Climate change, the energy transition and structural changes are the current keywords in social discussions. Even for experts, systems change quite unexpectedly and quickly. But at least the direction is clear: We want to establish a sustainable use of resources and leave a planet worth living in for future generations. Additionally, we want to use only renewable energies in all economic and everyday sectors.
In order to achieve this goal, social willingness, individual adaptability and, last but not least, technological innovations are required: By the many small and medium-sized technology companies that are opening up new markets, as well as by the “hidden champions” who bring their proven competencies to new applications.
A big task will be to integrate thermal energy into the energy transition process. Germany uses more than 50 % of its primary energy consumption for thermal energy provision for industry, trade, agriculture and heating. Most of this is required by the industry for process temperatures of 100 to 150 °C. Looking for sustainable resources, these can be found deep underground. Higher temperatures are encountered with increased depths. The average temperature increase amounts to roughly 30 degrees per kilometer. To supply thermal energy at 100 to 150 °C, relevant subsurface temperatures can be found at 3 – 5 km depth. As a result, the majority of all thermal processes relevant in industry, can be supplied by geothermal energy, e. g., district heating, chemical industry, agriculture, food production, metal, cement and construction industries and wood and paper processing, just to name a few.
Munich goes geothermal
Shallow geothermal energy has already proven its practical suitability and statistically provides heat to more than 650,000 two-person households heat – according to the figures of the Federal Geothermal Association. So far, however, only 37 further systems in Germany are in operation or planning, reaching depths of 400 m or more. Commonly, deep geothermal systems do consist of two boreholes, a production and an injection well. Hot water is being pumped up through one borehole to surface. Subsequently, the heat is being extracted from the water via heat exchangers, and the cold water is injected back into the earth via the second borehole. Thus, the cycle is being completed.
The city of Munich and its surrounding communities are the pioneers of geothermal energy in Germany. Over a dozen systems are already in operation and multiple more are planned. Munich plans to cover its district heating needs in a climate neutral manner by 2040 and will be therefore primarily relying on heat from geothermal energy. Limestone formations that extend from the Danube to the Alp are primarily used. These formations are usually fissured and therefore well permeable for water transport. The limestone formation is still at the surface in the Danube Valley. Whereas, the tectonic shift which developed the Alps upwards, pushed the limestone formation near the mountains deeper and deeper. Therefore, the rainwater in the Danube region can seep into the depths along the limestone layer for centuries. In Munich, the limestone formation is at a depth of approximately 3,000 m, while the water has a temperature of approximately 100 °C, which is the suitable temperature for urban district heating.
Thermal energy is everywhere
The Munich case seems to be an inimitable solution. Only a few regions such as the Upper-Rhine-Graben and parts of northern Germany seemed promising to gain sufficient thermal energy from deep underground. But what actually distinguishes them are the many years of oil and natural gas exploration and production have resulted in collection of extensive data about the subsurface. For the Rhineland, Ruhr, Central Germany and Lusatia districts these data are still missing. Due to the “shallow” coal mining, it has not been required to explore the greater depths in more detail. Now geologists are increasingly trying to close the knowledge gaps.
One of the largest geothermal reservoirs in Europe exists deep below North Rhine-Westphalia. It consists of limestone formations from the geological ages of the Devonian and Carboniferous, which today are found a few kilometers deep and are several 100 m thick (Figure 1).
The limestone formation had developed approximately 300 to 400 million years ago through reef growth in an extended, shallow and warm sea, which covered the areas as we know them as German Plain, the Benelux countries and the North Sea. Today, similar conditions can be found at the Great Barrier Reef in Australia. Through tectonic movement and stress as well as karstification, crevices and cavities developed in the rock over time, which are often very permeable to water. In places like the German Aachen or the Belgian Spa, thermal water reaches the surface. It has temperatures of up to 72 °C and was used in spas and for local heating of buildings in ancient times. These rocks are now being utilized and used for other energy exploitation purposes via deep boreholes in Belgium and the Netherlands. District heating networks, greenhouses, industrial companies and thermal baths benefit from climate-friendly energy from layers that carry thermal water. The coal phase-out in Germany offers the opportunity to develop this sustainable potential here as well.
