Cyclic performance assessment of medium-temperature cascade thermal energy storage

This study evaluates the charging/discharging behavior of seven different cascade thermal energy storages (CTES) configurations such as three single-stage storage systems, i.e., KNO3, NaNO3, and NaNO2, three two-stage CTES, i.e., KNO3/NaNO3, NaNO3/NaNO2, KNO3/NaNO2, and a three-stage CTES KNO3/NaNO3/NaNO2. The numerical modelling is done using the enthalpy porosity method. It is observed that two-stage NaNO3/NaNO2 CTES outperforms their other counterparts during charging. After 270 min of the charging operation, NaNO3/ NaNO2 CTES stores 35.72 % and 17.23 % more thermal energy than the KNO3/NaNO3 and KNO3/NaNO3/NaNO2 CTES, respectively. It is better to organize the PCMs in the CTES in-line order with the direction of HTF flow, which results in 30.76 % less charging duration. Moreover, dividing the shell equally for both the PCMs reduces the energy storage time of the NaNO3/NaNO2 CTES by 31.57 % and 10.34 % as compared to the NaNO3 and NaNO2 storages, respectively. The increment in the HTF inlet temperature from 600 K to 690 K results in the reduced charge cycle time of the storage by 66.07 %. The better-charging performance of NaNO2 is over-shadowed during discharging, which highlights the importance of the cyclic behavior study of CTES.