Experimental Study On Charging And Discharging Performance Of Multi-layer Packed Bed Phase Change Heat Storage Tank Under Solar Heat Source

As a clean and renewable energy,solar energy has broad prospects for development.In the process of solar thermal utilization,its volatility will cause the problem of mismatch between supply and demand.In order to solve this problem,a more effective way is to equip the system with a heat storage device to stabilize the fluctuation of solar energy and achieve stable output of energy.Multi-layer packed bed phase change heat storage system combines the thermocline single-tank heat storage with the phase change heat storage,which has the advantages of high heat storage density and temperature control capability.According to the phase change temperature cascading accumulation,the system makes full use of the latent heat of phase change and has good prospects.At present,research on its operational characteristics is mainly conducted through numerical simulation,but research on its performance under solar heat sources is very limited.Moreover,numerical simulation simulating solar radiation changes by changing the temperature of the charging inlet cannot correspond to the actual situation.It is necessary to study this issue through experiments.In this paper,a multi-layer packed bed heat storage experimental system is built,which uses solar collectors as the heat source of heat storage,uses paraffin phase change capsules with three different phase change temperatures,specific heat and latent heat as the main heat storage medium,and uses water as the secondary heat storage medium and heat transfer fluid.Based on previous experimental data and numerical simulation results in this paper,the dimensions of the paraffin phase change capsules were designed.The temperature measurement points of the original solar thermocline phase change heat storage system were rearranged,and some measurement points were set inside the paraffin phase change capsules to measure the temperature of the paraffin during the experimental process.The operation characteristics of single charging process and single discharging process in doublelayer and three-layer packed bed were studied and compared through experiments.The experimental results show that the short and violent fluctuation of solar radiation during the charging process has little effect on the heat storage power.The multi-layer phase change packed bed tank can effectively suppress the fluctuation of solar energy.The temperature response time of paraffin capsules is 7 min～22 min slower than that of water.The smaller the initial temperature difference between water and paraffin and the lower the charging inlet temperature,the longer the temperature response time of paraffin.The charging efficiency increases first and then decreases with the increase of charging flow rate.With the decrease of initial temperature,the double-layer packed bed tank shows an upward trend,while the three-layer packed bed tank is the opposite.In the discharging process,the smaller the diameter of the phase change capsules,the longer the time for the discharging outlet temperature to meet the heating demand temperature(67℃),and the phase change temperature of the phase change material determines the minimum temperature at which the discharging outlet temperature can be maintained before the time of rapid decline.The discharging efficiency of the double-layer packed bed tank changes little with the increase of the discharging flow,and the three-layer packed bed tank shows a significant downward trend.With the increase of the initial temperature,the discharging efficiency of both double-layer and three-layer packed bed tank increases significantly.The maximum charging/discharging efficiency of the three-layer packed bed tank is higher than that of the double-layer packed bed tank in both the charging process and the discharging process.