From tailor-made suit to standard clothing
Currently exist only a few standardized methods for geological exploration that are equally suitable for all potential locations. The framework conditions differ too much. Development work is still required, and many development paths now cross at the newly created Fraunhofer Institute for Energy Infrastructures and Geothermal Energy (Fraunhofer IEG). It was established to advance application-oriented research for the energy transition. One main component is the integration of the International Geothermal Center Bochum (GZB) and its many years of expertise, as well as the development of additional facilities. In Cottbus one division is researching future energy infrastructures, in Jülich the next one on sector coupling in quarters and areas, and another in Aachen on georesources and storage applications. The locations build a bridge between the regions in western and eastern Germany that are particularly hard hit by structural change. The research work at Fraunhofer IEG should run along the entire process chain from the idea to the operation of a geothermal system and even beyond, if it is about the conversion of existing heating infrastructures.
The regional geological exploration is step one, which compiles all available data about the underground, identifies promising geological formations and designs geoscientific and technical development concepts. Exposed rocks or rock walls, quarries, mines, geophysical measurements or boreholes in the region initially provide geological information. Together with Kabel Premium Pulp & Paper (KPPP) paper mill, Fraunhofer IEG is currently exploring the underground in Hagen. The paper manufacturer wants to convert the energy-intensive process step of paper drying at 100 to 200 °C from natural gas to sustainable geothermal energy in the next few years. A known fault zone runs through KPPP’s premises, which promises increased water permeability. Additionally, Devonian limestones are expected at an estimated depth of approximately 3,200 to 4,100 m according to the Geological Survey of North Rhine-Westphalia (GD NRW). This would represent a potential thermal reservoir similar to the conditions in Munich.
The results of the preliminary exploration are expanded through current measurements. Seismic methods are used, i. e. sound waves similar to those used in an ultrasound examination by a doctor, to get more detailed information about the underground. Truck mounted vibration generators gently vibrate the ground so that long-wave sound waves travel through the rock layers. Depending on their physical properties, the geological layers reflect the waves back to the surface. A network of sensitive microphones, so-called geophones, are recording these waves so that the information about all wave transit times are put together by computer to form an ultrasound image of the subsurface. Additionally, shallow exploratory boreholes provide samples of the relevant layers, which are examined in the laboratory (Figure 2).
Data from preliminary exploration, seismic investigations and the results from sample investigation of the rock laboratory enable the development of a geological model of the site.
The model can be used to estimate both, the probability of encountering usable thermal water and the effort involved in the actual drilling. The drilling effort can differ depending on the location. Loose sediments in the North German Plain can be drilled more quickly than the solid rock in the German low mountain range. At Fraunhofer IEG, a certified mobile 60 t double-head drilling rig including mud and high-pressure pumps as well as mud preparation offers the possibility of professionally drilling medium depth geothermal targets up to 1,500 m and thus conducting research in a practical manner (Figure 3). The cost of drilling accounts for the largest share of the investment budget in the development of geothermal power plant. One assumes 1 to 2 M €/km of borehole. Research and development which lower costs of drilling techniques have a great opportunity to increase the profitability of geothermal thermal power plants.
Drilling technology uses the best practices of the oil and gas industry. Roller cone bits or diamond bits are rotated into the depths from the derrick with a drill rod, whilst a drilling fluid cools the bit and transports loosened rock upwards. But the essential processes of this technology, which has been developed over 100 years, take place at great depths at high pressures and temperatures and thus elude simple experimental access. That is why Fraunhofer IEG has developed a new type of test stand called match.BOGS (Figure 4).
It should enable to simulate the processes during the drilling, the development and the usage phase of the reservoirs under real physical conditions in the laboratory. It thus forms the link between laboratory and real world application.
