| Phase change energy storage technology can solve the problem of mismatching of energy and time in partical application.However,compared with most phase change materials,it has the disadvantages of low thermal conductivity and large undercooling.Adding materials with high thermal conductivity and nucleating agents to phase change materials can effectively solve this problem.Cascade phase change energy storage technology is to arrange phase change materials with different melting points in the direction of energy flow,so it can provide stable heat flow and enhance the heat storage performance of phase change materials.Therefore,the characteristics of heat and storage in cascade phase change storage system were carried out in this paper by experimentally,and optimization study of cascade phase change storage system were also conducted.Melt blending method and Edison method was used to configure erythritol composite phase change material,sodium acetate trihydrate composite phase change material and sodium decahydrate sodium sulfate composite phase change material,respectively.DSC and step-cooling curves were used to characterize the thermal properties of the phase-change materials and explore the optimal scheme of material ratio.The results shown that the optimal mass fraction of erythritol and urea for erythritol composite phase change material is 6:4,which is added the graphene with a mass fraction of 5 %.The sodium acetate trihydrate composite phase change materials can show the optimal thermal performance with 3 % of graphene and 3 % of sodium pyrophosphate respectively.The sodium sulfate decahydrate phase change materials can show the optimal thermal performance with 3 % of graphene and3 % of borax respectively.The configured phase change materials were placed in energy storage boxes,and the volume control method was used to conduct experimental research on single-stage energy storage,double-stage energy storage,and three-stage energy storage.The results shown that the three-stage phase change energy storage system can provide the optimal thermal performance,with large energy storage,fast energy storage rate,and large energy storage efficiency.The heat storage rate of three-stage phase change energy storage system is 334.95J/s,the heat release rate is 186.37 J/s,the heat storage efficiency is 41.54 % and the heat release efficiency is 53.22 %.In addition,three-stage phase change energy storage system shows the economical efficiency based on the economic analysis.A phase change energy storage module was established in the TRNSYS software,and the phase change process was processed using the apparent specific heat capacity method.The average relative error,the maximum relative error,and the Bland-Altman consistency were used to varify the simulation results.The results shown that the numerical model is feasible and accurate.The supply temperature and flow rate of the three-stage phase change energy storage system were optimized.The results shown that the optimal flow rate of heat storage process is 3.0 L/min,and that of the exothermic process is 2.0 L/min when the focus is on the heat release rate.The optimal flow rate of the exothermic process is 1.4 L/min when the focus is on the energy efficiency of the system.For the optimization of supply temperature,the supply temperature of 100 ℃ is optimal in the heat storage process.The supply temperature of 9 ℃ is optimal when the heat release rate is emphasized in the exothermic process,and that of 15 ℃ is optimal when focusing on system energy efficiency.In summary,cascade phase change energy storage substantially improves the thermal properties of phase change materials,and this paper provides sufficient theoretical basis for further practical applications. |
