Transforming Coal Pits into Renewable Energy Sources: The Potential of Pumped Hydro Energy Storage in the Bełchatów Lignite Mine

The energy transition faces key challenges, including enhancing energy storage potential due to renewable energy sources' intermittency and repurposing abandoned coal mines. An approach that addresses both of these problems comprehensively is the concept of using mine pits and mine spoil heaps as sites for the location of Pumped Hydro Energy Storage (PHES). Poland, as one of European Union's member states and one of the most coal-dependent countries in Europe, is also obliged to fulfil postulates related to the transformation of electricity system. This work focuses on use of the Bełchatów Lignite Mine (KWB-B) for construction of PHES plant. It was assumed that the pits would take over a role of lower reservoir, while heaps could be used as upper reservoirs. This should be accomplished through appropriate earthworks, construction and prior analysis in terms of soil bearing capacity. In our work GIS tools have been used to determine key parameters of the PHES system, which are reservoir volumes, usable head and land slopes. The energy losses associated with required length of a penstock have been determined for each alternative. The Colebrook-White formula has been used to calculate a pressure height losses in the pipelines. In next step, another factor affecting reduction in energy efficiency has been determined, which is a loss of water from the upper reservoir through evaporation. The evaporation model has been created based on Penman-Monteith equation, which combines water evaporation related to aerodynamics and solar radiation. Finally, process of filling the reservoirs has been analysed, which under spontaneous groundwater filling conditions is estimated to take up to 60 years. For this reason, the concept presented in this study assumes additional reservoir recharge from the Warta River, around 29 kilometres away.  According to the results of the analysis done, in the most realistic scenarios, an energy storage potential is between 16.4 and 36.2 GWh per cycle, operating at around 75% efficiency, producing between 9.7 and 13.7 TWh of electricity per year, which is around 45% of the energy produced annually by the nearby Bełchatów lignite-fired power plant. The proposed concept enables 2,5-5,5-fold increase of the closed loop PHES storage capacity in Poland. With plans to phase out this lignite-fired power plant by 2036, development of a hybrid renewable energy source is a promising alternative that could be implemented in this area, especially if the filling of reservoirs would be accelerated through additional supply from nearby rivers.