The central element of match.BOGS is a large autoclave. This heatable pressure vessel is able to take rock samples with a length of 3 m and 25 cm in diameter and expose them to the pressure and temperatures that prevail at depths of up to 5 km – i. e. about 1,250 bar and 180 ° C. Special feedthroughs in the autoclave allow testing of borehole equipment such as pumps. Flow trough experiments in the autoclave are possible, even with corrosive fluids. Acoustic, optical and thermal sensors are attached along the rock sample in order to enable recording of processes in the reservoir in detail. Drilling tools can be tested on rock blocks inside of the chamber under simulated downhole conditions with a feed force of up to 400 kN and a torque of up to 12 kNm. The installed measurement, control and regulation technology enables fully automated drilling processes to be carried out with changing rock properties.
Measurements with the match.BOGS test rig under real conditions can help to further develop proven drilling techniques in the laboratory. The wear of the drill bit is a constant research topic for geothermal energy, because replacing parts of the bottom hole assembly during the drilling process is very time consuming, especially in deep wells. Additionally to bit edges, the moving parts in percussion bits are susceptible to wear and tear, such as the valves of the hydraulic system. In the EU project Geo-Drill, Fraunhofer IEG and its partners are developing a purely liquid-based valve system without any moving parts and based on the Coanda effect, i. e. the tendency of currents to cling to surfaces (Figure 5).
The fluidic switch (see picture on the left in figure 5) consists of an inlet (1) and two outlets (5), which are connected to the upper and lower chambers of the percussion piston. Due to the Coanda effect, the fluid flows along the inner surface of the mixing chamber (2) to one of the two outlet openings. The feedback loops (3) cause the liquid flow to alternate between outlets as soon as a chamber is filled. A filled chamber is emptied through one of the outlet openings (4). The simulations from the computer are verified with the first prototype from the 3-D printer (illustration on the right).
Additionally, Fraunhofer IEG is testing completely new drilling concepts. The rock can be weakened with electricity, plasma or laser beams to support the grinding effect of the drill bit. It can also be advantageous to drill mutliple horizontal boreholes from an existing vertical mother borehole to increase the flow rates of the thermal water. For this purpose, focused water jets or small turbine-driven drill bits could be used in the future, which are currently being researched in detail in laboratory and field tests.
“The way ahead is unknown”
The properties of the geological formations can vary greatly from place to place and also within a borehole, even if the essential processes of rock formation are well understood and the preliminary exploration has collected valid data. Therefore, the driller is always checking the operating parameters of his equipment, such as weight on bit, flowrate, torque and the quality of the cuttings, in order to verify the drilling progress. In addition to his experience, he can rely on sophisticated logging and measurement tools. Inclinometers, compass and thermometer are often utilized during drilling operations. Pressure, vibrations, flow rates, natural radioactivity and electrical resistance of the rock can be recorded. The driller can adjust the operation as soon as something unexpected happens. The latest research also deals with the connection of optical fibers as a sensor. Coupled light scatters in the fiber and allows conclusions to be drawn about temperature, vibration or pressure in the vicinity of the glass fiber with a high spatial and temporal resolution. If the optical fiber is installed appropriately, these scatter signals also provide information about drilling operations, cementation and the properties of the geothermal reservoir during development and operation.
Noise of signals
In parallel to measuring devices in the borehole, seismometers are also set up on the surface around the borehole. The natural seismicity is documented as a reference signal in advance of the drilling process. The most sensitive seismometers can still detect the waves of the Atlantic Ocean from a distance of 500 km, not to mention the car traffic on the neighboring country road. Hardly noticeable earthquakes for humans below magnitude 3 on the Richter scale occur worldwide around 1,500 times a day. During drilling, the operation of the bit and the reaction of the surrounding rock can still be very sensitively tracked with the seismo-meters. The operation of a geothermal system, which means the conveying and pressing of thermal water, can also be found as a characteristic signal in the spectrum of seismic waves. If you know the characteristic shape of the various (interference) signals, the signal can be separated from the noise. Here, the current research paths are aimed at interlacing the data from as many different sensors as possible in order to filter the observations more closely and more quickly into relevant findings for operation. For this purpose, manual evaluation steps are increasingly being automated and machine learning methods are being tested in order to narrow the increasing complexity to the relevant findings and to provide those responsible for operations with the best analyzes in real time.
