Study of the degradation mechanisms of a thermal energy storage system based on molten metal

Abstract

Energy storage systems are an essential part of the energy transition towards more sustainable sources. In comparison to electric batteries, thermal energy storage can be a more competitive option depending on the characteristics of the system (applications involving heating, solar thermal energy, heat waste recovery, etc.). The quest of storing the biggest amount of energy possible goes through storage at higher temperatures. Metallic Phase Change Materials (mPCMs) can present high energy densities and high thermal conductivity which make them suitable for their use in high temperature energy storage systems, especially when the volume available for the storage is limited, or fast charge and discharge are required. Many other criteria need to be considered when designing a high temperature thermal storage system beside the thermophysical properties of the storage material. Compatibility between the mPCM and the container must be ensured throughout the lifetime of the components. This also implies that the storage must be able to withstand a high number of charge/discharge cycles without compromising its performance. In the present work, a methodology to estimate the lifetime of the components of a thermal storage system is developed. For that purpose, systems based in Al-12.5wt.%Si used as storage material and several iron-based container materials have been tested. The reaction rates between the mPCM and its housing were calculated by keeping the alloy molten for different durations inside a crucible of the container material. Post-mortem analyses were used to determine the extent of the reaction taking place. In parallel, the reaction rates when the system is submitted to numerous melting/solidification cycles were also studied, allowing to estimate the influence of cycling in the degradation mechanisms. This influence was also assessed by measuring the variation in the storage capacity of the mPCM using Differential Scanning Calorimetry. The combination of these experimental results served to establish a methodological first approach for the prediction of component lifetime in latent-heat based thermalenergy storages.