Pumped-storage power plants are an efficient option for energy storage to address short-term variances. In general, pumped-storages are dependent on high differences in altitude and therefore located in mountainous areas. An advantageous variation is the installation of pumped-storage power plants completely or partially underground. Three variations are feasible: usage of the infrastructure of closed mines, usage of the infrastructure of operating mines, and usage of unexploited rock. Feasibility studies for the installation of pumped-storages in closed and operating mines have been presented and discussed. Pumped-storage power plants are crucial for the restructuring of Germany´s energy system but until they can be economically implemented it is up to the politicians to encourage the realisation of such projects.
Within the last years wind power has gained in importance in Germany. Characteristic for wind power is that it is generated in the northern and eastern parts of the country, especially close to the coast, but the power is mainly needed in the western and southern parts of Germany. Furthermore, the ability to generate wind power does not always correspond with the energy consumption. Therefore, a sufficient long-term storage and transport concept is needed to balance supply and demand of wind energy. Storage capacities are necessary for the considerable variations over time – daily, monthly and yearly variations – as well as storage options for short-term load and generation fluctuations. So far for the long-term variations no large-scale technologies are established, whereas for the short-term variations pumped-storage power plants provide a potential workaround (1). Drawbacks for this technology are the required differences in altitude as well as the considerable space requirements causing a huge impact on the environment. Already in 2007, a research group of Energie Forschungszentrum Niedersachsen (EFZN, energy-research centre of Lower Saxony), a scientific institute of Clausthal University of Technology (TU Clausthal), Clausthal-Zellerfeld/Germany, has started to work on these problems. One approach is the subsequent use of closed mines for underground storage systems of generated wind power which is looked at by a team of experts from the fields of mining, mechanical and electrical engineering as well as economic, social and legal sciences. In the meantime, the EFZN is hosting conferences to enable an interdisciplinary dialogue on pumped-storage solutions.
2 Underground pumped-storage power plants
A pumped-storage power plant is used to store surplus electric energy generated by wind power by pumping water from a low-altitude reservoir to a high-altitude reservoir (Figure 1). The electrical energy is transformed to potential energy whereby the stored energy in the water is proportional to the mass of the pumped water and the difference in altitude. When energy is demanded the water from the high-altitude reservoir is released and led through a turbine which operates a generator to generate electrical energy with an overall efficiency of 75 % (2).
Installing a pumped-storage power plant underground – or at least installing the low altitude reservoir underground – allows the usage of locations independent of natural differences in altitude and decreases the impact on the environment.
Three types of underground pumped-storage power plants can be distinguished (3):
- pumped-storage power plants in closed mines;
- pumped-storage power plants in operating mines; and
- pumped-storage power plants in unexploited rock.
The relevant advantages and disadvantages of each type are discussed below. Generally, it can be said that as sooner a pumped-storage power plant is included in the planning process of a mine the more economical and sustainable is this subsequent use.
2.1 Pumped-storage power plants in closed mines
In 2011 the EFZN presented in Goslar/Germany a project investigating the suitability of old mining regions as pumped-storage power plant locations. Metal mines proved to be the most suitable ones due to their stable geological conditions and the considerable depth of the mines. However, most of these mines are very old and cannot be used completely for modern pumped-storages. Potash and rock salt mines provide large excavations and workings but due to the solubility of the deposit fresh water cannot be stored underground. Likewise, brines are not applicable because of the temperature dependent crystallisation behaviour and the used machine technique. Furthermore, a long-term impermeability and mechanical stability of the excavations cannot be ensured. By comparison, old spar mines provide suitable stability but the excavations in these mines are normally backfilled (1). Closed collieries do not have sufficiently large excavations and would require the drivage of new excavations with additional impermeable lining. Also, it would be mandatory to install explosion protected electrical equipment which causes additional costs. The most suitable closed mines for a pumped-storage project are in the Harz Mountains, the Erz Mountains, and the Siegerland region. However, all possible mines require high efforts to be suitable for such a project. The EFZN project evaluated among others the amalgamated mine Bad Grund as a model mine.
The whole development needed to use this mine as a pumped-storage power plant was simulated in detail. The existing drifts of the mine were not suitable and the construction of a new low altitude and high altitude reservoir was included in the planning. As a first step the renovation of the shaft was considered to enable transport of machinery and material. A roadway system of parallel drifts was chosen because of its economic and mechanically advantageous development. Generally, the influence of the drift design is neglectable, only ventilation must be ensured, e. g. by ducts, to allow the air to escape when the water in the drifts is rising. For the development of the drifts the planning software VULCAN was used. The simulation of the water flow was realised with the software ANSYS. Significant is a large central connecting drift. The simulation showed that the flow velocities are manageable, and a maximum value of 3.7 m/s can be reached. This study – conducted mainly by researchers from TU Clausthal – proved the feasibility of such a project. The capital costs for a planned storage capacity of 400 MWh with a power output of 100 MW are estimated below 2,000 €/kW (1).
