Geothermal energy

Author: Florian Schwinghammer, tewag Technologie - Erdwärmeanlagen - Umweltschutz GmbH (As of: February 2016)

How and where geothermal energy is generated

The earth consists of a liquid core several thousand degrees hot with a solid earth crust that has grown over millions of years since its formation. The slow cooling of the earth from the outside to the inside causes heat to be transported in the opposite direction to the surface. In addition to this heat conduction, there are also so-called "hot spots" where convection in the upper mantle contributes significantly to heat transport to the earth's surface. Depending on the location, various geological factors or parameters are decisive for the usable heat recovery potential. In principle, however, the deeper, the higher temperatures can be expected. At geothermally favorable locations at tectonic plate boundaries, such as in Iceland, temperatures greater than 100 °C can already be reached at a depth of just a few hundred meters. At unfavorably located sites, in contrast, several thousand meters must be drilled to develop temperatures around 100 °C.

Use of geothermal energy

Due to the different temperatures in the various depths of the earth's crust, correspondingly diverse forms of use of this stored heat are possible. A general distinction is made by the division into near-surface and deep geothermal energy. There is no strict boundary here, but a common limit between near-surface and deep geothermal energy is given at 400 m. This makes sense because the heat stored in the earth's crust can be used in many different ways. This makes sense in so far, since due to the drilling depth different techniques must be used, as well as other geological and hydrogeological parameters for the utilization of the heat energy are decisive.

Near-surface geothermal energy

In the near-surface range one differentiates between closed and open systems. In closed systems, a heat transfer medium is circulated within a closed loop, absorbing ambient heat and transferring it to the user at the surface via a heat exchanger. In "shallow" applications up to 400 m, subsurface temperatures are usually not reached that are sufficient for direct use for heating and hot water generation in residential, industrial or office buildings. Therefore, in order to achieve a sufficient temperature level for building heating, a heat pump must be installed that can operate efficiently even with low subsurface source temperatures of 8 to 20 °C. Various systems can be used to tap the thermal energy stored in the near-surface area. The most commonly used method is the installation of a geothermal probe, which is inserted into a borehole down to its lowest point. The existing annular space between the geothermal probe and the borehole wall is then sealed with a cement suspension in order to separate any different groundwater levels from each other again and to achieve a good thermal connection to the surrounding rock. The installation depth of geothermal probes ranges from about 20 to 400 m.

Furthermore, there is the possibility of heat via geothermal collectors or geothermal baskets, which are installed only a few meters below the surface to win. Here it must be considered that the soil temperature and thus the usable heat energy depends strongly on the seasonal course of the outside air temperature. In fact, the solar energy stored in the surface soil is used for the most part. In contrast to a borehole heat exchanger system, the area occupied by these systems is significantly larger, since a lower extraction rate is possible per m² or per meter of pipe length. Another option is thermally activated foundation piles (so-called energy piles). These are thermally activated foundation piles that can be installed cost-effectively, for example, as part of a new construction project that requires a pile foundation anyway.

Another option for using geothermal energy is open systems. In these, water with an average temperature of 8 to 13 °C is extracted from an aquifer via an extraction well and heat is extracted (or supplied) via a heat exchanger. Subsequently, the cooled (or heated) and otherwise unchanged water is returned to the aquifer via an absorption well. An advantage of borehole heat exchanger wells and well systems is that building cooling is also possible during the summer in the case of cooling, i.e., heat is dissipated from the building into the subsurface or into the aquifer. In the case of geothermal probes, this heat can be stored in the rock to a certain extent and used again in winter. Furthermore, there is virtually no risk of discovery (in closed systems), since the subsurface has always stored low-temperature energy.

Deep geothermal

With increasing drilling depth, subsurface temperatures rise, so that higher subsurface temperatures are encountered at drilling depths of up to several thousand meters. Depending on the geological conditions (e.g. geothermal anomalies such as volcanically active areas), warm or hot water, steam/water mixtures or dry, superheated steam can be extracted.

