Biochar - Salt Hydrate Composites for Sustainable Low Temperature Heat Transformation

Abstract

The rapidly growing global demand for heating and cooling is largely met by greenhouse gas-emitting sources and will place increasing strain on future power grids. To meet growing demand and diversify energy sources, novel technologies and sustainable materials are essential. Adsorption heat transformation presents a promising solution for heating, cooling, and heat storage by harnessing sustainable low-temperature or waste heat. This work explores the use of salt hydrates as thermochemical materials and general adsorbents, overcoming their stability challenges by embedding them within porous biochar derived from waste streams. Biochars based on residual wood and chestnut shells were obtained as by-products from two gasification plants and chemically activated with KHCO3 and urea to enhance its pore properties. This treatment significantly increased the specific surface area and pore volume of the chestnut shelland woodbased biochars to 1576 ± 91 m²/g and 0.505 ± 0.01 cm³/g, and 1356 ± 101 m²/g and 0.622 ± 0.05 cm³/g, respectively. The wood-based biochar retained structural features of the original biomass and exhibited pronounced mesoporosity, whereas the chestnut shell-derived biochar predominantly displayed micropores. This structure seems to influence the vacuum impregnation with 80wt.% K2CO3 hydrate, as the structure of wood is still visible and salt is deposited in the nanopores, while chestnut shell biochar appeared coated with a salt layer, determined by SEM/EDS. The resulting composites (subscript ‘ad’) underwent multiple hydration–dehydration cycles to evaluate the stability of the salt hydrates within the carbon support. Initial testing revealed that water uptake and adsorption kinetics improved with cycling, with the wood-based composite showing superior kinetics. This performance is attributed to enhanced vapor transport facilitated by its mesoporous network and large-scale channels inherited from the wood precursor. Long-term cycling of the wood biochar-K2CO3 composite demonstrated stable water uptake of 275 ± 5 kgH2O/kgad over ten cycles, corresponding to a specific water loading of approximately 2.8 molH2O/molK2CO3 . This exceeds the equilibrium uptake of pure K2CO3, indicating altered equilibrium phases due to salt confinement in nanopores. Solution formation was found to significantly influence theoretical power output and energy density of the wood-based composite. Experimental data showed a specific maximum power output exceeding 1 kW/kgK2CO3 for at least ten cycles, and a theoretical energy density of about 1.63 GJ/m³, surpassing that of the pure salt. These findings suggest strong potential for domestic thermochemical energy storage. However, investigation of such systems in residential settings revealed major challenges, including limited efficiency and the availability of more efficient alternatives. In addition to heat storage, the potential for adsorption-based refrigeration was investigated. A composite of wood-based biochar impregnated with 80wt.% CaCl2 hydrate showed enhanced hydration performance, with a maximum water uptake of 1.12 kgH2O/kgad, resulting in a substantial specific cooling effect of up to 2736 kJ/kgad. However, salt solution leakage during hydration was observed, which may limit its use in packed-bed configurations. While the K2CO3 composite delivered a lower overall cooling effect, it demonstrated high specific cooling power, exceeding 550 W/kgad at short cycle times, due to its rapid reaction kinetics. Overall, porous biochar proves to be a promising support matrix for salt hydrates, enabling stable and reversible hydration reactions. The versatility of salt types offers potential not only for novel thermal energy storage systems but also for enhancing existing technologies such as adsorption refrigeration and desalination, particularly by utilizing sustainable low-temperature or waste heat sources