Research On Experiment And Multiobjective Optimization Of Transcritical CO₂ Heat Pump In Ultra-low Temperature Environment

The heating demand for supplying high-temperature hot water?above 75°C?in a low-temperature environment?below-20°C?was proposed in‘heating by electricity instead of coal'policy in northern China.However,the existing air-source heat pump?ASHP?,which is widely used in low-temperature environments,has many defects.On the one hand,the low-temperature environment required above cannot ensure the efficient operation of the single-stage ASHP system.Moreover,the steam injection ASHP system and the cascade ASHP system are not applicable to the popularization of civilians due to their low operating stability and large investment costs.On the other hand,the refrigerants used in the ASHP system are chlorofluorocarbons that have a certain negative impact on the environment.Fortunately,transcritical CO₂ heat pump systems with natural environmental protection characteristics have shown great potential for heating high-temperature hot water in low-temperature environments.Therefore,the successful development of a transcritical CO₂ heat pump hot water device in an ultra-low temperature environment has practical significance for solving the winter heating problem in northern China.A transcritical CO₂ heat pump experimental device for heating high-temperature hot water in an ultra-low temperature environment was built in this paper,and the performance of the heat pump system for heating 75°C hot water at an ambient temperature of-23.1 to-4.4°C was tested.Furthermore,considering that the transcritical CO₂ heat pump water heater has problems such as low economy and practicability,the NSGA-II algorithm is used to improve the design of transcritical CO₂heat pump water heaters in the case of typical ultra-low temperature in this paper.The main work has been carried out:?1?Considering the thermodynamic principle,component design,and economic evaluation of heat pump systems,detailed mathematical models of transcritical CO₂ heat pumps in ultra-low temperature environments are established.?2?By researching the ultra-low temperature transcritical CO₂ heat pump experiment,the heating capacity and Coefficient of performance?COP?can reach9.4k W and 1.74,respectively,at an ambient temperature of-23.1°C and an outlet temperature of 75°C.?3?A multi-objective optimization model of a transcritical CO₂ heat pump with a total revenue requirement?TRR?and environmental emissions?EE?as the objective function is established from the perspectives of investment cost,operating cost,environmental impact and practicability.?4?Based on a typical ultra-low temperature environment case,the NSGA-II algorithm is used to solve the transcritical CO₂ heat pump optimization model.the three optimized design schemes A,B,and C are obtained.The scheme A has the smallest EE?7077.1kg?and space occupation?150L?,and compared with the non-optimized design scheme,the EE is reduced by 18.6%,which is the most energy-saving,environmentally friendly and practical.The EE and TRR of scheme B are 4.36%and 11.3%lower than the non-optimized design scheme,respectively,and their comprehensiveness is the best.The TRR of scheme C is 25.1%lower than the non-optimized design scheme,which is the most economical.