Thermodynamic irreversibility and exergy destruction mapping in a series-tube thermal energy storage unit

This study presents a detailed exergetic assessment of a shell and tube latent heat thermal energy storage (LHTES) system having a five-tube heat exchanger configuration with paraffin-based phase-change material. The impact of heat transfer (HTF) mass flow rates (0.0067 to 0.02 kg s-1) on charging kinetics and thermodynamic sustainability was evaluated. The three investigated flow rates exhibited different melting characteristics, indicating that the system response depends on the operating flow rates. While the highest flow rate (0.02 kg s-1) achieved the maximum overall exergetic efficiency of 49.3% and the sustainability index (SI, defined as a thermodynamic indicator of resource-use effectiveness based on exergy efficiency) of 1.97, an intermediate flow rate (0.0133 kg s-1) demonstrated superior kinetics, reaching a fully liquid state and peak exergy accumulation more rapidly. This kinetic paradox suggests that moderate flow rates optimize power density through efficient merging of melting regions, whereas higher flow rates are essential for minimizing the exergy destruction ratio, which decreased from 0.68 to 0.506. Thermal profiles confirmed that the current geometry of the TES unit effectively triggers natural convection surges, significantly reducing time-integrated irreversibilities. These findings provide a framework for balancing charging rate and thermodynamic performance for LHTES systems, identifying distinct operational regimes for maximizing either charging speed or resource sustainability in renewable energy applications.