System Design And Performance Analysis For Heat Pump Adopted Natural Working Fliud

Heat pump is an efficient energy saving technology, but usually the output temperature of heat pump is relatively low which limits its application, so investigating how to improve the output temperature is important to expand the utilization of heat pump. And because the conventional working media of heat pump like Freon usually lead to the ozone hole and global warming, it is necessary to find out an environment-friendly and high-efficiency working medium with high heat pump output temperature. So in this work, high temperature heat pump with natural working medium is designed and analyzed, which is meaningful for improving the energy using efficiency and protecting the environment.As a kind of natural working media and because of the high cycle performance and environment protecting characteristic, CO₂ has been a working medium drawing more and more attention. And because of the low critical temperature of CO₂,transcritical cycle is usually adopted in the CO₂ heat pump cycle. However, CO₂ transcritical cycle heat pump with single compressing stage is usually of low cycle performance, and other problems like high compressor outlet temperature and high system pressure also exist usually, which have been proved by the theory and experiment analysis.In this work the CO₂ transcritical cycle heat pump with double compressing stages were investigated. The computational analysis of CO₂ transcritical cycle heat pump with double totally cooling compressing stages shows that like the single compressing stage cycle, the double compressing stage cycle also has an optimum pressure that maximizes the cycle performance. An improvement of 23-40% on the cycle performance coefficient is obtained with the double compressing stage cycle compared to single compressing stage cycle; the double compressing stage cycle has a lower compressor pressure ratio, and the compressor outlet temperature was decreased too? lower than 83 ? in this work?. But the CO₂ double compressing stage cycle is not proper for utilization in the the high temperature area because the outlet temperature of it is low.To apply the natural medium heat pump in the high temperature area, and considering the performance of R290 in the engineering application, a cascade heat pump cycle combing CO₂ transcritical cycle with double compressing stage and R290 cycle with single compressing stage is established. And the computational analysis of this cascade cycle shows that the outlet temperature of this cycle is increased after being heated twice, and the cycle can be used to high temperaturearea. To increase the cycle performance of this heat pump system and considering the throttling loss and cascade temperature difference, an expansion machine is used in the CO₂ transcritical cycle with double compressing stage, and a regenerator is used in the R290 cycle with single compressing stage. The performance coefficient is increased by 8.6-12.5%, and the expansion recovered power is lower than 6.5% of the total power consumption when the low pressure throttle valve of the CO₂ cycle is replaced by the expansion machine. Just adding the regenerator in the R290 cycle gives little increase on the system performance, and when the superheat degree is10?, the system performance is increased only 1.3%.