Heat pump technology is a new type of energy-saving refrigeration and heating technology. It has been mainly used in the heating and air-conditioning of buildings for a long time. Due to the good performance of heat pump heating in energy saving and environmental protection, more and more heat pump systems are used as heat sources in sanitary hot water supply systems. The air source heat pump, which uses outdoor air as the heat source, has a simple structure, does not require a dedicated engine room, and is easy to install and use. It has irreplaceable advantages in terms of sanitary hot water supply. In addition to the relatively large air source heat pump hot water system, Multiple brands of small household air source heat pump water heaters are also on the market. However, one of the major drawbacks of air source heat pumps is that the heating capacity and the coefficient of heat performance decrease as the outdoor air temperature decreases, so its use is limited by the ambient temperature and is generally applicable to areas with minimum temperatures above -10°C.
Combining heat pump technology with solar energy to supply domestic hot water, many domestic and foreign researches have been conducted in this area. There are mainly two methods. One is to directly use an air source heat pump as auxiliary heating equipment for the solar energy system, and the other is to use solar heat. Water is a low-temperature heat source or a solar heat pump system that uses a solar collector as a heat pump evaporator. The former mainly uses direct solar heating to supplement the air source heat pump to solve the problem of the continuity of solar heating, but still can not get rid of the impact of the ambient temperature on the heat performance of the heat pump; the latter fully uses solar energy as the heat source of the heat pump, greatly improving the solar energy. The use of efficiency, but the lack of solar energy resources still need to increase other auxiliary heat sources, and the heat pump heat capacity is limited by the amount of solar energy gathering, the scale is generally relatively small.
In a large-scale solar central hot water system, an air source heat pump is undoubtedly an ideal auxiliary heating device. In order to improve the performance of an air source heat pump in a low-temperature environment, it expands its use area and combines domestic and foreign solar heat pumps. Based on the advanced experience in the 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 combined with a solar thermal collector system. The solar energy and heat pumps are mutually auxiliary heat sources, which maximizes the use of solar energy, solves the problem of rainy weather and low ambient temperature in winter when solar energy resources are insufficient. Hot water supply guarantee rate, so that hot water can be supplied throughout the year and all-weather.
1 Solar-heat pump central hot water system
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-assisted heating air source heat pump units. Other auxiliary equipment is the same as the conventional central hot water system, including solar energy circulation pumps, hot water heating ring pumps, and heat exchange Equipment, hot water tanks and controllers.
1.2 Solar Auxiliary Heating Air Source Heat Pump Unit
1.2.1 Working principle of solar-assisted heating air source heat pump unit
In order to make the air source heat pump operate efficiently, stably, and reliably in a low-temperature environment, many research institutes and production companies at home and abroad have conducted research and development and improvement. 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, to increase the temperature of the heat pump heating water by electric heating, to use burners to heat the outdoor heat exchanger, and to apply phase change heat storage materials around the compressor to increase the temperature at low temperatures. Heating operation output, etc.; 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 with intermediate air-filling circuit, etc.; third, the use of frequency conversion system Under low temperature conditions, the compressor is allowed to work at high speed to increase the circulation rate of the working fluid, and at the same time, the compressor working chamber is sprayed 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 above-mentioned first method, as shown in FIG. 2 . An auxiliary heat exchanger was added to the evaporator of the unit. When the heat pump operates in a low-temperature environment, the solar hot water passing above the ambient temperature flows through the auxiliary heat exchanger, and exchanges heat with the outdoor air that will enter the evaporator to increase its temperature, so that the refrigerant is relatively high. Evaporation absorbs heat in the environment, improves the evaporation temperature, and improves the working conditions of the compressor.
2. Solar-assisted 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, the solar-assisted heating air source heat pump unit has the following obvious features when operating at low temperature conditions:
(1) Significant increase in COP At the same ambient temperature, solar-assisted heating has increased the evaporation temperature of the refrigerant system, and the unit's heating performance coefficient has been significantly improved compared to ordinary air source heat pump units.
(2) Preventing frosting of the evaporator and reducing the defrosting time Due to the heating effect of the auxiliary heat source, the temperature of the air entering the evaporator is increased to reduce the possibility of frost formation, so that frost on the surface of the evaporator can be prevented, making it The high heat exchange efficiency is maintained. At the same time, the number and time of defrosting of the unit are greatly reduced, which can save a large amount of electric energy and ensure 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 air-conditioner. When the ambient temperature is low, the compression ratio of the air-conditioning compressor increases sharply, and the compressor's exhaust temperature often exceeds the allowable working range of the compressor, resulting in compression. Frequent start-stops of the aircraft do not work properly, and in the long run, the overall performance of the compressor will be impaired and the service life of the air-conditioning equipment will be reduced. The use of solar energy as an auxiliary heat source to increase the evaporation temperature of the system indirectly improves the working environment of the compressor, and not only solves the problem that the compressor cannot work normally in an external low-temperature environment, but also can effectively extend the service life of the entire heat pump unit.
