar heating of domestic hot water will work under this working condition on most sunny days with good sunshine. At this time, the work of the solar circulation pump is controlled by the system controller according to the temperature of the solar collector and hot water tank, and the system will continue to operate continuously. The heat collected by the collector is transported to the hot water tank through the intermediate heat exchanger.

　　(2) Solar-assisted heat pump unit heats domestic hot water. On cloudy or cloudy days, when the solar heat collection temperature is lower than the hot water tank water temperature and is insufficient to directly heat domestic hot water, the heat pump unit starts and uses air as a heat source to heat the hot water tank. Domestic hot water inside. In autumn and winter, when the ambient temperature is lower than the economic operating temperature of the heat pump, the heating efficiency of the heat pump unit decreases and the evaporator surface is frosted. At this time, the heat pump auxiliary heating cycle starts, and low-temperature solar hot water higher than the ambient temperature enters the heat pump unit. In the auxiliary heat exchanger, the passing air is preheated to improve the efficiency of the heat pump and prevent frosting on the evaporator, which can save the power consumption of the heat pump unit.

　　(3) Solar energy and heat pump units heat domestic hot water at the same time. On a sunny day with good sunshine, if the heat consumption of the hot water system is greater than the effective heat supply of the solar collector system or the number of solar collectors is small, the hot water system cannot be satisfied. If the heat demand is high, the solar energy and heat pump units will work simultaneously to provide heat to the hot water system.

　　(4) The heat pump unit directly heats domestic hot water. In continuous rain and snow weather, the heat required by the hot water system is completely provided by the air source heat pump unit. At this time, the solar system is in standby mode and the heat pump unit works alone to heat the hot water tank.

　　3Solar energy-heat pump central hot water system design

　　3.1 Determination of the power of solar-assisted heating air source heat pump unit

　　In the solar-heat pump central hot water system, the solar auxiliary heating air source heat pump unit serves as the auxiliary heat source equipment of the solar heat collection system in fine weather. When solar resources are insufficient or in rainy weather, it serves as the main heat source of the system to ensure the normal supply of hot water. Therefore, The heating power should be determined according to the design heat load of the entire hot water system. For a full-time central hot water system, the power of the heat pump unit is determined based on the design hourly heat load of the hot water system. For a part-time central hot water system, the power of the heat pump unit should be determined based on parameters such as maximum water consumption, hot water tank volume, and heating time. OK, please refer to the literature for details. The rated heating power of the heat pump unit is not less than the design load of the central hot water system. In areas with relatively cold winters, the model of the unit can be appropriately increased so that it can operate during periods of relatively high temperatures throughout the day and in shorter periods of time. meet the system's heat demand within a certain time.

　　3.2 Determination of solar collector area

　　In the solar-heat pump central hot water system, the area of the solar collector should be based on the design heat load of the hot water system or the solar heat supply determined according to the actual situation, and the solar collector area per unit area of the project location should be analyzed and calculated. The average daily effective heat gain determines the installation area of the solar collector.

　　In hot water projects, solar collectors are generally installed at fixed angles. The effective daily heat gain per unit area changes with the seasons and the daily solar radiation intensity, and is not a fixed value. The influencing factors mainly include the installation angle of the collector, system operating conditions, local meteorological parameters and solar irradiation. Different collector types have different heat collection efficiencies and their effective heat gain is also different. Therefore, in practical applications, analysis and calculation are generally based on the performance parameters such as collector heat collection efficiency and solar irradiation data provided by the collector manufacturer. Take the annual average.

　　3.3 Solar thermal collection system form

　　For solar-heat pump central hot water systems, the solar heat collection system serves as both the main heat source for hot water heating and the auxiliary heat source for the heat pump unit. It should be able to withstand lower ambient temperatures, so a closed system should be used to circulate the working fluid in the system. Use antifreeze solution.

　　4. Practical significance of solar energy-heat pump central hot water system

　　4.1 Operation reliability analysis

　　As the main components of the solar-heat pump central hot water system, solar energy and air source heat pumps are both energy-saving and environmentally friendly products with mature technology. The large-scale utilization of solar energy in domestic hot water systems has a history of more than 20 years, and the large-scale application of air source heat pumps has a history of decades [10]. The solar heat pump central hot water system organically combines solar energy with air source heat pump technology. Without affecting the original operating functions of the two, the operating efficiency is significantly improved, thereby ensuring stable and reliable operation of the system and saving conventional energy consumption of the hot water system.

　　4.2 Energy saving benefit analysis

　　According to the solar radiation data in most areas of northern my country, the collector area of the solar hot water system is determined based on the average heat consumption of the sanitary hot water system and the average daily heat gain of the solar collector. In the solar-heat pump central hot water system, solar energy Direct heating can meet 60-80% of the heat demand of the hot water system throughout the year, and the remaining 20-40% of the heat is supplied by the solar-assisted heating air source heat pump unit. The average COP of the heat pump can reach 3.0, that is, more than 65% of the heat supplied comes from The solar energy and air heat energy that the collector cannot directly utilize. During the entire system operation, the solar energy absorbed by the collectorThe energy utilization rate is close to 100%, and the power consumption of auxiliary heating only accounts for 7-14% of the total energy consumption of the system. Compared with conventional energy hot water systems, it can save at least 85% more energy. From the above analysis, it can be seen that the solar-heat pump central hot water system is a reliable, environmentally friendly and energy-saving hot water system. This system only uses solar energy and a small amount of electric energy, and does not cause any pollution to the environment. It can be promoted and applied in most areas of my country where the daytime minimum temperature is above -15°C and the solar radiation is good. System and hot water supply, saving more than 60% of conventional energy during heating operation in winter. In most areas of my country with cold winters and hot summers, this system can also be combined with building heating and air conditioning systems to maximize energy saving.

　　5. Project application

　　In 2004, the solar-heat pump central hot water system was the main research content of the Chinese Academy of Sciences Olympic Science and Technology Project's Olympic Village and Olympic Venue Solar Heat Pump Central Hot Water System Demonstration Research Project. It was mainly used as a demonstration project at the Beijing Yuetan Sports Center Comprehensive Training Hall. The function is to supply centralized domestic hot water and provide abundant heat for auxiliary heating of the swimming pool. The project uses an 800m2 DC vacuum tube solar collector installed horizontally on the roof, and a solar-assisted heating air source heat pump unit with an output power of 300kW. After debugging in December 2004, the system entered the trial operation stage. According to preliminary test data, the solar thermal collection system of the system can meet about 90% of the system's designed heat supply on sunny days in winter. The heat pump unit operates smoothly on rainy days, using low-temperature solar energy of about 10°C. Hot water heat pump auxiliary heat source, heating performance is significantly improved. According to the meteorological conditions and solar radiation data in the Beijing area, the system's annual comprehensive energy saving rate can achieve the expected goal of more than 90%.

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