Geothermal energy is an important renewable energy source. Geothermal energy is a promising use of heat sources and also heat storage in the natural underground. The natural underground becomes about 3°C warmer (on average) per 100 meters’ depth. This heat can be used for heating. While an average groundwater temperature of a shallow aquifer (in the sense of not far below the surface) is about 10 to 15 °C on an annual average, we already find 70 to 80 °C hot water at a depth of two kilometers. Examples are the thermal springs in southern Germany. There are areas in Germany where the geothermal gradient is even greater, the best known in Germany is in the Upper Rhine Plain.
Cool groundwater can be used to cool buildings in summer. In this case, cool water is extracted and, after it has warmed up in the buildings, fed back into the aquifer. Certain temperatures must not be exceeded. It is conceivable that in urban areas several operators of geothermal energy plants may influence an aquifer. It is therefore necessary to forecast the mutual interference as well as the overall impact on the aquifer in order for the permitting authorities to approve, reject or restrict such projects.
Interactive simulation on geothermal energy
The interactive geothermal energy simulation shows such an example: several geothermal wells are active and, depending on the groundwater flow and the injection rates, the temperature plumes created in the groundwater can be mapped.
The interactive simulation is an example of “shallow geothermal energy”, because it takes place at a shallow depth of about 100 meters. “Deep geothermal energy” only occurs at a depth of two kilometers. It is therefore fundamentally distinguished from “shallow geothermal energy”. “Deep geothermal energy” is much more complex, because the borehole goes to depths of up to 2-3 kilometers. The rock at this depth is normally very impermeable. Therefore, it must first be hydraulically fractured so that water can circulate, which is extracted hot and then fed back in cold.
While “shallow geothermal energy” typically takes place in pore aquifers, “deep geothermal energy” is characterized by flow in strongly fissured rock. This is a starting point for the SFB1313 and its research area B “Fracture propagation and fluid flow“.
If we take a look at this fictitious city, we can see the different earth and rock layers underneath. There is groundwater in every layer. The deeper the layer, the higher the temperature. In Germany, regardless of the season, it gets warmer by about 3 °C on average per 100 m of depth. That means that also the groundwater temperature increases accordingly.
The temperature of the groundwater and the underground rocks increases in average by 3°C per 100 m depth. The temperature is therefore higher than the average ambient temperature, which is about 10°C in Germany.
Average ambient temperature + depth x 3 °C
At a depth of 2 km the temperature is already between 70 and 80 °C.
How does geothermal energy come to the surface?
Using groundwater for heating or for cooling requires appropriate technical systems. Production of geothermal energy up to a depth of 400 m is referred to as shallow geothermal energy while in greater depths it is denoted as deep geothermal energy.
How can the heat be used?
The underground heat can be used as energy source
For heating (winter): the groundwater can be used for heating buildings.
For generating electric current
Heat also can be injected or stored
For cooling (in summer): Groundwater can be used to cool buildings, streets or squares.
Carbon Capture and Storage (CCS) in the natural underground
Another example of the use of the natural subsoil is CO2 storage. Humans produce CO2 as a byproduct of energy production from fossil fuels, such as in powerplants, heating, airplanes and (non-electric) cars. We even breathe it out. When the CO2 is released in the atmosphere, it accumulates, and reduces radiation of energy back into space like in a greenhouse, which in turn may affect the climate.
This experiment has been created by the “Porous Media Group” of the University of Bergen in Norway. In the experiment, the researchers have created an illustrative cross-section of the North Sea, and show how injection of CO2 gas leads to accumulation under almost impermeable layers of fine sands. Over time, much of the CO2 dissolves into the water which always exists in subsurface sand layers.
Geothermal energy is a technology with which renewable energy can be obtained from geothermal energy.