Air-source heat pump is a device that uses air as a high-temperature (low-temperature) heat source for heating (refrigeration). Compared with other heat pump types, the research on air source heat pump in China started earlier and has more research contents. Taking ambient air as a low-grade heat source can be inexhaustible, available everywhere and free of charge. Air source heat pump is flexible to install, convenient to use and relatively low in initial investment, which is more suitable for home installation. At present, domestic indoor air conditioners mostly adopt this form. This also makes the market of air source heat pump cold and hot water units in China unprecedentedly prosperous, with relatively formed production and research, complete product specifications and diverse brands. According to the relevant investigation, the manufacturers of air source heat pump cold and hot water units in China have developed from more than a dozen in 1995 to more than forty now. According to incomplete statistics, there are more than 45 brands of units sold in China, of which domestic units account for about 25%, and the rest are joint venture products, Taiwan-funded products and imported products.
In order to better understand the development trend of air source heat pump in China, this paper will analyze the research of air source heat pump in China in recent years, and point out the existing problems and the direction to be improved on this basis, hoping to further promote the research and application of air source heat pump in China.
1 Research Status of Air Source Heat Pump in China
With the increasingly extensive application and in-depth research of air source heat pump in China, it is of great significance to understand the research status of air source heat pump in China for subsequent research. The following focuses on the technical progress of air source heat pump in China in recent years.
Study on 1 Frosting and Defrosting. 1 air source heat pump
Because air source heat pump uses air as heat source in winter, with the decrease of outdoor temperature, its evaporation temperature also decreases, and the evaporator surface temperature even drops below 0℃. At this time, when the outdoor air flows through the evaporator and is cooled, the moisture contained in it will precipitate and adhere to the surface of the evaporator, forming a frost layer. Frosting is extremely unfavorable to heat pumps. With the formation of frost layer, the heat transfer resistance of evaporator increases, the evaporation temperature decreases, the performance of unit decreases, the working condition deteriorates, and the heating capacity also decreases, which seriously affects the overall performance of compressor and heat pump. At the same time, the extra cost brought by defrosting will also reduce the economy of air source heat pump, which is also the reason why the application of air source heat pump in cold and humid areas is limited. Therefore, the frosting mechanism and defrosting method have always been the key and difficult points to be solved in the research and application of air source heat pump.
At present, there are many researches on the operation of air source heat pump units in winter. This paper mainly analyzes the frosting, dewing and dry cooling characteristics of wet air on the surface of air coil under different working conditions during heating, and carries out experiments and simulations in combination with the frosting process, and analyzes the influence of parameters such as head-on wind speed, environmental temperature and humidity, fin spacing, and number of tubes on frosting performance and a series of possible consequences. The main purpose of understanding the frosting mechanism is to solve the problem of how to defrost. Traditional defrosting control methods mainly include: timing defrosting method, time-temperature (pressure) method, air pressure difference control defrosting method, frost layer sensor control defrosting method, sound vibrator control defrosting method, maximum average heat supply control defrosting method, optimal defrosting time control method and so on. These methods have their own advantages and disadvantages and need to be improved.
In recent years, due to the development of computer technology, fuzzy control technology has been introduced into air source heat pump defrosting, and as an advanced and feasible new technology, people have gradually paid attention to it. This is mainly because the frosting problem of air source heat pump is multi-factor and nonlinear, and the advantage of fuzzy control technology lies in dealing with multi-dimensional, nonlinear and time-varying problems. In this way, the fuzzy control technology is introduced into the defrosting control of air source heat pump. Through the analysis of the system response in the defrosting process, the defrosting control can automatically adapt to the changes of the working environment of the unit and meet the control requirements of intelligent defrosting. For detailed research in this field, please refer to the literature. In addition, there is an intelligent defrosting control method with dual temperature sensors considering the change of environmental working conditions.
Regarding the start-stop characteristics of heat pump, some researches have analyzed the mechanism of refrigerant migration and vapor-liquid separation caused by system pressure balance when the heat pump is stopped, and pointed out that refrigerant migration and vapor-liquid separation will lead to the decrease of system COP when the heat pump is started.
The application of 1 Computer simulation of air source heat pump system
With the continuous popularization of computer technology, the application of computers in HVAC is also increasingly extensive. Many of the studies mentioned above also apply computer technology, but the representative applications of computer simulation in HVAC mainly include the following aspects:
① Calculation and Simulation of Compressor Neural Network is used to simulate the operating characteristics of screw compressor in air source heat pump in winter, and error back propagation is used for adjustment. The results show that the modeling and simulation of compressor by this method can meet the requirements of high precision. The simulation results are in good agreement with the experimental results. In addition, the calculation method of ε2NTU method (efficiency-heat transfer unit number method) for heat and mass transfer of finned tube of air source heat pump evaporator under dry condition, wet condition and frosting condition is also discussed [33].
