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Simplified representation of the Wiemannsbuchtschacht mineshaft
© M. Schmidt

Factors influencing the investment decision for pumped storage power plants
© EFZN
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Mines used as pumped storage power stations?

Taking a closer look at model mines

Based on two specific case studies, an interdisciplinary team of mining professionals, mechanical engineers, electrical engineers, lawyers and businessmen examined the complex problems that have to be solved when constructing an underground pumped storage system. They selected the former ore mine Pöhla in the Erzgebirge and the Wiemannsbuchtschacht mineshaft in the abandoned ore mine Grund in the Harz as model mines. For these locations, they explored technical, legal, planning and economic aspects that will be presented here using the Grund mine as an example. In conclusion, they consider both mines viable.

Grund mine: Storage construction scenario

Geologically, the Grund ore mine is part of the northwestern Upper Harz. Various metal ores, especially silverrich lead ores and their accompanying ores, were mined here. The Wiemannsbuchtschacht mineshaft along with three other shafts comprise the mining claim of the ore mine. It reaches down to a depth of 761 m and its inner diameter is at least 3.5 metres. It is currently sealed by a concrete plug down to a depth of 130 m. Accumulating pit water has flooded the lower cavities up to the height of the Ernst-August Gallery. In order to retrofit the mine, the researchers developed the following scenario: After removing the concrete plug and the water inside the mine workings, the shaft is then to be restored. At the same time, drivage and securing of the reservoirs and the hydroelectric machine caverns begins. The storage tanks are not designed as a single contiguous cavity but as a linked system with a cross-section of 5 m by 7.5 m and with a spacing of 15 m. This way, a usable storage capacity of about 240,000 to 260,000 m3 can be created with an average drop height of 700 m.

Additional shafts and drifts are constructed as transport and escape routes. Depending on the output of the power plant, a cable channel for high or extrahigh voltage cables may also be required. For grid connection on a voltage level of 110 kV, approximately 6 km need to be bridged up to the 110-kV overhead power line of Harz Energie Netz GmbH.

The sub-assemblies of the generating set are brought into the hydroelectric machine cavern via the shaft. For power generation, the water flows from the upper reservoir into the lower reservoir through a Francis turbine that drives an electric generator. While pumping, the generator operates as the motor of a two-stage pump that returns the water.

Economic efficiency

Depending on the scenario, the specific investment costs of the pumped storage power plant at the Grund site are within the approximate range of 1,800 euros/kW at a storage capacity of 400 MWh. For a pilot plant, the expected costs would amount to a total of around 180 million euros. The largest cost item is underground work with 60 %. The expected cost for mechanical engineering is 26 % and around 12 % for the grid connection. With a hybrid solution, i.e. the construction of an aboveground reservoir, further savings are possible.

Underground work increases the specific investment costs significantly more than aboveground pumped storages (between 650 euros and 1,435 euros per kW). However, it is unknown to which extent new locations for surface pumped storage power plants are even eligible for approval or would be publically accepted. Adding to that, the costs of mitigation and compensation measures may be significantly higher than in the past.

After comparing costs with possible revenues, it becomes clear that trading on the spot market exclusively is by far not enough for an amortisation. Reactive power provision or marketing black-start capability as a service is not enough. This could change if a majority of the revenue could be generated by pure power provision alone. The researchers therefore see the uncertainties in how the legal framework for the electricity market might develop rather than technical issues as the biggest obstacle to a specific investment decision.

Approval and acceptance

An underground pumped storage power plant has not yet been realised in Germany. Therefore, there is no experience in terms of its legal classification. Issues investigated were in particular emission control, mining law, water law, construction law, waste legislation, energy industry provisions and environmental impact assessment. An approval seems to be generally possible under the current regulatory framework. A consideration of each specific case is necessary because the approval-related legal situation depends on the location. Given the scale of pumped storage schemes, joint processing of approval requirements in one administrative proceeding would be desirable.

Acceptance risks arise mainly from the competitive relationships with other goods, such as water protection zones, the preservation of the country’s historical heritage and damage to the environment or persons. For example, the Ernst-August Gallery has recently been declared part of the World Heritage of the “Upper Harz Water Management System”. The shaft buildings are listed national monuments. The nearby water protection zones and landscape conservation areas and a site protected under the Habitats Directive must also be considered when planning.

The locations of Bad Grund and Pöhla exhibit highly complex characteristics, so that no general statements on acceptance can be made based on individual case assessments. In addition, the factors influencing acceptance can indeed be identified qualitatively, but their actual weight in the population can only be discerned after implementing the pumped storage power plant.

Conclusion

The researchers reached the conclusion that the construction of a pilot plant at the Wiemannsbuchtschacht mineshaft is in principle possible in technical, legal, environmental and economic terms given a careful consideration of environmental consequences and acceptance. In further steps, they aim to draw a detailed plan for a pilot plant and evaluate it with a cost-benefit analysis. The preliminary investigations could be completed in 2014. A first small pilot plant for research purposes could be realised in the period between 2015 and 2018 at the earliest.

Projektinfo 18/2013:
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