Transcritical Carbon Dioxide Heat Pump Water Heating System Dynamic Characteristics

As a kind of natural fluid, carbon dioxide is a promising alternative refrigerant in the refrigeration systems due to it's eco-friendly performance and good thermal properties. When CO₂ is used in the heat pump water heater system in transcrital cycle, a gliding temperature will appear, which contributes to a good temperature adaptation. It is important that finding a proper way to enhance the performance of refrigerant system, which is also one point of the study of heat pump water heater system all over the world. In this work, adopting the method of computer simulation to study heat pump water heater system is convenient for the system analysis.Firstly, cycle simulation model has been developed to analyze and caculate the transcritical carbon dioxide heat pump water heater system, based on the first and second law of themodymamic analysis. The analysis reveals that the factors affect the system performance of the second law are the heat rejection pressure, the outlet temperature of gas cooler, the evaporation temperature, the efficiency of internal heat exchanger and the performance of the compressor. Furthermore, the performance of the second law varies with the heat rejection pressure, and a maximum performance of the second law occurs at an optimal heat rejection pressur Popt₂, which is different from the optimal heat rejection pressur of COP.Secondly, a numerical model was built to simulate the transient state behavior of a transcritical CO₂ heat pump water heater system. Lumped models were built for the evaporator, gas cooler and internal heat exchanger respectively based on lumped parameter method and moving boundary method. Quasi-steady state models were applied for the compressor and expansion valve with empirical formula for their small thermal inertia. In this study, the cycle dynamic simulation model was based on the balance of conversation of energy, conversation of pressure and conversation of mass. The results show that the transient state behavior of a transcritical CO₂ heat pump water heater system is great influenced by the change of the parameters of inlet water of the gas cooler, control valve opening and the variable displacement compressor. All these had an influence on the design of control system.The results of this work are of significance for optimization of performance and control mode.