Feature Report: Geothermal Energy, the Earth’s Heat

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What Is Geothermal Energy?

As you descend deeper into the Earth's crust, underground rock and water become hotter. This heat can be recovered using different geothermal technologies depending on the temperature. But the heat resources in geothermal reservoirs are not inexhaustible.

Geysir, a natural source of steam in Iceland. The word geyser is derived from its name. © AFP

Thermal Gradient

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...). It covers all techniques used to recover the heat that is naturally present in the Earth’s subsurface, particularly in aquifers, the rock reservoirs that contain groundwater. About half this thermal (or “heat”) energy comes from the residual heat produced when the planet was formed 4.5 billion years ago and about half from natural radioactivity. 

3°C: the average rate at which temperature rises per 100 meters of depth

 

The temperature of geothermal water increases with depth, depending on the thermal gradient — the average rate at which the temperature rises with depth — of the region where it is found. The average value of the gradient worldwide is 3°C per 100 meters of depth, but it varies between 1°C and 10°C per 100 meters depending on the physical conditions and geology of the region.

The Different Types of Geothermal Energy

Geothermal technologies differ with the temperature of geothermal water, which determines what can be done with it:

The different temperature ranges are general, and practices may vary according to the economic conditions of the particular location.

 Availability of Geothermal Resources

This heat varies in different areas. The average geothermal heat flow — the energy available for any given surface area and period — on the surface is low. It averages 0.06 watts per square meter per year, or 3,500 times less than the solar energy flow received in a single year by the same surface area. This is why priority is given to using heat resources in those areas that are most likely to provide significant amounts of energy. These “geothermal reservoirs” are found in all the Earth’s sedimentary basins, but high-temperature geothermal energy is most likely to be found near volcanoes. In volcanic areas, geothermal heat flow can reach 1 wattThe watt (symbol W) is the derived unit of power (see definition) in the International System of Units (SI)... per square meter per year.

Geothermal reservoirs tend to be depleted with use, some faster than others. Their replenishment capacity depends on:

  •  Heat sources within the Earth’s crust, mainly radioactivity and residual heat.
  •  Energy from outside the reservoir (solar heat) for very low-temperature applications using heat pumps. Ensuring that these reservoirs will be reheated is especially crucial for geothermal heat pumps: external factors, such as low winter temperatures, cool the subsurface, meaning that less heat is available to be harnessed.
  •  The circulation of groundwater that is reheated on contact with heat sources located away from the reservoir before returning to the reservoir.

Therefore, these heat resources must be replenished to use a reservoir in a sustainable manner. This involves capping the amount of heat used and putting a time limit on the operation of the site. 

In addition, the availability of geothermal energy is geographically limited. Significant losses occur when heat is transported over long distances. This can cause problems, because production sites cannot always be located close enough to the place of consumption to meet energy needs.