Research On CO₂ Transcritical Water-water Heat Pump And Two-rolling Piston Expander

Global warming and ozone depletion urge people all over the world to work cooperatively to limit the use of traditional refrigerants, and their alternatives, natural referigerants such as carbon dioxide (CO2), revive. Based on theoretical analysis, mathematical simulation and experimental research, this dissertation focuses on the study of CO2 trans-critical water-water heat pump and two-rolling piston expander. A trans-critical water-water heat pump system using newly-designedheat exchangers was established, a two-rolling piston expander was developed, theoretical analysis and mathematical model were carried out, and experimental results were obtained.Internal heat exchanger can be used in basic CO2 trans-critical cycle to improve the efficiency of the system. Effects of the internal heat exchanger on the CO2 trans-critical cycle are first discussed in the dissertation. An evaluation criterion is got to judge the influence of the internal heat exchanger on the coefficient of performance (COPh) of the system. Analyses and experimental results show that the internal heat exchanger can help to improve COPh of the system. Two approaches can be employed to improve the efficiency of the basic CO2 trans-critical cycle, namely, multi-stage compression and the recovery of power in the throttling process through the use of expander. Therefore, this dissertation also includes analyses of trans-critical two-stage and tri-stage compression cycles.In the expansion process of the CO2 trans-critical cycle, there are various energy consumptions when super-critical CO2 changes to the gas-liquid state. According to the molecular aggregation theory, the energy consumed by molecules of the fluid in the degregation process and that consumed by the formation of bubbles should be considered in the expansion process. The suction control structure of single-stage rolling piston is analyzed. On the basis of single-stage rolling piston expander, single-stage two-cylinder expander is presented, which functions to great advantage in larger massflow rate and common refrigeration system using traditional refrigerants.The two-rolling piston expander was designed, its working process was analyzed, and main parameters were defined. The movement pattern and the connection between first-stage and second-stage expanders were analyzed. The model on the expander is founded to simulate the operating process, getting main thermodynamic parameters, irreversible losses, and further analysis of the performance of the expander.Devices such as the newly-desinged evaporator and lubricant cooler are adopted in the CO2 trans-critical water-water heat pump system. Experiments have been done with the new CO2 trans-critical water-water heat pump system using newly-designedheat exchangers. Experimental results justified the influence of the internal heat exchanger on the system and that of newly-designedheat exchangers with different evaporation areas on the system.Performance of the newly-developed two-rolling piston expander was tested in the CO2 trans-critical water-water heat pump system, and experimental results were obtained, revealing that the efficiency of expander is 28~33%. The expander operates calmly. Modified directions are discussed in order to further increase the efficiency of the expander.