(3) Research on system simulation By establishing the mathematical model of transient simulation when the heat pump of room air conditioner is running, we can get the curve of refrigeration system parameters and room temperature changes when the heat pump of room air conditioner is running [34], which is of great significance to realize the automatic control of air source heat pump system.
④ System energy consumption analysis software The development and application of the annual energy consumption analysis software of air source heat pump are introduced in relevant literature [35]. When solving the annual energy consumption of heat pump cooling, the software comprehensively considers three factors: air conditioning cooling load, outdoor dry bulb temperature and heat pump outlet temperature, and also considers the outdoor air relative humidity when calculating the energy consumption of heat pump heating, which makes the calculation of heat pump heating energy consumption more accurate and provides a good analysis method for the application of air source heat pump.
1.5 recently, in order to meet the needs of household heat pump measurement, someone put forward a new idea of measuring heat generated by heat pump by ratio method on the basis of comparative analysis of traditional measurement methods, and gave an application evaluation combined with actual measurement [36]. The application of frequency conversion technology in air source VRV heat pump is a new technology. Through a series of experimental studies on the energy-saving characteristics of Daikin VRV air conditioning system with frequency conversion control heat pump in summer, the operating characteristics of part load in summer are obtained. Compared with the common air source heat pump, it is proved that the air source heat pump unit with frequency conversion technology is more energy-saving than the common unit [37]. Bus air conditioning is also a major aspect of air source heat pump application. The feasibility and economy of applying air source heat pump to air-conditioned passenger cars, as well as the problems of defrosting, have been discussed and experimentally studied [38, 39, 40, 465, 438+0]. Specifically, there are related documents on the influence of compressor speed on the refrigeration capacity, input power and COP performance of locomotive heat pump air conditioning system [42]. In addition, from the introduction of some engineering application examples of air source heat pump in some public places, large and medium-sized shopping malls, wool mills and other large buildings, it can be seen that air source heat pump is not only relatively mature in theoretical research, but also widely used in practical projects in China.
2 air source heat pump problems to be solved and improvement direction
For air source heat pump, there are many shortcomings and problems to be solved besides various advantages. The performance of air source heat pump is greatly influenced by the change of outdoor climate conditions, and it deteriorates with the deterioration of outdoor environment. In summer, with the increase of outdoor temperature, the cooling load increases, but the condensation temperature of heat pump system increases, the temperature difference of heat pump increases, and the overall efficiency of the unit decreases. In winter, the temperature decreases, the heating load increases, while the evaporation temperature decreases and the temperature difference of heat pump increases, resulting in the overall efficiency of the unit decreasing. At the same time, with the bad outdoor conditions, the working performance of heat pump drops sharply, which further aggravates the bad outdoor environment. Further research should consider taking corresponding measures to improve the performance of the unit reasonably. Another outstanding problem of air source heat pump is the frost problem of evaporator in winter. This will not only lead to a sharp decline in the heating performance of the system, but also have a negative impact on important components such as compressors (such as ice plugs). In severe cases, the compressor will be damaged and the system will not run normally. At the same time, frosting will also increase the operating cost of the unit. Although China has done a lot of research work in this field, the research on frost control measures and defrosting technology needs further in-depth research and experimental demonstration. For example, some studies think that the average performance level of air source heat pump is the key to control the frosting process and give full play to the technical performance of heat pump system. At the same time, it is pointed out that how to obtain the best average performance level, the shortest defrosting period and the best working efficiency through the optimal coupling of equipment performance and cycle parameters in the design of heat pump system [3 1]. However, there is a lack of detailed explanation and in-depth research on how to achieve the best coupling.
In addition, how to optimize the design of the unit itself, reduce frosting, and how to adopt a better defrosting method to improve the operating efficiency of the air source heat pump and save the unit cost is still worth discussing. At present, most air-source heat pump units use refrigerants with ozone destruction potential ODP or global warming potential GWP, which has a great negative impact on the environment. In addition, according to the Montreal Protocol, countries will restrict the use of halogenated hydrocarbons, CFCs or HCFCs, ODP and GWP, and set a timetable for their complete prohibition by 2030. From this point of view, it is necessary to study the characteristics of new environmental protection alternative refrigerants (such as R4 10A, R 134a, R744, etc.). ), accordingly, the best matching of the system after adopting new refrigerant should also be tested and studied in detail. Because the outdoor air temperature fluctuates greatly all year round, it is a great challenge to realize the automatic control of the whole air source heat pump system. The research in this field is rare and needs to be explored and improved step by step. In addition, the feasibility of air source heat pump application in northern China needs to be demonstrated and analyzed in more detail according to the specific meteorological conditions in each region.
3 Conclusion
This paper comprehensively analyzes the research progress of air source heat pump in China in recent years from various aspects. Generally speaking, the research and application of air source heat pump in China has made some achievements, but there are still some shortcomings, such as energy saving by defrosting, replacement of new working medium and new cycle, and intelligent control of the system. In the future research, efforts should be made to solve these problems in order to promote the wider application of air source heat pumps in China.