Comparative investigations of sorption/resorption/cascading cycles for long-term thermal energy storage

Long-term thermal energy storage is one of the potential and critical solutions to solve the mismatching between thermal energy supply and demand. Even though single-stage sorption cycle, resorption cycle and multi-stage cascading cycle have been proposed for long-term thermal energy storage, the results are difficult to be utilized for selecting optimal cycles because most of them are investigated under different conditions and their performances are not compared under the same criterion experimentally. Herein, the experimental comparative study of different energy storage cycles is implemented for a wide range of heat source temperature and output temperature. For cycles with CaCl2/expanded nature graphite treated by the sulphuric acid (ENG-TSA) sorption bed as the output side, CaCl2/NH4Cl-NH3 cascading cycle has distinct advantage on effective discharging time. Under the general working conditions with relatively high heat source temperature and low output temperature, CaCl2-NH3 sorption cycle is the optimal choice. For cycles with MnCl2/ENG-TSA sorption bed as the output side, MnCl2-NH3 sorption cycle has the maximum thermal energy storage density, thermal energy storage efficiency and temperature lift of 420 kJ.kg(-1), 0.51 and 15.0 degrees C, respectively, at high heat source temperature of 175 degrees C. Under the critical conditions such as the heat source temperature lower than 155.C, MnCl2/CaCl2-NH3 cascading cycle is more suitable and shows optimum adaptability to unsteady heat source. The experimental results exhibit the suitable working range of heat source temperature and output temperature for different cycles, and the guidance is revealed for selecting optimal cycles for different types of low-grade thermal energy.