Feature Report: Geothermal Energy, the Earth’s Heat

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Low-Temperature Geothermal Energy: Heating

Low-temperature geothermal energy is used to supply heating for homes, apartment and office buildings, public facilities and farms. Medium- and high-temperature geothermal energy, above 90°C, is used to generate power 

Geothermal energy can be used to heat facilities such as greenhouses, like this one in Aigueperse in France's Auvergne region. © STF / AFP

Low-temperature geothermalDescribes the technology used to tap subsurface heat to produce energy... energy uses the heatIn the field of statistical thermodynamics today, heat refers to the transfer of the thermal agitation of the particles making up matter... of the Earth's subsurface or hot groundwater, between 20°C and 90°C (see Close-Up: "What Is Geothermal Energy?"). The heat contained in the rocky upper layers of the Earth’s subsurface (down to about 150 meters) can be captured for direct heating by means of heat pumps and ground source collectors, consisting of buried pipes (“loop”) or vertical boreholes. This is referred to as very low-temperature geothermal energy (see Close-Up: "Very Low-Temperature Geothermal Energy: Heat Pumps and Ground Source Collectors").

Saline or sulfate-containing geothermal water must be injected back into its original reservoir.

Worldwide heat production, or low-temperature geothermal energy, installed capacityThe power generation capacity of a particular plant. It is usually expressed in megawatts (or sometimes even gigawatts)... is estimated at 27,000 MW. In 2005, more than 70 countries, including Japan, China, Iceland, the United States and various European countries, used geothermal energy to produce heat (2,500 MW of installed capacity).

Drilling Geothermal Wells

Geothermal engineers start by drillingThe process of boring a hole into the ground using special equipment... one or more wells to reach the underground areas or hot water reservoirs whose heat they want to harness. They then insert casings, or hollow cylinders, to support the walls.

In the case of low-temperature geothermal energy, groundwater reservoirs, or aquifers, are tapped. They generally lie at depths of 1,000 to 2,500 meters and contain water whose temperature ranges between 40°C and 90°C.

There needs to be enough pressure to ensure a regular, sufficient flow of water. The scenario varies depending on the location:

  • When the pressure in the reservoir is higher than the atmospheric pressure, water is forced out of the well. This is known as an artesian well. If the water is the right temperature for heating (approximately 60°C) and the pressure is constant and not too strong, it can be fed directly into radiators.
  • If water pressure is not high enough, a motor-driven pump is installed. The pump draws the hot water up from underground and feeds it into the heating system.
  • If water pressure is not high enough, a motor-driven pump is installed. The pump draws the hot water up from underground and feeds it into the heating system.

Corrosion, the Enemy of Systems 

Geothermal water contains dissolved salts and gas. If there is too much salt or gas in the water, this can lead to corrosionThe gradual degradation of a material by a physical or physicochemical reaction... in the pipes and cause environmental problems. Geothermal engineers have developed a number of technical solutions to deal with these problems:

  • If the water is not too corrosive, chemicals that inhibit corrosion are injected at the bottom of the well. In addition, pipes made of composite materials are used instead of standard steel pipes, because they are less prone to corrosion.
  • If the water is very corrosive, it cannot be used directly in the heating system on the surface. After being pumped up through the well, the geothermal water transfers its heat through a heat exchanger installed between the geothermal loop and the heating pipes. In this way, the geothermal water heats the municipal water circulating in the heating pipes without coming into contact with them. A system like this, with a flow of 200 cubic meters of water an hour, can heat 2,000 to 3,000 homes.

If geothermal water is not pure enough, it cannot be discharged with wastewater. In this case, it is injected back into the source reservoir via a second well, in what is known as a double well installation, situated a sufficient distance from the production well. 

The Various Uses of Geothermal Heating

Most low-temperature geothermal technologies are used in public and residential buildings, where they supply heat to radiators and underfloor heating systems or heat domestic hot water.

There are also other applications including heating swimming pools, spas, leisure centers, greenhouses, mushroom farms, fish-farming ponds, and wood drying facilities.