Thermal Contact Resistance: Implications for Thermal Energy Storage Systems

Abstract

In thermal energy storage systems a major characteristic for charge and discharge rates is the overall thermal resistance between heat sink and heat source. Precise knowledge about thermal resistance is necessary for designing and simulating storage components. However, the thermal resistance is often simplified to bulk resistance of the materials whereas the thermal contact resistance at their interface is neglected. In this study we determine thermal contact resistances relevant to thermal energy storage systems and operating conditions. It is shown that the thermal contact resistance can have a clear effect on overall heat in- and output depending on the design of the storage component. Thermal contact resistances between storage material and container material were determined via a modified stationary cylinder method. The examined storage material was the metallic Phase Change Material Al-12wt%.Si and the examined container material was the Ceramic Matrix Composite C/C-SiC. The thermal contact resistance dependence on temperature (25 - 300 °C) and pressure (0 - 4 MPa) was found. The thermal contact resistance between storage and container material was found to decrease asymptotically with increasing temperature and increasing pressure. For pressures over 3 MPa the contact resistance was reduced to under 1 cm²K/W. It is concluded that thermal contact resistance can have a major effect on charge and discharge rates, especially for storage designs where several layers of materials lie between heat source and heat sink. Storage designs that take advantage of pressure between storage material and heat transport components due to solidification or thermal expansion/contraction could improve functionality. Thus, thermal contact resistance is recommended to be considered in the design of thermal energy storage systems.