CFD and Experimental Validation of a Gas-Based Thermocline TES System for CSP Applications with Nanofluid Modeling Perspective

Abstract This paper presents an integrated experimental and numerical study of a thermocline-based packed-bed thermal energy storage (TES) system employing nitrogen as the heat transfer fluid (HTF), designed for use in Concentrated Solar Power (CSP) applications. A full-scale laboratory setup was developed at the University of Cagliari to investigate charging behavior under various nitrogen flow rates. Axial temperature evolution was measured using an array of 16 thermocouples and compared against predictions from a three-dimensional CFD model developed in ANSYS Fluent. The CFD model incorporates a porous media approach with Ergun-based resistance parameters and was validated against experimental temperature profiles and pressure drop data. Detailed plots of temperature and velocity distributions confirm strong thermal stratification and accurate thermocline formation. The model captures the progression of the thermal front across the packed bed and agrees closely with experimental sensor data, confirming the accuracy of the simulated heat transfer dynamics. In parallel, a mathematical model was formulated to evaluate nanofluid inclusion, using CuO nitrogen mixtures at low volume fractions. The model predicts enhanced thermal conductivity and energy storage potential. Based on mathematical modeling, the inclusion of CuO nanoparticles in nitrogen is projected to increase thermal conductivity and convective heat transfer coefficient, while reducing pressure drop. These predicted enhancements indicate the potential for sharper thermocline formation, improved energy storage efficiency, and reduced hydraulic losses; paving the way for more compact and effective TES systems in solar thermal applications.