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Climate-Friendly Heating with Heat Utilisation from Mine Water in Post-Mining

With 5.5 million inhabitants, the Ruhr Area is one of the largest urban centers in Europe. In times of the industrialization, the Ruhr Area became an industrial conurbation. The coal mines which are now part of the RAG Aktiengesellschaft, Essen/Germany, provided work.

Due to the underground coal mining in depths of more than 1,000 m, there is an area with several mines and chambers in the underground of the Ruhr Area. Rainwater seeps into the resulting cavities. On the way through the rocks, minerals dissolve in the water. The rainwater becomes mine water. In the past, pumping the accumulated mine water was necessary to protect the active underground tunnels and shafts. In 2018, all coal mines in and around the Ruhr Area will be closed. Nowadays, the most important goal is to ensure that no rising mine water reaches the groundwater level. Therefore, continuous pumping of the mine water is required even after active coal mining.

The aim of the project Grubenwasser-Ruhr (GW-Ruhr) is using the thermal energy in mine water to supply geothermal heating energy. The mine water has a temperature between 15 and 30 °C, depending on the location of the pump and the depth of the lifting. The research objective is the use of geothermal energy. Supporting technologies such as heat pumps change the temperature level of the mine water and enable heating and cooling. The use of heat from mine water is an environmental friendly alternative to fossil fuels such as oil and gas.

The selection of the appropriate system for using the heat from mine water takes place with the help of economic and ecological aspects. They form the basis for investments and the implementation of heating by mine water heat at several locations. In addition to the technical aspects, there will be an exchange and cooperation with potential heat consumers in the relevant area, which will be crucial for future success. A low exergy network (LowEx network) connects all consumers and delivers the thermal energy of the mine water. This system transports the mine water temperature to the customer with little loss. A heat pump changes the temperature to meet individual customer requirements.

The project will present viable concepts and locations for mine water utilization in 2019 at the end of the first phase. There are three different locations under development. Following the first project phase, implementation and operational monitoring is part of the following project phase.

The Chair of Energy Systems and Energy Economics (LEE) of the Ruhr-Universität Bochum (RUB), Bochum/Germany, manages and coordinates the project. The company Eimer Projekt Consulting (EPC), Berlin/Germany, is responsible for the strategic contact of potential heat consumers. RAG as the mine owner and DMT GmbH & Co. KG, Essen/Germany, will support the implementation and investment planning.

Authors: Tobias Reiners M. Sc., Prof. Dr.-Ing. Hermann-Josef Wagner, Dr.-Ing. Nils Penczek, Lisa Altieri M. Sc., Michael Gross M. Sc., Lehrstuhl Energiesysteme und Energiewirtschaft (LEE), Ruhr-Universität Bochum (RUB), Bochum  

1  Initial situation

With the ending of coal mining at the end of 2018, an era will end in the Ruhr Area. However, the tasks of RAG Aktiengesellschaft, Essen/Germany, are not yet completed. RAG will fulfill its so-called eternity task at six sites in the future and pump millions of cubic meters of warm mine water from the old shafts.

Fig. 1. Origin of mine water. // Bild 1. Entstehung des Grubenwassers. Source/Quelle: RAG

Since the coal mines in the north of the Ruhr are in depth up to 1,400 m, the underground is traversed by a variety of tunnels and shafts. In these cavities, the dripping rainwater collects heat energy (Figure 1). As the amount increases, the so-called mine water continues to rise. In the past, pumping out this water was necessary above all to protect the tunnels and shafts still active, as the mines are connected underground. However, after the gradual closure of the individual mines, the pumps cannot stop. Otherwise the mine water would eventually penetrate into the groundwater used as drinking water and mix with it. Because of the high salt content in mine water this must be prevented.

So far, RAG has been channeling the mine water unused into the rivers. Since it is warm between 15 and 30 °C, depending on the depth, it can also help to heat buildings. The hot mine water offers an enormous heat potential, which is undeveloped to 99.9 %. Due to the high proportion of metropolitan areas in the Ruhr Area and the increasing demand for regenerative heat supply, the heat source and sink can unite in a unique way. With the end of the coal industry in the Ruhr Area in 2018, the so-called eternity tasks will place mine water use in the medial focus.

