The German coal industry will leave an extensive infrastructure behind when it finally comes to a close at the end of 2018. Mine shafts up to 1,200 m in depth, a huge network of underground workings and extensive water drainage and pumping facilities create real prospects for all kinds of after-use projects. The development of underground pumped hydro storage power plants (UPHS plants) at existing mine sites could provide an answer to the energy storage problem. A collaborative project currently under way seeks to address all the aspects involved on a result-oriented basis and has chosen Prosper-Haniel colliery, the last of the Ruhr mines, as a reference point. The colliery will cease operations at the end of 2018 and with the end of active coal production this site offers prospects for after-use after more than 200 years of mining tradition in the Ruhr.
The starting position in the context of the energy transition
The politically driven expansion of renewable-energy capacity and its deeper integration into Germany’s energy supply system has now become a priority issue for the energy sector. However, the problem of how to store the energy remains unresolved and existing storage capacity is insufficient to compensate for the resulting energy fluctuations. At the same time current market conditions have almost put a stop to the planning of new pumped-storage power projects. This often renders them commercially unviable (1). In addition to the application and development of new storage technologies the use of new sites with tried and tested engineering can make a further contribution to the much needed extension of storage capacity. Underground pumped hydro storage power plants (UPHS plants) follow this rationale and this paper will seek to discuss the fundamental opportunities and demands arising from this approach.
After-use and redevelopment of the coal industry’s extensive infrastructure above and below ground
The coal industry of North Rhine-Westphalia will leave a vast infrastructure behind. Mine shafts up to 1,200 m in depth and several hundred kilometres of underground roadways, some still in use, stand witness to the scale of the mine workings that exist in the Ruhr. There are extensive tracts of land and infrastructure elements of exceptional quality available above and below ground. In addition to the collieries themselves the coalfield also includes water drainage and pumping facilities that are located directly beneath the entire region, these constituting a significant part of the industry’s infrastructural legacy. The extensive extraction of coal below the water catchment areas of the Ruhr, Emscher, Lippe and Rhine rivers meant that action had to be taken in the early days of the industry to establish a cross-regional water management regime for all mining sites around the Ruhr Area. This wide-ranging networked system was developed over more than 150 years and now extends from Kamp-Lintfort to Hamm and Bergkamen (2). Water routing was systematically practised during the mining era with the result that transfers are still possible today between the individual pumping stations. In order to prevent any undesirable rise in the ground-water level in the Carboniferous strata it will be necessary to maintain the pumping regime after the last colliery closes at the end of 2018. This operation is mainly aimed at protecting the drinking-water supply, a state of affairs that has been designated an “eternity task’” And therein lie the prospects for using these historically evolved structures for UPHS operations. However, as there is just one active colliery remaining in the Ruhr coalfield, namely Prosper-Haniel mine in Bottrop, it is this location that has become the focus of consideration.
The flow of mine water also provides further options in that this medium has a temperature of between 20 and 30 °C. This water, which amounts to some 80 m m3/a, provides a constant heat flow and could be pumped to the surface in order to generate a sustainable energy supply. Further research projects are now under way, including a study currently being undertaken at the Faculty of Energy Systems and Energy Management (LEE) at the Ruhr University Bochum, with a view to examining the opportunities available here. Numerous other projects are also being carried out into the use of geothermal energy from below ground and it remains to be seen what can actually be achieved in this area according to the prevailing economic criteria. In this respect the entire technology is being examined by the project partners in a very carefully structured way.
All kinds of ideas are being contemplated for the after-use of coal-industry sites and infrastructure facilities above and below ground and these are now being driven forward in many areas of action around the region. A good compilation of the various concepts being proposed for different energy-based after-use projects can be found in Fischer (3).
A pumped-storage plant below ground – is such a thing possible?
The operating principle of pumped-storage installations can be described in simplified terms as storage based on the conversion of electrical energy into gravitational energy (potential energy). Water is transferred by a pump (energy input) from a lower level to a higher level. The storage reservoir is charged and the resulting gravitational energy is recovered by allowing the water to flow down from the upper level through a turbine. This turbine converts the kinetic energy into electrical energy via a generator and then delivers it into the grid. On the whole, more energy is consumed than generated. Installations of this kind can achieve efficiency rates of up to 80 %, which is high in relation to other storage systems. What is more, they are backed up by more than 100 years of proven technical application.
