Adsorption thermochemical battery-based heat transformer for low-grade energy upgrading

While thermal energy storage promises to mitigate the temporal mismatch between intermittent renewable energies and time-varying user loads, it fails to tackle the low-grade issue that causes the underutilization of energy sources at low temperatures. Heat upgrading with favorable storage performance can be accomplished by utilizing the salt hydrate-based adsorption thermochemical heat transformer (THT). Herein, the thermochemical materials cascaded thermochemical energy storage (C-TCES) is proposed to improve the system performance. The dynamic characteristics during charging and discharging for the regular TCES (R-TCES), pressurization-assisted TCES (P-TCES), and C-TCES cycles are comparatively analyzed. Moreover, the influences of heat input, heat output, and heat sink temperatures on the system's performance are systematically studied. Results suggest that the C-TCES cycle accomplishes notable intensification in temperature upgrading capability and energy storage performance, while the performance enhancements achieved by the P-TCES cycle are not significant under some conditions. The P-TCES and C-TCES systems yield output heat of 110.8 kWh and 165.3 kWh, respectively, with smooth heat duty output of 5.45 kW and 5.97 kW at output and input temperatures of 95 °C and 65 °C. Compared with the R-TCES system, the performance indexes are strikingly boosted by the C-TCES system, e.g., the energy storage density, energy storage efficiency, and exergy efficiency are escalated from 89.5 kW/m3, 53.2 %, and 31.4 % to 136.4 kW/m3, 69.8 %, and 53.1 %, respectively. Replacing MgCl2∙6H2O with FeCl2∙2H2O as storage material in the C-TCES system further considerably increases the performance indexes. These findings offer valuable insights into advancing efficient thermochemical batteries and underscore the potential of TCES systems in optimizing the utilization of low-grade thermal energy.