Feature Report: What Is Geothermal Energy?

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What Is Geothermal Energy?

As you descend deeper into the subsoil and the Earth’s crust, underground rock and water become hotter. This heat is referred to as geothermal energy, and can be recovered at various depths and temperatures using a range of techniques. It represents a renewable energy source that can heat – or cool – houses, apartment blocks, factories and farm buildings. If used to drive a turbine, the heat can also be converted into electricity.

fr-Qu’est-ce que la géothermie ?
Geysir, a natural source of steam in Iceland. The word geyser is derived from its name. © AFP

Groundwater and Aquifers

The adjective “geothermalDescribes the technology used to tap subsurface heat to produce energy... ” comes from the Greek words ge (earth) and thermos (heatIn the field of statistical thermodynamics today, heat refers to the transfer of the thermal agitation of the particles making up matter... ). Underground heat is stored in shallow groundwater, and, deeper down, in aquifers – rock reservoirs in the Earth’s subsurface containing water – and granite rocks. About half this thermal energy comes from the residual heat produced when the planet was formed 4.5 billion years ago and the other half from natural radioactivity.


Geothermal Gradient

The geothermal gradient of each region measures the average rate at which the temperature rises with depth. In Europe, the average value of the gradient is 3°C per 100 meters of depth, or an increase of 30°C for 1,000 meters. This average depends on the physical conditions and geology of the region. For example, the temperatures in the subsurface of a volcanic area rise very quickly.


The Different Types of Geothermal Energy

Different types of geothermal energy require different technologies, depending on temperature, depth and how the heat is used.

There are three main categories:

- Very low-temperature geothermal energy. Harnessing temperatures under 30°C at depths between 10 and 200 meters, this type of energy is used to heat or cool single-family homes, apartment blocks or commercial buildings. The energy is recovered by pumping groundwater to the surface or by installing horizontal or vertical “geothermal borehole” systems in the ground. Heat pumps are used to raise the temperatures for heating or lower them for cooling. Find out more.

- Deep low-temperature geothermal energy. Technologies in this category use water at temperatures between 30°C and 90°C collected at depths ranging from 200 to 2,500 meters. Water from aquifers is pumped up to the surface and then reinjected underground, using two wells forming a geothermal “doublet”. Connected to an aboveground heating system, this technique can be used to heat entire neighborhoods with thousands of inhabitants or industrial parks. Find out more.

- High-temperature geothermal energy. Working with temperatures over 150°C, the main method used in this area is to draw on wells of steam far below the surface to produce electricityForm of energy resulting from the movement of charged particles (electrons) through a conductor... . Another technique involves injecting water into hot, dry rock deep underground, then recovering it back at the surface. High-temperature geothermal energy can also be used in cogenerationThe simultaneous production of both heat (thermal energy) and power... to recover heat at the same time as generating powerIn physics, power is the amount of energy supplied by a system per unit time. In simpler terms, power can be viewed as energy output...Find out more.


In short:

 

Temperature

Depth

Targets

Methods

Very low-temperature geothermal energy

Less than 30%

Between 10 meters and 200 meters

Single-family homes, buildings, shopping malls

Horizontal networks, vertical boreholes, groundwater recovery (with or without a heat pump)

Deep low-temperature geothermal energy

 

 

Between 30°C and 90°C

Between 200 meters and 2,500 meters

Neighborhoods, industrial parks

Doublets (two vertical boreholes in aquifers)

High-temperature geothermal energy

More than 150°C

Between 1,500 meters and 5,000 meters

Power plants

Deep boreholes in aquifers, or injectionSecondary oil recovery method in which water or gas is injected into a deposit to increase pressure in the reservoir... of water into deep rock

 

Advantages and Limitations of Geothermal Energy

Geothermal plants have the advantage of not taking up much space on the surface, not producing pollution and not making a lot of noise. They do not consume any water, instead reinjecting it back underground. And after the investment phase, operating costs are low.

However, geothermal reservoirs tend to deplete after 30 or 50 years of use. Deep boreholes need to be monitored to make sure they do not cause seismic movements. And at the surface, transporting heat over long distances entails significant losses, making it preferable to use the heat in the immediate vicinity or install a network to distribute it in the area.


Find out more:

See the French Renewable Energy Syndicate (SER) site (in French).