【作者】 刘旭；

【导师】 方贵银；

【作者基本信息】 南京大学 ， 制冷及低温工程， 2011， 硕士

【摘要】 蓄能技术具有转移电网高峰用电量、平衡电网峰谷差的功能,提高了电网的安全运行性能,充分利用分时电价差节省运行电费。相对于以水为介质的显热蓄能技术,复合材料相变潜热蓄能具有储能密度高与运行温差小的优势,利用热管换热可以实现高效节能。本文提出了相变潜热复合蓄能材料的制备方法,并对复合蓄能材料的特性进行了分析。阐述了热管蓄能系统的结构和工作原理,并对热管蓄冷和蓄热系统进行了模拟和分析,讨论了热管液膜厚度对蓄热过程的影响。1.复合蓄能材料制备及特性采用膨胀石墨吸收不同质量比的液态硬脂酸(1：1、3：1、5：1)制备了硬脂酸／膨胀石墨复合蓄能材料,其中硬脂酸为相变材料,多孔网状膨胀石墨作为基体。热分析结果表明复合蓄能材料与硬脂酸有相同的相变特征,不同掺杂比的复合材料相变潜热与理论计算值差别不大。显微结构分析表明膨胀石墨多孔网状结构中毛细管力和表面张力的作用使得硬脂酸紧密分散在多孔网状结构中,避免了硬脂酸的因过高的温度而产生的渗漏,其中硬脂酸掺杂质量比为83%的样品的凝固温度为54.28℃,凝固潜热为155.70 kJ/kg；融化温度为53.12℃,融化潜热为155.50kJ/kg；且热扩散率为硬脂酸的10倍。采用溶胶凝胶方法制备了不同掺杂浓度(3g、5g、10g)的月桂酸/二氧化硅定形复合蓄能材料,月桂酸为相变材料,多孔网状二氧化硅为基体。扫描电子显微镜(SEM)的结果表明月桂酸已均匀分散在二氧化硅多孔网状结构中,示差扫描热量仪(DSC)测得月桂酸质量比为64.8%的样品的融化潜热为117.21 kJ/kg,热重分析(TGA)表明该复合蓄能材料具有较好的热稳定性,可以在余热回收系统和太阳能蓄能系统中反复利用。2.分离式热管蓄冷系统动态性能研究建立了分离式热管蒸发段充冷过程的数理模型,分析了在不同热管介质入口温度下热管蒸发段管外冰层厚度、热管介质出口温度、热管外蓄冷介质温度、单位时间蓄冷量以及总蓄冷量随时间的变化关系,研究结果表明在热管蒸发段长度和管径一定的情况下,降低热管介质入口温度可以提高热管蒸发段单位时间蓄冷量、减小热管充冷时间。建立了热管蓄冷系统中热管冷凝段传热过程的数理模型,该模型考虑了热管内气液两相之间的剪切力,以R134a作为热管工作介质,计算了液膜厚度、剪切力及热管换热系数。讨论了液膜剪切力对液膜厚度的影响,分析了热管冷凝换热系数随热管管径和热流密度的变化规律以及热管倾角和热流密度对冷凝换热系数的影响,并将该模型所得结果与Nusselt模型的结果进行了比较,研究结果表明该模型所得的液膜厚度要比Nuseelt模型计算的液膜厚度要薄,而其冷凝换热系数要大。建立了分离式热管蓄冷系统有效能分析模型,比较了盘管式冰蓄冷系统和热管式冰蓄冷系统的有效能,计算结果表明：热管式冰蓄冷系统的有效能效率比盘管式冰蓄冷系统提高了9.55%。两种冰蓄冷系统中冷凝过程的有效能损失在整个循环过程中是最高的,减少热交换过程中的热阻和传热温差或者回收利用冷凝器散发到环境中的热能可以降低冷凝过程中的有效能损失；采用涡旋式或螺杆式压缩机可以提高压缩机的等熵压缩效率,从而也就降低了压缩过程中的有效能损失；增加进入节流阀前制冷剂的过冷温度或者降低冷凝压力和蒸发压力的压差,同样可以降低节流过程中的有效能损失；在蒸发过程中,降低热交换过程中的热阻和传热温差同样能降低其有效能损失。3.热管蓄能装置动态蓄热特性研究建立了热管蓄能装置动态特性数理模型,分析了在不同热管介质入口温度和相变蓄热材料起始温度下的热管管外相变蓄热材料厚度和温度、热管介质出口温度、热管蓄能装置总蓄热量和蓄热率随时间的变化关系,研究结果表明在相同的热管长度和半径情况下,提高热管介质入口温度或降低相变蓄热材料起始温度可以使热管蓄能装置的总蓄热量和蓄热率增加。考虑了液膜努赛尔模型和剪切力模型对蓄热过程的影响,分析了不同的液膜厚度下的热管管外相变蓄热材料厚度和温度、热管介质出口温度、热管蓄能装置总蓄热量和蓄热率随时间的变化关系,研究结果表明在相同的热管长度和半径情况下,提高热管入口热流密度或者增加剪切力都可以使热管蓄能装置的总蓄热量和蓄热率增加。更多还原

【Abstract】 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.更多还原