Pumped-storage plants have been successfully tried and tested in regions with sufficient differences in elevation. In topographical terms Germany has few areas with this kind of potential. The storage lakes that are required (both the upper and the lower reservoirs) often take up a significant area of land and landscape and this factor frequently causes major social acceptance problems.
Figure 1 shows the operating principles behind a UPHS plant. Most of the technical installations and the lower storage reservoir are located below ground. While the upper reservoir would obviously be clearly visible, it could be sited within the confines of the colliery. The entire development would also include additional energy-related operations based around the mining facilities.
From idea to implementation – but how?
When assessing the feasibility of such a project it is essential to examine closely the environmental, economic, legal and social factors involved. The study based on Prosper-Haniel mine therefore involved the following stages:
- Ascertainment of the technical requirements for creating a high-performance UPHS plant at Prosper-Haniel in the Ruhr coalfield.
- Profitability appraisal against the background of market dynamics in the energy sector, including an assessment of how the follow-up project can contribute to reducing the long-term liability costs.
- Estimation and quantification of the environmental and energy-policy implications.
- Development of plans for ensuring occupational and industrial safety.
- Determination of social and political acceptance in the survey area, including a stakeholder dialogue on the regional and structural opportunities.
- Investigation of measures and costs relating to option securement, including the legal conditions and an assessment of the legal framework.
- Industry involvement and operator models.
Eleven partners from five different establishments have joined forces to address the wide range of issues associated with this joint project, true to the motto “from the region for the region”. This consortium includes the Ruhr regional universities of Duis-burg-Essen and Bochum, mine operators RAG from Essen, the mining consultants and specialists DMT GmbH und Co KG, also from Essen, and for matters to do with social acceptance and policy consultation the Rhine-Ruhr Institute for Social Research and Policy Consulting (RISP), which is affiliated to the University of Duisburg-Essen. A total of some fifty experts are currently working on the assessment process. The Mining Control Authority, the “renewable energies” task force, the NRW Energy Agency and the relevant departments of the ministries concerned have all been involved in the overall project. The description of the geological conditions presented below will be followed by a summary of selected analysis results.
The geological conditions present at the various mining sites in the Ruhr valley are characterised by the submergence of the coal-bearing sedimentary rock of the Carboniferous System beneath the younger, non-folded sedimentary strata of the Upper Cretaceous (Figure 2). A slight tilt in the section of crust of up to 5° north is the reason why the coal seams lying in this direction are to be found at an increasing depth. The stratigraphic sequence of the Carboniferous is characterised by intensive interbedding of coal-bearing and barren units and is structured into wide-spanning anticlines and synclines within the Hercynian orogenesis. The flat-lying seams of the major synclines are most suited for mechanised mining. The synclines and anticlines are dislocated at trough and horst structures by a series of tectonic faults that are mainly vertical in direction. The reason for this vertical tectonic geology lies in the parallel shift faults that run at intervals of 4 to 8 km in a line west-northwest to east-southeast (5) and which were probably created mostly in the Mesozoic Era. The shallow dip of the Cretaceous beds to the north increases the thickness of the overlying rock to more than 400 m in the centre of the coalfield and to as much as 1,100 m in the Hohe Mark area north of the river Lippe. In the western part of the Lippe trough the overburden is only 300 to 500 m thick in places due to a tectonic elevation, with the result that the coal-bearing Carboniferous beds in this area can be accessed within a stratigraphic sequence of more than 1,000 m (as is the case at Prosper-Haniel colliery). Around Prosper 1 shaft the Cretaceous/Upper Carboniferous boundary is at a depth of about 135 m, while at number 10 shaft it is at 304.5 m. The direction of dip of the Carboniferous surface from south to north meant that over the years mining operations tended to reallocate in the same direction and as a result the depth of the workings increased significantly (Figure 3). The fact that the coal was now being mined at much deeper levels also meant an increase in the lowering depth of the mine-water pumping system, which was needed in order to be able to extract the coal free from the effects of water pressure.
