摘要
探讨了利用多孔材料增加相变和热化学储热功率的可行性,研究了金属泡沫和膨胀石墨添加在储热材料中对储放热过程的影响.低温相变材料采用的是石蜡,高温相变材料采用的是硝酸钠,热化学储热工质对采用的是镁/氢化镁.实验研究了两种相变材料在开孔金属泡沫中的储放热过程.对膨胀石墨质量分数分别为3%,6%和9%的石蜡/膨胀石墨复合相变材料也进行了研究,并与采用金属泡沫的复合相变材料进行了对比.结果表明,金属泡沫具有更好的传热能力,主要因其具有膨胀石墨所不具备的内部连通结构.但是,金属泡沫会抑制液态区的自然对流,特别是对于低黏性的相变材料,因此导致了在固态区、固液共存区和液态区具有不同的传热特性,表明金属泡沫并不是在每个区域都能增强换热.数值研究了热化学储热材料在加入金属泡沫前后的放热过程.在热化学储热中加入金属泡沫,可使反应床平均温度下降,明显提高放热功率.同时,由于放热反应存在最佳反应温度,在一定氢气压力和壁温条件下,通过加入不同孔隙率的金属泡沫对放热功率进行研究,表明反应床加入的金属泡沫存在最佳孔隙率,使得床层整体温度更接近最佳反应温度,实现放热功率最大化.
Thermal energy storage plays an important role in energy conservation and reducing CO_2 emissions. Thermal energy storage involves sensible heat storage, latent heat storage and thermochemical heat storage. Compared with sensible heat storage, latent heat storage and thermochemical heat storage benefits of their high energy storage densities, which helps to reduce the initial cost of the construction of heat storage systems. However, the thermal conductivities of the phase change and thermochemical reaction materials are usually lower than 1 W m^（-1）K^（-1）, which impedes the development and further applications of the corresponding energy storage systems. Porous materials, e.g. metal foams and expanded graphite, combining with other materials to form composites is an effective method for heat transfer enhancement. In this paper, the feasibility of using metal foams to enhance the heat transfer characteristics of heat storage materials in thermal energy storage systems was assessed. Heat transfer in solid/liquid phase change and thermochemical reaction of porous materials（metal foams and expanded graphite） was investigated. Organic commercial paraffin wax and inorganic calcium chloride hydrate were employed as the low-temperature materials, whereas sodium nitrate was used as the hightemperature materials in the experiment. Heat transfer characteristics of these PCMs embedded with open-cell metal foams and expanded graphite were studied. Composites of paraffin and expanded graphite with a graphite mass ratio of 3%, 6%, and 9% were prepared. The heat transfer performances of these composites were tested and compared with the results using metal foams. It is shown that heat transfer can be enhanced by adding these porous materials. Metal foams have better heat transfer performance due to their continuous inter-connected structures than expanded graphite. However, porous materials can suppress the effects of natural convection in liquid zone, particularly for PCMs with low viscosities, thereby leading to different heat transfer performances at different regimes（solid, solid/liquid, and liquid regions）. This implies that porous materials do not always enhance heat transfer in every regime; thereby an optimal metal foam structure or expanded graphite fraction can be developed using PCMs for the overal thermal energy storage performance. For thermochemical heat storage systems, the feasibility of using metal foams to enhance the heat transfer capability of heat storage materials was assessed. Reversible reaction MgH_2→Mg＋H_2 was used as thermochemical heat storage reaction. The effective thermal conductivities of metal foams with various porosities（0.88–0.98） were estimated with Boomsma Poulikakos model. A two dimensional mathematical model for the Mg/MgH_2 system was estabilished to study the transient heat and mass transfer process. Heat release characteristics of chemical reaction in fixed beds with/without metal foams were compared to illustrate the effects of metal foams. Various factors influencing the reaction time for fixed reaction beds with metal foams were analyzed. The results show that metal foams shorten the reaction time and increase the output power by decreasing the average temperatures of the fixed beds. After adding metal foams with a porosity of 0.92, a 40% reduction of the reaction time and 60% promotion of the exothermic power can be achieved. The parametric study shows that there exists an optimal porosity of metal foams for the highest output power under a certain reaction condition. The cooling fluid temperature and hydrogen pressure are confirmed to have a more significant impact on the reaction rate when metal foams are embeded in fixed beds. In general, as heat transfer is coupled to phase change and chemical reaction processes in latent heat storage and thermochemical heat storage, the effects of porous materials on these heat storage systems are complex. The porous materials need to be carefully selected in order to optimizing the performance of heat storage systems.
出处
《科学通报》
EI
CAS
CSCD
北大核心
2016年第17期1897-1911,共15页
Chinese Science Bulletin
基金
国家重点基础研究发展计划(2013CB228303)资助
关键词
传热
储热
相变材料
热化学储热
自然对流
多孔介质
heat transfer
thermal energy storage
phase change materials
thermochemcial heat storage
natural convection
porous media
作者简介
联系人，E-mail：changyingzhao@sjtu．edu．cn
