The problem of geothermal energy production is in fact partly the drilling technique, where the subsoil is hot enough only at a very high depth (for the normal citizen) to produce steam for a steam turbine according to the above-mentioned method.The average geothermal gradient in Germany is 3°C per 100m. [1 In order to be able to start with a turbine at all, you would need at least a temperature of 80°C (with the so-called Organic-Rankine-Cycle).So we are already talking about a borehole with a depth of theoretically 2700m.
But the real problem still lies.The most important element of geothermal energy is not so much the temperature as the temperature gradient and the enthalpy of the environment. This, in turn, leads to the problem of using only one heat probe at the bottom of a borehole. This will cool the surrounding rock/gap water, and then effectively only provide as much energy as can flow into the environment of the heat probe.
For this reason, the variant with two holes and a possible one is used in large scale.artificially generated rift water conductors between them, which significantly increases the “cooled area” of geothermal use, and thus also the heat afterflow, which is effectively usable for geothermal energy. So you need not only 80°C warm rock, but effectively rock, which with a corresponding cooling still keeps at this temperature level, because magma in the surrounding area, for example, heats the rock. Or you would have to drill and expand even deeper.
However, nature is not so nice and indicates a suitable geological structure with a shield on the earth’s surface, and the work at such depth, not to mention the problem with the pumps and the energy to pump the medium upwards. , makes the whole thing very uneconomical quickly when you get deeper.As a result, so-called high-enthalpy deposits are used to generate electricity. In addition, there are deposits that, due to their proximity to volcanoes or volcanically active areas, provide access to hot rocks at much lower depths. The first geothermal power plant, for example, was built in Larderello, Tuscany, in 1913, and today produces 500 to 750 MW of electrical energy using several steam wells (the data vary).
Instead of generating electricity, however, geothermal energy is worthwhile for heat generation, especially in the context of housing construction.The operation of a heat pump in combination with a large heat storage system allows the use of a single geothermal probe with a deeper bore. However, I was told that certain companies would rather swarm two heat probes on their customers, and then drill two holes to avoid the approval process at the Mining Authority, and twice as much for the drilling rig and two geothermal probes. If you get the permit, drilling at a depth of 120m is still cheaper (drilling companies calculate the build-up and the running meter) and more effective (because of the geothermal gradient) than drilling twice, for example, at 60m depth. In addition, one avoids the necessary distances between the two probes, which can lead to problems (even with the geothermal neighbors) on a smaller plot.
By the way, using the Earth’s interior itself fails at the moment due to the problem that at the deep boreholes, at some point the environment of the borehole becomes too plastic.Then it’s like drilling through hot pudding, pressing on the machines with a few thousand bar. [2