## 4 Results and Discussion
### Compressor Outlet Pressure  
Figure 2 illustrates the effect of the charging pressure on the RTE, ERTE, and SPC. It is noticed that the RTE increases from 45.62% to 47.72% as the charging pressure increases from 10 MPa to 12 MPa, and then the RTE shows a downward trend. The ERTE reaches its maximum value of 69.74% at the charging pressure of 12 MPa and drops to 69.17% as the charging pressure rises to 18 MPa. When the charging pressure increases from 10 MPa to 18 MPa, the compression power keeps increasing from 30,546 kW to 35,649 kW (by 16.71%), the liquefaction ratio increases from 83.44% to 86.49% and then gradually decreases to 84.97%, and the SPC rises from 0.1830 kWh\({}_{\text{e}}\cdot\)kg\({}_{\text{LA}}\)\({}^{-1}\) to 0.2098 kWh\({}_{\text{e}}\cdot\)kg\({}_{\text{LA}}\)\({}^{-1}\). The total net output power of the proposed system, which takes into account the power produced by the ATs, GT, and CT, as well as the power consumed by the LAP and LNGP, increases from 24,250 kW to 27,369 kW as the charging pressure varies from 10 MPa to 12 MPa. As \begin{table}
\begin{tabular}{l c} \hline \hline
**Parameter** & **Value** \\ \hline Service life (SL) & 20 years \\ Discount rate \((i)\) & 3\% \\ Inflation rate \((f)\) & 2\% \\ Annual interest rate \((R_{\text{ai}})\) & 5\% \\ Annual operation and maintenance factor \((\alpha)\) & 5\% \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Parameter interpretation of the economic evaluation indicators [30; 38; 39; 40; 41; 42]. the charging pressure further increases to 18 MPa, the total net output power decreases to 25,961 kW. Figure 3 shows the materials investment cost and LNG fuel purchase cost per year. Figure 4 shows the investment in the equipment. The initial investment cost \(Z_{\text{IIC}}\) is USD 29.88-30.99 million, and the investment cost in the LPTs (USD 8.98-10.25 million), the ACs, ATs, and LOTs accounts for the main part of the \(Z_{\text{IIC}}\). Capital investment in the materials accounts for at most 8.5% of the total capital expenditure. As the charging pressure increases, the compression power consumption and the compression heat increase, resulting in higher investment costs for the ACs and thermal oil. The liquefaction temperature of the compressed air also increases with rising charging pressure, decreasing the cold energy requirement for the air condensation process, and reducing the investment costs in propane and the LPTs. Figure 3: Effect of charging pressure on materials investment cost and LNG purchase cost. Figure 2: Effect of charging pressure on the RTE, ERTE, and SPC. Figure 5a shows that the NPV increases, as time goes by, during the life span of the system. The maximum NPV of USD 55.81 million and the minimum NPV of USD 40.98 million occur at the charging pressures of 12 MPa and 18 MPa, respectively. Therefore, it is indicated that the optimal lifetime economic benefits of the project occur with a charging pressure of 12 MPa. When operating at a charging pressure of 10 MPa, the NPV of the system is USD 41.09 million, which is USD 109 thousand higher than the NPV at 18 MPa. This is due to the similar total annual profit for the two different conditions (USD 4,770,187 at 10 MPa and USD 4,769,784 at 18 MPa). Both the DPP and LCOE show a decreasing trend before increasing with the rising charging pressure, according to Figure 5b. When increasing the charging pressure to 12-16 MPa, the DPP can be shortened to 5.73-6.20 years. The LCOE with the charging pressure of 12 MPa is 0.0965 USD\(\cdot\)kWh\({}^{-1}\), which is 0.0101 USD\(\cdot\)kWh\({}^{-1}\) less than that with the charging pressure of 18 MPa. It is because the largest net output power of the system occurs at the charging pressure of 12 MPa (see Table 5), allowing the system to generate the highest total annual revenue. Figure 4: Effect of charging pressure on component investment cost. Figure 5: Effect of charging pressure on the economic indicators, (**a**) NPV and (**b**) DPP and LCOE.