2  Objectives of the project

Due to the considerable quantities of water – currently about 61 m m3 are pumped to the surface every year at twelve locations – the project Grubenwasser-Ruhr (GW-Ruhr) has set itself the goal of realizing an environmentally friendly energy supply. This should be done by reusing existing mining infrastructure. For this purpose, decision-ready implementation concepts are being developed for the sites investigated that will be implemented in a follow-up project.

Since the protection of drinking water reservoirs is indispensable and the mine water must be lifted “forever”, the future of this type of heat supply would be secured in the long term. The six future active mine waterworks in the Ruhr Area alone offer a technically usable potential of more than 800 GWh/a of thermal energy. This corresponds at least to the heat requirement of 125,000 residential units. The large mine water quantities with a temperature level of 15 to 30 °C are ideal for “cold” heat networks (LowEx networks). Investigations in the project have shown that mine water can supply entire city quarters at suitable locations.

Instead of using fossil fuels such as oil or gas, e. g., residential buildings or other surrounding customers can alternatively be heated with the heat of the mine water. Auxiliary techniques, such as heat pumps, increase the temperature level of mine water, so that end users can use the mine heat for heating purposes. In addition to the use of mine water for heating, the possibility of using the energy source for cooling is also being investigated. In particular, the optimized interaction of the individual components in LowEx local heating networks represents a special opportunity for a sustainable and environmentally friendly energy supply.

3  Walkthrough

In a first step, the local heat potentials are first identified and then addressed to the promising locations of municipalities and energy suppliers in order to then develop decision-ready implementation concepts. At the end of the first project phase after three years of operation, the structural implementation of the concepts should take place.

Through the merger of several actors, the project goes beyond purely technical aspects. This will also focus on socio-economic issues; lower or at least constant energy prices are of particular importance to the end consumer. Pure environmental benefits in the form of lower CO2 emissions rarely outweigh the economic motives. In addition, the security of supply for the possible heat consumers, which must be realized by backup systems, plays an important role.

Auxiliary technologies, such as heat pumps, are necessary to provide the required energy supply temperatures. Since these also consume energy, it is investigated whether the use of heat from mine water is still economical. Without auxiliary techniques, the temperature level of mine water can be used for cooling and, e. g., used in new housing estates with passive houses.

As part of the energetic and technical analysis at the local level, different approaches to the use of mine water heat are defined. These are created with the simulation software TOP-Energy. To map the technical components within the simulation environment and link them with business data, they are kept in contact with manufacturers of technical systems. The technical possibilities must be continuously adapted to the interests of the stakeholder.

4  Technology

The energetic use of mine water begins with the development of the source. Here the thermal energy is extracted from the mine water. The potential of mine water heating is determined at each location. Basically, the sources should be subdivided into active mine water stations and safe havens with access to depth. One-day mine dwellings provide direct access to the heat source. If there is access to the depth at a safety location, e. g., the heat is extracted from the pit with a closed system. For the technical feasibility of heat extraction, the selection, dimensioning and material choice the heat exchanger is crucial.

Fig. 2. LowEx-heating network for using mine water. // Bild 2. LowEx-Nahwärmenetz zur Nutzung von Grubenwasser. Source/Quelle: RUB

The Mine water use is tied to heating networks to transport the heat energy. A heat exchanger transfers the energy to a LowEx system with temperatures below the mine water temperature (Figure 2). At network temperatures of about 20 °C, heat is transported to the consumer with low losses at a low exergy level. If required, it can be lifted to the desired temperature level with heat pumps. This system can be used for heating as well as air conditioning.

Consumers in the household, commercial, trade and service sectors are supplied with heat in a network. In the properties, the temperature level of the heating network is used either directly (factories, warehouses) or decentralized with heat pumps to the required temperature level. As this is a near conversion area near the shaft, high energy standards and low temperature heating systems are expected in the new construction sector. Due to the thermal mine water use the construction standard according to KfW40 can be achieved without additional measures. The demand for cooling systems, which occurs at high isolation standards, is also taken into account.

This approach combines proven technical components into a unique, innovative concept. LowEx networks with a temperature level between 20 and 30 °C are currently unique. This temperature level is ideal in combination with conventional heat pumps that provide low temperature heating. Fossil fuels can be eliminated in direct operation, resulting in a low-CO2 heat supply.

