Underground pumped storage hydropower (UPSH) induces hydrochemical changes when water evolves to reach equilibrium with the atmosphere (in the surface reservoir) and with the surrounding medium (in the underground reservoir). These hydrochemical changes may impact the environment and the efficiency of the system (i.e., the UPSH plant), especially in coal mine environments where the presence of sulphide minerals is common. For this reason, it is needed to assess the variables that control the behavior of the system in order to establish criteria for the selection of abandoned mines to be used as underground reservoirs in future UPSH plants.
Coupled hydro-chemical numerical models are used for investigating the influence of hydraulic parameters on the hydrochemical changes when pyrite is present in the surrounding medium. Results show the role of the hydraulic conductivity and the porosity on the system behavior, which is helpful for selecting those abandoned mines where the hydrochemical changes and their associated consequences will be less.
Underground pumped storage hydropower (UPSH) is an alternative energy storage system (ESS) for flat regions (Pujades et al., 2016; Pummer and Schüttrumpf, 2018). UPSH plants consist in two reservoirs, one is underground while the other is located at the surface (Barnes and Levine, 2011). The excess of electricity generated during low demand energy periods is used for pumping water from the underground to the surface reservoir, and when the demand of energy increases, water is released into the underground reservoir through turbines for generating electricity. Although there are not bibliographical evidences of UPSH constructed plants, this technology has been investigated in different parts of the world: the Netherlands (Min, 1984), Singapore (Wong, 1996), USA (Allen et al., 1984; Severson, 2011), Germany (Beck and Schmidt, 2011; Zillman and Perau, 2015; Alvarado et al., 2016), Belgium (Bodeux et al., 2016; Poulain et al., 2018), Spain (Menéndez et al., 2017) and South Africa (Winde and Stoch, 2010a, b; Khan and Davidson, 2016; Winde et al., 2017), Finland and Australia (Academy of Science of South Africa, 2016).
Although it would be possible to drill the underground reservoir, the alternative considered in this paper, which may be more efficient and have positive effects for local communities after the cessation of mine activities, would consist in re-using abandoned mines. The main concern of UPSH using abandoned mines is the water exchanges between the underground reservoir and the surrounding porous medium because they can affect the environment and the efficiency of the UPSH plant. Most studies focused on water exchanges consider flow related issues (Bodeux et al., 2017; Pujades et al., 2017a). However, recently, Pujades et al. (2017b) have suggested the importance of considering hydrochemical changes induced by UPSH plants. These changes may impact on the environment and affect the efficiency of the plant.
General view of the problem
Water is aerated when it is pumped, discharged and stored in the surface reservoir. As a result, its chemistry evolves to reach equilibrium with the atmosphere. Similarly, when water is discharged into the underground reservoir, its chemistry evolves to reach equilibrium with the surrounding porous medium. These hydrochemical changes may produce pH variations, especially in coal mine contexts where pyrite is a common mineral. Its oxidation leads to pH lowering. Low pH values would affect the environment (decreasing the quality of groundwater and surface water bodies) and the efficiency of the plant (corroding UPSH facilities such as pipes, turbines, pumps or concrete structures).
Although the general behaviour of the system has been previously stated (Pujades et al., 2018), there is not any study in which the influence of the aquifer hydraulic parameters on a UPSH system is assessed. To establish the role of aquifer hydraulic parameters will be meaningful for selecting the most suitable places where constructing future UPSH plants. Thus, the main objective of this work is to investigate the importance of the hydraulic parameters on the pH variations occurring when abandoned coal mines (with presence of pyrite) are used for UPSH.
A 200 m thick domain with an underground reservoir in the middle is
considered (Fig. 1a). The reservoir (
Frequency of pumping and discharging phases is chosen according to day/night
cycles (i.e., 12 h pumping and 12 h discharging water). Pumping and
discharging rates are 43 000 m
It is assumed that the modelled cavity belongs to an abandoned coal mine. Coal deposits usually contain sulphide minerals, whose oxidation may entail important consequences for water chemistry. Pyrite is the most common sulphide mineral in this kind of deposits (Akcil and Koldas, 2006), and thus, it is assumed that the porous medium contains 1 % pyrite. Reaction rates for the other minerals (e.g. silicates) are assumed very low (White and Brantley, 1995), and are neglected.
