Two scale finite element thermomechanical assessment of solid media thermal energy storage systems operating conditions

Abstract

One of the main challenges in concentrated solar power (CSP) plants is the storage of thermal energy during daylight hours for its subsequent release during the night time. In this sense, thermal energy storage (TES) systems have gained much attention from the renewable energy sector due to its tremendous potential in lowering the levelized cost of energy (LCOE) and, thus, becoming a real economic alternative to conventional generation. As a matter of fact, many different options for storing energy have arisen, and these can be classified, depending on the nature of the heat storage, into sensible, latent, and thermochemical systems [1]. Storage of thermal energy in the form of sensible heat is a cheap and simple alternative to complex latent heat or thermochemical storage systems. Moreover, the storage capacity of solid media TES devices largely depends on the material density, specific heat, and conductivity. However, when subjected to thermal cycling, these thermophysical properties may suffer degradation, threatening the overall thermomechanical performance of TES systems. In this work, the author presents a two-scale thermomechanical approach for the analysis of solid media TES systems under operating conditions, focusing on the durability of the material level (i.e., the mesoscale) and the long-term performance of at the component level (i.e., the macroscale). When subjected to high temperature, materials such as concrete, rock, or ceramics may experience mechanical damage as the presence of cracks becomes more relevant, promoting a loss of conductivity in the medium. However, although mechanical damage at the mesoscale has been analyzed by other authors [2], there is little information on how thermal properties are degraded by the presence of such damage at both the meso- and macroscale. Therefore, this analysis is carried out herein by means of a coupled thermomechanical continuum damage-based finite element model at the material scale, providing the evolution of the mechanical (e.g., stiffness) and thermal (e.g., conductivity) properties in terms of the temperature history at the component level. Finally, the presented framework can be used to compare different solid media TES system alternatives based on their material properties, and estimate their thermomechanical performance under operating conditions.