Numerical Simulation Research On Thermal Energy Storage Characteristics Of Packed Bed With Spherical Phase Change Capsules

Thermal energy storage technology is one of the valid methods to coordinate the balance of energy sources supply and demand and settle the matter of the time and space mismatching of renewable energy in the future.Among multitudinous thermal energy storage technologies,the packed bed thermal energy storage(PBTES)technology has the superiorities of simple construction,low investment,large heat exchange area,good heat transfer performance,and low equipment requirements.The use of numerical simulation methods to study the performance of PBTES has become the mainstream and focus of research work in recent years because it reduces cost and avoids time waste.To sum up,this article has carried out the following research work.Construct a physical model of a PBTES device encapsulating spherical phase change capsules.Based on Schumann’s model and considering the influence of heat transfer resistance between spherical phase change capsules and HTF and heat storage tank and surrounding environment during the charging process,a one-dimensional two-phase mathematical model of PBTES device with double energy equation is established and verified.The performance evaluation indicators such as thermal energy storage density,average thermal energy storage rate,exergy efficiency and average exergy storage rate are put forward and their formulas are given.Based on the model,MATLAB mathematical software is used to carry out numerical simulation calculations for the PBTES device,and the effects of Re,Ste,porosity of the packed bed and effective thermal conductivity of PCM on the temperature evolution trend and thermal performance of PBTES device are investigated by single factor analysis.In addition,the structure of the PBTES device is optimized,a cascade PBTES device filled with three spherical phase change capsules is established,and the encapsulation PCMs are selected.Using the above mathematical model,compare and analyze thermal energy storage characteristics of the singlelayer PCM packed bed and the cascade packed bed based on thermal performance indicators such as temperature distribution,thermal energy storage capacity,thermal energy storage density,average thermal energy storage rate,effective thermal energy storage time,and exergy efficiency during the charging process.The effect of a single factor on the thermal performance of the PBTES device shows that during the charging process,the thermal energy storage density can be increased by increasing Ste and reducing the porosity;and by increasing Re and Ste,the porosity can be reduced.Average thermal energy storage rate and average exergy storage rate are increased by increasing Re and Ste and decreasing porosity,that is,Re and Ste are helpful to increase the thermal energy storage capacity of PBTES device.Increasing the porosity can increase the exergy efficiency and improve the heat transfer characteristics.However,the effective thermal conductivity of PCM has little effect on the thermal energy storage characteristics of PBTES device.The comparison and analysis results of the cascade packed bed and the single-layer PCM packed bed show that the cascade PBTES device can store higher thermal energy storage capacity and thermal energy storage density during the charging process.Compared with single-layer PCM packed bed,the effective thermal energy storage time is significantly reduced,and the average thermal energy storage rate is significantly higher than that of single-layer PCM packed bed.Exergy efficiency was much higher than that of single-layer PCM packed bed.In conclusion,the cascaded PBTES device exhibits better thermal performance.