Research On The Thermal Performance Of CO₂ Heat Pump Under Low Temperature Conditions

Considering environmental protection and energy conservation,realizing the sustainable development of building and building a clean and low carbon energy system have become the public focus of attention.Under the guidance of "13th Five Year Plan","coal to electricity" and a series of policies,reusing natural working CO₂ as refrigerant has been a significant measure.Because CO₂ trans-critical cycle heat pump system is energy efficient and environment friendly,it is followed with interests.However,CO₂ heat pump has potential problems under low temperature condition which means research on the thermal performance of CO₂ heat pump system in operation under low temperature is of great importance.In this context,experiment of CO₂ trans-critical cycle heat pump system was proposed,thermal performance under specific conditions was systematically studied.In order to avoid the loss of the system under low temperature working condition and ensure the system works in a stable and efficient state,this paper introduces four technical approaches respectively,including increasing the enthalpy/inspiratory pressure of the inlet of CO₂ gas cooler,increasing the evaporation temperature,increasing the inspiratory quality and using the regenerative cycle.In the same time,the thermodynamic analysis was carried out.Under low-temperature climate conditions,the compressor suction pressure and temperature are reduced owning to the decrease of the temperature of the inlet water and the outlet CO₂ of the gas cooler in the trans-critical cycle CO₂ heat pump system.When the compressor suction pressure is less than the lower limit of the suction pressure,it leads to the unstable operation of the system.In order to overcome this drawback,a method of mixing water at the inlet of cold water in the gas cooler is presented.Proper quantities hot water in hot water tank is supplied to the cold water inlet of the gas cooler by a by-pass pipe.Adjusting the three-way valve to change the proportion of mixing water,the inlet temperature of gas cooler is increased to make the system run steadily under low-temperature climate conditions.Test results show that the method of mixing water could not only ensure the system of CO₂ air source heat pump hot water system works steadily under low-temperature conditions,but also increase operation time of the system and reduce the frequency of frosting.However,the heating power and COP（coefficient of performance）of the system declines slightly.Considering the operation stability and thermal performance simultaneously,the suitable temperature of mixing water for this system is 12-18 ℃ when ambient temperature is-20 ℃ and heating temperature is 60 ℃.Regarding the fixed frequency CO₂ heat pump system can’t run in the condition of wide temperature and wide load,the variable frequency compressor CO₂ heat pump system which is based on PLC on sub-critical cycle is putted forward.According to the theoretical analysis and experimental testing,the impact frequency of the suction pressure,discharge pressure,input power,heating capacity and the COP under low-temperature conditions were analyzed.The results show that:when the frequency increases,the compressor suction pressure drops,but the discharge pressure and input power rises;heating capacity increases while there is sharp rising status point;also the COP system has optimal frequency value.Taking into account the situation of heating capacity and COP,there is an optimal system adjustment frequency which is between 55 and 60Hz.The temperature of the inlet and outlet water of the gas cooler,the frequency of the compressor and the temperature of the dry bulb were determined by the orthogonal experiment.The heating efficiency COP and the system heating capacity were investigated at the same time.Through testing and analyzing of the thermodynamic performance of CO₂ heat pump,optimizing the operating parameters of CO₂ heat pump under low temperature condition,experimental evidence for the efficient and stable operation of carbon dioxide heat pump in cold and severe code region of China is provided.