5  Site selection

On the heat source side, concrete locations are being searched for, which are to be further investigated. For this purpose, a rough review of the active drainage areas in the Ruhr Area is carried out. The aim is to determine the exact temperature level and the flow velocity at these locations.

Table. 1. Location categories. // Tabelle 1. Kategorien der Standorte. Source/Quelle: RUB

Each access to the depth is categorized and assigned to four categories (Table 1). This classification classifies the connectivity of the heat source and the availability of heat sinks. They describe the relevance to the project in descending order. The first category defines the locations where the long-term mine water is actively pumped out.

On the heatsink side, business clipping letters were created for the sites, which included general information about each actor and a ranking of probably the largest heatsinks to identify key stakeholders for the stakeholder approach. Individual temperature levels, flow rates and mine water increase after 2020 were simulated.

6  Status of the project

The comprehensive investigation of the Ruhr Area has identified at the present time 21 sites with a high theoretical potential. For this purpose, the shaft locations were divided into four categories. The largest energy potentials are offered by the pit locations with future pit drainage of category 1. The shaft locations with future function as a category 2 backup site offer easy access to the deep (Figure 3).

Fig. 3. Shaft locations with future function in the Ruhr Area. // Bild 3. Zukünftige Grubenwasserstandorte im Ruhrgebiet. Source/Quelle: RUB

At three of these locations, as a result of structural change in the Ruhr Area, new energy needs are being created on conversion areas. In Bergkamen the water town of Aden is being built on the former industrial area of the hard coal mine Haus Aden. In Bochum, a new industrial estate is being built at the former Robert Müser mine and in Essen the new residential district ESSEN 51 is being built on the former Krupp belt at the Amalie shaft.

6.1  Location Haus Aden – active mine water lifting: new use

Fig. 4. Concept of water town Aden. // Bild 4. Konzept der Wasserstadt Aden. Source/Quelle: RUB

The active pit drain house Aden in Bergkamen in the northern Ruhr Area (Figure 4) is one of the six permanent elevation sites. Starting in 2023, approximately 24 m3/min of mine water will be continuously pumped from a depth of approximately 600 m at a temperature between 25 and 29 °C. The surrounding coalfield at Haus Aden will be converted into a city quarter. The resulting new water town of Aden stands as a symbol of structural change in the Ruhr Area and, in this context, mine water heat offers the opportunity for environmentally friendly supply of this urban development.

In the pilot project supply of a city district with mine water heat, a novel cold district heating network has priority. This 9 km long network connects the heat consumers with the central heat extraction at the mine water heater, whereby the temperature level fed in here for direct heating of the consumers is too low. 35 °C are necessary for the heating of new buildings to KfW40 standard with a surface heating. Therefore, heat pumps are used to supply the customers as needed. This concept could cover up to 80 % of the heat demand of new buildings from mine water heat. In the pilot project, the combination of mine water extraction, a low-temperature heat network and individual heat pumps at the customer would be novel and worthy of research. Ultimately, the primary goal is to realize the opportunity of a former mining infrastructure towards the most efficient use of ecological heat.

6.2  Location Robert Müser – active mine water lifting: expansion of heat utilization

Fig. 5. Possible extension of mine water use at Robert Müser site. // Bild 5. Mögliche Erweiterung der Grubenwassernutzung am Standort Robert Müser. Source/Quelle: RUB

The mine water collection at the Robert Müser site in Bochum is one of the six active water abstraction stations in the Ruhr Area (Figure 5). Since 2012, an average of about 14 m3/min of mine water has been lifted from a depth of approximately 600 m at a temperature of 18 to 20 °C in this central part of the Ruhr Area. At the location, a first use of mine water has already been realised. The mine water is pumped to the surface by up to three pumps and collected in a water lock. At the water lock, a heat exchanger is connected, which transfers the heat to a heat network. This network leads to two schools and a fire station. The water with a temperature of 18 °C from the pit thus achieves the system technology of the respective boiler rooms. The existing buildings, however, require higher temperatures of more than 70 °C for heating. The necessary increase in temperature requires a combination of different heating techniques. Stadtwerke Bochum GmbH operate a stratified storage tank for this purpose to operate a gas boiler, a combined heat and power unit, combined gas absorption and compression heat pumps. Due to this concept, only a limited part of the heat demand of the existing buildings can be covered by the mine water heat.

