**1. Introduction**

Renewable energies, such as solar or wind, may not be sufficiently efficient since they are intermittent and random, and consequently, their production of electricity is not adapted to the demand [1–4]. For this reason, they must be combined with energy storage systems (ESSs) [5] that allow for balancing the production and the demand [6]. ESSs are useful to store the surplus of electricity during periods of low demand and to generate electricity when the demand increases. Pumped storage hydropower (PSH) is the most

**Citation:** Pujades, E.; Poulain, A.; Orban, P.; Goderniaux, P.; Dassargues, A. The Impact of Hydrogeological Features on the Performance of Underground Pumped-Storage Hydropower (UPSH). *Appl. Sci.* **2021**, *11*, 1760. https://doi.org/10.3390/ app11041760

Academic Editors: Jorge Loredo and Javier Menéndez Received: 21 January 2021 Accepted: 13 February 2021 Published: 17 February 2021

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**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

worldwide used EES [7] because it allows for the storage and production of large amounts of electricity [8]. For example, about 95% of the utility-scale energy storage in the United States is PSH [9], and up to 99% in the European Union [10].

PSH plants consist of two reservoirs placed at different elevations (upper and lower reservoirs). The excess of electricity during low demand periods is stored in the form of potential energy by pumping water from the lower to the upper reservoir. Later, during high demand periods, electricity is produced by discharging the water through turbines from the upper to the lower reservoir [11]. Despite its extensive use, PSH has limitations [12–14], the most important being that a specific topography is required as both reservoirs must be located at different elevations [15]. Consequently, PSH plants can only be installed in relatively steep areas [16].

Underground pumped storage hydropower (UPSH) [17] is an opportunity to increase the capacity of managing the electrical production in areas where a conventional PSH is not possible. In addition, UPSH avoids some of the adverse environmental impacts related to PSH, and to hydropower in general, such as modifying the flow discharge in a river or changing the seasonal flow regime [18,19]. UPSH uses an underground cavity as the lower reservoir (underground reservoir) and constructs the upper reservoir at the surface [20] or, alternatively, at a shallow depth.

While the underground reservoir can be specifically excavated [21], the most inexpensive (i.e., efficient) option can be to take advantage of abandoned underground mines [22,23]. In addition, there are numerous mines that could be potentially used for UPSH. For example, in France, there are up to 4710 active mines and 101,616 abandoned mines [24], and, in Belgium, there are 964 active mines and more than 5000 abandoned mines [25]. Clearly, not all of these mines are suitable for constructing an UPSH plant; however, the objectives of electricity production and storage could be reached by using only a small portion of them.

For example, in France, these objectives could be reached by using 0.1% of the total available mines [26], and, in Belgium, it would be possible to obtain up to e 4896 MWh considering only mines with suitable characteristics for UPSH [27]. However, since mines are generally not waterproofed, it is expected that water exchanges will occur between the underground reservoir of UPSH plants and the surrounding groundwater systems [28]. This fact may entail negative consequences in terms of the environmental impacts [29–31] and for the efficiency (*η*) of UPSH [32]. We refer to *η* as the ratio between the energy used for pumping water from the underground reservoir and the energy generated when water is discharged from the upper reservoir under ideal conditions. Thus, energy losses due to conversion issues are not considered.

Recently, researchers observed that water exchanges may progressively fill the underground reservoir, reducing *η*. Occasionally, a volume of pumped water could actually not be fully discharged into the underground reservoir because the latter has been partially filled by underground water exchanges [33]. In addition, this volume of water must then be discharged into surface water systems, which could alter their quality because mine water is often not of an appropriate quality. If the released water is to impact the quality of the water bodies, it must be treated before its release to fulfill the current regulations concerning the water quality, such as the Water Framework Directive [34].

