Study On The Performance Of Low Ambient CO₂ Air-source Heat Pump System

At present, environmental problem is increasingly prominent, Global warming and ozone depletion are concerned by more and more people. With the development of economy, people put forward higher requirements for thermal comfort. ASHPs have been widely used in the area of Yangtze River, with the features of high efficiency, energy saving, safety and environmental protection and easy to use. But there are some troubles in its popularization in cold regions of northern China. Main reasons include: when the ASHP is at a low ambient temperature, its heat performance and COP are at a very low level, the compressor is very difficult to start, the compressor shuts down to protect itself from the high exhaust temperature, and the evaporator frosts frequently in high humidity areas, and it will reduce the heat performance again. CO₂ is a kind of natural refrigerant which is not consumed by the ozone and the global warming potential is very low（ODP=0, GWP=1）. The two stage compression with air compensating in the middle system can efficiently solve the problem of the too large compression ratio and the exhaust of too high temperature,the flash of middle air can also reduce the degree of under cooling of the refrigerant at the entrance of evaporator, and reduce the degree of superheat of the refrigerant at entrance of the higher pressure compressor, therefor, it can furtherly increase the COP of the system.This paper aiming at ambient temperature-20℃, gas cooled inlet and outlet temperature of respectively 5 and 65℃, influent flow 70L/h. evaporator send wind speed of 3m/s conditions, based on performance of coupling between the first law ofthermodynamics and components of a double level compression model of CO₂ heat pump water heater system and the established model of the environmental temperature, air inlet temperature and the effect of water flow on the system characteristics, providing some bases for the design and optimization of the system.The study found that the ambient temperature is-20℃, the temperature of the water inlet temperature of 5 ℃, the water flow rate of 70L/h, the wind speed 3m/s,the system heat is 4.74 k W, COP is 2.23, the water temperature is 61.8℃. When water flow, water temperature is below 10℃, the system heating capacity and COP with inlet temperature increases little change, showed a decreasing trend, with inlet temperature, outlet temperature was elevated, the growth trend and is slow with inlet temperature in the inlet water temperature higher than 10 ℃.When the water temperature is constant, the heat and COP of the system increases with the increase of the influent flow rate, and the water temperature increases with the increase of the influent flow rate.