Abstract: The research status of air-conditioning condensation heat recovery in China is introduced in this paper. The feasibility of air-conditioning condensation heat recovery, the classification of recovery, the form of recovery, the problems in application and so on are reviewed. Research direction of heat recovery. Key words: condensation heat recovery 1 Introduction Energy is one of the five major elements on which mankind depends for existence and is an important strategic material for the development of the national economy and society. The rapid economic development must be based on the supply of energy, especially electricity. China's energy structure, a large proportion of raw coal, up 75%, China's coal consumption accounts for 27% of the world's total coal consumption, is the world's only coal-based energy consumption power, extensive use of coal and lack of Effective governance has caused serious environmental pollution. The energy utilization rate is relatively low. At present, China's energy utilization rate is still over 10% lower than that of advanced industrial countries. Its energy consumption per unit of GDP is 6 to 10 times higher than that of advanced countries, and its energy consumption per unit of production is 50% higher than that of other countries % ~ 100% [1]. The structural and inefficient use of energy not only affects our country's economic construction and development, but also affects the surrounding environment on which we depend for existence. HVAC project as one of the main energy-consuming technologies, we must base ourselves on the rational use of energy and effective energy-saving measures. 2 air conditioning condensation heat recovery feasibility Conventional air-conditioning system is mainly composed of a refrigerant cycle, a cooling water cycle, a chilled water cycle, and an air cycle. In the refrigerant cycle, the gas refrigerant is compressed in the compressor, the temperature increases, the pressure increases; through the exhaust pipe, high pressure gas refrigerant into the condenser is cooled by cooling water, into a high pressure liquid; through Throttling valve, the pressure is reduced, the high-pressure refrigerant into a low-pressure gas-liquid mixture containing a small amount of gas; thereafter the refrigerant in the evaporator at constant pressure (low pressure) absorbs a large amount of heat in the evaporator of chilled water evaporates into a low pressure gas Refrigerant; gaseous refrigerant back to the compressor through the suction line. In the cooling water circulation, the cooling water absorbs the heat of the refrigerant in the condenser, and then is pumped to the upper part of the cooling tower for heat and moisture exchange with the counter-current (rising) air to lower the cooling water temperature. Cooling water is then pumped to the condenser for heat exchange with the refrigerant, the temperature rises, and so on. Air conditioning room cooling load (ie, heat) into the evaporator through the evaporator cycle, into a part of the condensation heat, and then through the cooling water circulation to the atmosphere [2]. Therefore, for the conventional air conditioning refrigerator, its main role is air conditioning, air conditioning system, the heat of condensation directly discharged to the atmosphere is not used. The refrigeration unit emits a large amount of condensing heat to the atmospheric environment when operating under air conditioning conditions, usually with the condensing heat up to 1.15 to 1.3 times the refrigeration capacity [3]. A large amount of condensation heat directly into the atmosphere, lost in vain, resulting in greater energy waste, the heat of the distribution of the surrounding environment and then the temperature increases, causing serious environmental pollution. If the condensation heat released by the refrigerator is used to heat domestic hot water and production process hot water, not only the pollution caused by condensation heat can be reduced, but also an energy-saving method that turns waste into treasure. In recent years, more and more studies have been done on the heat recovery of condensation heat removal in air conditioning systems. 3 China's air conditioning condensing heat recovery status With the development of our national economy and the improvement of people's living standard, the popularization rate of air conditioners in our country has been increasing rapidly. At the same time, due to the change of people's living habits and the requirement for cleanliness and hygiene, residential buildings pay more and more attention to the supply of sanitary hot water . At present, most of the hot water supply needed by domestic families in daily life is provided by special hot water heaters. This has further exacerbated the energy shortage and environmental pollution in the world and has drawn great attention from all countries. Therefore, in recent years, more and more studies have been done on the preparation of domestic hot water from the heat recovery and condensation heat recovery of the air conditioning system in China. 3.1 condensation heat recovery classification Condensing heat utilization can be divided into direct and indirect. Direct refers to the refrigerant from the compressor into the heat recovery device directly after the heat exchanger with tap water to prepare domestic hot water. Indirect refers to the use of conventional air conditioning condenser side of the high-temperature air or 37 degrees of water to heat the preparation of domestic hot water. Indirect due to more equipment to be added, the heat transfer efficiency is relatively low, so the technology is not easy to promote. Direct and can be divided into two categories, one is the use of compressor outlet steam sensible heat, steam sensible heat generally accounts for about 15% of the total condensation heat [4] [5], in accordance with the demand for hot water and heat Draw the number of heat recovery unit, the other condensation heat is taken away by the cooling water in the condenser; the other is to use the full condensation heat. The two comparison as the former only use the steam sensible heat, heat recovery pressure drop is relatively small, making the condenser pressure is relatively stable on the impact of refrigeration is relatively small. 