Solar-driven MnSO4/MnO water-splitting cycle for hydrogen production and energy storage

The solar-driven MnSO 4 /MnO water-splitting (WS) cycle is the subject of a thorough thermodynamic performance analysis in this work to evaluate its solar-to-fuel energy conversion efficiency ( η s o l a r − t o − f u e l ) under various process circumstances. Using the thermodynamic database and tools from HSC Chemistry software, equilibrium and energy analysis were carried out. Two redox processes are involved in this cycle: (a) MnSO 4 is thermally reduced between 1366 and 1488 K, and then MnO is reoxidized at 500 K. Reactor surface losses (20%), heat recuperation ( ε g g = 0.5; ε s s = 0.0), and inert sweep gas flow rate ( n ˙ i n e r t = 10 to 100 mol/s) were all thoroughly examined. The thermal reduction temperature ( T r e d ) decreased by 127 K when n ˙ i n e r t was increased from 10 to 100 mol/s. However, because of increased separation and other heating penalties, the total solar heat demand ( Q ˙ s o l a r ) increased from 1059.5 kW to 1462.9 kW. Auxiliary heating demands for inert gas increased from 24.2 kW to 238.4 kW, while the overall separation energy requirements increased from 210.8 kW to 348.7 kW (SO 2 separation) and 124.8 kW to 195.7 kW (O 2 separation). With reradiation losses of 92–100 kW, the cycle's solar energy absorption efficiency ( η a b s o p r t i o n ) ranged from 90.7% to 93.5%. As a result, at 10 mol/s, 1488 K, η s o l a r − t o − f u e l dropped from 26.7% to 19.4% (at 100 mol/s, 1366 K). These findings support the MnSO 4 /MnO WS system's thermodynamic viability as a high-efficiency, moderate-temperature (≤1490 K) WS cycle.