This decision should be taken based on the chemical composition of the water pumped from the mine and the expected reactions when mixing with surface water. If a treatment is needed, the additional investment required negatively affects the overall efficiency of the UPSH. Therefore, water exchanges with the surrounding geological medium are of paramount importance and must be investigated. Theoretically, these water exchanges depend on the local hydrogeological characteristics. Therefore, these features should play an essential role in the performance of UPSH influencing *η* and the potential environmental impacts. However, no studies were found that were focused on analyzing how hydrogeological properties influence water exchanges and their associated consequences. This information, however, appears to be crucial to define screening methodologies and to

determine the best locations, in terms of *η* and the environmental impacts, where future UPSH plants could be constructed. where future UPSH plants could be constructed. Thus, the objective of this paper was to determine the role of hydrogeological features

how hydrogeological properties influence water exchanges and their associated consequences. This information, however, appears to be crucial to define screening methodologies and to determine the best locations, in terms of *η* and the environmental impacts,

*Appl. Sci.* **2021**, *11*, x FOR PEER REVIEW 3 of 16

Thus, the objective of this paper was to determine the role of hydrogeological features (i.e., hydraulic conductivity and piezometric head) on the groundwater exchanges occurring in the context of UPSH and how they influence the efficiency of UPSH plants and their associated environmental impacts. This objective was reached by comparing the numerical results of different simulated scenarios based on an abandoned mine in Belgium that potentially could be used for constructing an UPSH plant. (i.e., hydraulic conductivity and piezometric head) on the groundwater exchanges occurring in the context of UPSH and how they influence the efficiency of UPSH plants and their associated environmental impacts. This objective was reached by comparing the numerical results of different simulated scenarios based on an abandoned mine in Belgium that potentially could be used for constructing an UPSH plant. The objective of this investigation was not to ascertain the system behavior at a spe-

The objective of this investigation was not to ascertain the system behavior at a specific site. The final goal was to provide a set of criteria to be considered during the design of future UPSH plants in these types of mining exploitation, to increase their efficiency and decrease the potential environmental impacts. Therefore, although the investigation was based on a real abandoned mine, the numerical models were purposely simplified to allow for determining the role of the different variables in the system behavior and extrapolating the main findings. cific site. The final goal was to provide a set of criteria to be considered during the design of future UPSH plants in these types of mining exploitation, to increase their efficiency and decrease the potential environmental impacts. Therefore, although the investigation was based on a real abandoned mine, the numerical models were purposely simplified to allow for determining the role of the different variables in the system behavior and extrapolating the main findings. The main novelty of this work is that we investigated how the *η* of UPSH plants, and

The main novelty of this work is that we investigated how the *η* of UPSH plants, and their associated environmental impacts vary depending on the hydraulic conductivity (*K*) of the surrounding medium and on the relative elevation of the piezometric head. This information, which has not yet been considered, will be crucial for designing future UPSH plants by taking advantage of abandoned mines. their associated environmental impacts vary depending on the hydraulic conductivity (*K*) of the surrounding medium and on the relative elevation of the piezometric head. This information, which has not yet been considered, will be crucial for designing future UPSH plants by taking advantage of abandoned mines.

## **2. Materials and Methods 2. Materials and Methods**

### *2.1. Problem Statement 2.1. Problem Statement*

The groundwater model was based on the characteristics of an abandoned mine located in Martelange in south-east Belgium (Figure 1). This abandoned mine could be used for the construction of a UPSH plant. The groundwater model was based on the characteristics of an abandoned mine located in Martelange in south-east Belgium (Figure 1). This abandoned mine could be used for the construction of a UPSH plant.

**Figure 1.** General view of Europe with Belgium highlighted with a red line (on the left) and a detailed view of Belgium (on the right) indicating the location of the considered mine (Martelange). **Figure 1.** General view of Europe with Belgium highlighted with a red line (on the left) and a detailed view of Belgium (on the right) indicating the location of the considered mine (Martelange).

