The geothermal principle in a nutshell
The center of the Earth is a solid core (due to pressure) of metal at nearly 5000°C, largely the result of the disintegration of fissile atoms contained in rocks. This 1200km-radius sphere is surrounded by a 2300km-thick layer of liquid metal at a temperature of 4000°C. The “mantle”, composed of a layer 2250km thick and a layer in contact with the earth’s crust, at the limit of which the temperature is close to 1000°C. The latter is 5-10km deep at ocean level and 30-70km deep at continental level. The deepest borehole is 12km.
The deeper we dig, the higher the temperature. This varies from zone to zone: some are closer to the magma than others. This is known as the “thermal gradient”. The great limitation of this energy is its availability: not all zones are equal. Iceland, for example, is very conducive: you don’t need to dig very deep to get close to the magma zone (of course, there are a lot of volcanoes) and therefore high temperatures. Conversely, it’s more complicated in France, which isn’t really a seismic zone.
Volcanic zones are particularly well-suited to the task: while in general, going down to a depth of 100m brings a gain of 3 to 4°C, in these areas the increase is 2 to 3 times greater.
The deeper you go, the more heat you’ll bring up, but the heavier and more expensive the installation.
The different types of geothermal energy
There are 3 types of geothermal energy: high, medium and low temperature.
High-temperature geothermal energy / deep geothermal energy
Deep geothermal energy draws water from deep aquifers at temperatures of between 150 and 350°C, kept in liquid form by pressure. Pumping the water transforms it into steam, the pressure of which drives a turbine, which in turn drives an alternator, transforming DC power into AC, which is then fed into the grid via a transformer. The water is then returned to the subsoil. This is the principle of a geothermal power plant.
In some cases, water can be injected deep underground to create this reservoir. This is the technique of extracting heat from rocks.
In France, there are currently only two geothermal power plants:
- The 20 MW Bouillante plant in Guadeloupe, which has been in service for some twenty years.
- The Soultz-sous-Forêts power plant is a prototype designed to perfect the technique of extracting heat from rocks. It has a capacity of 1.7MW and can produce 12GWh/year.
Low-temperature/low-energy geothermal energy
It is also possible to extract heat at lower temperatures, up to 150°C, by descending to a depth of 2,000 meters. This technique is used for direct heating: heat is brought up and used directly for heating. We can use it to heat greenhouses, swimming pools or urban heating networks.
One method is to extract water from aquifers.
Heat pumps are not yet widely used for this type of installation. In fact, there’s no need to extract heat to bring it to the desired level: it’s already there.
Superficial geothermal energy / very low temperature / very low energy
This category includes geothermal heat pumps.
However, I haven’t yet considered this to be relevant, as it’s only one category of heat pump. When we talk about geothermal energy, we tend to think of deep geothermal energy, the extraction of heat from the depths of the earth. However, surface geothermal energy doesn’t so much extract heat from underground as it does from the insulating power of a few meters of earth.
We can also think of other technologies, such as the passive air-conditioning of the puit provençal and the puit canadien.
Advantages and disadvantages of geothermal energy
Geothermal energy has some very fine qualities: it’s literally inexhaustible, clean and (absolutely) safe… Nevertheless, it has one major problem: availability and cost. It’s a single problem: the deeper the heat has to be extracted, the more expensive the system. What’s more, while the system is cheaper in seismic zones, it is also riskier: earthquakes can damage it. This is a crucial point, and one that has led to the low level of development of this type of energy today.
Another problem is that drilling can cause micro-earthquakes. This is said to have been the case in Switzerland, where two earthquakes (3.4 and 3.6) were attributable to this activity.
Possible geothermal applications
Geothermal energy has two sets of applications: electricity generation and heat production.
Geothermal energy to generate electricity
Only high-temperature geothermal energy can be used to generate electricity.
Geothermal energy to generate heat
It is also possible to recover heat for direct use in heating networks to heat buildings. If the heat is sufficient, it could also be used for industrial processes, but I haven’t seen any mention of this.
Geothermal energy worldwide
In the world is produced 88.9 TWh of geothermal energy in 2018 (I do not know if it is electric or thermal).
Geothermal energy plays an important role in electricity production in some countries:
- 44% for Kenya (5.2 TWh)
- 30% for Iceland (6 TWh)
- 18% for New Zealand (7.9 TWh)
- 11% for the Philippines (10.7 TWh)
Several geothermal projects around the world illustrate the potential of this renewable energy source:
- The Hellisheidi geothermal power plant in Iceland: one of the world’s largest geothermal power plants, it generates electricity and heat for the Reykjavik region.
- The Geysers project in California (USA): the world’s largest geothermal power plant complex, with a generating capacity of over 1,500 MW.
- The Larderello geothermal power plant in Italy: in operation since the early 20th century, it continues to generate electricity for the region.