Studies On Transcritical CO₂ Cycles Adopted In Heat Pump Water Heater

At present, environmental problems have become a global one, among them ozone depletion and green house effect which are closely related to the people's health and life have drawn a great attention. Discharge of CFCs and HCFCs widely used in refrigerating equipments is one of the major reasons for ozone depletion and green house effect as well, and will be phased out. Alternative refrigerant HFCs is not a proper one. For long term consideration, natural refrigerant is the most satisfied substitutes.The feasibility investigation around the world showed that transcritical CO₂ cycles is promising on heat pump water heater system, for no ozone depletion effect and less global warming effect. Since the investigation just begins, much work needs to be done.This paper studies the CO₂ which one of natural refrigerant, analyzes its thermal properties, the character of CO₂ cycle, to make transcritical CO₂ cycle more perfect, and analyzes the economic factors that transcritical CO₂ cycle used in heat pump water heater.First, this paper analyzes the signification, the purpose, the circumstance of alternative refrigerant in global, the emphasis of substitution, then refer to that the natural refrigerant is the only kind way to environment. This paper also introduces the history of CO₂ as refrigerant.In view of the fact that the evaluation of a refrigerant only by its simple thermodynamical cycle analysis is an one-sided approach, the comprehensive evaluation of CO₂ refrigerant is conducted in terms of environment, safety, thermophysical characters, compressor efficiency, price and so on.Based on air conditioning cycle simulations, the influence of the system evaperation temperature and gas cooler outlet temperature on the cycle COP in transcriptical CO₂ is analyzed. It is shown that an optimal heat rejection pressure exits to furnish maximum COP. Then, the paper analyzes how to derive the optimal heat rejection pressure.The feasibility, the advantage, the economical factor are analyzed by comparison with other kinds of water heater, such as natural gas, oil-fired, and so on. Then, analyzed the exergy efficiency of the system.