*2.1. Preliminary Energy Balance*

The storable amount of electricity in UPSH systems depends on the reservoirs' capacity and the hydraulic net head [26]. Figure 1 shows the stored energy per cycle in a closed coal mine considering water masses between 0.1–0.5 Hm<sup>3</sup> and net heads between 100–600 mH2O. A Francis turbine efficiency of 90% in turbine and pump modes has been considered [29]. Finally, the power output and the power input of the turbines depends on the time at full load. The energy storage reaches 480 MWh cycle−<sup>1</sup> considering a gross head of 450 mH2O and a useful water capacity of 450,000 m<sup>3</sup> . and the power input of the turbines depends on the time at full load. The energy storage reaches 480 MWh cycle−1 considering a gross head of 450 mH2O and a useful water capacity of 450,000 m3.

*Appl. Sci.* **2020**, *10*, x FOR PEER REVIEW 3 of 13

*Appl. Sci.* **2020**, *10*, x FOR PEER REVIEW 3 of 13

**Figure 1.** Storable amount of energy of an UPSH plant, considering a turbine efficiency of 90%. **Figure 1.** Storable amount of energy of an UPSH plant, considering a turbine efficiency of 90%.

**Figure 1.** Storable amount of energy of an UPSH plant, considering a turbine efficiency of 90%.

### *2.2. Geology 2.2. Geology*

The Asturian Central Coal Basin (ACCB) is a coal mining area located in NW Spain. It is precisely in the ACCB where the terrigenous carboniferous sediments are best represented, reaching a thickness close to 6000 m between the Namurian B and Westfalian D. The succession is divided at large scale into two sectors: one lower denominated Lena Group, characterized by limestones and thin seams of coal, and another superior called Sama Group, in which limestones are less abundant and there are levels of sandstones, shales and seams of exploitable bituminous and hard coal [8]. The Asturian Central Coal Basin (ACCB) is a coal mining area located in NW Spain. It is precisely in the ACCB where the terrigenous carboniferous sediments are best represented, reaching a thickness close to 6000 m between the Namurian B and Westfalian D. The succession is divided at large scale into two sectors: one lower denominated Lena Group, characterized by limestones and thin seams of coal, and another superior called Sama Group, in which limestones are less abundant and there are levels of sandstones, shales and seams of exploitable bituminous and hard coal [8]. *2.2. Geology*  The Asturian Central Coal Basin (ACCB) is a coal mining area located in NW Spain. It is precisely in the ACCB where the terrigenous carboniferous sediments are best represented, reaching a thickness close to 6000 m between the Namurian B and Westfalian D. The succession is divided at large scale into two sectors: one lower denominated Lena Group, characterized by limestones and thin seams of coal, and another superior called Sama Group, in which limestones are less abundant

### *2.3. Underground Hydroelectric Power Plant 2.3. Underground Hydroelectric Power Plant* and there are levels of sandstones, shales and seams of exploitable bituminous and hard coal [8].

UPSH systems consist of two water reservoirs; the upper reservoir is located above ground, while the lower reservoir is underground. The scheme of an underground hydroelectric power plant and lower water reservoir is shown in Figure 2a. In the present work, a subsurface water reservoir conformed by a new network of tunnels with an arched roof and straight walls cross-section of 30 m2 (Figure 2b) is considered. The water reservoir consists of a central tunnel connected to the ventilation shaft, and 200 m length of transversal tunnels with a distance between them of 20 m. To avoid water leakage, the tunnels' surface is covered with an impermeable high-strength membrane. UPSH systems consist of two water reservoirs; the upper reservoir is located above ground, while the lower reservoir is underground. The scheme of an underground hydroelectric power plant and lower water reservoir is shown in Figure 2a. In the present work, a subsurface water reservoir conformed by a new network of tunnels with an arched roof and straight walls cross-section of 30 m<sup>2</sup> (Figure 2b) is considered. The water reservoir consists of a central tunnel connected to the ventilation shaft, and 200 m length of transversal tunnels with a distance between them of 20 m. To avoid water leakage, the tunnels' surface is covered with an impermeable high-strength membrane. *2.3. Underground Hydroelectric Power Plant*  UPSH systems consist of two water reservoirs; the upper reservoir is located above ground, while the lower reservoir is underground. The scheme of an underground hydroelectric power plant and lower water reservoir is shown in Figure 2a. In the present work, a subsurface water reservoir conformed by a new network of tunnels with an arched roof and straight walls cross-section of 30 m2 (Figure 2b) is considered. The water reservoir consists of a central tunnel connected to the ventilation shaft, and 200 m length of transversal tunnels with a distance between them of 20 m. To avoid water leakage, the tunnels' surface is covered with an impermeable high-strength membrane.

**Figure 2.** (**a**) Scheme of an UPSH plant with a lower water reservoir and an air shaft in a closed underground coal mine; (**b**) cross section of tunnels. **Figure 2.** (**a**) Scheme of an UPSH plant with a lower water reservoir and an air shaft in a closed underground coal mine; (**b**) cross section of tunnels.
