Air source heat pump is the use of air as a high temperature (low temperature) heat source for heating (cooling) device. Relative to other types of heat pumps, our research on air source heat pump started earlier, more research. With the ambient air as a low-grade heat source, you can inexhaustible, everywhere, for free access. Air source heat pump is installed flexible, easy to use, the initial investment is relatively low, and more suitable for household installation, at present, most of our indoor air conditioners use this form. This also makes China's air-source heat pump hot and cold water units unprecedented prosperity in the market, the production has been more developed, complete product specifications, a wide range of brands.
According to the survey shows that at present China's air-source heat pump hot and cold water unit manufacturers from 1995 to more than a dozen have now developed more than 40, according to incomplete statistics, the domestic sales of units has more than 45 brands, including domestic About 25% of the crew, the rest for joint ventures, Taiwan-funded products and imported products. In order to get a better understanding of the development of air source heat pump in our country, this article will analyze the research of air source heat pump in our country in recent years and point out the problems existing in air source heat pump and the direction to be improved on this basis. This will further promote the research and application of air source heat pump in our country.
1 Research status of air source heat pump in our country With the application of air source heat pump in our country is becoming more and more extensive and the research is becoming more and more in-depth, it is of great significance to understand the research status of air source heat pump in our country. The following highlights our country in recent years on the air source heat pump technology advances.
1. 1 air source heat pump frosting, defrosting issues As the air source heat pump using air as a heat source in winter, so as the outdoor temperature decreases, the evaporation temperature also decreases, the evaporator surface temperature decreases, or even Below 0 ℃. At this time, when the outdoor air is cooled by flowing through the evaporator, the moisture contained therein will precipitate and adhere to the evaporator surface to form a frost layer. Frosting is extremely detrimental to the heat pump. With the formation of the frost layer, the heat transfer resistance of the evaporator is increased, the evaporation temperature is decreased, the performance of the unit is decreased, the working conditions are deteriorated, and the heating capacity will also decrease, which will seriously affect the performance of the compressor and the heat pump as a whole. Meanwhile, The additional costs will also reduce the economy of air-source heat pumps, which is why air-source heat pumps are limited in cold and wet areas. Therefore, the frosting mechanism, defrosting method has been the research and application of air-source heat pump to solve the key and difficult. At present, there are quite a few studies on the operation status of air source heat pump units in winter [1, 2, 3]. The main work is to analyze the frosting, condensation and dry cooling characteristics of the air coil on the air coil under different operating conditions. The frosting process was tested and simulated. The influence of on-site wind speed, ambient temperature and humidity, fin spacing and tube row number on the frosting performance and the possible consequences were analyzed. The main purpose of understanding the mechanism of frost is to solve the problem of how to defrost. The traditional defrosting control methods mainly include: time defrosting method, time-temperature (pressure) method, air pressure control defrosting method, frosting sensor control defrosting method, sound oscillator control defrosting method, maximum average heating Quantity control defrosting method, the best defrost time control method. Each of these methods has its pros and cons and needs to be improved. In recent years, due to the development of computer technology, the introduction of fuzzy control technology into the air source heat pump defrosting research as an advanced and feasible new technology has gradually attracted people's attention. This is mainly due to the air-source heat pump frost problem is multi-factor, nonlinear, and fuzzy control technology is the advantage of dealing with multi-dimensional, nonlinear, time-varying problems. In this way, the fuzzy control technology is introduced into the defrosting control of the air source heat pump. By analyzing the system response of the defrosting process, the defrosting control can automatically adapt to changes in the working environment of the unit to meet the control requirements of the intelligent defrosting. For a detailed study on this topic, see [4, 5, 6, 7]. In addition, there are intelligent temperature defrost control methods that take into account changes in environmental conditions [8]. Although the air source heat pump has many advantages, it is also limited by the outdoor environment, which is the main reason why the air source heat pump is widely used in the areas south of the Yellow River in our country at present. In the north of the Yellow River, the application of air source heat pump has its own special requirements depending on the region. At present, the actual application of air source heat pump in Xi'an, Jiaodong and cold areas has been studied [9, 10, 11, 12], and some corresponding improvement measures have been put forward on the specific problems encountered, such as excessive compression ratio , Can be used as a reference in the practical application of the corresponding area. In addition, in order to quantify the losses caused by frost defrosting of air-source heat pump, the concept of average defrosting defrosting loss coefficient in different regions and different conditions of use has been proposed. The average frost defrosting loss coefficient The larger the area, the less economical the use of air source heat pumps. Accordingly, the area where air source heat pump is used is divided into four categories according to the average frost loss coefficient: low temperature frost area: such as Jinan, Beijing, Zhengzhou, Xi'an, Lanzhou, etc .; light cream areas: such as Chengdu, Guilin, Frost area: such as Changsha; general frost area: such as Hangzhou, Wuhan, Shanghai, Nanjing, Nanchang, Yichang, etc. [13,14]. These can be used as an important heat pump design in the future reference.
