Thermal Process Characteristics On High-temperature Thermocline Hybrid Thermal Energy Storage With Molten Salt Materials

The intermittent and unsteady characteristic of solar energy cannot meet the requirements of the large-scale industrialized continuous energy supply, so the low-cost heat transfer and thermal storage media and the high efficient thermal energy storage (TES) technologies should be developed to solve the solar energy conversion, storage and transportation problems. To study the preparation theory and transmission mechanism of the high-performance heat transfer and thermal storage media not only concerns the heat absorption, heat storage and heat transfer efficiency, but also directly influences the reliability and stability of the system. A new type of high-temperature (>200℃) thermocline hybrid thermal storage with molten salt materials has been put forward. Comparing with other TES system, the thermocline hybrid TES system will not only increase the thermal storage capacity, but also simplify the molten-salt liquid charge and discharge structures. Moreover, the hybrid thermocline TES system can be used in the production processes (such as the chemical industry, metallurgy, thermodynamic engineer and nuclear industry) and other areas of renewable energy utilization, and the research results have better theoretical significance and practical applications.The high-temperature thermocline hybrid thermal storage with molten salts process is a typical heat and mass transfer process with multi-phase and multi- field driven structure. The theoretical analysis, numerical simulation and experimental investigation on the heat storage and heat transfer characteristics of the high-temperature thermocline hybrid TES system have been presented in this thesis. Based on the local thermal equilibrium (LTE) assumption, the heat transfer and storage simulation model of the single-phase flow through porous media in the molten salt thermocline system was first provided, and the influences of the thermal-physical parameters of the molten salt and porous media, the pore structure of the porous media and the operating conditions on the TES performance were analyzed. The simulation results showed that 1) comparing with the single-phase flow molten salt thermocline system (with non-filled porous media), to fill appropriate porous media would decrease the thickness of thermocline layer and improve its shape; 2) the inlet velocity, operationing temperature difference, length & diameter ratio, heat capacity per unit volume of porous media ( )ρcs and porosityφwere main factors to affect the thickness of thermocline layer, namely, the effective thermal storage capacity; 3) it was more appropriate to control the inlet velocity at the level of 0.001m/s, the length & diameter ratio to be 2:1, the ( )ρcs to be larger than the ( )ρcp fand the porosity value to be under 0.4; and 4) The mean Nusselt number Nu increased with the increase of heat capacity per unit volume of porous media ( )ρcs, inlet velocity, operational temperature difference, initial temperature, but decreased with the increase of operating time, ambient temperature, porosityφ, equivalent particle diameter of porous media ( d p). Then a concentric circular tubes model for the heat exchanger of shell and tubes with PCM (phase change material) was presented, and the influences of the tube inlet velocity and the melting point of PCM on the liquid fraction and the total melting time were analyzed under the natural convection conditions. The results indicated that 1) the total melting time decreased, and the decreasing rate was obvious when considering the natural convection; and 2) the mushy region was no longer parallel to the axis to move, the top of the PCM heat exchanger melted earlier than the bottom, and the mushy region developed in the shape of the cone.The testing platform for the heat transfer and storage with molten salt materials was set up. The experimental investigation on the heat transfer and thermal storage characteristics of single-phase flow molten salt thermocline system (with non-filled porous media), the single-phase flow through porous media in the molten salt thermocline system and the PCM heat exchanger of shell and tubes were respectively carried out under the given conditions, and the axial temperature distribution for the single-tank and the temperature of the shell side in the PCM heat exchanger were measured. The results comfirmed that 1) the thickness of thermocline layer increased with the passage of time at the initial stage and then the increasing rate dropped gradually; 2) adopting the porous media whose heat capacity per unit volume is larger than that of the molten salt is favorable to the TES, and 3) it was reasonable that the spent time when the temperature of the specific measuring point in the PCM region exceeded the melting point of PCM and rose close to the tube inlet temperature was used as the evidence to judge the melt ending.