## 2 Research Background  
### Mechanism of Power-To-Gas System (P2G)  
The novel P2G conversion system has emerged in recent years. Researchers and system operators (SOs) are also very willing to deploy this renewable-based energy-storage system, the so-called P2G conversion facility, in integrated electricity and natural gas systems [14; 15]. However, before utilizing the P2G facility in various networks, perfect recognition of its process will help deploy efficiently and reduce operating costs.  
Two main processes exist in generating synthetic natural gas (SNG), namely electrolysis and methanation. At first, delivering electricity from the energy market or wind power unit to the P2G system splits water into hydrogen and oxygen via the electrolysis process indicated in (1). Therefore, the generated hydrogen reacts with carbon dioxide (CO\({}_{2}\)), according to the methanation procedure, to produce SNG described in (2), which is known as the Sabatier reaction. The Sabatier reaction is mainly performed by using chemical catalysts, in which the most common catalyst is nickel with aluminum oxide [30]. Until today, some of the practical projects have used catalytic methanation, such as CO\({}_{2}\)-SNG, in Poland [31]; Jupiter, in France, with the capacity up to 1000 kW [32]; Exytron demonstration project in Germany, with the capacity of 21 kW [33]; etc. Moreover, this reaction can be conducted with a standard metabolic procedure in the production of biogas, and biological reactors can be taken into account [34]. The Power to Flex joint project in Germany and the Netherlands [35], Symbio in Denmark [36], and Wijster in the Netherlands [37] are a examples of practical projects that have used biological reactors. The optimal condition to perform the reaction is 10 bar pressure and 300 \({}^{\circ}\)C, which, in doing so, over 98% CH\({}_{4}\) can be produced [34]. Furthermore, the required CO\({}_{2}\) can be available from many sources, e.g., biogas, refineries, fuel oil, etc.; however, biogas is the best source for the P2G procedure [38]. As depicted in References [39; 40], these two presented processes are principle steps in progressing the P2G system, which is illustrated in Figure 1. The energy conversion efficiency of the whole P2G system mostly about between 50% and 60%. However, producing hydrogen from the first stage of SNG is more productive than the whole P2G procedure.  
\[2\mathrm{H}_{2}\mathrm{O}\to 2\mathrm{H}_{2}+\mathrm{O}_{2} \tag{1}\]  
\[4\mathrm{H}_{2}+\mathrm{CO}_{2}\to 2\mathrm{H}_{2}\mathrm{O}+\mathrm{CH}_{4} \tag{2}\]  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Reference**} & \multirow{2}{*}{
\begin{tabular}{c} **Bidding Strategies** \\ **of GenCos** \\ \end{tabular} } & \multicolumn{3}{c}{**Technologies**} & \multicolumn{1}{c}{**Uncertainty**} \\ \cline{3-5}  & & **GFU** & **WPU** & **P2G** & **Management** \\ \hline
[16] & ✓ & ✓ & & & IGDT \\
[17] & ✓ & ✓ & & & IGDT \\
[18] & ✓ & ✓ & & & IGDT \\
[19] & ✓ & ✓ & & & Deterministic \\
[25] & ✓ & ✓ & ✓ & & Robust \\
[1] & ✓ & ✓ & & ✓ & Stochastic \\
[24] & ✓ & ✓ & & ✓ & Stochastic \\ Current work & ✓ & ✓ & ✓ & ✓ & Robust \\ \hline \hline \end{tabular}
\end{table}
Table 1: Comparison of the literature.