Hydrochemical changes induced by underground pumped storage hydropower and their associated impacts

Abstract

Underground pumped storage hydropower (UPSH) using abandoned mines is an alternative system to manage electricity production in flat regions. Water from an underground reservoir is pumped to a surface reservoir to store electricity in the form of potential energy. Later, water is discharged through turbines into the underground reservoir to produce electricity when demand increases. During this operation, the water hydrochemistry continuously evolves. It varies in order to reach chemical equilibrium with the atmosphere (in the surface reservoir) and with the surrounding porous medium and groundwater (in the underground reservoir). The hydrochemical variations may lead to reactions in the reservoirs and in the surrounding porous medium, causing potentially negative consequences for the environment and the system efficiency, especially when pyrite is present in the surrounding porous medium. In this case, pyrite oxidation leads to a decrease in pH and the precipitation of goethite or schwertmannite in the surface reservoir. The decrease in pH is mitigated when calcite is present in the porous medium. However, other concerns may arise, such as slight increases in pH, the precipitation of ferrihydrite and calcite in the surface reservoir, and the oxidation of pyrite and dissolution of calcite in the surrounding porous medium. Understanding the pH variations and the precipitation/dissolution of minerals is of paramount importance in terms of the environmental impacts and system efficiency. For this reason, with numerical modelling, this work investigates the main hydrochemical changes and their associated consequences when abandoned deep mines are used for UPSH. The main objective is to highlight the importance of considering hydrochemical aspects when designing future UPSH plants.

Parametric assessment of hydrochemical changes associated to underground pumped hydropower storage

“…Impacts of the water exchanges on the groundwater flow conditions and on the UPHS efficiency evolution have been also investigated (Poulain et al, 2016;Pujades et al, 2016;Bodeux et al, 2017;Pujades et al, 2017a). Finally, Pujades et al (2017bPujades et al ( , 2018 have investigated quantitatively potential hydrochemical impacts induced by UPHS when abandoned mines are used as underground reservoirs. They have simulated chemical changes associated to UPHS as water tends to evolve chemically to reach chemical equilibrium with the atmosphere (in the surface reservoir) and with the surrounding porous medium (in the underground reservoir).…”

“…Hydrochemical changes vary depending on the chemical composition of the porous medium. Pujades et al (2018) studied them under three different scenarios that differed in the chemical composition of the porous medium. They consider the presence of only pyrite, pyrite and calcite or only calcite.…”

“…The actual future frequency of pumping and discharge phases cannot be known in advance as it depends on numerous factors (Pujades et al, 2018). Therefore, day/night cycles are considered defining regular pumping and discharge phases allowing to obtain representative results for prediction.…”

“…This assumption is reasonable because water is not stagnant and is mostly moving in the surface reservoir. Anyway, if water pO 2 and pCO 2 do not reach completely chemical equilibrium with the atmosphere in the upper reservoir, chemical reactions are the same but their consequences are mitigated as shown in Pujades et al (2018).…”

“…The chemistry of the discharged water depends on the previously pumped water that is aerated in the surface reservoir before to be discharged. Thus, successive iterations with increasing simulated periods are performed following the same procedure than Pujades et al (2018). The chemical characteristics of the discharged water are derived from the previous iteration results and are reintroduced in each iteration.…”