High-temperature multiphase thermochemical heat storage systems: Thermodynamic analysis and operating strategy based on phase evolution

Existing studies have demonstrated the potential of introducing crystallization into conventional gas-liquid absorption thermochemical heat storage systems to overcome the storage-density limitation imposed by intrinsic p–T–x properties. However, the key factors governing heat release performance remain insufficiently understood, hindering the development of effective operating strategies. To address this issue, a high-temperature multiphase thermochemical heat storage system was proposed in this study. A thermodynamic model was built, and performance indicators incorporating chemical potential were developed. By combining thermodynamic analysis with experimental investigations of phase evolution, an operating strategy was established. Results demonstrate that selecting dehydration products (e.g., anhydrous CaCl2) as the charging terminal state enables a higher discharge temperature of 145 °C. Compared with the saturated solution, the anhydrous CaCl2 terminal state increases heat storage density by 144.5% and exergy storage efficiency by 25.9%, while achieving an energy storage efficiency of 51.8%. Increasing the evaporation temperature from 60 °C to 80 °C further reveals a trade-off between heat quantity and heat quality. The discharge exergy increases by 21.4%, and the exergy storage efficiency increases by 3.70%, while the total heat release decreases by only 1.68%. These findings confirm that phase evolution is the key link between operating conditions and system performance, providing a thermodynamic basis for applying multiphase thermochemical heat storage to industrial waste heat recovery and high-temperature heat supply. This may contribute to industrial decarbonization by reducing reliance on fossil fuels.