When constructing the underground installations required for such a project it is advantageous to have access to undisturbed areas of homogeneous and stable rock. However, these are only to be found sporadically in the vicinity of mine sites. A detailed study of the stratigraphic sequence will form the basis for selecting the sites for the generator and transformer caverns, whose dimensions and resulting stability requirements will tend to favour sequences with thick sandstone beds. The same will apply to the location of the proposed storage gallery, whose route should avoid the old underground workings as far as possible
The findings – what is suitable and what is not? And what else is important?
Producing a technical plan for an UPHS plant first meant evaluating all the installations in place at Auguste Victoria colliery in Marl and at Prosper-Haniel mine in Bottrop. Mine workings and installations that have been decommissioned are not suitable for use as UPHS sites. In the case of active mines the prevailing conditions will be fully known and understood right up until the coal industry closes down at the end of 2018, plus the three to four years needed for salvage and final decommissioning in accordance with the general operating plan. This is an important factor in the design of the storage concept as it allows the planners to identify permanently stable and available/usable components for the UPHS plant.
Different schemes were then studied, whereby a basic distinction was made between:
- a closed system (with a defined upper and lower reservoir) and
- an open system using the mine-dewatering circuit (need for run-of-river or connection to a water body, the stored water to be raised and discharged at another point).
In the overall analysis it is the closed system that is the preferred model. The open system presents a certain number of imponderables.
An exemplary concept was subsequently prepared and technically assessed in order to examine the technical feasibility of the scheme. This 200 MW plant was notionally located and conceptualised at the Prosper-Haniel mine site (Figure 4). With the closure of the colliery and the cessation of coal mining all know-ledge of the site, and especially of its geology, will be lost and by and large will never be reactivated. With the aid of the present mine infrastructure it would be possible to use four existing points of access to the underground workings and then to excavate a new 15 km long ring storage chamber to serve as an underground water reservoir. This would be a more favourable solution than converting the existing mine roadways. The generator and transformer caverns and the engineering equipment would be laid out in the conventional way. The size and scale of the installations will be limited by the geological conditions (stratification and fault zones). The upper reservoir will be created using a rockfill dam. The site already has a connection to the 110 kV mains supply, with the 220 kV and 380 kV voltage network only a short distance away. Any thoughts of using the existing roadways have to be ruled out because it would be more expensive to strengthen these than to drive new tunnels under controlled conditions. This exercise essentially confirmed the basic feasibility of such a scheme. The geological conditions prevailing in the Ruhr Basin, with fault zones and a highly changeable stratification, do however impose limits on the size and scale of the installation.
The investments needed for such a project were established in their entirety. This was set against the revenue situation for different market mechanisms. The results showed that a UPHS plant of this kind would not be cost-efficient at the present time. However, the same can currently be said of all pumped-storage power plants. Yet storage systems are needed and therefore market changes and energy-policy adjustments will have to be made. The investment costs will largely depend on the development of the lower reservoir.
The legal framework for such a UPHS plant was also examined and it was essentially found that no legislative action would be needed. The plant could in all probability be approved on the basis of the existing framework as it applies to both mining and water legislation. One of the most significant factors here is to have a separate planning permit in place once mining operations are finally concluded. The existing responsibilities of the higher mining authorities could be incorporated in a sensible and expedient way. The same would also apply to the operational perspective of a UPHS installation.
A representative national survey established that there is a high degree of general acceptance for a mining follow-up scheme of this kind. This point needs to be emphasised, as such a favourable reception is not often given to conventional pumped-storage projects. It can be assumed that this acceptance was based on the fact that the sites concerned are already undergoing re-development and the new scheme is set to occupy a fairly small amount of space. The level of approval was also higher than that normally given to conventional energy generating installations, with the survey showing that the closed system was clearly preferred to the open system.
Table 1 presents the relevant data for an UPHS plant to be built at the Prosper-Haniel mine site.