1.2.3 Design of Solar Auxiliary Heat Exchanger
The auxiliary heat exchanger is located on the outside of the heat pump evaporator. As a component of the heat pump unit, it is designed and produced synchronously with the heat pump unit. The finned tube heat exchanger is the same size and material as the evaporator. The heat exchange area of ​​the auxiliary heat exchanger, the air passing through the temperature rise and the distance between the air and the heat pump evaporator shall be based on the parameters such as auxiliary heat, solar water temperature, ambient temperature, evaporation temperature of the heat pump unit and exhaust air amount that the solar collector can provide. Design Calculation.
1.3 Solar Collector
The current solar collectors commonly used in solar water heating projects include flat solar collectors, all-glass vacuum collectors, U-tube vacuum collectors, heat-tube vacuum collectors, and DC vacuum collectors. Heater five. For the relatively large-scale solar central hot water system used throughout the year, it is required that the solar collector should have 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, all-glass vacuum tube collectors have high heat collection efficiency and high market share, but they are not suitable for pressure operation and are prone to cracking. They are not suitable for large-area solar water heating systems. Most of them are used as heat-collecting components for domestic solar water heaters. The other four types of solar collectors are metal heat sinks that can operate under pressure and are suitable for use in large solar hot water projects.
Flat plate collector is a kind of solar energy collector used earlier. It has been the leading product in the world solar energy market. It is widely used in various low temperature hot water heating areas, but with the appearance of vacuum tube solar collectors, The limitation of its own structure has no advantage in the heat collection efficiency. Because the antifreeze problem and the heat collection performance are greatly affected by the seasons and the environment, it is mainly applied in areas with high winter temperatures in the south, and the winter cold operation in northern regions is not effective. It is not recommended for large-scale hot water projects.
U-tube vacuum tube collector, heat tube vacuum tube collector and DC vacuum tube collector are products developed on the basis of all-glass vacuum tube collectors. The common features of the three are relatively high heat collection efficiency. The metal acts as a heat sink and can be operated under pressure. However, it is comprehensively evaluated from the aspects of heat collection efficiency, leakage prevention, anti-scaling, durability, safety, reliability, and difficulty in installation and maintenance. The heat pipe vacuum tube collector is the most suitable The type of solar collector used in the central hot water supply system was followed by a U-tube vacuum tube collector and a DC vacuum tube collector. Heat pipe vacuum tube collector uses heat pipe heat transfer, dry connection, no water in the pipe, small heat capacity, fast heat transfer, freeze resistance, thermal shock resistance, strong pressure resistance, good heat insulation, no leakage, easy maintenance, etc. Advantages: The U-tube vacuum tube collector and the direct current collector use the vacuum tube inner concentric sleeve to directly heat the working fluid. In addition to the characteristics of high operating temperature, strong pressure bearing capacity, and good thermal shock resistance, the The thermal efficiency is higher than other types of collectors, and can be installed horizontally, simplifies mounting brackets, reduces installation site area, avoids collectors affecting building appearance, and has strong adaptability in terms of solar energy and building integration, but its installation procedure is better than The heat pipe type vacuum tube collector is complicated and has many interfaces. It has hidden troubles in the operation and the system maintenance cost is relatively high.
2 Solar-heat pump central hot water system works
The combination of solar energy and solar-assisted heating air source heat pump serves as the heat source of the central hot water system. Its purpose is to complement each other and complement each other. It serves as a backup for each other. When solar energy is sufficient, it is preferred to use solar energy to heat hot water and use solar collectors to generate heat. The low-temperature hot water serves as an auxiliary heat source for the solar-assisted heating of the air source heat pump, thereby improving the operating conditions of the heat pump and improving its heating performance. This kind of combination makes both of them work under conditions that are relatively stable and highly efficient, ensuring the system's all-weather hot water supply throughout the year. The air source heat pump heating process is essentially the use of the solar thermal energy contained in the air. According to the heat pump's operating characteristics, only one of the heat supplied by the heat pump unit as an auxiliary heat source operates during the entire hot water system operation. A small part comes from electrical energy, so the solar-heat pump central hot water system greatly improves solar energy utilization and reduces the consumption of primary energy.
There are four main operating conditions for the solar-heat pump central hot water system:
(1) Solar heating domestic hot water In most sunny days with good sunshine, the system works according to this working condition. 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 the hot water tank, and the source is continuously The heat collected by the collector is sent to the hot water tank through the intermediate heat exchanger.
(2) Solar-assisted heat pump unit heating domestic hot water When cloudy or cloudy weather, when the solar collector temperature is lower than the hot water tank temperature is not enough to directly heat the domestic hot water, the heat pump unit starts, using air as a heat source to heat the hot water tank Domestic hot water. In autumn and winter, when the ambient temperature is lower than the economic operating temperature of the heat pump, the heat efficiency of the heat pump unit decreases and the surface of the evaporator frosts. At this time, the heat pump auxiliary heating cycle starts, and the low temperature solar hot water above the ambient temperature enters the heat pump unit. In the auxiliary heat exchanger, preheating of the passed air increases the efficiency of the heat pump and cuts it to prevent frosting of the evaporator, which can save the power consumption of the heat pump unit.
