Thermal Performance Assessment Of Multi-layer Integrated Receiver-storage System

As a new type of the concentrating solar power(CSP)system,the beam-down CSP system has the characteristics of high photothermal conversion efficiency and low initial investment cost,and has great potential in the field of medium and high temperature heat utilization.The multi-layer integrated receiver-storage(MLIRS)unit applied to the beam-down CSP system is proposed in this paper.Parametric analysis and the overall thermal performance assessment of the MLIRS unit under long prriod are studied.The main research contents and conclusions are as follows:(1)The numerical calculation method of the MLIRS unit is proposed.The thermal performance evaluation parameters of the MLIRS unit are based on the first and second laws of thermodynamics.The expressions of the absorbing efficiency,charging efficiency,discharging efficiency,overall efficiency,solar-to-exergy conversion ratio and average outlet air temperature are summarized.(2)The numerical heat transfer model of the porous ceramic layer is established.Based on the Matlab software platform,parametric analysis on the porous ceramic layer is studied.The results show: The effect of equivalent pore diameter and porosity on the thermal performance of porous ceramic layer is small.When the equivalent pore diameter decreases from 0.27 mm to 0.03 mm,the absorbing efficiency only increases by 0.8%,and when the porosity decreases from 0.8 to 0.4,the absorbing efficiency only increases by 0.2%.When the air mass flow rate decreases from 0.9 kg/s to 0.3 kg/s,the absorbing efficiency decreases from 83.1% to 62.9%.High temperature outlet air under different mass flow rates serves as the inlet boundary condition of the layer of rocks.(3)The numerical heat transfer model of the layer of rocks is established.Based on the Matlab software platform,the transient thermal performance of the layer of rocks under long period is analyzed,and parametric analysis on the layer of rocks is studied.The results show:The layer of rocks reaches a steady state after 30 cycles.When the porosity decreases from 0.8to 0.4,the charging efficiency increases by 2.7%,the dischanrging efficiency decreases by 4.9%,the overall efficiency decreases by 2.3%,the solar-to-exergy conversion ratio decreases by 0.02,and the average outlet air temperature decreases by 17 K.When the particle diameter decreases from 0.09 m to 0.03 m,the charging efficiency increases by 1.9%,the discharging efficiency increases by 1.5%,the overall efficiency increases by 3.2%,the solar-to-exergy conversion ratio increases by 0.02,and the average outlet air temperature increases by 25 K.When the air mass flow is in the range of 0.3 kg/s~0.9 kg/s,the maximum solar-to-exergy conversion ratio of the rock layer reaches 0.49 when the air mass flow is 0.6 kg/s.(4)On the basis of the parametric analysis of the MLIRS unit,the overall thermal performance of the MLIRS unit is analyzed,and the thermal performance is compared with that of the single-layer integrated receiver-storage(SLIRS)unit and other photothermal units using single-tank packed bed for heat storage.The results show: On the basis of parametric research,at the end of 30 cycles,the absorbing efficiency,charging efficiency,discharging efficiency,overall efficiency,solar-to-exergy conversion ratio and average outlet air temperature of the MLIRS unit are 79.1%,99%,95.3%,94.4%,0.49,and 887 K,respectively.Compared with the SLIRS unit,the MLIRS unit reduces the top solid temperature of the packed bed by 392 K by using the porous ceramic and has the higher absorbing efficiency,solar-to-exergy conversion ratio and average outlet air temperature.When the average outlet air temperature is about 822 K,the MLIRS unit has superior thermal performance compared with other photothermal units that use the single-tank packed bed for heat storage.