The mine of Martelange was developed to extract metamorphic slates from lower Devonian formations in the Ardenne anticlinorium. Specifically, from the "Formation de La Roche". The formation of these fractured slates started at the Lower Devonian, when the transgressive seas of the lower Devonian were at their maximum and clays and silts were deposited. Afterward, the clays and silt deposits were affected by different stages of The mine of Martelange was developed to extract metamorphic slates from lower Devonian formations in the Ardenne anticlinorium. Specifically, from the "Formation de La Roche". The formation of these fractured slates started at the Lower Devonian, when the transgressive seas of the lower Devonian were at their maximum and clays and silts were deposited. Afterward, the clays and silt deposits were affected by different stages of deformation and metamorphism to become a dark fractured slate containing a thin bed of quartzites. The main slate cleavage (schistosity) was induced orthogonally to the main

stress conditions during metamorphism phases but was not actually parallel to the bedding plane. The specific storage coefficient was 10−4 m−1, the saturated water content was 0.05, and the residual water content was 0.01. These parameters are typical of slate mines [22,37]. When the mining activities ceased, the piezometric head recovered, flooding the

deformation and metamorphism to become a dark fractured slate containing a thin bed of quartzites. The main slate cleavage (schistosity) was induced orthogonally to the main stress conditions during metamorphism phases but was not actually parallel to the bed-

The exploitable layers had a dip between 55° and 66° [35]. Concerning the hydrogeological characteristics of the site, reference data was derived from previous works since, unfortunately, we did not have the opportunity to carry out hydraulic tests. According to previous works, these slates have a low global *K* (≈10−7 m/s [36]), and groundwater flows through preferential flow channels in multiple fractures. Thus, the hydrogeological behavior of the formation depends strongly on the aperture, density, and connectivity of the

*Appl. Sci.* **2021**, *11*, x FOR PEER REVIEW 4 of 16

ding plane.

fractures.

The exploitable layers had a dip between 55◦ and 66◦ [35]. Concerning the hydrogeological characteristics of the site, reference data was derived from previous works since, unfortunately, we did not have the opportunity to carry out hydraulic tests. According to previous works, these slates have a low global *<sup>K</sup>* (≈10−<sup>7</sup> m/s [36]), and groundwater flows through preferential flow channels in multiple fractures. Thus, the hydrogeological behavior of the formation depends strongly on the aperture, density, and connectivity of the fractures. mine because its natural position is near the top of the mined cavities. The terms "hydraulic head" and "piezometric head" are used from this point forward to refer the water head inside the underground reservoir and the groundwater head, respectively. The underground cavity roughly consists of nine adjacent and vertical chambers (CH) that are connected through galleries. The volume of the chambers varies as they have different heights. Their width and length are, approximately, 15 and 45 m, respectively, while their heights vary from 70 to 110 m. The top of all chambers is located at the same depth (40 m below the surface), whilst their base depth decreases progressively from CH1

The specific storage coefficient was 10−<sup>4</sup> m−<sup>1</sup> , the saturated water content was 0.05, and the residual water content was 0.01. These parameters are typical of slate mines [22,37]. When the mining activities ceased, the piezometric head recovered, flooding the mine because its natural position is near the top of the mined cavities. The terms "hydraulic head" and "piezometric head" are used from this point forward to refer the water head inside the underground reservoir and the groundwater head, respectively. to CH9, with a decrement of about 5 m (Figure 2). Thus, the bases of chambers CH1, CH2, CH3, CH4, CH5, CH6, CH7, CH8, and CH9 are 150, 145, 140, 135, 130, 125, 120, 115, and 110 m deep, respectively. A vertical 170-m-deep extraction shaft connects the base of CH1 with the surface [38]. Approximately, we calculated that a volume of 400,000 m3 could be potentially used for UPSH. This value was obtained by considering that (1) the top of the chambers is not exceeded by the hydraulic head, and (2) 10% of the maximum available volume is not

The underground cavity roughly consists of nine adjacent and vertical chambers (CH) that are connected through galleries. The volume of the chambers varies as they have different heights. Their width and length are, approximately, 15 and 45 m, respectively, while their heights vary from 70 to 110 m. The top of all chambers is located at the same depth (40 m below the surface), whilst their base depth decreases progressively from CH1 to CH9, with a decrement of about 5 m (Figure 2). Thus, the bases of chambers CH1, CH2, CH3, CH4, CH5, CH6, CH7, CH8, and CH9 are 150, 145, 140, 135, 130, 125, 120, 115, and 110 m deep, respectively. used (i.e., pumped) to avoid total emptying of the reservoir (the underground reservoir is not totally emptied to avoid the pumps and turbines being out of the water). Consequently, this mine has a high water capacity. If a surface reservoir was constructed strategically 500 m away in the northwest direction [38], it could be possible to reach a mean effective hydraulic head difference of 215 m between the underground and the upper reservoirs. Thus, a large amount of electricity could be stored and produced. Assuming an average pumping–discharge rate of 6 m3/s, the available power may reach up to 104 MW (this value may vary depending on the considered efficiency for the pumps and turbines). Figure 2 shows a simplified plain view (2a) and cross section (2b) of the modeled mine.

