Modeling and simulation of creep in a thermal energy storage unit

Abstract

Azelio is developing a solution for storing renewable energy in the form of a Thermal Energy Storage (TES) unit. The TES unit, which holds an aluminium alloy, is made out of the heat-resistant austenitic steel alloy 253MA. Due to the high temperatures and the weight of the aluminium, it is of interest to evaluate the creep strains that develop in the TES unit over the 25 year service life. A creep model is constructed using primary creep data from 316LN steel and three steady-state deformation mechanisms that describe the strain-generating movement of defects throughout the crystal lattice. Some model parameter values are taken from austenitic steel alloys, while some parameter values are obtained from fitting to 253MA secondary creep rate data. A limited validation of the creep model is performed, and the model is implemented in ANSYS through a custom creep subroutine. Creep is simulated in the TES unit over 25 years at the operational temperature of ~600 C. The analysis results show substantial stress relaxations, and that the largest creep strains develop in the suspension blade welds. The limited validation of the creep model against 1% creep strength data gave strain values in the vicinity of the ideal values. However, some model parameter values from other alloys are used, whose values may not apply to 253MA. The deformation mechanism models are simplistic, and may not accurately reflect reality. In order to gain a better understanding of creep in 253MA, and to properly calibrate the creep model, more experimental data on 253MA is required.