In the pilot project “Extension of the mine water use Robert Müser” the existing heat supply is to be extended. So far, only 10 % of the available capacity is called up for heating at full load. It makes sense to connect the newly planned business park Robert Müser also to the mine water use, thus expanding the existing buildings to new buildings. The business park would be provided by a cold heat network with the 18 °C warm water from the pit. For the planned commercial new buildings, the combined heating technology can be dispensed with and at the same time the proportion of mine water heat can be increased. Due to commercial use, e. g., in warehouses or industrial plants, it is also possible to use the pit heat at 18 °C directly, without additional heating technology. This could cover nearly 100 % of the heat demand from mine water heat. This option can be used, e. g., in warehouses, workshops and in production via surface heating systems. For office and social rooms, the use of a heat pump is necessary.

A particular challenge is to extend the existing pilot plant for the supply of existing buildings to an even more efficient infrastructure at this mine water collection site. The connected consumers should use the heat of 18 °C directly without auxiliary means or only in connection with a heat pump.

6.3  Location Amalie – extracting heat from the depth via an open shaft

Amalie is a former colliery, which is located near the center in Essen. In contrast to the pilot areas described above, the site is not intended for active drainage after the end of mining. This location provides only access to the depth and is operated as a backup location, which can be equipped at short notice with lifting pumps if needed. This site is to be used for mine water use, as the city district ESSEN 51 is being built on the surrounding coal mines (Figure 6), and mine water heat could become part of an innovative energy concept that is as CO2-neutral as possible. The sole supply of mine water heat will not be sufficient, since the heat requirement of the new urban district is very high.

Fig. 6. ESSEN 51 concept. // Bild 6. ESSEN 51-Konzept. Source/Quelle: Thelen Gruppe

In the pilot project “Integration of mine water heat in systems with several heat sources”, mine water is to be developed as a geothermal source at a depth of approximately 850 to 1,000 m. Although the shaft should be cohesively filled from the surface of the day to the level of the 8th level, it has sheaths as a securing location and is thus accessible to the mine water. In the shaft below the 8th level, a heat exchanger or probe system is to be introduced down to the level of the mine water. In a closed pipe system circulates a carrier medium, thus promoting the heat to the surface. This solution minimizes the technical requirements and the cost of promoting heat to the surface. Today, two volume flows are assumed at a depth of approximately 1,000 m, the mixed water of which has a mine water temperature of around 26 °C. For the promotion of heat only small losses are expected. Since the heat is available all year round, mine water use can take over part of the base load for the urban development project ESSEN 51 all year round. This secures the supply, if other regenerative forms of energy are not available.

7  Outlook

It is planned to bring the project into a second phase of the project. The aim of the project “Grubenwasser-Ruhr (GW-Ruhr) II” is to transfer the concepts developed in the first phase of the project into implementation and to accompany them scientifically. Investments and structural measures are made for the commissioning and the corresponding operation. By means of installed measuring technology an optimal working method with the highest possible efficiency should be determined. In addition, phase II will investigate whether further measures can be taken on the heat consumer side in order to increase the overall efficiency.

The actor network built up in phase I will continue to be used in order to consolidate it, to analyze different behaviors and, if necessary, to act in a network. In particular, aspects such as communication and knowledge sharing are also elementary building blocks in the second phase.

The results of the second phase of the project are a functional heat network system which includes a process monitoring. The scientific support builds on this and leads to positive and innovative results. Through the active implementation of the developed concepts, insights are gained that enable a large-scale development of the energy of the mine water. Everything should be bundled in a transfer of results and experience in order to ensure international transferability of the project results.

The project Grubenwasser-Ruhr is sponsored by the Federal Ministry for Economic Affairs and Energy of Germany (BMWi) 6th Energy Research Programme of the Federal Government

Authors: Tobias Reiners M. Sc., Prof. Dr.-Ing. Hermann-Josef Wagner, Dr.-Ing. Nils Penczek, Lisa Altieri M. Sc., Michael Gross M. Sc., Lehrstuhl Energiesysteme und Energiewirtschaft (LEE), Ruhr-Universität Bochum (RUB), Bochum