Once the target depth has been reached and the bit is back at the surface, the borehole is explored in detail with measuring probes and the rocks are characterized. Where necessary, the borehole will be cased and the annulus behind casing will be filled with cement. Once the amount and temperature of the thermal water has been determined, the operating concept of the geothermal system is finalized. It should be noted here that thermal waters could be full of dissolved salts due to their long time in the subsurface. This can enormously affect the power plant technology. On the other hand, the salts could also be economically interesting in the future, e. g., when it comes to the salts of rare or sought-after elements such as lithium.
Technology for extreme operation
The impact of salt and temperature on the pumps in the geothermal wells is higher than on the equipment in the oil industry. And also the power plant technology on the surface will not always be able to fall back on technologies from other industries. A central component are the high-temperature heat exchangers, which separate the salty reservoir water cycle and the reservoir power plant cycle. Depending on the application, it could make sense to use high-temperature heat pumps, operated with sustainable electricity, to increase the temperature level of the thermal water. The high temperature level and the necessary performance class usually require enormous know-how and tailor-made solutions from the system manufacturers.
At the interface of the EU projects DGE-ROLLOUT and HEATSTORE, Fraunhofer IEG is currently working with industrial partners to design a heat pump that can also bring water from the underground with “only” 60 °C to a temperature level of up to 120 °C and thus heat for a local district heating system. The flooded drift of an abandoned small colliery in the Ruhr area is to serve as a seasonal heat store. The colliery on the premises of Fraunhofer IEG in Bochum was drilled into a depth of 60 m for this very purpose. Parabolic solar collectors of a planned solar thermal power plant (Figure 6) are going to heat up the colliery, which is full of water, in the future with temperatures up to 60 °C.
In winter, the preheated mine water could then be used by a high-temperature heat pump as a heat reservoir in order to bring water to the operating temperature of the local district heating grid of over 100 °C.
Another example is the Dannenbaum colliery. It is currently being developed in the EU project D2Grids a heat and cold storage for the industry and knowledge campus “Mark 51°7”, which is being built on the former factory site of the car manufacturer Opel in Bochum. In the medium term, buildings with approximately 6,000 workplaces can be heated and in parallel cooling capacity is provided for commercial applications. It is estimated that 200 former hard coal mines in Germany could be utilized for geothermal and or storage purposes. In order to explore and develop their potential in the Ruhr area, Fraunhofer IEG operates the TRUDI demonstrator (Figure 7).
It is located in the 50 km2 field “Future Energy” in the southern part of Bochum. Within the laboratory test field drilling tests to depths of over 5,000 m are possible. Reference drilling and reservoir utilization techniques could here be carried out, in order to develop dense rock formations for usage of geothermal energy production.
Fraunhofer IEG is in parallel planning to set up a demonstrator with an observatory and a research power plant for deep geothermal energy in the Rhenish area (Figure 8). The location is characterized by the spatial overlap of the Aachen district heating grid on the surface with the assumed layout of the thermal water-bearing limestone sediments underground, whereby the Aachen district heating network was previously fed with waste heat from the Weisweiler lignite power station. The subsurface observatory is intended to monitor reservoir processes. Afterwards, the research power plant has to combine technological innovations with new value-added potential in the energy sector at one of the most ambitious research locations in the Rhenish District. On the one hand, the demonstrator is intended to pump hydrothermal water and provide heat for the regional district heating network. In particular, however, the system serves as a development environment and nucleus of the deep geothermal industry as well as a training and further education center for geothermal technologies.
Fraunhofer IEG accepts its social mandate and develops ideas, technologies and strategies for the next phase of the energy transition. It makes a significant contribution to opening up the markets for the application of geothermal energy systems, the storage of energy carriers and technologies for coupling the energy sectors of heat, electricity and transport in an even more targeted manner with its partners from business and science. Thus, Fraunhofer IEG is working towards a sustainable future.
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