Design and Numerical Analysis of Single-Tank Thermal Energy Storage System with Thermocline for Solar Thermal Applications

Abstract

This study presents the design and numerical evaluation of a single-tank thermocline thermal energy storage (TES) system for solar thermal applications aimed at improving energy access in rural Ethiopia through small to medium-scale concentrated solar power (CSP) integration. A two-dimensional model using Computational Fluid Dynamics (CFD) and the Finite Element Method (FEM) simulated unsteady, incompressible laminar flow and heat transfer in a vertical cylindrical tank modelled as an isotropic porous medium. The system used water as the heat transfer fluid and operated between 55°C and 98°C, with a height-to-diameter ratio of 1.2. The computational mesh included 3,938 domain elements and 192 boundary elements, with a maximum element size of 0.01 m. During charging, hot water introduced at the top formed a thermocline that moved downward, reaching a thickness of 0.263 m after 7,200 seconds. The flow remained laminar, with a peak velocity of 0.02 m/s and a Reynolds number of about 1,078, while a pressure gradient of 2.49 kPa developed from top to bottom. The performance index was approximately 0.95, indicating strong stratification and minimal mixing. During discharging, reverse flow from the bottom reached a velocity of 0.002 m/s with a Reynolds number near 769, and the thermocline rose to 0.25 m by 7,200 seconds, maintaining separation of hot and cold zones. Enthalpy increased from 2.25 J/m³ to 2.55 J/m³ during charging and decreased from 4.0 J/m³ to 3.75 J/m³ during discharging, reflecting efficient energy storage and release. Overall, the results confirm stable thermal stratification, high energy efficiency, and the suitability of the proposed water-based thermocline TES for cost-effective, off-grid solar thermal energy applications.