| Thermal energy storage (TES) is an important technology to improve the energyefficiency and protect the environment. In recent years, it has become a hot research topic inthe field of energy and materials. At present, the TES technology research mainly focuses onlow temperature applications, such as building energy saving, electronic device cooling andcivil products, etc. But the TES technology with medium to high temperature can be widelyused in solar thermal utilization and industrial waste heat recovery, it will become a researchemphasis in the future.In this paper, the mannitol was selected as medium temperature phase change material(PCM), the structure and thermal physical property of mannitol were characterized usingthermogravimetric analyzer, X-ray diffraction, differential scanning calorimetry and thermalconductivity analyzer. The results show that mannitol has a β-crystal structure and goodthermal stability within250°C. Solid phase thermal conductivity is0.6W·m-1·K-1. The solid-liquid and liquid-solid phase transition temperatures are164.6°C and127.87°C, respectively.The corresponding phase change latent heat is322.8J/g and266.8J/g. Mannitol has obvioussupercooling phenomenon.A pilot plant test system was set up to conduct an experiment research on the mannitolthermal energy storage performance. Thermal energy storage heat exchanger is made up oftwo identical structure of spiral coil heat exchangers, one is used for thermal energy charging,and another for thermal energy discharging. The high temperature heat conduction oil andcooling water are chose as thermal energy charging and discharging heat transfer fluids(HTF), respectively. The total filling quantity of mannitol is14Kg in the thermal storagesystem, and the volume of water storage tank is100L that is one of standard volume for thesolar water heaters. an initial temperature of30°C cooling water will be heated to55°C warmwater that can be used in the family life through the heat storage technology. In thisexperiment, for thermal energy charging process, the volumetric flow rate of heat conductionoil is330L/h,360L/h and390l/h, and the inlet temperature is200°C、210°C and220°C.For thermal energy discharging process, the volumetric flow rate of water is60L/h.Thermal energy charging process is divided into three stages, the first stage is thetemperature rise process of the solid mannitol with the sensible heat storage phase. Heattransfer is controlled by heat conduction, mannitol has a uniform temperature rise. In thesecond stage, heat transfer is controlled by natural convection, the density difference in solidliquid two phases creates buoyancy effect leading to the complicated solid-liquid interface motion and a temperature fluctuation. Solid-liquid phase change platform is not obvious.The third stage is that mannitol is completely melted to liquid phase, the temperaturecontinues to rise to close the temperature of the HTF. Similar three stages can be found inthermal energy discharging process, due to the liquid-solid phase change process is controlledby heat conduction, the temperature is stable, a significant platform for the liquid-solid phasechange can be seen. Heat conversion efficiency of mannitol thermal storage is higher than87%. Both increase the volumetric flow rate and inlet temperature of heat conduction oil,mannitol temperature can be improved in the thermal energy charging process. The heatenergy charging time is obviously reduced with increasing the inlet temperature.Thermal energy storage performance was simulated using commercial numericalsimulation software FLUENT. The simulated temperature field is consistent with theexperimental results. The dynamic characteristics of mannitol melting and solidificationprocess were obtained through the liquid phase rate change. The average deviation is5.7%and6.7%for the simulated temperature with experimental data for the thermal energycharging and discharging process, respectively. Simulation results are in good agreement withmeasured values. |
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