Another study evaluates the usage of excavations at the coal mine Prosper-Haniel for a pumped-storage power plant with a power output of 200 MW (4, 5). From 2012 until 2014 the feasibility of this project was evaluated by a group of eleven partners from five institutions including the University Duisburg-Essen and Ruhr University Bochum (RUB). The results of the study stated that closed collieries are not suitable and that a closed system should be preferred rather than an open system including underground water. For the development of the underground reservoir with a diameter of 8 m the usage of a full-face cutting machine is planned. The reservoir consists of a 16 km long circle drift which is lined completely with strong concrete segments. The high-altitude reservoir is constructed on the mine premises on surface which further reduces the costs as well as the usage of the existing shafts as accesses. All things considered, the capital costs are estimated up to 2,600 €/kW. Like other projects it is not yet economically feasible.
Similar projects are conducted abroad. China investigates possible subsequent usages of mines as pumped-storage power plants as e. g. at a cooper mine in the province Yunnan near Kunming. This promising mine is located in a mountainous area providing a natural difference in altitude. Furthermore, a near by river might be redirected and integrated in a run-of-river power station.
2.2 Pumped-storage power plants in operating mines
Another study of TU Clausthal evaluated the possibility of a pumped-storage power plant at the mine Wohlverwahrt-Nammen operated by Barbara Rohstofbetriebe at Porta Westfalica in North Rhine-Westphalia. The mine operates in a marine sedimentary iron oolite formation in the lower coral oolite. The steep dip of 20g to the North provides a sufficient difference in altitude for the two reservoirs which can be installed in the existing excavations. Pumping the water between the two reservoirs can be realised with pipes running in the existing drifts of the mine. Despite the low capital costs required to implement a pumped-storage power plant at the mine no investor has been found so far. A schematic sketch is shown in Figure 2.
2.3 Pumped-storage power plants in unexploited rock
A third variation is the drivage of excavations for reservoirs in unexploited rock. Different advantages can be achieved including the selection of locations with optimal rock mechanical conditions and the selection of locations in close proximity to wind farms. A disadvantage is the necessity to build completely new access elements and an underground infrastructure. If environmental aspects are in favour the high altitude reservoir can be constructed at the surface to keep the capital costs manageable.
All three variations of underground pumped-storage power plants are technically feasible. However, the expected costs are difficult to estimate – a problem shared with all tunnel projects for which cost overruns of 50 % are not uncommon. So far, an underground pumped-storage project has not been realised impeding a precise cost estimation. Nevertheless, for the restructuring of the energy system storage capacities are crucial and the implementation of storage projects lies within the duty of politics. The deposit of the Barbara Rohstoffbetriebe e. g. is ideal for such a project in both active mines and unexploited areas. Those areas, even for a larger project, are sufficiently available there.
Regarding the technical aspect, a pumped-storage power plant in unexploited rock is preferable due to the possible proximity to a wind farm. By comparison, the usage of closed or operating mines provides cost benefits as existing infrastructure can be used. However, costs for the renovation of shafts from closed mines should not be underestimated. A useful subsequent use of mining infrastructure can be economically achieved by including such projects in the planning process and preparing the required excavations during the operation of the mine. This approach requires only low additional costs and is described in the concept of “blue mining” developed in 2013 by the Institute of Mining of TU Clausthal (3,6).
A sufficient number of studies and concepts for underground pumped-storage power plants is available. Even the automobile industry, which is facing high energy demands for their production, as well as energy suppliers have conducted studies and the feasibility of pumped-storages in different formations has been proven repeatedly. The next step requires the actual implementation of such projects. If the implementation cannot be driven by the economy, it is the politics responsibility to fill in as they call for the realisation of the energy transition.
(1) Beck, H.-P.; Schmidt, M. (Hrsg.); Langefeld, O. e.a.: Windenergiespeicherung durch Nachnutzung stillgelegter Bergwerke. Abschlussbericht des EFZN, Goslar; 31.08.2011; ISBN 978-3-942216-54-8.
(2) Giesecke, J.; Mosonyi,E.: Wasserkraftanlagen Planung, Bau und Betrieb. 5. Aktualisierte und erweiterte Auflage – Berlin; Springer 2009.
(3) Langefeld, O.; Kellner, M.: “Blue Mining” – The future of Mining. Proceedings 6th International Conference on Sustainable Development in the Minerals Industry, 30 June – 3 July 2013, Milos Island, Greece.
(4) Niemann, A. e.a.: Entwicklung eines Realisierungskonzeptes für die Nutzung von Anlagen des Steinkohlenbergbaus als unterirdi
sche Pumpspeicherkraftwerke – zusammenfassender Abschlussbericht. Förderkennzeichen 64.65.69-PRO-0039; gefördert durch NRW und EU.
(5) Niemann, A.; Balmes, J. P.; Schreiber, U.; Wagner, H.-J.; Friedrich, T.: Ein untertägiges Pumpspeicherwerk am Bergwerk -Prosper-Haniel in Bottrop – Sachstand und Perspektiven. Mining Report Glückauf (154), Heft 3/2018; S. 214 – 223.
(6) Langefeld, O., Binder, A.: Responsible Mining. Mining Report Glückauf (154), Heft 1/2018; S. 20 – 27.