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Starting one year ago, several buildings in Bochum-Werne are being heated using heat from a mining shaft.
© Stadt Bochum, Presse- und Informationsamt
Geothermal energy in former mines
Projektinfo 13/2013

The diagram shows the structure of the system employed for the thermal use of pit water.
© Stadtwerke Bochum Holding GmbH
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Thermal heat from pit water

The coal mining company RAG annually pumps over 100 million cubic metres of water to the surface from a depth of up to 1,000 m to secure the stability of the ground and the safety of drinking water in former mining regions. For one year now, the municipal utility of Bochum has been heating several public buildings with that pit water. The pilot project demonstrates how to use old mines for an eco-friendly heat supply of surrounding buildings.

The Ruhr region will see the extraction of a resource with an energy potential beyond 2019. Pit water, up to 50 °C hot, will be pumped up through former mines for safety reasons. In a pilot project, the municipal utility of Bochum demonstrates how pit water can be used for geothermal applications. A primary school, a comprehensive school with a canteen and a swimming pool and the main fire station of Bochum are being supplied with heat based on this energy source since the autumn of 2012. The fuel requirement of the existing buildings is to be reduced by approximately 1,200 MWh per year along with 245 tonnes of saved carbon dioxide emissions. 35 % of the total heat demand is covered by pit water.

The experience gained in the project will serve as a basis for the extensive thermal use of pit water. Other RAG sites alone will see the extraction of a further 90 million m³ of pit water annually. With this usable energy potential, heat can be provided to the equivalent of 5,000 to 10,000 households according to RAG estimates.

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Warm pit water with a temperature of 20 °C is extracted from the Arnold pit of the Robert Müser mine in Bochum-Werne for the pilot project. At this location, RAG annually extracts about 10 million m³ of pit water from a depth of 570 m. The electrical energy consumption is about 25,000 MWh. The pit water is pumped up through a pressure pipe.

Pit water as a "Ewigkeitsaufgabe"

Pumping out pit water is one of the so-called “Ewigkeitsaufgaben“ (tasks to be carried out indefinitely) of the German hard coal mining industry which is assumed by RAG together with a number of water management associations such as Emschergenossenschaft and Lippeverband in the Ruhr region. Annual drainage costs amount to around 100 million euros.

In order to mine hard coal in the Ruhr region, the pit water must be pumped out. It accumulates in the pits because water from the aquifers seeps into the tunnels. The pit water is first collected in pools at the lowest point of the mine. From there it is pumped out and fed through a discharge system into the surrounding water bodies. Pit water pumped from the Robert Müser mine flows into the Ruhr River.

After the coal mining operations will have ended in 2019, water will still need to be pumped out. Drainage prevents an uncontrolled increase of the pit water level. With an increase in the pit water level, the risk that the ground surface rises or collapses increases as well. In addition, pit water could push methane gas to the surface at an increased rate.

Another important function of water drainage is to protect drinking water. If pit water containing salts or iron were to mix with groundwater in the overlying rock, this water would be contaminated. Pit water from the Robert Müser mine contains, amongst other substances, chloride, heavy metals, hydrogen carbonate and phosphorus. The salty components in the pit water can lead to pipe corrosion.

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Addresses

Project implementation
Stadtwerke Bochum Holding GmbH

Extraction of pit water
RAG Aktiengesellschaft

Heat customer
Stadt Bochum