Thermodynamic evaluation of solar assisted ZnO/Zn thermochemical CO₂ splitting cycle

The solar thermochemical CO₂ splitting (CDS) is scrutinized via a redox ZnO/Zn cycle. The second law efficiency analysis is carried out by acquiring the required thermodynamic data from HSC Chemistry software. The main focus of this study is to explore the influence of reduction temperature (T_red), molar flow rate of inert sweep gas (n˙_inert), and energy required for the gas separation on the solar-to-fuel energy conversion efficiency (η_{solar-to-fuel}) of the ZnO/Zn cycle. All the calculations are conducted at a constant gas-to-gas heat recovery effectiveness (ε_gg) equal to 0.5. n˙_inert required is recorded to be too high (5050 mol/s) at T_red equal to 1500 K and moderately low (15 mol/s) for T_red equal to 2000 K. The amount of thermal energy required to heat the inert/O₂ gas mixture (from CDS temperature to separator-1 temperature) and inert sweep gas (from separator-1 temperature to reduction temperature) has a significant impact on the total thermal energy requirement of the cycle (Q˙_TC). The rise in T_red from 1500 K to 2000 K shows a considerable decline in Q˙_TC from 77417.5 kW to 1161.8 kW, respectively. Consequently, the highest η_{solar-to-fuel} (17.0%) is recorded for T_red equal to 2000 K.