Numerical analysis on the improved thermo-chemical behaviour of hierarchical energy materials as a cascaded thermal accumulator

Highlights: • Local thermal non-equilibrium model is adopted. • Inhomogeneous porosity is considered. • Cascaded heat storage materials stabilise output temperature. • Numerical system storage capacity and COP are 10.8 kWh and 0.71. The present study aims to improve the thermo-chemical conversion behaviours, including reactive transport processes and output performances of an open thermochemical energy storage (TCES) unit. The local thermal non-equilibrium (LTNE) model and the effect of non-uniform porosity are adopted and considered to better elucidate the conversion processes. Cascading the reaction sub-units filled with different thermochemical materials (TCMs), i.e., zeolite, salt hydrate-based composite sorbent, and pure salt of SrBr₂·6H₂O, to form an integrated storage bed ameliorates the output performance. The numerical results indicate that the maximum temperature difference ranging from 3.5 to 4.9 °C between heat transfer fluid and solid reactants exists during desorption, and the realistic non-uniform porosity facilitates the reactant conversion near the wall compared to the uniform porosity assumption. The cascaded scheme promotes the charging and discharging processes compared to the cases filled with sole TCM, the time required for charging this 10.8 kWh storage model is 16 h. Increasing the charging temperature from 100 °C to 145 °C, the charging time reduced to 6.5 h, saving 59.4%. Boosting the inlet velocity of airflow also accelerates the charging rate. The cascaded storage unit significantly stabilises the output temperature during discharging, warming up the airflow from 20 °C to 35 °C for 24 h with a tiny temperature fluctuation. Airflow with higher relative humidity facilitates hydration but shortens the stable period. Overall power and thermal efficiency of the "thermal accumulator" in charging are 598 W and 92.8%, 164 W and 92.4% in discharging, with a total COP of 0.71. The satisfying performances suggest that the cascaded TCES unit may provide a strategy and reference in the design and promotion of the low-grade energy storage system.