Theoretical And Experimental Study On Varying Composition Heat Pump System

The widespread application of air-source heat pump systems conforms to the environmental protection and energy-saving development trend. However, it is difficult to provide enough heat in winter because of the ultra low suction pressure under low ambient temperatures. Thus the indoor comfort can not be kept. The capacity control is an effective way to solve this problem. The capacity control passed through several stages, such as on-off of the compressor, joint-operation regulation of the multi-compressor, speed regulation of the compressor and mixed qualitative capacity adjustment of refrigerant mixtures. Though the adoption of the 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. The solution may be achieved by using refrigerant mixtures, for pure refrigerant is difficult to meet the needs of changing heating or cooling capacity of a heat pump. In this thesis, a new capacity-controlled heat pump system using refrigerant mixtures and a normal constant-speed compressor was put forward.An experiment apparatus for this heat pump system was designed and set up. The distillation device was installed between the condenser and compressor exits and throttle valve entrance. The discharged gas from the compressor directly works as the heat source that the distillation process needs without any additional energy input. The system stores most of the low boiling point working fluid so that lubricants can return to the cycle system together with the high boiling point working fluid. In this way, the problem of oil return can be solved. The liquid storage tank with low boiling point working fluid located in the high pressure section of the system, and no special insulation is needed.By conducting the theoretical model and simulating the new system, the relations of some important performance parameters changing with the inlet temperature and the low boiling point component's concentration of the evaporator can be obtained, such as heating capacity, COP, power consumption of compressor, etc. The problems which should be paid attention to were obtained by predicting the performance of new system with refrigerant mixtures.The system was used to study the changes of cycle performance with concentration of R32/R134a. Analysis of the COP changes of the system with concentration and the inlet temperature of the evaporator were given. The capacity adjustment and the temperature changing range of R32/R134a were obtained. The reasons of the changing trend of the system performances were analysed by comparing with the simulation results.R407C is operated on the new heat pump system, and the effects on the performance of the system due to different refrigerant mixture concentrations are analyzed. The performances using R407C are compared with R32/R134a. The range of the variable concentration of R32/R134a is greater than that of R407C. It provided an important experimental basis for the use of R407C in the varying composition capacity-controlled heat pump system.The experimental results confirmed that the new capacity-controlled system is feasible. In addition, the direction to improve the system is pointed out through the analysis of the system design and operation.Finally, some advice are put forward for further research.