Since the Industrial Revolution, fossil fuels have been the dominant energy sources, both in electricity generation and heating. However, in recent years, renewable energy sources have gained importance in order to comply with the guidelines set out in the 2015 Paris Agreement and the goal of zero carbon emissions by 2050. To reach this goal, energy-related global emissions should be reduced to 30% below 2019 levels by 2030, and 75% by 2040 (BloombergNEF 2022). On the other hand, global energy demand is expected to grow by more than a quarter by 2040 and the current situation due to the COVID-19 pandemic and the war in Ukraine has only made the situation more difficult. This is expected to be achieved by promoting the accelerated development of clean, low-carbon renewable energy sources and improving energy efficiency, as set out in Directive (EU) 2018/2002. An outlook based on the current configuration shows a scenario in which fossil fuel demand slows to a plateau in the 2030s and then falls slightly by 2050, as almost all the growth in energy demand will come from low-emission sources (IEA 2022).
Among these renewable sources, only hydroelectric and geothermal systems use continental water to generate energy, so they are covered in this study in order to find clean alternatives for the energy use of mine water.
Hydro energy can be defined as the energy harnessed from flowing water through a turbine to generate electricity, in such a way that an electric generator converts the mechanical energy of rotation into electric power. A reservoir is required to store water before it moves through the hydraulic turbine. During periods of lower demand, the water is pumped to the upper reservoir and during times of high demand, the water flows back through turbines to produce electricity. The use of hydroelectric generation systems is being promoted throughout the world, considering that water is a renewable, free and abundant energy resource. This type of technology is very well known and proven, since it is over a century old. Over time, there have been relevant developments in its design (structures, components and distribution systems), and it is one of the largest renewable energy source for electricity generation and energy storage in the world (Marahatta et al. 2022; Kadiyala et al. 2016; Wagner and Mathur 2011). Mine water can be used to generate hydro power (Jardón et al. 2013) and store energy, for example by means of the underground pumped hydroelectric energy storage systems (UPHS) (Álvarez et al. 2021).
Geothermal energy is that stored in the form of heat below the solid surface of the Earth, which is transmitted from its inner layers and accumulates in rocks and groundwater (Barbier 2002). It is considered a renewable energy (the Earth's heat is unlimited on a human scale and will be available for future generations), economical (the investment cost of a geothermal installation for heating is superior to conventional systems, but its maintenance costs are lower and its performance is higher), efficient (it is considered the best for heating and cooling by the US Environmental Protection Agency), available (it is continuous throughout the year and accessible in all countries, not depending on external factors) and clean (it does not contribute to the emission of greenhouse gases). Actually, installations that use heat pumps for heating, cooling and the production of Domestic Hot Water (DHW) consume electrical energy for the operation of electric compressors, circulation pumps and building fans, but the emissions are much lower than those of conventional systems. Additionally, geothermal systems are well accepted and most people are supportive of them, since they are not intrusive and in many cases they are almost invisible or with low impact, due to the compact installations (Sundell and Rämä 2022; Tester et al. 2021; Rosen and Koohi-Fayegh 2017; Glassley 2015; Gupta and Roy 2007; US EPA 1997). The US Environmental Protection Agency (US EPA) estimated that geothermal heat pumps could reduce energy consumption by up to 72% compared to conventional electrical heating and air conditioning. CO2 emission reductions from 15–77% were achieved through the use of Ground-source heat pumps (GSHPs) in comparison to residential fossil fuel heating systems (US EPA 1997; Omer 2008). The geothermal potential of mine water, particularly from closed coal mines, is well recognized all over the world, due to its stable and often high temperature. Even low-temperature (low enthalpy) mine water resources might provide significant direct-use geothermal energy for space heating (Preene and Younger 2014). Geothermal is the most common energetic use of mine water, particularly at coal mines, and there are applications in quite a few countries in Europe and North America (Banks et al. 2022; Menéndez et al. 2020; Andrés et al. 2017; Peralta et al. 2015; Watzlaf and Ackman 2006; Islam et al. 2004).
Notwithstanding, attention should be paid to the mine water quality. Iron sulphides, which are often present in coal deposits, can be exposed and oxidize in mine voids when the mine is active, so when they are flooded, water quality can deteriorate. It is normally assumed that this leads to acidic waters, rich in iron and sulfates, or in the presence of carbonates, the waters can be hard and encrusting. These mine water characteristics might compromise its geothermal use, but it largely depends on the case (Wolkersdorfer et al. 2022; Loredo et al. 2017; Wolkersdorfer et al. 2008; Younger et al. 2002). Mine waters have variable flow and quality, depending on the hydrogeology, the type of mining exploitation and the geological materials involved. Mine water can be used also as a water resource for agricultural, industrial, livestock, environmental or domestic purposes, provided that its hydrochemical characteristics meet the specific requirements demanded. Many mine waters, particularly the drainages from mountain mining, could be used for human consumption without requiring more intensive treatment than usual (Loredo et al. 2017; Ordóñez et al. 2012; Wolkersdorfer et al. 2008).
The circular economy strategy in Europe proposes to reduce the generation of waste, increase reuse, improve efficiency in the use of water and reduce the emission of greenhouse gases. In particular, the sustainable use of mine water makes it possible to transform what is normally considered waste into a renewable energy resource, giving it a second life, which is of particular interest in old mining areas in decline. The objective of this work is to study the possibilities of energy use of the mine water upwellings of the Laciana Valley (León, NW Spain) and to categorize them according to an ample analysis, in order to provide a development alternative to a former mining area, currently economically and demographically disadvantaged.