Thermal Storage Characteristics And Performance Regulation Of Packed Bed System With Spherical Encapsulated Phase Change Materials

The medium-high temperature heat storage technology stores thermal energy such as solar heat,industrial heat,and waste heat to solve the problems caused by the uneven distribution of energy in time and space or the mismatch between the supply side of renewable energy and the demand side of energy consumption.It can significantly improve the energy utilization rate and energy supply stability of the energy system,and reduce the system cost.Therefore,the development of efficient medium-high temperature heat storage systems and new heat collection and heat storage methods are of great significance for the development and efficient utilization of new energy.However,due to the low thermal conductivity and strong corrosiveness of phase change materials,the medium and high temperature packed bed latent heat storage system develops slowly in terms of thermal performance improvement,structural optimization design,and high temperature energy acquisition,which seriously restricts the development of heat storage technology.In this thesis,the optimization design of the packed bed heat storage system,the experimental test of heat storage materials,and the high-temperature heat storage of the new particle heat absorber are studied.A transient two-dimensional axisymmetric numerical model of the packed bed latent heat storage system is constructed,and the phase change process of the phase change materials is simulated based on the enthalpy method model.The natural convection in the liquid phase process of phase change materials is considered in the form of effective thermal conductivity.Combined with the theory of porous media flow and heat transfer,the optimal design of the internal heat storage unit structure of the heat storage system is carried out.And the correctness of the model is verified by experiments.A structure optimization method for the radial gradient arrangement of phase-change heat storage spheres is proposed,and the influence of different structural parameters such as the arrangement of heat storage units in the system,packing density,and unit size on the thermal performance of the system is analyzed.The influence of the radial and axial gradient arrangement of the phase change heat storage spheres in the packed bed heat storage system on the thermal performance of the system.The research results show that the radial cascade arrangement of the heat storage system can improve the heat storage and release the power of the heat storage system.Compared with the axial cascade arrangement,the radial cascade arrangement can significantly reduce the pressure drop of the system,thereby improving the overall system.It is found that the high-temperature energy utilization rate of the packed bed latent heat storage system with radially stepped arrangement can be improved by 6.5%.Based on the numerical model,a new type of experimental system for medium-high temperature latent heat storage that can realize free series-parallel heat storage is designed and built.Macro-encapsulated phase change heat storage spheres are prepared by using nitrate as the phase change materials.The heat and mass transfer characteristics of the system during the charging process,the static heat preservation process,and the discharging process are revealed through experimental research.The comparison between the experimental results and the numerical prediction results proves that the transient twodimensional numerical model established in this paper can provide more accurate prediction results for the latent heat storage system.In addition,the influence of air temperature and airflow rate on the pressure drop of the packed bed heat storage system is given through experimental research,and the airflow rate and the system pressure drop have a quadratic relationship.The heat acquisition of the solar high-temperature heat storage system is the key to the whole heat storage cycle system.Therefore,a novel solar particle receiver is proposed.Combined with Monte Carlo Ray Tracing method and Finite Element Method,a twodimensional transient numerical simulation of the particle receiver is carried out.Based on the coupled model,the effects of different particle sizes,radiation fluxes,porosity,and particle residence time on the temperature distribution and heat transfer performance of the particle receiver are revealed.It is found that the transient heat transfer process inside a single particle has less effect on the average temperature at the outlet of the receiver and the solar energy conversion efficiency.It is given that the solid particles of 0.2 ～ 0.4 mm can obtain a higher outlet temperature and heat conversion efficiency of the solar collector.Based on the solar heat absorption system,a coupled dynamic theoretical model of the solar absorber model and the packed bed latent heat storage system is constructed.The influence of the inlet boundary condition on the dynamic heat transport of the heat storage circulatory system is analyzed.The effects of mass flow on the total heat exchange,total exergy input,exergy storage,and exergy efficiency of the heat storage system under steady and unsteady inlet boundary conditions are obtained.It is found that the steadystate and unsteady inlet boundary conditions show significant differences in the thermal performance of the latent heat storage system.