Public acceptance of major projects plays a huge part in their feasibility and realisation. Schemes that are being implemented without the approval or against the wishes of the public usually face massive resistance. The Rhine-Ruhr Institute for Social Research and Policy Consulting (RISP) has studied the local response to the proposed underground pumped-storage project in a representative population survey. This representative poll asked respondents about the level of information available and the degree of acceptance for the plant in the area (7). The findings indicated that barely 40 % of the population were aware of the proposed scheme. Generally speaking it was found that people were already showing a greater interest in projects connected with coal-industry after-use and redevelopment. The same poll also asked whether a UPHS plant could be considered as worthwhile. More than 80 % of respondents thought that it was, a level of acceptance that is now rarely encountered when it comes to conventional pumped-storage power projects. This high degree of acceptance may be connected with the fact that a number of former mining sites have already been successfully developed and that a relatively small amount of space is required for a UPHS installation. This result testifies to the anticipated high level of public acceptance for constructive after-use projects based around the region’s existing coal-industry infrastructure. Local stakeholders are now being consulted on this issue and discussions are being held with political representatives, the professional public and potential operators.
The entire process has always been kept as open as possible and so offers room for ideas from third parties. The positive attitude to pumped-storage installations in general, and especially towards the underground variety, has also been reflected in a series of talks that have been held with political figures at state and local level. The UPHS feasibility project has received a positive response from the different parliamentary groups at all political levels and is seen as an innovative step forward. At the same time the implementation of the UPHS scheme is being regarded as significant not just for the success of the energy transition pro-cess but also for raising the profile of the region as a centre for the environmental economy. However it remains to be seen whether such a forward-looking project can be executed under the current market conditions. The concept is a fascinating one and the technical feasibility of the scheme has now been confirmed on site. Yet various economic questions also need to be resolved before the project can move forward. The current discussions are now focused on option securement and the dismantling and decommissioning operations being planned at the Prosper-Haniel site.
Future prospects and the international dimension
Fossil resources have never been more in demand than they are today. The mining industry globally is producing coal, ores and rare earths on a scale never previously recorded in the history of the world. But a trend reversal is also becoming more evident – away from fossil-based energy generation and towards a low-emission lifestyle, a development that is being driven increasingly by issues such as air pollution and health protection. At international level former coal-based regions are now embracing structural change as never before and so it is only logical that the theme of “post-mining landscapes” should be developing into a scientific discipline in its own right. The European Union has recently identified 41 regions in Europe that will reach this status within the next 15 years. And with the end of many years of coal mining now in sight the Ruhr valley, with its large body of experience and many pilot projects, has much to bring to the table in this area. This includes the re-development of mining sites for UPHS schemes. Sustained and comprehensive media coverage has created wide national and international interest in this topic and delegations from European coal producing countries, such as Poland, Slovakia, Belgium and France, have been following the project closely. International delegations from China, South Korea, Australia and the USA have also visited the proposed site. The reasons are readily apparent: the withdrawal from coal mining attracts global interest. No installations of this kind exist anywhere in the world. The plant would therefore receive huge attention as a proactive project for shaping the post-mining landscape – regardless of the fact that this particular region has committed itself to the post-mining era. In this respect it should only be a matter of time and market circumstances before a scheme of this kind becomes a reality.
We should like to express our gratitude to the many establishments, sponsors and backers whose support has made this interesting and forward-looking project possible. More than 50 experts, scientists and project institutes have been engaged at one time or another in preparing the specifications needed for this fascinating venture and the results that we now have available would not exist without their commitment and involvement.
- Projekt „Unterflur-Pumpspeicherwerke – Nutzung von Anlagen des Bergbaus zur Speicherung regenerativer Energien“. Stiftung Mercator, MERCUR, Förderkennzeichen: Pr-2011-0022.
- Projekt „Entwicklung eines Realisierungskonzeptes für die Nutzung von Anlagen des Steinkohlebaus als unterirdische Pumpspeicherkraftwerke“. MKULNV NRW, Projektträger ETN, Förderkennzeichen: PRO 0039, NRW-EU Ziel 2-Programm 2007–2013.
- Projekt „Machbarkeit eines untertägigen PSW am Bergwerk Prosper-Haniel in Bottrop in der Bergbaufolge“. MKULNV NRW, Projektträger ETN, Förderkennzeichen: PRO 0063 sowie BMWi, Projektträger Jülich, Förderkennzeichen 03E T6100.
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