Geothermal Energy
Beneath our feet glows a furnace that has been burning for billions of years. Geothermal heat is the only major energy source that has nothing to do with the sun.
Almost everything that warms us ultimately comes from the sun: wind, water, plants, coal. Geothermal heat is one of the few exceptions. It comes from the interior of our planet, partly from the residual heat of its glowing formation, partly from the decay of radioactive substances deep in the rock. This heat is vast and, by human standards, inexhaustible; it is only waiting to be tapped cleverly.
A bit of history
Where geothermal heat comes to the surface by itself, people have used it since time immemorial: the Romans bathed in hot springs, and in Iceland people have heated for centuries with what steams out of the ground. Electricity from geothermal heat is younger: in 1904, in Larderello in Italy, electric current was generated for the first time from hot earth-steam, and the power plant there runs in principle to this day. Countries on volcanically active ground, like Iceland, Kenya or the Philippines, now cover a considerable part of their needs geothermally.1
How it works: two very different depths
Geothermal heat is used in two fundamentally different ways, and they are easily confused.
Near-surface, at five to about a hundred metres depth, the ground is a constant roughly 10 to 15 degrees warm all year round, in winter warmer than the air, in summer cooler. This is what the heat pump uses: it fetches this mild earth-heat and "pumps" it up to heating temperature with a little electricity; in summer the game runs the other way round, for cooling. This is no glowing depth but simply a very even temperature store, and that is exactly why it works under almost any house.
Deep geothermal, by contrast, goes hundreds to thousands of metres down, to where the rock is really hot. Roughly: per hundred metres of depth the temperature rises by about 3 degrees; in the Earth's core it reaches several thousand degrees. At a few kilometres' depth the water is hot enough to heat whole city districts or even to drive turbines for electricity.
Today
Near-surface geothermal heat via heat pumps is currently experiencing a boom, because it is one of the cleanest ways to heat buildings without oil and gas. Deep geothermal, by contrast, is still in its beginnings in Germany: there are individual successful installations, for example in the Munich area, but the great breakthrough is a long time coming. The reason is simple: drilling deep is expensive and risky.
Advantages
Geothermal heat has an unbeatable advantage over sun and wind: it is always there. It knows neither night nor lull nor season, but delivers evenly around the clock, which makes it the ideal base load. It needs hardly any surface area, causes almost no CO₂ in operation and is practically inexhaustible.
Disadvantages
The great catch is money. A deep drilling costs millions, and no one knows with final certainty whether enough hot water really flows down there until they have drilled. This exploration risk deters investors. Added to this: if you press water into deep rock layers, it can trigger small earthquakes. In Basel and in Pohang in South Korea, geothermal projects led to noticeable quakes and had to be stopped, a loss of trust that still has aftereffects today.
The critical look
Geothermal heat is an instructive case, because it shows how much the usability of an energy source depends on the place. In Iceland, where the heat lies almost at the surface, it is a blessing; in flat northern Germany you have to drill kilometres deep for it. "Inexhaustible" does not, after all, mean "worthwhile" everywhere.
Whoever wants to judge in a self-determined way guards here against two exaggerations. Some praise geothermal energy as a miracle solution that solves all problems at one stroke, and keep silent about costs and risks. Others demonize it across the board because of a few quakes. The sober truth lies in between: the mild earth-heat under the house is already today one of the cleverest heating systems there is, while the deep, hot variant remains a promising but still expensive and location-dependent promise. The question is not whether the heat is there, but whether it is worth fetching at the particular place.