Preparation Of Thermal Energy Storage Composite Materials And Study On Dynamic Performances Of Thermal Storage Systems With Heat Pipe

Thermal energy storage technology, which plays a significant role in conserving available energy, improving its utilization and correcting the mismatch that occurs between the supply and demand of energy, can improve the safety of power plant and decrease the electricity expenses. Comparing to the sensible heat thermal energy storage system using water as thermal storage medium, the latent heat thermal energy storage system using phase change material has the advantage of high thermal energy storage density and small temperature change range. In this paper, preparation of thermal energy storage composite materials was presented, and characteristics of thermal energy storage composite materials were analyzed. The structure and working principle of thermal storage systems with heat pipe were described. The dynamic performances of thermal storage systems with heat pipe were simulated and analyzed, and the effect of the thickness of film on heat storage process was also discussed.1. Preparation and characterization of thermal energy storage composite materialsStearic acid/expanded graphite composites with different mass ratios (1:1,3:1,5:1) were prepared by absorbing liquid stearic acid into the expanded graphite. In the composite materials, the stearic acid was used as the phase change material for thermal energy storage, and the expanded graphite acted as the supporting material. The thermal analysis results indicated that the materials exhibited the same phase transition characteristics as the stearic acid and their latent heats were approximately the same as the values calculated based on the weight fraction of the stearic acid in the composites. The microstructural analysis results showed that the stearic acid was well absorbed in the porous network of the expanded graphite, and there was no leakage of the stearic acid from the composites even when it was in the molten state. The stearic acid/expanded graphite composites composites solidify at temperature of 54.28℃with latent heat of 155.70 kJ/kg, and melt at temperature of 53.12℃with latent heat of 155.50 kJ/kg when the mass percentage of the stearic acid in the composites is up to 83%. The thermal diffusivity of the composites is 10 times higher than that of the stearic acid.Form-stable lauric acid (LA)/silicon dioxide (SiO2) composite phase change materials with different mass ratios (3g,5g, 10g) were prepared using sol-gel methods. The LA was used as the phase change material for thermal energy storage, with the SiO2 acting as the supporting material. The SEM results showed that the LA was well dispersed in the porous network of SiO2. The DSC results indicated that the melting latent heat of the form-stable composite phase change material is 117.21 kJ/ kg when the mass percentage of the LA in the SiO2 is 64.8%. The results of the TGA showed that these materials have good thermal stability. The form-stable composite phase change materials can be used for thermal energy storage in waste heat recovery and solar heating systems.2. Study on dynamic performances of cool storage system with separated heat pipeThe mathematical and physical model of evaporation unit of separated heat pipe during charging process was built. The thickness of ice layer outside the evaporation unit of separated heat pipe, outlet temperature of heat pipe medium, the temperature of cool storage material outside heat pipe, the cool storage capacity per unit time and the total cool storage capacity of the evaporation unit of separated heat pipe with three different inlet temperature of heat pipe medium during the charging process were analyzed. The result indicated that the cool storage capacity per unit time of the evaporation unit of separated heat pipe can be increased, and the charging time of heat pipe can be decreased when the inlet temperature of heat pipe medium decreased at the same length and diameter of the evaporation unit of separated heat pipe.The condensation heat transfer characteristics model of condensation part of heat pipe in cool storage air-conditioning system was presented. In this model, the shear stress between gas and liquid two phase flow was considered. R134a was taken as working medium of heat pipe. The liquid film thickness, shear stress and heat transfer coefficient of heat pipe were calculated. The effect of shear stress on the film thickness was discussed. The variation of heat transfer coefficient with heat pipe diameter and heat flux was analyzed. Influences of heat pipe incline angle and heat flux on heat transfer coefficient were also discussed. The results indicated that the film thickness based on this model is thinner than that based on Nusselt model, and condensation heat transfer coefficient is larger than that based on Nusselt model.The analytical model relating to the exergy loss amount and loss rate was developed in each component of the ice storage air-conditioning system. The exergy analysis results of the ice storage air-conditioning system with heat pipe were compared with those of the ice storage air-conditioning system with ice-on-coil. The results showed that the exergy efficiency of the ice storage air-conditioning system with heat pipe is 9.55% higher than that of the ice storage airconditioning system with ice-on-coil. The exergy loss in the condenser is the largest in all components.The exergy loss in the condenser may be reduced by reducing thermal resistance and the temperature difference of heat transfer or utilizing exergy of heat rejected by the condenser. The compressor with the higher isentropic compression efficiency may reduce exergy loss, so scroll or screw compressors are usually chosen in the ice storage airconditioning system. The exergy loss in the expansion valve is reduced through increasing supercooling temperature of the refrigerant before entering into the expansion valve or decreasing pressure difference between the condensing pressure and the evaporating pressure. The exergy loss in the evaporator is also decreased through reducing thermal resistance and the temperature difference of heat transfer. 3. Study on dynamic charging characteristics of heat storage device with heat pipeThe dynamic charging characteristics model of heat storage device with heat pipe was presented. It was analyzed that influences of inlet temperature of heat pipe medium and initial temperature of heat storage material on the thickness and temperature of heat storage material, outlet temperature of heat pipe medium, total heat storage capacity and heat storage rate of heat storage device. The results indicated that total heat storage capacity and heat storage rate of heat storage device increased when the inlet temperature of heat pipe medium increased or the initial temperature of heat storage material decreased.It was analyzed that influences of film for Nusselt model and shear stress model on the thickness and temperature of heat storage material, outlet temperature of heat pipe medium, total heat storage capacity and heat storage rate of heat storage device. The results indicated that total heat storage capacity and heat storage rate of heat storage device increased when the inlet heat flux of heat pipe medium increased or the shear stress of heat pipe medium increased.