Geoenergy & utilization
The terms geothermal energy and geothermal heat refer both to the thermal energy stored naturally and technically below the earth's surface and to its exploration and use. The thermal energy can be used for heating and cooling as well as for generating electricity. Active heat storage makes it possible to compensate for seasonal fluctuations in demand. Heat utilization and electricity generation can also be combined in combined heat and power plants.
Geothermal energy is available regardless of the time of year and time of day and, unlike many other renewable energies, is therefore suitable for operating base load systems. Geothermal energy is considered renewable, as heat flows into the underground reservoir due to the temperature conditions and transport processes.
Near-surface geothermal probes and groundwater utilization make geothermal energy usable for residential buildings or individual industrial plants. The GECKO project relates to the use of geothermal energy from deep underground and thus to the supply of larger units, such as municipalities, urban districts and industrial estates.
In Germany, deep geothermal energy comprises the following utilization systems:
- Hydrothermal systems
- Enhanced geothermal systems (EGS)
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Deep geothermal probes
The geothermal resources in Germany are distributed across the following regions:
- Molasse basin of the Alpine foothills
- Upper Rhine Graben
- North German basin
Example of the Upper Rhine Graben - Geological structure
The Upper Rhine Graben in its current extent from Basel in the south to Frankfurt am Main in the north and in its current form is a tectonic graben. The sequence of layers that prevailed until the Jurassic (ended around 145 million years ago) has been broken up since the Paleogene (began around 66 million years ago). Along the fault lines (tectonic faults), the sediment layers successively sank, in some cases by several thousand meters. At the same time, the edges of the Vosges and the Black Forest rose. The resulting trench was in turn filled with new deposits of different origins. Today, lateral thrusting often takes place at the faults created at that time due to pressure from the south.
Positive temperature anomaly
The faults are still relevant for the underground heat balance today. Flow paths for deep water can develop along the fault zones. The deep water from deep layers transports stored heat to higher layers. This is called convective heat transport. As a result, the temperature increases with depth from usually three degrees Celsius per 100 meters in the Upper Rhine Graben to five degrees Celsius per 100 meters. The temperature increase (geothermal gradient) in the Karlsruhe area and especially in the region around the KIT Campus North is one of the highest in Germany at more than ten degrees Celsius per 100 meters. The location is therefore particularly suitable for geothermal use.
Geothermal energy in the deep underground at KIT Campus North
In addition to the favorable thermal conditions, the subsurface of the North Campus also has fluid-bearing units at various depths:
- The layers of the old Leopoldshafen oil field have been proven to have good storage properties.
- The temperature anomaly indicates natural fluid circulation in the deep subsurface.
The subsurface therefore allows the development of a
- heat storage in the water-bearing edge areas of the old oil field and a thermal water
- thermal water extraction from the areas where natural circulation (convective heat transport) prevails.
These systems are usually operated via at least two wells. Such a geothermal doublet consists of a production well and an injection well. The production well is used to bring the thermal water up from underground. After cooling in the above-ground facility, the water is fed back into the reservoir at depth through the injection well. Excess heat can also be stored in a heat storage tank in summer.
Since April 2019, KIT has been exploring the deep underground for both options as part of a joint exploration permit for geothermal energy and brine with EnBW Energie Baden-Württemberg AG. While the creation of a high-temperature heat storage facility is still a purely scientific project, there are already a number of successful examples of thermal water extraction in the Upper Rhine Graben. The approval of the geothermal project or a borehole is not yet included in the exploration permit.
Examples of use
The projects described below for the use of deep geothermal energy are geologically and technically comparable to the planned infrastructure at KIT Campus North. They show that deep geothermal projects have been in existence in the Upper Rhine Graben for decades.
In Riehen (Switzerland), a hydrothermal doublet operated by Wärmeverbund Riehen AG has been supplying heat to parts of the town since 1994 and to the municipality of Stetten in Germany on a cross-border basis since 1997. A borehole with a depth of 1,547 meters supplies thermal water with a temperature of approx. 65 degrees Celsius. At moderate flow rates of up to 25 liters per second, the water is injected back underground at a temperature of approx. 25 degrees Celsius in the second borehole at a depth of 1,247 meters. Above ground, a geothermal heat exchanger provides 1,550 kilowatts of thermal energy, which is extracted from the thermal water. Around 8,500 residents now draw district heating from it, which is transported in a district heating network over 37 kilometers long.
The municipality of Riehen holds 73 percent of the shares in Wärmeverbund Riehen AG. It is currently planning a second deep borehole under the project name "geo2riehen", which is due to go into operation in 2025. A further 4,000 residents will then receive heat from it. Transparent communication and participation of the population of Riehen are declared goals of the project operators. The "geo2dialogue" format, organized and moderated by the neutral Risk Dialogue Foundation, provides regular, detailed information on the individual project stages. The operators also take on board concerns and feedback from the public and, where possible, take them into account as the project progresses.
In Bruchsal, deep geothermal energy has already been contributing to the town's heat and electricity supply for several years. The project, which has been operated by EnBW Energie Baden-Württemberg AG since 2005, consists of a hydrothermal doublet (i) with drilling depths of approx. 2,500 meters for the production well and approx. 1,900 meters for the injection well. At a production rate of 24 liters per second, the thermal water at a temperature of 120 degrees Celsius is brought to the surface. The thermal power plant process supplies the surrounding area with 5.5 megawatts of heat via a local heating network. In addition to the heat, electricity with an output of 550 kilowatts is generated from the thermal water.
Bruchsal borders the KIT Campus North permit field to the east and is only around ten kilometers away from the KIT Campus North. The planning for the KIT North Campus can therefore benefit from the important experience gained in Bruchsal.