Heat transfer of phase change materials (PCMs) and thermochemical heat storage in porous materials

Abstract

Thermal energy storage plays an important role in energy conservation and reducing CO2 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 high- temperature 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 MgH2↔Mg+H2 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/MgH2 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.