At some 6,000 degrees Celsius (11,000 degrees Fahrenheit), Earth's core is about as hot as the sun. Though not comparable, even at 2,000 to 5,000 meters (6,500-6,000 feet) beneath the surface of the planet, it can be a scorching 60 to 200 C. In volcanic regions, even surface temperatures can reach 400 C.

That makes for a lot of potential heat-based energy. Our ancestors were no strangers to the power of geothermal energy. In the first century AD, Romans living in the western German cities now known as Aachen and Wiesbaden heated their houses and thermal baths with hot spring water. In New Zealand, the Maori people cooked their food using the Earth's heat, and in 1904, geothermal energy was used to generate electricity in Larderello, an area in central Italy.

Volcanic areas turn geothermal energy into electricity

These days, some 400 power plants in 30 countries generate electricity using steam generated beneath Earth's surface, producing a total capacity of 16 gigawatts (GW).

This method of generating electricity is particularly important in volcanic regions along the Pacific Ring of Fire, including the United States, Mexico, El Salvador, Iceland, Turkey, Kenya, Indonesia, the Philippines and New Zealand. But on a global level, geothermal energy only accounts for 0.5% of electricity generation.

Heat from deep geothermal energy is available everywhere

Across the world, geothermal energy is mainly used for heating swimming pools, buildings, greenhouses and for urban heating systems. Water up to 200 degrees C is pumped from boreholes up to 5,000 meters deep. The heat is then extracted and the cooled water is pumped back in through a second bore.

This method of heat capture is feasible worldwide, inexpensive and increasingly popular in countries that lack volcanic activity. According to assessments by the Renewables Global Status Report, the installed capacity of geothermal heat plants is around 38 gigawatts worldwide — more than double the capacity of geothermal power plants that generate electricity.

To date, China (14 GW), Turkey (3 GW), Iceland (2 GW) and Japan (2 GW) are the leaders in developing deep geothermal energy, heating more and more city districts and greenhouses. In Germany, the city of Munich enjoys inexpensive geothermal heating and has set its sight on using the technology to make the sector climate neutral by 2035.

The German government is also looking at further developing deep geothermal energy to create a nationwide climate-neutral heat supply by 2045.  According to studies, deep geothermal energy could generate around 300 terawatt hours of heat annually from an installed capacity of 70 GW — more than half the future heat demand of all buildings.

Using heat pumps to extract heat from the earth's surface

Increasingly, however, geothermal energy is also being harnessed from sources close to the earth's surface using heat pumps. In boreholes just 50 to 400 meters deep, a closed pipe system carries water from the surface to underground and then back, heating it 10 to 20 degrees C. A heat pump then uses this energy to output water at 30 to 70 degrees C, which is then used to heat buildings.

Researchers believe using this shallow geothermal energy in Germany offers heating potential similar to deep geothermal energy. In Germany, these two technologies alone could satisfy the entire future heating demand for buildings.

How much does heat from deep geothermal energy cost?

According to analysis by six German research institutes, generating heat with deep geothermal energy costs less than three euro cents ($0.03) per kilowatt hour (kWh).

The kind of geothermal energy technology used so far pumps hot water from aquifers or water-bearing zones deep underground to the surface and uses the heat to warm homes, for instance.  

But now the world's first commercial geothermal plant that doesn't rely on water pumped from aquifers is being built in the town of Geretsried in the German state of Bavaria.   

The new technology could allow countries to exploit geothermal energy in locations where it wasn't previously possible to do so, according to Professor Rolf Bracke, head of the Fraunhofer Research Institution for Energy Infrastructures and Geothermal Systems.  

Before Russia's attack on Ukraine, natural gas could generate heat even cheaper than this for many municipal utilities in Europe. That made it unattractive to invest in the construction of deep geothermal energy plants. Since Russia's invasion, however, sharply rises in gas prices have pushed that cost to more than 12 cents per kWh, changing the calculation. Municipal utilities are now showing great interest in deep geothermal energy for heat supplies.

Can geothermal energy completely satisfy the demand for heat?

No. The heating demands of the world's buildings can be met by the near-unlimited potential of deep geothermal energy and near-surface geothermal energy.

But industrial applications sometimes require temperatures of over 200 degrees, which, with present technologies, are generally unattainable with geothermal energy. For such high temperatures, heating with electricity, biogas, biomass and green hydrogen are the climate-friendly alternatives.

How quickly can deep geothermal energy start supplying heat?

Over the past century, the oil and gas industries in particular have amassed considerable knowledge of the earth's subsurface, on drill techniques, how to train personnel, and have developed sophisticated technology. Professor Rolf Bracke, head of the Fraunhofer Institute for Energy Infrastructure and Geothermal Energy (IEG), told DW he is confident that geothermal energy can be expanded rapidly "if the oil and gas industries turn their attention to geothermal energy."

But he says if those companies continue to focus on oil and gas production because it generates more money, there would be insufficient personnel and drilling technology to rapidly expand geothermal energy. According to Bracke, it takes two to three years to develop geothermal heat sources if approval is granted quickly, and about three times longer than that in Germany due to bureaucratic delays. The German government now wants to speed up this process and increase heat energy production tenfold from the current production of 1 terawatt by 2030.

Can deep geothermal energy cause earthquakes?

Yes. In regions with seismic activity, geothermal energy can trigger small earthquakes when water is injected into the subsurface at too high a pressure, triggering existing stresses. In some cases, the tremors have resulted in cracks in buildings and public opposition to this technology.

According to Bracke, there have been no reports of earthquakes in regions without underlying stresses. Meanwhile, geothermal techniques have also been improved: surface tremors can now be avoided with lower underground water pressures and more sophisticated monitoring methods,

But compared to oil, gas and coal extraction, geothermal is far less risky, Bracke emphasized, and "by far the safest source of energy from our Earth."

This article was originally written in German. It was first published on January 1, 2023 and was updated on August 24 to include news on the latest geothermal technology.