3.2 condensation heat recovery form In recent years, research and application of condensation heat recovery in China mainly in the form of the following: 3.2.1 double condenser heat recovery technology Rong Guohua [6] put forward: summer use of chiller condenser heating tap water can provide hot water, reduce energy consumption. To use chillers to heat tap water, chillers with water-cooled condensers should be used. When tap water is used as cooling water, the amount of water should be matched with the cooling load of the building and the cooling water volume of the refrigerator to ensure the safe and normal operation of the refrigerator. However, the variation rules of cooling load and cooling water quantity of all kinds of buildings vary with the regularity of their tap water consumption, and it is more difficult to achieve the best match. To solve this problem, Sun Zhi Gao, Li Shuhong [7] in the air conditioning system energy recovery energy-saving analysis envisaged the deployment of heat storage device condensing heat, cooling water, hot water consumption imbalance. Wu Xianzhong et al [8] for the characteristics of the steam compression refrigeration unit, put forward to directly meet the hot water consumption tap water into the heat recovery heat exchanger, the compressor exhaust heat and partial condensation latent heat to heat it, high temperature heat Water is stored in the water tank for use. Gong Qicai, Chang Shijun and others [9] combined with the previous research based on the analysis of the exergy from the perspective of heat recovery condensate heat and proposed a double-condenser heat recovery technology (Figure 1), the compressor and the condenser A heat recovery (condenser) is added to recover the heat of condensation. The refrigerant coming out of this additional heat exchanger is either a vapor-liquid mixture or gaseous and the remaining condenser absorbs the remaining heat. This Figure 1 double condenser heat recovery cooling schematic Technology can be directly recovered according to the requirements of the refrigeration unit of the steam sensible heat or sensible heat plus some of the latent heat to a one-time heating or heating cycle to the specified temperature of water. This form is mainly used in central air conditioning chillers. 3.2.2 household air conditioners commonly used form of heat recovery Household air conditioners are widely used in our country, and the number is large, is one of the important direction of heat recovery. Lin Hong [10] discussed the recycling and utilization of domestic air-conditioning condensing heat, and pointed out: Domestic air-conditioning condensing heat recovery system as an effective method to recover low-grade energy can be widely applied to the transformation of ordinary household air-conditioning system, its application and Energy saving prospects. Harbin Institute of Technology, Jianghui Min, Wang Yang et al [11], put forward in the study of their own household air conditioners for heat recovery form (Figure 2). In this form, the high-temperature and high-pressure refrigerant vapor discharged from the compressor in the air conditioner is injected into the hot water heat exchange device for heat exchange to heat domestic hot water. If the heat exchange capacity of hot water heat exchanger can independently bear all the condensation heat, you do not need to use the air-cooled condenser, on the contrary, you must use both air-cooled and water-cooled condenser to bear all the condensation load. Figure 2 with a hot water supply air conditioner heat recovery schematic diagram 3.2.3 heat pump recovery technology Yu Yingjun, Wang Mengyun [12] on the condensing heat recovery methods and feasibility were discussed. As the air-conditioning cooling water temperature is generally 30 ~ 38 ℃, is a low-grade heat, in order to fully recover the need for heat pump technology, by the refrigerator and heat pump unit to form a combined heat recovery unit. The device is connected to the heat pump evaporator and cooling water circuit, more suitable for the air conditioning cooling water system in the transformation, control is relatively easy to achieve. Yin Yingde, Zhang Ling and others [13] proposed this typical form of indirect heat of condensation after heat recovery (Figure 3). When the chiller and heat pump work at the same time, the cooling water return temperature can be controlled by controlling the start-stop of the cooling tower fan. Through the electric three-way valve to control the cooling tower cooling water flow and heat pump evaporator flow ratio, the heat pump evaporator water temperature below 32 ℃, in order to ensure the normal operation of chillers. This kind of way is in the original system parallel set of heat pump units, the condensation heat as a heat pump heat source to prepare hot water. Figure 3 uses a heat pump heat recovery schematic 3.2.4 phase change material recycling air conditioning condensation heat Xi'an Jiaotong University Liu Hongjuan, Gu Zhaolin [14] proposed the use of phase change material recycling air conditioning condensation heat recovery form. Regenerative heat accumulators replace double condensers. The condensers at the compressor outlet in the heat recovery technology are connected in series with conventional air-cooled condensers (or cooling towers), using cooling towers to remove the residual heat that the heat recovery system can not store. The temperature of the phase change material in the heat recovery heat accumulator varies with the condensation temperature. Initially, the conventional air-cooled condenser (or cooling tower circuit) is closed and the heat recovery heat accumulator utilizes the sensible and latent heat of condensation of the superheating section of the refrigerant to heat the phase-change material, at which point the condensation pressure increases with the heat recovery heat accumulator Phase change material temperature rises and rises. When the system condensing pressure reaches the limit value, turn on the air-cooled condenser to release the latent heat of condensation of the refrigerant and reduce the condensing pressure of the system. In this case, the heat recovery regenerator can still utilize the sensible heat exothermic heating phase change material of the gaseous refrigerant overheating section flowing through the heat accumulator tube to further improve the temperature of the phase change material. When the phase change material temperature reaches a certain set value (which can be measured by the phase change material thermostat), the system resumes the condensing mode of the original condenser (cooling tower). Figure 4 water source heat pump schematic diagram of a heat recovery In addition, Han Huimin of Shandong Shengjian Machinery Factory proposed a kind of heat recovery form of water source heat pump [15] (Fig. 4). There is obvious deficiency in this kind of form. Yang Xiaolin [16] proposed a refrigerator condenser using double beam or double tube condenser, but the heat exchange efficiency is lower. 