The Flow And Transport Mechanism For High Temperature Phase-change Thermal Storage Processes

Solar energy is the most abundant, clean and widely available renewable energy, andhigher efficient use of solar energy will change existing China's energy shortage andunreasonable energy resources structure. The heat flux of solar energy has the characteristicsof intermittent and instability, the sunlight hours just less than half of a day and the cloudcovering which increase the difficulty of large-scale industrial application of solar energy.Therefore, the solar radiation energy needs to be stored in order to make the system runing inlonger time. However, the mechanism of thermal storage process and devices design is stilllack of investigation in the utilization of solar energy. This paper established the thermalstorage method and system using spherical packed bed to restore sensible and latent heat ofmolten salts (phase change material) in higher-temperature. It can reduce the thermal storagecosts, improve the efficiency and thermal storage capacity, and it can be used not only forthermal storage in solar thermal power generation, but also for the high temperature thermalstorage in the industrial production process.The flow and heat transfer phenomenon was studied by theoretical and experimentalmethod during thermal charging process for the high-temperature molten salt spherical packedbed storage system. The transient two-phase non-equilibrium heat transfer model of thepacked-bed sensible storage system is established. The model of temperature distribution,thermal storage capacity, entropy generation, and efficiency were analysed and tested. Theinternal mechanism and reason of different change law led by different parameters was foundout and the main physical parameters of different filler materials that affect the thermalstorage performance were also determined. This paper set up the evaluation criteria accordingto thermal storage capacity, entropy generation, efficiencyη1based on outlet temperature,and efficiencyη2based on thermocline thickness to describe the charging efficiency andperformance of the thermal storage system. The results show that when the filler material haslarger volumetric heat capacity, the storage system has higher storage capacity and chargingefficiency, but the time for finishing the charging process is longer and the entropy generationis also larger; when the filler material has lower thermal conductivity, the charging efficiencyis higher and the entropy generation of the system is smaller; the porosity has little effect onthe thermal storage system, when the porosity is lower, the storage capacity is higher, but thecharging efficiency is lower and the entropy generation of the system is larger; when thediameter of filler material is smaller, the charging efficiency is higher and the entropy generation of the system is smaller. There is a certain temperature difference between the hightemperature molten salt and the porous filler material. The maximal value of temperaturedifference is bigger at the initial charging time, with the increase of charging time, themaximal value of temperature difference decreases gradually,but the axial length oftemperature difference is increased. The temperature difference increases with the increase ofmolten salt flow rate and fill material particle diameter.The heat transfer model of high-temperature molten salt spherical packed bed latentstorage system was established using the heat transfer oil and sodium nitrite as the heattransfer fluid and the phase change storage material separately. The heat transfercharacteristics during the phase change process were analyzed using the numerical simulationmethod. The temperature distributions of molten salts (phase change material) in packed-bedchanges along the axial height in different charging time, the temperature change of phasechange material over time at different positions, the temperature changes of phase changematerial and heat transfer fluid at different charging time and the change of thermal storagecapacity over time were obtained. The specific heat, inlet temperature and inlet velocity ofheat transfer fluid, the particle diameter of phase change ball and latent heat value of phasechange material which affect the performance of latent heat storage system were analyzed todetermine the optimum operating parameters of the system.The experimental study about the high-temperature molten salt spherical packed bedsensible and latent storage system in specific conditions was carried out and the overallperformance of the thermal storage system was tested. The heat transfer laws of thetemperature distribution, charging efficiency, thermal storage capacity and phase changeprocess were studied during the charging process. The results show that the trend of theexperimental data and numerical calculation values are basically consistent, and the error issmaller. After verifying the accuracy of the mathematical model, the results can providetheoretical and experimental basis for the design of thermal storage system.