## 4 Conclusions  
The necessity of decarbonize the industrial and residential sectors is motivating a huge increment in renewable electricity production. In order to manage the mismatching between renewable production and consumption, it is essential to develop energy storage \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline  & **K** & **TAC** & **CAPEX (€)** & **a** & **OPEX** & **P** & **V** & **Cr** \\  & **(€/Year)** & **(€)** & **(€/Year)** & **(€/Year)** & **(****bar)** & **(****m\({}^{3}\))** & **Cr** \\ \hline \multicolumn{1}{c}{(a)} & & & & & & & \\ \hline Optimization base & 4-SC & 60,068 & 847,505 & 56,966 & 3102 & 72.47 & 15.49 & 2.92 \\ Decrease COE 20\% & 4-SC & 59,164 & 847,505 & 56,966 & 2198 & 72.49 & 15.48 & 2.92 \\ Increase COE 20\% & 4-SC & 60,972 & 847,505 & 56,966 & 4006 & 72.46 & 15.51 & 2.92 \\ \hline \multicolumn{1}{c}{(b)} & & & & & & & \\ \hline Optimization base & 4-SC & 83,734 & 1,121,000 & 75,380 & 8354 & 67.64 & 193.70 & 4.00 \\ Decrease COE 20\% & 4-SC & 81,329 & 1,121,000 & 75,318 & 6012 & 69.89 & 187.80 & 4.12 \\ Increase COE 20\% & 4-SC & 86,116 & 1,121,000 & 75,463 & 10,653 & 65.51 & 199.80 & 4.00 \\ \hline \hline \end{tabular}
\end{table} 
Table 10: Sensitivity analysis varying electricity price in comparison to optimized base case. (a) CO\({}_{2}\); (b) H\({}_{2}\). \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline  & **K** & **TAC** & **CAPEX** & **a** & **OPEX** & **P** & **V** & **Cr** \\  & **(€/Year)** & **(€)** & **(€/Year)** & **(€/Year)** & **(€/Year)** & **(****bar)** & **(****m\({}^{3}\))** & **Cr** \\ \hline \multicolumn{1}{c}{(a)} & & & & & & & & \\ \hline Optimization Base & 4-SC & 60,068 & 847,505 & 56,966 & 3102 & 72.47 & 15.49 & 2.92 \\
96 h/week (4 days/week) & 4-SC & 63,170 & 847,505 & 56,966 & 6204 & 72.46 & 15.51 & 2.92 \\
168 h/week (7 days/week) & 4-SC & 67,824 & 847,505 & 56,966 & 10,858 & 72.44 & 15.53 & 2.92 \\ \hline \multicolumn{1}{c}{(b)} & & & & & & & \\ \hline Optimization Base & 4-SC & 83,734 & 1,121,000 & 75,380 & 8354 & 67.64 & 193.70 & 4.00 \\
96 h/week (4 days/week) & 4-SC & 91,974 & 1,121,000 & 75,318 & 16,258 & 61.18 & 213.40 & 3.94 \\
168 h/week (7 days/week) & 4-SC & 103,935 & 1,121,000 & 75,463 & 27,388 & 53.31 & 243.80 & 3.76 \\ \hline \hline \end{tabular}
\end{table} 
Table 11: Sensitivity analysis varying the operational hours in comparison to the optimized base case: (a) CO\({}_{2}\); (b) H\({}_{2}\). alternatives. Power to gas is a promising option to take advantage of renewable H\({}_{2}\) by converting it, together with the captured CO\({}_{2}\), into renewable gases, in particular renewable methane. It allows the temporal displacement (storage) in the use of renewable energy. Storage vessels for H\({}_{2}\) and CO\({}_{2}\) are necessary to smoothly run a methanation installation and then a new research line about the feasible operational variables that minimize the energy requirements in this storage is required. The objective of this research was reached through a techno-economic analysis of the feasible layout and storage pressures that minimize the capital and operation cost of H\({}_{2}\) and CO\({}_{2}\) gas compression and storage vessels for power-to-gas applications. The main findings of the analysis are highlighted:  
1. Four compressor stages for CO\({}_{2}\) storage at a pressure of 72.5 bar minimize the annual storage cost. This value is just below the supercritical pressure. Increasing pressure above this value increases the compressor, installation, and operational cost, without reducing significantly the storage tank value and cost. 2. In the case of H\({}_{2}\), the minimum cost is found with a storage pressure of 67.6 bar, slightly lower than in the case of CO\({}_{2}\) and with one compressor stage less. This value is in agreement with the usual pressures to store and distribute natural gas. 3. In both cases the value of the pressure that minimize the cost remains practically unchanged despite the number of compressor stages.