# Economic Feasibility of Underground Pumped Storage Hydropower Plants Providing Ancillary Services
## 3 Results and Discussion
### Design Considerations of UPSH Plants  
The proper dimensioning of a UPSH plant is highly important for optimizing its economic feasibility. The net head and the amount of water moved must be maximized for increasing the storable amount of energy and the economic feasibility. Although the amount of energy remains constant, different dimensioning of UPSH could be carried out depending on the DTC. When the DTC increases, the output power of the FT decreases, consequently reducing the investment costs. DTCs greater than 7 h would be suitable for UPSH plants. The profitability also depends on the lifetime considered. Therefore, the profitability analysis was carried out considering lifetimes of 50 and 75 years. In addition, when the lifetime is increased, the amortization costs decrease and the EBIT increase.  
Table 5 shows the NPVs, IRRs and PBs for HPS 1, HPS 2 and HPS 3 when considering the construction of new tunnels or caverns as a lower reservoir. The NPVs and IRRs of HPS 1, HPS 2 and HPS 3 increase with respect to the scenario that considers a lifetime of 35 years. However, IRRs lower than 5% and NPVs lower than 167.88 M\(\upepsilon\) are reached for 75 years, and therefore the projects are not economically feasible.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**EBIT and Profitability** & **HPS 1** & **HPS 2** & **HPS 3** \\ \hline
**EBIT (k\& year\({}^{-1}\))** & 3115.57 & 5847.65 & 12,368.74 \\
**NPV (k\&)** & 44,614.77 & 86,094.49 & 185,702.73 \\
**IRR (\%)** & 6.66\% & 6.90\% & 7.10\% \\
**PB (years)** & 15 & 15 & 15 \\ \hline \hline \end{tabular}
\end{table}
Table 4: Profitability analysis of HPS 1, HPS 2 and HPS 3, making use of existing infrastructure and considering DTCs of 8 h and a lifetime of 35 years.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**EBIT (k\& year\({}^{-1}\))** & 554.85 & 1352.20 & 3335.28 \\
**NPV (k\&)** & \(-\)41,570.22 & \(-\)65,194.66 & \(-\)118,307.90 \\
**IRR (\%)** & 2.43\% & 2.61\% & 2.75\% \\
**PB (years)** & 25 & 25 & 24 \\ \hline \hline \end{tabular}
\end{table}
Table 3: Profitability analysis of HPS 1, HPS 2 and HPS 3, considering the construction of new tunnels or caverns, DTCs of 8 h and a lifetime of 35 years.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**EBIT and Profitability** & **HPS 1** & **HPS 2** & **HPS 3** \\ \hline
**EBIT (k\& year\({}^{-1}\))** & 554.85 & 1352.20 & 3335.28 \\
**NPV (k\&)** & \(-\)41,570.22 & \(-\)65,194.66 & \(-\)118,307.90 \\
**IRR (\%)** & 2.43\% & 2.61\% & 2.75\% \\
**PB (years)** & 25 & 25 & 24 \\ \hline \hline \end{tabular}
\end{table}
Table 3: Profitability analysis of HPS 1, HPS 2 and HPS 3, considering the construction of new tunnels or caverns, DTCs of 8 h and a lifetime of 35 years.  
Table 6 shows the profitability analysis for lifetimes of 50 and 75 years in UPSH plants where the existing infrastructure is used as a lower reservoir. The NPV and the IRR reach 471.89 Mk and 8.13% when the lifetime of the UPSH increases to 75 years. The NPVs of HPS 3 for a lifetime of 75 years are increased by 284% and 108% in comparison with the values obtained for HPS 1 and HPS 2, respectively. Finally, the PB remains constant in each HPS.