## 4 Discussion  
The methanation process is a strongly exothermic reaction and requires external energy to initiate the reaction. Subsequent to initiation, the reaction can be maintained by the heat released. As a result, the energy requirements for the methanation system decrease significantly after reaction initiation. Energy consumption of the methanation system is a significant issue for the efficient conversion of solar  
Figure 6: Total power consumption of the reactors under (**a**) a sunny conditions (15 th November 2019) and (**b**) overcast conditions (6 th December 2019). energy to gas. It is important to note that a highly efficient StM system is a result of highly efficient CPV modules, highly powerful DC/DC converters, and highly active catalysts. The StM conversion system performed well and achieved high efficiency under variable weather conditions. This was a result of efficient CPV modules and active catalysts. Utilizing methanation, StM is a chemical energy storage technique where renewable energy is stored as gas for later use in a gas grid infrastructure. The photocatalytic conversion of CO\({}_{2}\) and H\({}_{2}\) into artificial CH\({}_{4}\) might be a useful approach with applications in transportation and the current gas grid infrastructure. Potentially, CH\({}_{4}\) could be stored safely and distributed widely as natural gas in the future. The StM method could be utilized to produce CH\({}_{4}\) as a renewable energy. The stored chemical energy of CH\({}_{4}\) should be considered in StM conversions because it is directly used in power generation via combustion. Figure 7 shows a schematic illustration of combustion energy of CH\({}_{4}\). CH\({}_{4}\) has a combustion energy of \(-802\) kJ/mol. Solar to methane efficiency (\(\eta_{\text{StM}}\)) is defined as shown in Equation (3):  
\[\eta_{\text{StM}}=\frac{\text{Combustion energy of produced methane}\text{Xholes of produced methane}}{\text{Input energy}+\text{Electrical energy required to heat reactors}}, \tag{3}\] \[=\frac{\left|\Delta\text{H}^{\circ}\right|\text{xamount of CH}_{4}}{E_{\text{DNI}}+E_{\text{ reactors}}},\]  
where the stored chemical energy of produced methane yields the combustion energy (\(\Delta\text{H}^{\circ}=-802\) kJ/mol). The integrated irradiance is denoted as \(E_{\text{DNI}}\), while the term \(E_{\text{ reactors}}\) represents the energy consumed by the reactors in the methanation system. Table 1 shows the summary of power input and output for our system. The CH\({}_{4}\) generation rate was calculated based on mole fractions and generation rate of H\({}_{2}\) input into the reaction. The calculation assumes that the CH\({}_{4}\) generation rate corresponds to one-quarter times the input of H\({}_{2}\). Subsequently, the CH\({}_{4}\) generation rate was multiplied with the calculated conversion of CO\({}_{2}\) to CH\({}_{4}\) (97.6%). The rate of H\({}_{2}\) input into the StM system was evaluated by MFM. The conversion of CO\({}_{2}\) to CH\({}_{4}\) was analyzed and assessed by QMS. For sunny conditions (on 15 th November 2019), an efficiency of 13.8% was achieved as shown in Table 1. In contrast, the system could not generate electricity from CPV modules under an overcast condition because the CPV modules use only the direct-beam component. Therefore, the hydrogen generation and Sabatier process did not operate under an overcast condition. Figure 7: Schematic illustration of the combustion energy contained in methane. Table 2 shows the one-day elemental efficiencies focused on solar to hydrogen conversion. The elementary efficiencies are defined as follows: \(\eta_{\text{CPV}}\) is the CPV module efficiency; \(\eta_{\text{DCDC}}\) is the conversion efficiency of the DC/DC converters; \(\eta_{\text{EC}}\) is the energy conversion efficiency from DC electricity into the free energy of hydrogen at the electrolyzer [18]. Especially, \(\eta_{\text{EC}}\) decreased in comparison with previous work [18].