Study on thermal characteristics of composite heat storage units made by impregnation of phase change material into porous materials. For cases of porous ceramics and porous metals. Cooperative research

Temperature changes in heat transfer fluid can be effectively reduced by a heat storage unit of phase change material (PCM) when effective thermal conductivity of the unit is improved by impregnating PCM into porous materials with high thermal conductivity. This technique can be applied to heat utilization system connected to High Temperature Gas Cooled Reactors to improve safety and also it can be applied to many industries such for supplying constant temperature heat source. Composite heat storage units were made by impregnating palmitic acid into porous ceramics of ZrO2 (porosity 45.3%) and porous metals of nickel-chrome (92.6%) and nickel (95.3%). Time-dependent and spatial temperature characteristics of the composite heat storage units were studied by performing temperature varying experiment and numerical simulation. Results show that temperatures in the composite units are more uniform and temperature is kept at melting temperature for longer time during cooling for the composite heat storage units of porous metals than that of porous ceramics. These come from the reasons that effective thermal conductivity is higher for metal composite than for ZrO2 composite and more amount of PCM is impregnated in the metal composite than in ZrO2 composite. Results of numerical simulation agree relatively well with experiments. In order to improve the accuracy, precise effective thermal properties of the composites, precise phase change characteristics during melting and solidification and natural convection analysis for high porosity heat storage unit would be required. It can be concluded that metal composite is more effective than ceramic composite for the temperature reducing heat storage unit. (author)