3.3 condensation heat recovery technology problems in the application 3.3.1 double condenser heat recovery technology and home air conditioners commonly used forms of heat recovery in the application of the existing problems: 1. Heat recovery to be used special high-performance heat exchanger. Due to the strong permeability of Freon and its placement in front of the condenser, this point is more prone to leakage at Freon's high pressure (1.44Mpa) and overheating (about 75 ° C). Therefore, the heat exchanger material and manufacturing processes have special requirements, if inadvertently, not only fail to save energy, but damage to the refrigeration unit. (2) In addition to ensure that the heat exchanger with the modified head of the heat transfer area, but also must have a lower resistance will not affect the original condition of the refrigerator, otherwise it will reduce the output. 3. Because it uses the heat generated by the refrigeration cycle, the heat transfer difference is much smaller than the original hot water system, only a small flow of hot water can be continuously prepared, it is impossible to provide a large amount of heat such as steam heaters or hot water heaters for a short period of time Water, therefore, the system should be equipped with sufficient capacity hot water tank. Usually calculated according to the specific system 1 hour water consumption [17]. 3.3.2 phase change material recycling air conditioning condensing heat problems: 1. At present, the regenerator has been widely used in foreign countries, and gradually developed in our country. Waste heat and solar energy and other resources of the effective use of the urgent need to design and development of high efficiency, low cost phase change heat accumulator. However, there is no uniform method for the design and calculation of heat accumulator. Therefore, the design of heat accumulator is also in the groping stage. 2. Requirements for heat storage material: large heat capacity, strong thermal storage capacity, good chemical stability, low melting point, harmless to the human body, animals and plants, and low price. China's development of thermal storage materials to be further strengthened. 3. The refrigeration unit should be operated at a lower condensing temperature as much as possible. The condensing temperature selected in the design is a fixed value, while the actual operating condensing temperature is changing. Condensation temperature and the ambient temperature, not only with the seasons, but also day by day changes are constantly changing. Therefore, the design should take full account of various unfavorable factors, select the appropriate condensation temperature to ensure that the refrigeration unit in high efficiency and energy saving operation. 3.3.3 The use of heat pumps to recover condensate heat problems: 1. More suitable for the existing air conditioning system in the transformation of the application, but a larger investment, high operating costs. 3 China's air conditioning condensation heat recovery trend In order to achieve the strategy of "sustainable development", China has formulated a series of energy and environmental policies. Energy saving and environmental protection have become one of the most important research topics in the field of air conditioning. At present, China's HVAC scholars have gradually realized the importance of the application of waste heat recovery system. Some developers have also had some interest in this project. In Beijing [18], Guangzhou [19] [20], Shanghai [21] , Some hotels in northeastern China, and artificial ice rinks and swimming pools [22] have applications and energy-saving effect is very good. At present, the research on the heat recovery system of condensing heat and exhaust heat in China is only limited to the theoretical analysis, and is in the initial stage of exploration, and lacks practical and in-depth systematic research. Judging from the domestic research situation, there are still a series of problems. The direct heating of tap water in the recovery system has the following problems: First, air-conditioning system and hot water operation period of time difference. Second, there is also a problem of unsynchronization between the amount of domestic hot water and the heat of condensation. Therefore, the installation of heat storage device is necessary, it can store at noon air conditioning at full load hot water to supplement the evening air conditioning low load water production deficiencies. At present, there is a big problem in domestic research of thermal storage tanks. The problems of dynamic simulation of thermal storage devices in system operation and system forms are also problems that we should solve in future work. Condensation heat recovery depends on the work of the main equipment, and sometimes may not get the desired results. In terms of condensing heat recovery, its effect depends on the operating conditions of the air conditioning system, its purpose is only from the perspective of energy saving and environmental protection recycling waste heat, but not the cart before the horse in order to gain heat to arbitrarily change the condition of air conditioning. Therefore, due to the influence of air-conditioning operation time (usually summer operation only), the auxiliary heat source in the heat recovery system is necessary under most air conditions. China in the development of heat recovery equipment, but also the lack of optimization of the entire equipment components matching design, computer simulation of this research. Through the comparison with the experimental results to optimize the matching of the whole machine. In addition, optimizing the system itself and the control system, reducing the initial investment of equipment, and forecasting and evaluating the application prospect of the product in China are also one of the important research directions.