(3) Solar and heat pump units heat domestic hot water at the same time. When the sunshine is good on a sunny day, if the heat consumption of the hot water system is greater than the effective heat supply of the solar heat collection system or the number of solar heat collectors is small, the hot water system cannot be satisfied. With the heat demand, the solar and heat pump units work simultaneously to supply heat to the hot water system.
(4) Heat pump unit directly heating domestic hot water In the 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 point, the solar system is in standby mode, and the heat pump unit works alone to heat the hot water tank.
3 Solar-heat pump central hot water system design
3.1 Determination of the power of the solar-assisted heating air source heat pump unit
In the solar-heat pump central hot water system, the solar-assisted heating air source heat pump unit serves as an auxiliary heat source device for the solar heat collection system in clear weather. When the solar energy resource is insufficient or rainy weather is the main heat source of the system, normal supply of hot water is guaranteed. 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 heat pump unit power is determined according to the hourly heat load of the hot water system design. For non-full-time central hot water systems, the power of the heat pump unit should be based on parameters such as maximum water consumption, hot water tank volume, and heating time. Determine, see 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 the colder regions in winter, the model of the unit may be appropriately increased to make it operate as much as possible during the day when the air temperature is relatively high. Meet the system's heat requirements in 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 thermal load of the hot water system or the solar energy supply determined based on actual conditions, and the solar collector of the unit area where the project is located should be analyzed and calculated. The average daily effective heat, so as to determine the installation area of ​​solar collectors.
In the hot water project, the solar collector is generally installed at a fixed angle, and its effective daily amount of heat per unit area varies with the season and the daily solar radiation intensity is not a fixed value. The influencing factors mainly include the installation angle of the collector, the operating conditions of the system, the meteorological parameters of the location and the solar irradiation. Different collector types have different heat collection efficiency, and their effective heat gain is also different. Therefore, in practical applications, analysis and calculation are generally performed based on performance parameters such as collector heat collection efficiency and solar radiation data provided by a collector manufacturer. Take the annual average.
3.3 Solar Collector System Form
For the solar-heat pump central hot water system, 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, and it should be able to withstand lower ambient temperatures. Therefore, a closed system should be used. Use antifreeze solution.
4 The practical significance of solar-heat pump central hot water system
4.1 Operation Reliability Analysis
As a major component of the solar-heat pump central hot water system, both solar and air source heat pumps are mature energy-saving and environmentally friendly products. The large-scale utilization of solar energy in domestic hot water systems has a history of more than 20 years. The large number of applications of air source heat pumps also has a history of several decades. [10] The solar water heat pump central hot water system organically combines solar energy and air source heat pump technology. Without affecting the original operating functions of the two, the operating efficiency is significantly improved, which can ensure the stable and reliable operation of the system and save the conventional energy consumption of the hot water system.
4.2 Energy Efficiency Analysis
According to the data of solar radiation in most parts of northern China, the area of ​​the collector of the solar hot water system is determined according to the average heat consumption of the sanitary hot water system and the average daily heat gain of the solar collectors. In the solar-heat pump central hot water system, solar energy is used. 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, over 65% of the supplied heat comes from Solar collectors cannot directly use solar and thermal energy. In the whole system operation, the solar energy utilization rate of the solar collector is close to 100%, and the auxiliary heating power consumption only accounts for 7-14% of the total energy consumption of the system, and at least 85% more than the conventional energy hot water system.
The above analysis shows that the solar-heat pump central hot water system is a reliable, environmentally friendly and energy-saving form of hot water system. The system only uses solar energy and a small amount of electrical energy, and does not pollute the environment. In the daytime, the minimum temperature of -15 °C or more, the sun is well irradiated in most areas of China can be applied. The system and the hot water supply will save more than 60% of conventional energy during winter heating operation. In most cold winter and hot summer areas of China, this system can also be combined with the building heating and air conditioning system to maximize the energy saving effect.
5, project application
In 2004, the solar-heat-pumped central hot water system was the main research content of the Olympics Science and Technology Project of the Chinese Academy of Sciences as a demonstration project of the Central Hot Water System for Olympic Heat Pumps in Olympic Villages and Olympic Stadiums. It is a demonstration project in the comprehensive training hall of Beijing Yuetan Sports Center. The main function is to concentrate on the domestic hot water supply, and the rich heat is auxiliary heating for the swimming pool. The 800m2 DC vacuum tube solar collector is installed horizontally on the roof of the project, and the output power of the solar-assisted air source heat pump unit is 300kW. In December 2004, the commissioning was completed and the trial operation was completed. According to the preliminary test data, the system was able to meet the design requirement of about 90% of the system's heat supply during the sunny days in winter, and the rainy heat pump unit was running smoothly, using a low temperature of around 10°C. Hot water heat pump auxiliary heat source, heating performance has improved significantly. According to the meteorological conditions and solar radiation data in Beijing, the system's annual comprehensive energy-saving rate can achieve the expected goal of more than 90%.
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