Since the beginning of humanity, humans have been able to take advantage of geothermal energy, particularly thanks to hot springs. In summary, what is geothermal energy? It is a renewable energy which consists of exploiting the earth's heat to heat buildings or produce electricity. Focus on the mode of operation, the advantages, disadvantages and definition of geothermal energy.



The definition of geothermal energy

Geothermal energy is the exploitation of the earth's heat by humans. Earth, rocks or even groundwater are all sources of energy that can be recovered by geothermal energy. About 10 meters deep underground, the constant temperature is around 15°C. It is this heat that is used to power a geothermal heating system. In this case, it is so-called “very low energy” geothermal energy, which consists of extracting heat at a shallow depth on earth using a heat pump.

What are the three types of geothermal energy?

We cannot simply answer the question: “What is geothermal energy?” because geothermal energy actually encompasses three very distinct types of geothermal energy which do not have the same applications:

1. Shallow geothermal energy

Also called very low energy geothermal energy, it is that which is used for an individual geothermal domestic heating system, and which exploits the surface heat of the ground. Indeed, at approximately 10 meters depth, the earth benefits from the heat of the sun and the mild temperature of rainwater which infiltrates into the heart of the water tables. The temperature is stable, since it is constantly estimated at around 15°C. This heat is recovered using a system of sensors and heat pump. This system could be compared to preheating in winter, and to a cooler in summer, because this temperature can also be used to cool the ambient air in a home during high summer temperatures. This is possible if you choose to install a so-called “reversible” heat pump.

2. Medium geothermal energy

We are talking about average geothermal energy beyond 400 meters depth. Earth's heat is recovered by drilling, sometimes by hydrothermal drilling in aquifers, and the earth's temperature reached oscillates between 40 and 130 °C. A pump collects hot water from the ground, then the thermal energy present in this water, which is then injected into a geothermal fluid which is itself sent into a collective or urban heating network. Here again, this temperature is increased using a heat pump. Finally, the residual water from the drilling is evacuated into a watercourse or reinjected into the subsoil. Nearly 200,000 homes in the Île-de-France and Aquitaine regions currently benefit from these facilities

3. Deep geothermal energy

Deep geothermal energy concerns drilling 4000 to 6000 meters underground, where the temperature reaches around 200°C. The first step consists of drilling rock which is then fractured using water under very high pressure, with the aim of improving its permeability. The second step of the geothermal process is to inject water into the previously created fractures. This water heats up on contact with the rock, then it is pumped again to be brought to the surface. A liquid, called “working liquid” is then heated and then transformed into gas under pressure by the thermal energy thus recovered. A turbo generator then comes into action to produce electricity. The pumped water is reinjected into the ground while the residual heat is sent into a heating network. This technology is called SGS, Stimulated Geothermal System. The danger of this technology is that its application exposes the site to a risk of earthquake, caused by the fracturing of the rock at very great depths. In fact, drilling takes place several kilometers underground, and these operating techniques are comparable to those of oil platforms.

For the moment, drilling is only taking place a few kilometers underground, but the heat in the bowels of the Earth is still largely under-exploited, because it is out of reach with our current technologies. Electricity production by geothermal energy is therefore still marginal and in its infancy. But its potential is enormous, and we should expect great future technological progress in this area.

What is vertical or horizontal geothermal energy?

Vertical or horizontal geothermal energy is characterized by the sensors, which are therefore vertical or horizontal. Although they have the same purpose, these sensors are very different. In all cases, before you decide on geothermal energy, whether vertical or horizontal, it is necessary to carry out a serious and careful feasibility study upstream.

Vertical sensors

Vertical sensors do not require a particular surface area. However, as the probe can go down to a depth of 100 meters, it is necessary to plan for significant drilling. Vertical sensors are more efficient than horizontal sensors because the power recovered is 40W per m². The installation of a vertical geothermal system in an individual dwelling requires 1.3 m² of vertical collector surface per m² of living space. From a legal point of view, vertical collectors are considered drilling for domestic use. As with horizontal sensors, work requests must be made. In addition, a declaration must be previously filed with the services of the DRIRE , Regional Directorate of Industry, Research and the Environment.

Horizontal sensors

Horizontal sensors must be buried at a depth ranging from 60 centimeters to 1.80 meters. The power recovered is 35W per m² of land on average, which must be flat and without planting. The surface area required for the installation of a horizontal geothermal system can therefore be more or less significant depending on the surface area of ​​the housing to be equipped and the power required. This element can constitute an obstacle if the land adjoining the home does not have an adequate surface area, or if it is hilly or wooded. Legally, horizontal sensors must be the subject of a request for work at the town hall, in the same way as vertical sensors. They must also comply with municipal and prefectural decrees.

The advantages of geothermal energy

Geothermal energy is today considered to be the most efficient of all renewable energies because:

  • It does not depend on any external conditions such as the weather, unlike solar energy; 
  • Its exploitation does not undergo any seasonal variation;
  • The thermal losses caused by the transport of heat are tiny because the basement in which it is located acts as insulation; 
  • Geothermal exploitation can be implemented anywhere in the world, even if certain sites are more favorable, such as volcanic areas, which benefit from a higher temperature underground;
  • The energy produced from geothermal energy appears to be relatively clean, since it does not require the combustion of polluting materials such as coal, uranium or even oil and geothermal exploitation produces little waste.
  • On an individual scale, a home that uses geothermal energy produces 2 tons less carbon dioxide per year;
  • Geothermal energy seems to represent a source of energy, if not renewable, at least inexhaustible;
  • Finally, the savings made in a home of 150 to 180 m² are around 70%

  • The disadvantages of geothermal energy

    Geothermal energy still has some disadvantages:

    • The main thing is that this technology is still largely unknown. For example, there are currently less than a hundred large geothermal operations in France;
    • Methods for harnessing underground energy are still very expensive and extraction is a long process. 
    • Individual geothermal installation is not possible if the subsoil is rocky, whether vertically or horizontally; the candidate for an installation of this type is therefore dependent on the nature of the ground on which his home rests;
    • Installing a geothermal power plant is almost impossible in large cities;
    • This system is still relatively expensive, between 13,000 and 15,000 euros, although the energy savings are immediate;
    • Geothermal energy, in the current state of technological advances, cannot yet be used as a main source of heat;
    • We can fear a long-term environmental impact because the increase in drilling and deep fracturing risks causing soil deterioration. This is already the case in large geothermal exploitation infrastructures;
    • Toxic hydrogen sulfide emissions may occur in the event of corrosion or leakage. The introduction of protective substances into the pipes where the water passes can help prevent this type of incident.

    This article may also interest you: Advantages and disadvantages of renewable energies

    As we have already said, geothermal energy is almost inexhaustible, clean and does not depend on any meteorological or seasonal hazards. Its weakness therefore lies mainly in its very restricted accessibility, which considerably increases its implementation cost. This is the main reason why this energy still remains marginal in terms of heat and electricity production. However, we can predict its evolution over the course of technological advances in prospecting and sampling. The involvement and financial support of States remains for the moment the major asset in the development of geothermal energy. But we will still have to rely on fossil fuels for a while, because even renewable energies such as photovoltaic solar or wind power do not cover all of the energy needs of populations, although much progress has been made in these areas