Investigation on performance of multi-salt composite sorbents for multilevel sorption thermal energy storage

- Novel multi-salt composite sorbents are developed and investigated.
- Sorption hysteresis can be alleviated and even eliminated by multi-salt composite sorbent.
- The highest energy storage density of reaction heat is about and 1949 kJ/kg.
- Multilevel STES technology reveals the great potential for utilization of variable heat source.

Novel bi-salt and tri-salt composite sorbents are developed, and expanded natural graphite treated with sulfuric acid (ENG-TSA) is integrated as the matrix with different mass ratios for heat transfer intensification. Tri-salt composite sorbent is mainly composed of Manganese chloride (MnCl2), Calcium chloride (CaCl2) and Ammonium chloride (NH4Cl) whereas bi-salt composite sorbent comprises Calcium chloride (CaCl2) and Ammonium chloride (NH4Cl). Sorption characteristics under non-equilibrium condition are investigated and compared with that under equilibrium condition. Results show that the sorption hysteresis can be alleviated by bi-salt composite sorbent and even eliminated by tri-salt composite sorbent. Based on testing results, multilevel sorption thermal energy storage (STES) is analyzed, which can greatly enhance the versatility and working reliability. It is also worth noting that the highest energy storage density of reaction heat is 1802 kJ/kg and 1949 kJ/kg for tri-salt and bi-salt composite sorbents, respectively. Performance of bi-salt composite sorbent is relatively close to the theoretical data, which indicates three main stages. Comparably, performance of tri-salt composite sorbent shows continuous variation with the increment of reaction temperature. The promising multilevel STES reveals the great potential for energy utilization of variable heat source such as solar power when compared with conventional heat storage methods.