# Economic Feasibility of Underground Pumped Storage Hydropower Plants Providing Ancillary Services
## 3 Results and Discussion  
### Investment Cost of UPSH Plants  
In this section, the investment costs of UPSH plants are estimated for the case of using existing underground infrastructure and the case of digging new tunnels or caverns as a lower reservoir. Currently, there are no research works where the investment costs of UPSH plants are analyzed. However, several authors have studied the investment cost of conventional PSH plants. Connolly  
Figure 3: Technical data of HPS 1, HPS 2 and HPS 3, considering daily turbine cycle times at full load (DTCs) between 4–10 h day\({}^{-1}\). (**a**) Power in turbine and pump modes; (**b**) water flow rate in turbine and pump modes.  
et al. [18] proposed a range between 0.47 and 2.17 ME MW\({}^{-1}\) from projects in some countries in the European Union. Steffen [31] proposes a range between 0.77 and 1.28 ME MW\({}^{-1}\) from projects of PSH in Germany and Luxembourg. An increase between 7 and 15% is produced when the PSH plant is equipped with variable speed units. The investment cost of a UPSH plant depends strongly on the facility's location and type of underground infrastructure.  
The investment costs of a UPSH plant in a closed mine considering the case of excavation of new tunnels and the case of using existing infrastructure are shown in Table 1. A Francis pump-turbine (FT) with a maximum output power of 219 MW and a maximum input power of 208 MW, a water flow rate in turbine mode of 55.56 m\({}^{3}\) s\({}^{-1}\) (turbine cycle of 8 h at full load) and a water reservoir capacity of 1.6 Mm\({}^{3}\) have been considered. In addition, in both scenarios, due to the dimensions and weight of the hydropower equipment (FT and motor-alternator), the excavation of a new access tunnel from the surface to the powerhouse is required. This tunnel would be used during the construction and operation phases. The investment cost reaches 687.34 ME, which represents a unit cost per MW of installed power of 3138 \(\upxi\) kW\({}^{-1}\). The main cost corresponds to civil works, reaching 525.69 ME and representing 76.48% of the total investment cost. The cost of excavating the new tunnels as a lower reservoir reaches 453.33 ME, which is 86.23% of the total civil works cost. Conversely, the construction and waterproofing of the upper reservoir reach 30.73 ME. More details can be found in Appendix A, where the capital cost of a UPSH plant are detailed.  
When the existing infrastructure is used as a lower reservoir, the investment costs are reduced to 366.96 ME, representing 46.66% less than the previous investment cost. The existing underground infrastructure would be secured with a reinforced shotcrete layer and waterproofed. Consequently, the unit cost per MW of installed capacity is also reduced to 1675 \(\upxi\) kW\({}^{-1}\). The cost of civil works decreases down to 218.08 ME, which is 59.40% of the total cost. The cost of conditioning the lower reservoir reaches 145.73 ME, representing 66.82% of the total civil works cost. Electrical grid connection cost includes the electric substation located in the UPSH plant and the electric power line. The excavation materials of the upper and lower reservoirs could be used for restoring the open pit mines existing in the study area. As indicated in Figure 2b, both upper and lower reservoirs could be underground. In this scenario, where the environmental impact is reduced, the investment cost reaches 1076 ME.