1.2 Air source heat pump energy-saving research Exergy is the quality and quantity of the system can be a comprehensive assessment. Exergy analysis of the system reveals the location, type, and amount of exergy losses in the system in an effort to reduce these losses. By exergy calculation and analysis, only 20% of the compression work is utilized, while 80% is lost, of which, the compressor losses are 30.7%, the condenser accounts for 2014%, the evaporator accounts for 1715% and the capillary accounts for 10% %[twenty three ] . Therefore, we can come out, the main part of energy-saving air-source heat pump system is the compressor to improve the compressor's own technical indicators, is to improve the efficiency of the entire system, the key to the exergy, condenser and evaporator exergy optimization measures are mainly to try to reduce Heat transfer temperature difference. Of course, the system of energy-saving improvement and economy are mutually constrained, and only from the energy efficiency analysis has some limitations. From this perspective, the researchers proposed the concept of the optimal economic equilibrium for heating, with a view to minimizing the initial investment and operating costs of the entire heating system (heat pump + auxiliary heat source) at the best economic equilibrium temperature Reasonable heat pump optimization [15, 16]. In addition, through the air source heat pump units and water-cooled cold water + boiler units, lithium bromide absorption unit (+ boiler) of these three programs, the economy can be drawn that the air source heat pump relative to the other two forms, the economy has Significant superiority [17].
1.3 computer simulation in the air source heat pump system with the continuous popularization of computer technology, the computer in the HVAC applications are also increasingly widespread. Many of the studies mentioned above also apply computer technology, but the typical applications of computer simulation in HVAC are as follows:
(1) Calculation of Compressor The neural network method was used to simulate the winter operating characteristics of the screw compressor in air-source heat pump, and combined with the error back propagation algorithm (BP algorithm), the results showed that the proposed method was applied to compressors Modeling simulation can achieve higher accuracy requirements. The simulation results agree well with the experimental results [30].
(2) Calculation of Evaporator Through the dynamic simulation calculation of evaporator frost formation problem of air source heat pump, the effects of frost evaporator and refrigerant charge on system performance can be analyzed in detail [31, 32]. In addition, the calculation of heat and mass transfer of air-source heat pump evaporator finned tubes under dry conditions, wet conditions and frost conditions is also discussed using ε2NTU method (efficiency-heat transfer unit method) ].
â‘¢ System Simulation Research By establishing the mathematical model of the transient simulation of the room air conditioner heat pump operation, we can draw the curve of the cooling system parameters and the room temperature changes during the operation of the room air conditioner heat pump [34], which is helpful to realize the air source heat pump system The self-control has great significance.
â‘£ system energy analysis software on the air source heat pump annual energy analysis application software development and application described in the literature [35], the software solution heat pump cooling throughout the year when the energy consumption, taking into account the air-conditioning cooling load , Outdoor dry-bulb temperature, heat pump water temperature of these three factors in the heat pump heat energy, the outdoor air humidity will also take this important parameter into account, which makes the calculation of heat pump heat energy more accurate, but also The application of air source heat pump provides a good analytical method.
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