Study On Application Performance Of Transcritical CO₂ Air Source Heat Pump In Heating Area

In recent years,the most concerned environmental issue is the haze problem,which has negative effect on people's health.The coal combustion is the largest source of haze problem in China,which is directly attributed to the coal combustion for heating.Therefore,in order to alleviate the air pollution,Chinese government put forward the policy of using electricity to replace coal for heating in 2015,where using air source heat pump for heating is one of the most promising methods.Nonetheless,it would bring a potential threat to air source heat pump future for the policy of replacing HCFC or limited majority use of HFCs.As natural refrigerant,CO₂ is reasonably to be used in a heat pump to produce hot water,due to its excellent thermophysical properties.In order to both popularize the use of heat pump technology for domestic heating and follow the principle of reducing the use of synthetic refrigerants,promoting CO₂ heat pump cycle for home heating is a promising choice.Thus,it is necessary to evaluate its system efficiency and applied performance before widely application.A mathematical model was developed to comprehensively evaluate the energetic,economical and environmental performances of CO₂ heat pump system.The heating problem was abstractly considered as different conditions featured by varied temperature distributions in different cities,where a special city was represented by a special temperature distribution.Each temperature bin was set as 1?C.For the residential heating condition,the corresponding CO₂ outlet temperature was relatively fixed due to the nearly constant water inlet and outlet temperature,which ensures the accuracy of the model.A mathematical simulation model specialized for transcritical CO₂ air source heat pump was also introduced for the required data calculation of each temperature bin.In addition,to compare with traditional heating ways directly,with the inspiration provided by the calculation of IPLV in refrigeration,a heating load calculation model was also introduced.To verify the accuracy of the simulation model specialized for CO₂ heat pump,the corresponding experimental study was carried out firstly.The experimental platform used in this study was designed based on the simulation model,its results show that COP of the system increases with the increase of the ambient temperature or the decrease of the outlet temperature of CO₂ in the gas cooler.Within the range of heating temperature,the corresponding CO₂ outlet temperature in gas cooler was relatively fixed due to the nearly constant water inlet and outlet temperature under the optimal discharge pressure condition,What's more,for system optimal discharge pressure,the influence of the outlet CO₂temperature in gas cooler is higher than ambient temperature.Taking temperature range of-15-10?C for example,when water inlet temperature and outlet temperature is 30?C and55?C,respectively,all its optimal discharge pressures are concentrated near 8.5MPa,which imply thermal expansive valve may could be used for CO₂ heat pump heating system to guarantee the optimal discharge pressure.Based on the simulation model specialized for CO₂ heat pump whose accuracy verified by the experimental data before,the techno-economic analysis results indicate that,under the optimal discharge pressure condition,the optimal SPF of CO₂ heat pump would reach2.52 in Harbin,2.73 in Shenyang,2.94 in Dalian,2.96 in Beijing and 3.24 in Shanghai,where renewable energy contribution ratio of CO₂ heat pump is higher than 60 percent.In terms of environmental emission,emission value of CO₂ heat pump is lower than that of direct electric heating or coal fired boiler,but slightly higher than that of wall-hanging gas boiler.Therefore,CO₂ heat pump system is advanced in energy saving and energy protection.In hot summer and cold winter region,taking Shanghai as example,annual operation cost of CO₂ heat pump is 18.8 percent lower than that of coal fired boiler,20.1percent lower than that of wall hanging gas boiler due to its higher ambient temperature.However,for the cold region and severely cold region,the government operating subsidy would be within the range of 85⁹20RMB if CO₂ heat pump was used to substitute coal-fired boiler for heating.However,initial capital cost is a major reason restricting the application of CO₂ heat pump where compressor investment accounts for more than 70 percent.Key trend of CO₂refrigeration and heat pump industry implies that decrease of compressor cost will exceed30 percent where life cycle cost of CO₂ heat pump would be lower than that of the other three heating methods.As increase of system efficiency and decrease of equipment investment in the developing process of CO₂ refrigeration and heat pump industry,it is believe that CO₂ heat pump system is a promising way to be widely applied for residential heating in China,especial for the hot summer and cold winter region.