Research On Thermal Transfer And Thermal Storage/Release Characteristics Of Seasonal Underground Composite Thermal Storage System

Underground seasonal thermal storage can effectively solve the mismatch characteristics of renewable energy systems in time,space and intensity.In recent years,the technology has developed more and more rapidly and its application range has become more and more extensive.However,large thermal loss and low solar energy guarantee rate are the biggest technical problems facing it.At home and abroad,it has begun to pay attention to and increase research in this area.Through measures such as high temperature and large temperature difference thermal storage,increasing the thickness of the insulation layer and changing the backfill material to reduce thermal loss and improve the solar energy guarantee rate,but the actual effect is not very significant,and these studies are focused on the single thermal storage source utilization mode,and there are few studies involving multiple thermal storage source composite utilization,especially heat transfer in the composite thermal storage mode research on mechanism,coupled thermal action characteristics,and thermal storage and release characteristics still needs to be strengthened.In this study,two thermal storage methods,borehole thermal storage and water tank thermal storage,are combined to form a new underground seasonal composite thermal storage utilization system to study the composite utilization effect of multiple thermal storage sources.Through the combination of experimental studies and simulation calculations,the coupling thermal action characteristics,thermal diffusion effects,and temperature field distribution of the system are studied.At the same time,the thermal storage and release characteristics of the system during continuous operation are analyzed to reveal the temperature change law of the thermal storage body during the multi-year operation of the system,improve the thermal transfer mechanism and the theory of thermal storage and release characteristics of the seasonal thermal storage system to provide guidance for engineering applications.The research work mainly includes:Coupling the two methods of thermal storage with traditional hot water storage and borehole thermal storage,a new underground seasonal composite thermal storage system is proposed.The characteristics of the underground thermal transfer process are theoretically analyzed,and a three-dimensional unsteady numerical calculation model of the composite thermal storage system based on Fluent is established.The system boundary conditions,grid division and initial conditions are introduced in detail.In addition,an underground seasonal composite thermal storage test laboratory was built,and system thermal storage and release experiment was completed.By comparing the simulated data with the experimental data,the accuracy and precision of the model were verified.The analysis shows that the model can be used to analyze and predict the temperature change and evolution of the actual underground seasonal composite thermal storage system.The composite thermal storage model was used to complete the simulation calculations of the four stages of thermal storage,thermal preservation,thermal release and recovery,and the thermal diffusion in the soil was studied,including the temperature change characteristics,the radius of action,the delay characteristics,and the temperature of each stage of the system.The distribution and change of the field,and by comparing with the borehole thermal storage and the water tank thermal storage mode,the coupled thermal action characteristics of the composite thermal storage system are obtained,and the thermal transfer mechanism and evolution law of the system are revealed.Provide a basis for the design of the thermal storage system.On the basis of the above model,the multi-period and long-term operation simulation calculation of the underground seasonal composite thermal storage system was carried out,and the outlet temperature change of the composite thermal storage system,the characteristic point temperature change,and the temperature field of the composite thermal storage body were analyzed.The distribution and the law of system efficiency change are compared with the borehole thermal storage and water tank thermal storage modes,and the coupling thermal storage body thermal diffusion promotion and suppression are analyzed,revealing the storage and release characteristics and efficiency changes during the long-term operation of the system,to provide theoretical reference for practical engineering applications.