Energy–exergy analysis of sinusoidal-channel thermal energy storage system for high-temperature concentrated-solar applications using air as heat transfer fluid

Abstract

In this study, an energy–exergy analysis was performed on a thermal energy storage system (TESS) that uses air as the heat transfer fluid, flowing through a solid with a sinusoidal channels, a geometry optimized for heat transfer. Simplified energy conservation equations were proposed for the air and the solid, allowing different configurations of the TESS to be examined. The aerodynamic parameters were obtained by simulations on a full CFD platform to ensure reliable results. The model discretizes the domain into finite volumes and solves the energy equations, using the upwind scheme for the advective term and an implicit scheme in time. Temperature and mass flow were used to estimate exergy lost during the processes. Simulations of the charging, discharging and thermal-redistribution cycle were carried out for TESS under operational conditions based on real CSP plant. 3, 6 and 9-meters long TESS, with different mass flow rates were tested. The simulations revealed how geometric configuration and mass flow through channels affect destroyed and unused exergy from the absorbed solar energy. While 3-m long TESS was unable to sustain air temperature necessary to power the thermodynamic system, regardless of mass flow rate, the 9-m long with 0.035 m diameter channels and flow 0.2kg/m²s, presented best results with a total exergy loss of just over 5% of the storage capacity. However, a 6-m TESS showed a performance similar to 9-m and could be an option if compactness is valuable for the installation. Additionally, blower power is significantly influenced by the channel geometry and should therefore be carefully considered during the design of sinusoidal-channel thermal energy storage systems. Results of this study demonstrated that sinusoidal-channel porous media, could be an interesting option for high-temperature sensible-heat small storage system due to its enhanced heat transfer coefficient, low pressure loss and easy modular assembly, providing it with flexibility and cost effectiveness.