The code PHAST (Parkhurst et al., 1995; Parkhurst and Kipp, 2002) is used to
simulate the problem. This code solves multicomponent, reactive solute
transport in three-dimensional saturated groundwater flow (Parkhurst et al.,
2010). The watershed divide crossing the domain from the west to the east
boundaries (Fig. 1a) allows modeling only half of the domain without
affecting the results. The modeled “half-domain” is divided in 15 600
elements whose size ranges from 2 to 100 m (they are refined towards the
underground reservoir) (Fig. 1). Dirichlet boundary conditions (BCs) are
implemented in the west and east boundaries with head prescribed at 92.5 and
97.5 m depth, respectively. Flow-rate BCs are adopted in nodes located
inside the underground reservoir for simulating the pumpings and discharges.
The values of longitudinal (
Pujades et al. (2018) stated the main trends of the system. Dissolved oxygen increases when the water is pumped, discharged and stored in the surface reservoir. When this water is discharged in the underground reservoir and is exchanged with the surrounding porous medium, it oxidizes pyrite decreasing the groundwater pH (Fig. 2). Pyrite is oxidized until all available oxygen is consumed. Subsequently, water is pumped, discharged and stored in the surface reservoir and the dissolved oxygen increases again. pH in the underground reservoir decreases when it is filled with groundwater from the surrounding porous medium (Fig. 2). As a result, when this water is pumped to the surface reservoir, pH also decreases on it. In addition, minerals such as ferrihydrite, goethite and schwermannite may precipitate in the surface reservoir contributing also to the pH reduction.
pH evolution in the surrounding porous medium at 15 m from the
underground reservoir
Results show the differences in percentage between Sce1 and the scenarios Sce2 and Sce3. A positive difference means that computed results (for Sce2 and Sce3) are higher than those obtained for Sce1 while differences are negative when they are lower.
Figure 3 shows the results concerning the pH evolution in the surface (left)
and underground reservoirs (right). pH is higher for both scenarios (Sce2
and Sce3) than that computed for Sce1. pH decreases less for Sce2 and Sce3
than for Sce1 because the volume of groundwater reaching the reservoir from
the upgradient side, which is less affected by the pyrite oxidation,
increases when the values of
pH differences in the surface
Figure 4 shows the results concerning the pH evolution in the surrounding porous medium. pH is computed at a distance of 15 m from the underground reservoir (in the downgradient side). In Sce2, pH is lower than that for Sce1 because dissolved oxygen reaches faster the surrounding medium (dissolving more pyrite) and groundwater with low pH flows faster until the distance at which the pH is computed. Contrarily, pH decreases less for Sce3 than in Sce1 although the water exchanges increase and the dissolved oxygen reaches faster the surrounding medium. In this case (Sce3), the volume of water in the aquifer is higher than that of Sce1 and the pH reduction is buffered (i.e., there is a dilution effect).
pH differences in the surrounding porous medium. pH is computed 15 m far away from the underground reservoir in the downstream direction.
This work investigates the influence of the hydraulic conductivity and the porosity on the hydrochemical changes induced by UPSH. Results could be helpful for defining screening strategies, which should be used for the selection of potential abandoned coal mines to construct future UPSH plants.
Results show that pH decreases less in the reservoirs but more in the
surrounding porous medium when the value of
A different behaviour is observed when porosity is increased. In this case, pH decreases less in the reservoirs and also in the surrounding porous medium. In that particular case, the adverse effects of UPSH in the presence of pyrite would be mitigated. Thus, coal mines surrounded by materials with high values of porosity would be preferable for constructing UPSH plants.
Data containing the numerical results presented in this
article are openly available in Open Science Framework at
The authors declare that they have no conflict of interest.
This article is part of the special issue “European Geosciences Union General Assembly 2018, EGU Division Energy, Resources & Environment (ERE)”. It is a result of the EGU General Assembly 2018, Vienna, Austria, 8–13 April 2018.
Estanislao Pujades and Anna Jurado gratefully acknowledge the financial support from the University of Liège and the EU through the Marie Curie BeIPD-COFUND postdoctoral fellowship programme (2014–2016 and 2015–2017 “Fellows from FP7-MSCA-COFUND, 600405”). This research was supported by the Public Service of Wallonia – Department of Energy and Sustainable Building through the Smartwater project. The article processing charges for this open-access publication were covered by a Research Centre of the Helmholtz Association. Edited by: Luke Griffiths Reviewed by: Mauro Cacace and one anonymous referee