## 3 Optimization of Annual Costs  
Previous calculations were done for a fixed final pressure in the storage tank. Evidently, if this pressure is modified then the final results of the cost and consumption will change, and other process configuration may minimize the cost. There are several variables that could influence the results. On the one hand, the cost expression of the gas storage tank depends on the internal pressure, gas volume, and material from which the tank is constructed. The higher the gas storage pressure (compressor cost increases), the lower \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline  & **TAC** & **CC** & **HEC** & **TC** & **E** & **CAPEX** & **a** & **OPEX** \\  & **(6/Year)** & **(6)** & **(6)** & **(6)** & **(6)** & **(6)** & **(6/Year)** & **(6/Year)** \\ \hline
1 & 87,816 & 64,144 & 15,867 & 449,650 & 545,527 & 1,200,000 & 80,670 & 7147 \\
2 & 96,130 & 52,012 & 14,045 & 449,650 & 609,856 & 1,342,000 & 90,182 & 5948 \\
3 & 106,067 & 48,961 & 13,589 & 449,650 & 678,069 & 1,492,000 & 100,269 & 5797 \\
4-SC & 116,331 & 47,587 & 13,384 & 449,650 & 747,072 & 1,644,000 & 110,473 & 5858 \\
5 & 126,717 & 46,808 & 13,268 & 449,650 & 816,367 & 1,796,000 & 120,720 & 5997 \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Summary of costs per scenario considering a storage pressure of 20 bar, H\({}_{2}\). \begin{table}
\begin{tabular}{c c c} \hline \hline
**Cost Description Study Case** & **Cost CO\({}_{2}\) (6)** & **Cost H\({}_{2}\) (6)** \\ \hline FIXED COSTS & & & \\ E, Main equipment cost (Compressors and heat & 460,303 & 747,072 \\ exchangers) & & & \\ Equipment installation & 92,061 & 149,414 \\ Instrumentation and control systems & 73,648 & 119,532 \\ Gas piping & 156,503 & 254,004 \\ Electrical systems & 23,015 & 37,354 \\ Industrial warehouse & 23,015 & 37,354 \\ Service centre & 46,030 & 74,707 \\ TOTAL FIXED COSTS & 874,576 & 1,419,437 \\ VARIABLE COSTS & & \\ Engineering and supervision & 23,015 & 37,354 \\ Building costs & 46,030 & 74,707 \\ Legal costs & 13,809 & 22,412 \\ Administrative fees & 9206 & 14,941 \\ Contingencies & 46,030 & 74,707 \\ TOTAL VARIABLE COSTS & 138,091 & 224,122 \\ CAPEX & **1,012,667** & **1,643,558** \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Detailed description of CAPEX considering a storage pressure of 20 bar. Base case. \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline \multirow{2}{*}{**K**} & **TAC** & **CC** & **HEC** & **TC** & **E** & **CAPEX** & **a** & **OPEX** \\  & **(6/Year)** & **(6)** & **(6)** & **(6)** & **(6)** & **(6)** & **(6/Year)** & **(6/Year)** \\ \hline
1 & 40,117 & 51,582 & 14,058 & 173,737 & 253,435 & 557,557 & 37,477 & 2640 \\
2 & 49,941 & 47,208 & 13,366 & 173,737 & 321,617 & 707,557 & 47,559 & 2382 \\
3 & 60,196 & 46,016 & 13,176 & 173,737 & 390,839 & 859,846 & 57,795 & 2401 \\
4-SC & 70,551 & 45,467 & 13,087 & 173,737 & 460,303 & 1,013,000 & 68,067 & 2484 \\
5 & 80,945 & 45,152 & 13,036 & 173,737 & 529,861 & 1,166,000 & 78,353 & 2592 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Summary of the costs per scenario considering a storage pressure of 20 bar, CO\({}_{2}\). the volume required to store it (storage tank cost decreases), but the tank material must be stronger to withstand high pressures (storage tank cost increases). Thus, the inverse influence of these variables in the expression of the tank cost makes this optimization study essential. Equation (6) show the cost dependency on volume and pressure. Increasing gas pressure causes an increment in density and a reduction in (specific) volume. As these two variables are multiplying in Equation (6), the pressure that minimizes the capital cost is not evident. Pressure and volume are related through the density and the pressure that minimizes the cost depends on the variation in gas density with pressure. On the other hand, the work required by the equipment (overall consumption) also plays an important role in the optimization study, since it is directly related to costs.