The decisive factor in such hydrothermal systems is the amount of water that can be extracted and the temperature of the extracted water. If the temperature of the extracted groundwater is below about 80 °C, it can only be used for balneological purposes or for heat generation. If higher groundwater temperatures are reached, electricity can be generated via a steam turbine system in addition to thermal use. Geothermally generated steam can be fed directly into a steam turbine at temperatures of 150°C and above. After heat extraction, the extracted water is pumped back into the source horizon via a second well. Between 80°C and 150°C, geothermal wells are used to generate electricity via an intermediate circuit with a low-boiling organic fluid, for which a specially adapted turbine is used. These fluids are not without safety issues. This process is called Organic Rankine Cycle (ORC) in international parlance. Alternatively, the same basic principle is used for the so-called Kalina process, which works with a two-substance mixture consisting of ammonia and water. Geothermal power plants deliver predictable power, their partial load behavior corresponds to that of steam power plants.

In great depths, geothermal energy can also be extracted from (dry) dense rock. The term "hot dry rock" has been defined internationally for this process. Suitable measures, such as high-pressure water injections (up to several hundred bar), are used to hydraulically widen existing fractures in the rock bed or create new ones. This is intended to expand the reactive rock surface or the thermal exchange surface and thus enable efficient exchange between the heat transfer medium and the hot rock surface. Water or another heat transfer medium is injected into the resulting fracture system via a borehole, heated and conveyed to the earth's surface via a second borehole. After heat transfer via a heat exchanger into the power plant process, the heat transfer medium is again conveyed into the crack system via the downcomer. The boreholes must be so far apart that they cannot affect each other thermally. This type is very costly to develop, which is why it is only used on a large scale with high power plant capacity. Due to the artificial widening of fractures in the subsurface at high pressures, stresses may be released in the subsurface. These can continue to the earth's surface and lead to minor (induced) earthquakes. As a rule, the intensity of these earthquakes is below the sensibility limit, but earthquakes of magnitude 2 to 3 have also been measured at various locations. For example, the deep geothermal power plant in Landau in the Palatinate has now been temporarily shut down due to light tremors. On the other hand, there are also projects such as those in the Munich area (Unterhaching, Sauerlach, Dürnhaar) that have been running in full operation for several years without any major disruptions.

Economics, risks and project examples

Deep geothermal energy:

In Germany, geothermal energy enjoys feed-in priority under the Renewable Energy Sources Act (EEG). The feed-in tariff to be paid by the electricity customer per kWh for this technology, is adjusted with each new version of the EEG, as for all other renewable energies. These costs, as well as the costs of developing the heat source, must be taken into account in a profitability analysis. In deep geothermal energy, there is a certain risk of discovery if no groundwater is encountered in the target horizon or if the temperature and/or the bulk rate of the groundwater is too low. Since the investment costs of a well, some of which are several thousand meters deep, are relatively high, this economic risk is also increased. The components in contact with the deep groundwater are subject to strong corrosive attack in some cases, which means higher wear of the plant components and thus maintenance and repair costs. The heat output installed in Germany in 2015 was about 271 MW, and the installed electrical output was about 35 MW. An overview of existing plants is available at www.geothermie.de or www.tiefegeothermie.de.

Surface geothermal energy:

If the heating system is not properly or inadequate design of the heat source, the subsurface may be overstressed during its service life, i.e., subsurface temperatures drop, correspondingly reducing the heat pump's coefficient of performance and thus increasing the heat pump's power consumption. There is no discovery risk, except in the case of a well system. As of 2014, a total of approximately 316,000 heat pump systems were in operation in Germany, providing a total heat output of 3,931 MW. The newly installed number of systems in 2015 was 18,500 with a heating capacity of 196 MW. An overview of realized projects in Germany is available at www.erdwaermeliga.de or www.geothermie.de.

Conclusion

Risks cannot be ruled out, as with any construction project in both shallow and deep geothermal energy. In the case of deep geothermal energy, the plants are in most cases equipped with state-of-the-art measurement technology that registers and records the smallest events such as quakes and terrain shifts, some of which occur well below the threshold of perception. However, it cannot be ruled out that unforeseen slightly amplified earthquakes induced by the drilling operations may occur, even if they do not usually cause any damage. Even if a causal connection with the power plant cannot be established, the events fuel fears among the population. Citizens' initiatives are forming and exerting pressure on local politicians and authorities. Deep geothermal energy will probably continue to play a subordinate role in the German electricity transition, not least because of the dense population and limited site compatibility. By contrast, in the heat turnaround, there is significantly greater potential with cold or warm local heating networks. Accordingly, near-surface geothermal energy with space heating and water heating using heat pumps still has great potential in renovation construction and especially in new construction. The upward trend that has emerged in recent years is likely to continue, even with the recent decline in prices for petroleum products.