(b)

**Figure 2.** Schematic plan view (**a**) and cross section (**b**) of the mine in Martelange that is considered in this study. The black and red dashed lines in (**b**) indicate the natural position of the piezometric head considered in the two simulated scenarios. The black line indicates the scenarios called TOP, whilst the red line indicates the scenarios denoted as MIDDLE. The pictures taken inside the chambers can be checked at http://tchorski.morkitu.org/2/martelange-02.htm. **Figure 2.** Schematic plan view (**a**) and cross section (**b**) of the mine in Martelange that is considered in this study. The black and red dashed lines in (**b**) indicate the natural position of the piezometric head considered in the two simulated scenarios. The black line indicates the scenarios called TOP, whilst the red line indicates the scenarios denoted as MIDDLE. The pictures taken inside the chambers can be checked at http://tchorski.morkitu.org/2/martelange-02.htm.

డఏ

డ௧

evolves according with the following equations [43]:

SUFT3D [39,40] is the finite element numerical code we used to develop the groundwater numerical model. This code solves the groundwater flow equation (Equation 1) based on a mixed formulation of Richard's equation proposed by Celia et al. [41] using

where *t* is the time [T], ߠ is the water content [-], *z* is the elevation [L], *h* is the pressure head [L], *q* is a source/sink term [T−1], and ܭ is the hydraulic conductivity tensor [LT−1]

where ܭௌ is the saturated permeability tensor [LT−1], and ܭ is the relative hydraulic conductivity [-] that varies from a value of 1 for full saturation to a value of 0 when the water phase is considered immobilized [42]. In the partially saturated zone, the value of ܭ

> ሺఏೞିఏሻ ሺ್ିೌሻ

> > ߠ−ߠ

ߠ − ௦ߠ

where ߠ is the residual water content [-], ߠ௦ is the saturated water content [-], ℎ is the pressure head at which the water content is just lower than the saturated one [L], and ℎ is the pressure head at which the water content is the same as the residual one [L]. The ܭ varies linearly between the unsaturated and saturated zones as can be observed in Equations (3) and (4). This adopted linearity for defining the transition between saturated and unsaturated zones does not alter the results of the model, because this work is focused on processes that occurred only in the saturated portion of the soil, while this contributed to mitigate the convergence errors that are common when non-linear expressions are used.

ߠ=ߠ

= ሻߠሺܭ

(1) ,ݍݖሻߠሺܭ ∙ ℎሻߠሺܭ ∙ =

(2) ,ௌܭܭ = ܭ

ሺℎ−ℎሻ, (3)

(4)

*2.2. Description of the Numerical Model* 

the control volume finite element (CVFE):

2.2.1. Code

defined as

A vertical 170-m-deep extraction shaft connects the base of CH1 with the surface [38]. Approximately, we calculated that a volume of 400,000 m<sup>3</sup> could be potentially used for UPSH. This value was obtained by considering that (1) the top of the chambers is not exceeded by the hydraulic head, and (2) 10% of the maximum available volume is not used (i.e., pumped) to avoid total emptying of the reservoir (the underground reservoir is not totally emptied to avoid the pumps and turbines being out of the water). Consequently, this mine has a high water capacity.

If a surface reservoir was constructed strategically 500 m away in the northwest direction [38], it could be possible to reach a mean effective hydraulic head difference of 215 m between the underground and the upper reservoirs. Thus, a large amount of electricity could be stored and produced. Assuming an average pumping–discharge rate of 6 m3/s, the available power may reach up to 10<sup>4</sup> MW (this value may vary depending on the considered efficiency for the pumps and turbines). Figure 2 shows a simplified plain view (2a) and cross section (2b) of the modeled mine.
