Performance Analysis And Optimization Design Of A Single-tank Molten-Salt Thermal Energy Storage System In Packed-bed

The application of energy storage technologies can effectively reduce the conflict between the intermittency and fluctuation of the renewable energy and energy consumption demand of human society.Among various energy storage technologies,the medium-high temperature molten-salt single-tank thermal energy storage in packedbed has become a research hotspot in recent years,because of its wide range of applying temperature and cost-effectiveness.However,we still lack of comprehensive and profound understanding on the influences of design and operating parameters on the performance characteristics of the packed-bed heat storage at the present stage.Besides,there still exist rooms for improvements on the performance optimization and dimensional design of the storage system.Moreover,both of the application scenario and integration performance of the system in clean energy need further explorations.Based on the above,the present work carries out the following investigations.We developed and verified a one-dimensional transient thermodynamics model for the single-tank packed-bed thermal energy storage system with arbitrary packed-bed configurations based on dispersed-concentric model,enthalpy method and effective thermal conductivity method,and developed a relevant computational procedure.Using the model,we investigated and compared the charge and discharge characteristics of the practical-scale molten-salt packed-bed storage systems with different packed-bed configurations(sensible heat,single-layer latent heat,two-layer latent heat and three-layer hybrid heat)under different operating conditions.Besides,we studied the influences of key design parameters on the thermal performance of the systems.The results indicated that the introduction of latent heat packed-bed layers can improve the effective heat storage capacity and capacity factor of the packed-bed heat storage.Besides,partial charges can affect the effective heat storage capacity of the packed-bed heat storage.Furthermore,the increase in the capacity factor of the packedbed storage is always accompanied with a lower charging(discharging)power rate,a larger cut-off temperature margin,a larger packed-bed height,a larger packed-bed radius and a smaller filler size.Based on the above results,we analyzed the effect of packed-bed configuration on the thermal performance and unit capital cost of the single-tank molten-salt packed-bed storage system,and calculated the cost-optimized packed-bed configuration.The results show that cost reductions of 36.8% ~ 39.2 can be achieved for packed-bed storage systems under cost-optimized packed-bed configurations(with a filling proportion of the latent heat layers ranging from 14.1% to 14.7%,and a volume ratio of the high-temperature latent heat layer ranging from 61% to 67%),comparing with two-tank storage systems with equivalent scales.In addition,we proposed a fast and accurate size design strategy for the packed-bed storage system under arbitrary operating conditions,and evaluated the thermal performance of the well-sized packedbed storage systems under an actual continuous operating condition.The results indicate that the strategy can save 25% ~ 71% of computational costs by adopting a novel iterative initial condition.Moreover,when determining the packed-bed size of the sensible heat and single-layer latent heat packed-bed storage system,a certain design margin should be taken into consideration to offset the reduction of effective storage capacity caused by the fluctuation of heat storage under actual continuous operating conditions.In addition,we also presented a conceptual design of a nuclear-solar hybrid energy power system integrating with a hybrid heat packed-bed thermal energy storage.The operational performance of the hybrid system was compared with a concentrating solar power plant with an equivalent installed capacity.Effects of key design parameters on the performance of the hybrid system were evaluated through a parametric study,based on which the design parameters corresponding to the optimum composite performance were provided.The results show that the hybrid system can achieve a much higher heat generation utilization(85.3% vs.76.0%)and power supply efficiency(98.4% vs.61.5%)and than the concentrating solar power system.Besides,it is found that the increase of nuclear power capacity proportion can increase the power supply efficiency while decrease the effect of storage scale on the heat generation utilization of the hybrid system.The increase of solar multiple can lead to a decrease in heat generation utilization while an increase in power supply efficiency of the hybrid system.The increase of thermal energy storage scale contributes to increasing both heat generation utilization and power supply efficiency of the hybrid system.The hybrid system can achieve a 100% heat generation utilization and 100% power supply efficiency under the optimum design parameters(with a nuclear power capacity proportion of 50%,a solar multiple of 1.27 and a theoretical storage duration of 14.8 h).In summary,this paper enhances the understanding of the performance characteristics of the single-tank molten-salt thermal energy storage system in packedbed,and provides theoretical guidelines for its performance and cost optimization,and also offers an efficient solution for its size design,and finally presents a novel ideal for the development of its application scenario in clean energy.