| The capacity of an air-source compression heat pump system is usually controlled by on-off of the compressor. However, it is difficult to provide enough heat in winter because of the ultra low suction pressure under low ambient temperatures. Thus amenity of the indoor environment can not be kept. Though the adoption of inverter compressor makes it possible to regulate the heating capacity of the heat pump to some extent, it still can not solve those problems due to ultra low suction pressure, such as high compression ratio and the decrease of heating capacity. In this thesis, a new capacity-controllable heat pump system was put forward. It can effectively run at a wide range of ambient temperature. The regulation of heating or cooling capacity of the heat pump is realized by changing the composition of the environment-friendly refrigerant mixtures. Compared with the traditional compressor regulation and throttle device regulation, this capacity-controllable heat pump system can obtain a satisfactory effect with little additional energy loss and a low compression ratio, while only a normal constant-speed compressor is used.The main research contents of this thesis are as follows:1. The research history and present status of capacity-controllable heat pump system of refrigerant mixture at home and abroad were reviewed. The characteristics of serveral kinds of these systems which have been studied up to now are analyzed. The key points to practical utility of the system are pointed out. Based on the existed systems, we proposed a novel system in which composition- separation device is used at the low pressure section of the cycle. It has a simple structure and can easily be controlled.2. Based on the principles of thermodynamics, the relative volatility of refrigerant pairs, environmental protection and safety, a screen work of binary refrigerant pairs used in capacity-controllable heat pump is done, and theoretically perfect pairs suchas R32/R134a, R32/R227ea are obtained. Finally, R32/R134a is selected as the best refrigerant for further research.3. A set of software that can be used to simulate the performance of air-source heat pump system was developed. A steady state simulation of the performance of the variable composition capacity regulation heat pump system in winter weather is conducted. The relations between the main performance parameters and some variables such as air inlet temperature of evaporator, water inlet temperature of condenser, air inlet velocity of evaporator, water circulating amount of condenser, relative humidity of air, heat exchanger types of evaporator and throttling types are obtained. The effects on the performance of the system due to different structure parameters are analyzed. The main methods to improve system performance are found.4. The effects on the performance of the system due to different refrigerant mixture concentrations are analyzed. Trends of capacity-controllable heat pump system and the traditional system are compared at different ambient temperatures. All the state parameters of the system are analyzed, from which the important state parameters benefit to auto-control of system are determined.5. An experiment setup for this novel capacity-controllable heat pump system is designed and built. Comparisons of R22 conventional heat pump system, R32/R134a constant composition heat pump system with this new heat pump system are made. The experimental results show that the theoretical model is reliable and the novel capacity-controllable heat pump system is practical. Several problems in design and operation are also exposed. It gives a direction for further study of this system.6. Automatic control manners of capacity regulations for capacity-controllable heat pumps are discussed and semi-automatic control is realized. It lays a foundation for the application of this novel heat pump system.
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