| Cascaded air source heat pump(CASHP)can not only greatly improve the low temperature adaptability of the air source heat pump(ASHP),but also improve the heating efficiency of the ASHP,but it is also affected by frost formation.A special accumulator device can be added to a conventional CASHP system to store excess heat energy of the low temperature stage through the phase-change material to achieve the purpose of simultaneously providing low-temperature defrosting and high-grade heat supply.In this paper,a solid-liquid phase-change energy storage system is designed.Under different average evaporating temperature conditions,The heat-storage and discharge characteristics of the heat accumulator in the heat-storage mode and the heat release mode were analyzed and compared,and the analysis of the parameters such as the heat-storage efficiency,the enthalpy efficiency,and the high and low temperature level heat distribution were analyzed.The heat accumulator is the most important physical component in the phase-change thermal storage system.The phase-change heat transfer process of the phase-change material is the core part of the phase-change thermal storage system.Therefore,a detailed analysis of the phase heat transfer process is used to study the phase-change thermal storage device.The performance has great reference value.This phase-change heat transfer process involves the movement of the solid-liquid phase interface,the temperature change in the phase-change region,and the liquid phase rate change process.For the complex multi-dimensional model,the numerical simulation method is generally used to predict the heat transfer process.Therefore,the simplified two-dimensional circular cross section and the single tube heat accumulator are numerically simulated.Comparing with experimentally measured data,comprehensively analyze the heat release characteristics of the phase-change heat transfer process.The following are the main conclusions of experimental studies and numerical simulations:Experimental Study:(1)Calculate and analyze the heat-storage efficiency of the low-temperature refrigerant.The larger the outdoor average evaporation temperature,the lower the heat-storage efficiency.For the overall enthalpy-efficiency analysis of the cryogenic stage,the higher the outdoor average evaporating temperature,the lower the overall enthalpy efficiency;the calculation and analysis of the enthalpy efficiency of the cryogenic stage heat-storage and the high temperature(HT)and low temperature(LT)exothermic process respectively,and the enthalpy efficiency of the exothermic process is far greater than that of the thermal storarg process.(2)Calculate the heat-storage capacity of the heat accumulator in the three different groups of outdoor average evaporation temperature conditions.The higher the average outdoor evaporation temperature,the greater the total heat-storage capacity of the heat accumulator and the smaller the heat release.(3)Analyze the change of heat output of high and low temperature grades under the conditions of three different groups of outdoor average evaporation temperature,and calculate the percentage of total heat released by HT and LT heat release under different outdoor average evaporation temperature conditions,the proportion of total heat released by the low-temperature level of heat is getting smaller and smaller.Numerical Simulation:Heat-storage and discharge mode of circular section:(4)Analyze the distribution maps and temperature distribution maps of phase-change materials at 50 s,200s,500 s,and 900 s at the solid-liquid interface.The heat transfer mode in the initial stage is mainly heat conduction,and the heat transfer mode in the latter stage is mainly convection,resulting in copper.The upper phase-change material of the pipe is changed into a liquid phase prior to the phase-change material in the lower part of the copper pipe.When the low temperature fluid heats up,the high temperature fluid transfers heat and then drops,resulting in a liquid circulating flow.The entire heat-storage time lasts about 2100 seconds.(5)Analyze the solid-liquid interface profile and temperature field distribution map of 50 s,100s,400 s,and 600 s for high and low temperature grade phase-change materials.In the initial stage,the temperature change rate below the pipeline is faster than the top of the pipeline,and the solidification rate below is faster than the top of the pipeline.As the solidification proceeds,the melting rate of the phase-change material around the round pipe is consistent,and the entire time of HT and LT stage heat release last 1100 s and 800 s respectively.Heat-storage and discharge mode of single-tube:(6)Analyze experimental measurement of temperature change of phase change material in the direction of single tube flow,and analyze the distribution of the solid-liquid interface and temperature distribution of phase-change materials at 150 s,300s,800 s,and 1200 s.In the early stage of heat-storage,the phase-change material at the inlet of the refrigerant is heated first,and nearly 45% phase-change at 300 s.The material temperature reached the phase-change temperature(279.3 K),and the entire heat-storage time lasts 1500 s.Finally,the calculation results are compared with the experimental data.The correctness of the calculation model was verified.(7)Analysis of experimentally measured temperature changes of high and low temperature phase change materials in the direction of single tube flow,and analyze the distribution of the solid-liquid interface and the distribution of the temperature field during the 50 s,200s,400 s and 500 s of the exothermic time.At the beginning of the exothermic phase,the phase-change material at the inlet of the refrigerant is first cooled.Nearly 60% of the phase-change material temperature reaches the phase transition temperature(281.3K)at 200 s,and the entire time of HT and LT stage heat release last 700 s and 500 s respectively.Finally,the calculation results are compared with the experimental data.The correctness of the calculation model was verified. |
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