| Phase change heat storage technology is an important way to solve the instability of renewable energy output and improve energy utilization.As one of the potential phase change heat storage technologies,packed-bed latent thermal storage systems have the superiority of high heat storage density,high heat storage and release rate,simple structure,and small equipment volume.In order to improve the thermal storage performance of a packed-bed latent thermal storage system,based on the finite element software COMSOL Physics 5.6,this paper systematically studied the effects of the packing methods of phase change microcapsules in porous and macro scale heat storage tanks on the thermal storage performance of the packedbed latent thermal storage system,and comprehensively evaluated its performance based on indicators such as heat storage,instantaneous energy efficiency,heat storage rate,and thermal efficiency.Firstly,a two-dimensional model with a computing domain of 12.5 mm×15 mm was established at the pore scale to study the heat storage characteristics of the single-layered,diameter-changed two-layered and three-layered packing thermal storage system of phasechange microcapsules.The effects of microcapsule diameter,in-line packing and cross packing on the thermal performance of single-layered heat storage system were analyzed.The results show that reducing capsule diameter or using cross packing can shorten the heat storage time of the system.When the capsule diameter is reduced from 2 to 0.5 mm,the heat storage time can be shortened by 72.97%,but at the same time,the pressure drop of the thermal storage system is significantly increased by 15.5 times.In order to further optimize the structure of the thermal storage system,diameter-changed two-layered and three-layered packing models are constructed respectively.The results show that the heat storage performance of diameterchanged two-layered and three-layered heat storage system is better than that of single-layered thermal storage system.Compared with the single-layered heat storage system with capsule diameter of 0.5 mm,the diameter-changed stratified heat storage system can effectively reduce the pressure drop.When the capsule diameter of the single-layered heat storage system is 2 mm,the heat storage time of the diameter-changed two-layered and three-layered heat storage system is shortened by 16.21% and 24.32%,respectively,and the pressure drop is increased by1.56 times and 6.18 times,respectively.In addition,the diameter-changed stratified heat storage system can effectively shorten the thermocline thickness and has a high energy utilization rate.Secondly,based on the study of pore scale,a two-dimensional axisymmetric model is further established at the macro scale.The study shows that the property of a single-layered packed-bed latent thermal storage system is affected by the diameter of the phase change capsule,the temperature and volume flowrate of the heat transfer fluid(HTF)inlet.The specific conclusions are as follows: reducing the capsule diameter,increasing the HTF inlet temperature,and the inlet flow rate can effectively improve the heat storage rate and shorten the heat storage time.When the capsule diameter is reduced from 60 to 0.5 mm,the heat storage rate of the single-layered heat storage system is raised by 1.9 times,and the heat storage time is shortened by 65%.The exergy efficiency of latent heat storage system in single-layered packed bed increases first and then decreases with the decrease of capsule diameter.When the capsule diameter is 2 mm,the exergy efficiency is the highest,with a value of 79.97%.When the inlet temperature of HTF increased from 343.15 to 383.15 K,the heat storage rate enhanced by 1.01 times and the heat storage time reduced by 21.42%.When the inlet volume flowrate of HTF increases from 2 to 6 L/min,the heat storage rate is increased by 1.2 times,the heat storage time is shortened by 57.14%,but the pressure drop of the heat storage system is increased by7.68 times.Although the decrease of capsule diameter and the increase of HTF inlet volume flowrate improved the heat transfer performance of the single-layered packed bed latent heat storage system,it also significantly increased the pressure drop of the system and the energy consumption of the pump.Therefore,the diameter-changed stratified packed-bed latent heat storage system is proposed to optimize the design.The results indicate that compared with the single-layered packed-bed latent heat storage system,the diameter-changed two-layered and three-layered packed bed can shorten the heat storage time,and the pressure drop increases slightly.In addition,by comparing with the latent heat storage system of single-layered packed bed,the paper also studied the thermal storage performance of three phase change material(PCM)cascaded and multi-layered solid-PCM packed-bed latent heat storage system.The results showed that different cascaded and multi-layered solid-PCM packed-bed latent heat storage system could effectively increase the energy storage and exergy efficiency.Compared with the latent heat storage system of single stage lauric acid packed bed,the heat storage of cascade packed bed increased by 5.07%,exergy efficiency increased by 8.71%.Compared with the rock particle packed-bed heat storage system,exergy efficiency increased by 61.69% and14.85%.In summary,through the systematic study of different structures of latent heat storage systems in packed beds,the variation rules of related parameters of different heat storage are obtained,which can provide a theoretical reference for the optimal design of latent heat storage systems in medium-low temperature packed beds. |
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