Effects of cellular metals on the performances and durability of composite heat storage systems

Energy storage systems based on PCMs (phase change materials) are characterized by the relatively low heat transfer parameters, which reduce their effectiveness and performance. In this study in order to overcome the low thermal conductivity of PCMs and to design a stable and enduring energy storage system the metal alloy foam was impregnated with PCMs and subjected to several charging/discharging cycles. ZnAl foam with open porosity was produced by investment casting involving modification of polyurethane foam (10 PPI), mold burning and pouring of liquid alloy under vacuum. Porous structure was impregnated with paraffin or potassium nitrate KNO3 and an experimental test stand was built to study the heat transfer characteristics of composite PCMs. Further, they were subjected to several charging/discharging cycles, which allowed to evaluate the stability of the system and the foam endurance. Large thermal mismatch in phase changes of paraffin to metal foam produces high fatigue stresses, which can damage thin cellular structure. SEM observation of cell struts after dozens of cycles confirmed the nucleation of cracks. By using differential scanning calorimetry (DSC) KNO3 was characterized and identified and employed because of its low temperature phase transformation. Prepared composite was charged without melting of KNO3 to avoid stress generation in metal foam during solidification. Performed heating/cooling cycles confirmed the high transfer of heat and good stability of the system. (C) 2017 Elsevier Ltd. All rights reserved.