Influence of varying aspect ratios and flow rates in thermal energy storage tanks using phase change material

• Tank aspect ratio and flow rate strongly influence heat transfer and thermal stratification in TES systems. • Paraffin wax PCM improves heat transfer by 26% (1:1) and 76% (2:1) aspect ratio tanks. • A 2:1 aspect ratio reduces charging time by up to 31.8%, enhancing TES efficiency. • Optimizing tank geometry with PCM increases thermal stratification by over 60%. The Thermal energy storage (TES) systems play a crucial role in enhancing energy utilisation efficiency in renewable and industrial thermal applications. Among design parameters, the tank aspect ratio strongly governs stratification and heat transfer behaviour. This study experimentally investigates the combined effects of tank geometry and flow rate on the thermal performance of cylindrical TES units with and without phase change material (PCM) integration. Two configurations, with aspect ratios of 1:1 and 2:1, were examined at flow rates of 3, 4, and 6 L min −1 . Both 17 L tanks were insulated with 40 mm glass wool and used paraffin wax (melting range 69–74.4 °C) as PCM and water as the heat transfer fluid. Key parameters like instantaneous and cumulative heat transfer, stratification number, charging efficiency, and Richardson number were evaluated. PCM incorporation significantly improved thermal response, enhancing instantaneous heat transfer by 26% in the 1:1 tank and 76% in the 2:1 tank. Charging duration decreased by 31.8% and 26.1% for the 2:1 configuration without and with PCM, respectively. Stratification strength increased by 54% and 62% in the 1:1 and 2:1 tanks. These findings demonstrate that coordinated optimisation of aspect ratio and PCM distribution enhances heat storage efficiency and stratification stability in medium-temperature TES systems.