| The application of air source heat pump system in severe cold regions will be significantly influenced due to the insufficient heat transfer of the outdoor evaporator in low ambient temperature,which resulted in its decreasing heating performance.Latent thermal energy storage technology is able to carry out short-period heat charging/discharging processes for air source heat pump system owing to the large temperature difference between day and night in severe cold regions,which avoided the system operation in extremely low ambient temperature.It is one of the most potential solutions for the application of air source heat pump system in severe cold regions at this stage.However,the current research about the air source heat pump system based on latent thermal energy storage has found out several problems such as insufficient charging/discharging performance of latent thermal energy storage device in the system,unintelligible system dynamic operating characteristics,and lack of system feasibility analysis.As a result,this paper carried out the following work:(1)A high-efficiency latent thermal energy storage device—condensing heat storage,combined with an air source heat pump system,is designed based on direct heat transfer between refrigerant and phase change material.The thermodynamic analysis of the system cycle is carried out using the pressure-enthalpy diagram,and it is pointed out that the performance of the system will decay as the charging time increases.In order to maintain the efficient and stable operation of the system,it is reasonable to set a subcooler to control the subcooling degree of the condensing heat storage unit.The phase change material in the condensing heat storage unit is selected.Taking a R410A air source heat pump system and a low-temperature hot water floor radiation heating terminal as examples,paraffin with a phase change temperature of 48?can be used as the filled phase change material in the condensing heat storage unit.The structure of the condensing heat storage unit is also selected.The fin-and-tube heat exchanger is optimized and transformed by designing a single-pass tube into a double-pass tube,so that the refrigerant and the circulating water are able to conduct heat transfer in a counter-flow form.The prototype structure of the condensing heat storage unit is constructed by packaging and filling the optimized fin-and-tube heat exchanger unit.In addition,the charging/discharging performance of the condensing heat storage unit can be further enhanced by setting up multiple parallel refrigerant/water tubes.(2)The dynamic operating characteristics of the system are numerically and experimentally investigated;a safe and efficient operation method of the system,an optimized design method of the high-performance condensing heat storage unit,and an optimized criteria for the refrigerants used in the system are proposed.Numerical calculation of each component used in the system is carried out by using software Matlab.The dynamic characteristics of the system under continuous operating conditions in severe cold regions are studied.In addition,the structural parameters of the condensing heat storage unit are optimized,and the performance characteristics of different refrigerants used in the system are compared.The system can achieve its steady state under operating condition with an ambient temperature of-20?after continuous operating time for 6 days.The average heat storage power,COP and heat storage capacity are 7.2 k W,2.0,and 100.6 k W·h,respectively,after 14 hours of charging process carried out by the condensing heat storage unit in the system.The average heat release power of the condensing heat storage unit during discharging process is 10.1 k W,which can theoretically provide continuous heating for a room of164 m2 for 10 hours,which can achieve all-day continuous heat supply in severe cold regions.In addition,the smaller the fin spacing and tube spacing in the condensing heat storage unit are,the better the charging/discharging performance will be.Considering that R290 has a lower GWP value and a certain degree of low temperature adaptability,it becomes the best refrigerant used in the system which can replace R410A in severe cold regions.The condensing heat storage unit is developed,and an experimental platform of an air source heat pump system combined with direct latent thermal energy storage unit is established.The system dynamic operating characteristics are analyzed,the system model is verified by the experimental data,and the influence of ambient temperature on the system charging performance,and inlet water flow rate on the system discharging performance are investigated,respectively.In order to ensure the safe and efficient operation of the system,it should be guaranteed that there is still paraffin left without completing its phase change melting process in the condensing heat storage unit before the end of the charging process.In addition,the lower the ambient temperature is,the longer the charging time required by the system,the higher the average power consumption,the lower the average heat storage power and the average COP will be;the greater the inlet water flow rate is,the shorter the effective discharging time of the system and the higher the average heat release power will be.(3)A techno-economic analysis of the proposed system is carried out,and the performance differences between the system and other heating systems are compared from the aspects of energy consumption,environmental protection and economy,which can provide a feasible solution for the application of this system in severe cold regions.The techno-economic analysis of the system is carried out from the perspectives of primary energy consumption,primary energy ratio,carbon dioxide emissions,sulfur dioxide emissions,nitrogen oxide emissions,initial investment cost,running investment cost,and so on.The application effect of the proposed system is compared with a quasi two-stage air source heat pump,a coal-fired boiler,a wall-hanging gas boiler and a direct electric heating in Shanghai,Beijing,Shenyang and Harbin.The research results show that the proposed system has lower primary energy consumption and shows better energy-saving performance under low ambient temperature,compared with the quasi two-stage air source heat pump system.The heat pump heating systems migrate the CO2,SO2,and NOX from the residential district to the power plant.The dilution,desulfurization,and denitrification processes in the power plant will significantly reduce the emissions to the environment.As a result,the heat pump heating systems are more environmentally friendly than the traditional heating systems.The proposed system has a static payback period of about 10 years compared with the quasi two-stage air source heat pump system applied in Harbin,and the static payback period will be reduced to about 6 years if the discount rate of the condensing heat storage unit is set to 40%.In summary,the proposed system has a better low temperature adaptability than the quasi two-stage air source heat pump system from the perspectives of energy consumption,environment and economy,which can provide a practical feasibility for the large-scale application of the proposed system in severe cold regions. |
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