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There are Several Types of Geothermal Energy – How Do they Work , and What are the Risks?

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Recent news in the Rhine ditch reports human-induced earthquakes during the exploitation of geothermal energy.

The exploitation of the internal heat of the Earth, “geothermal energy,” is a very ancient practice that began with the use of thermal springs and which has experienced in recent decades a particular boom with the advent of the energy transition, which urges humanity to change its energy production and consumption patterns.

Indeed, while 99% of the Earth’s volume is at a temperature above 100 ° C, this heat is unevenly distributed on the scale of the globe. The great tectonic movements at the borders and inside the tectonic plates, volcanism, or hydrothermalism are all factors favoring heat circulation from the depths. Let’s take stock of this resource with its various facets, which remains little known despite everything.

Three types of geothermal energy

The use of heat from the basement is dependent on technological advances allowing us to recover this heat, either by the use of heat pumps or by hydraulic stimulation, for example, which we will discuss later. Depending on energy demand and the amount of heat available in the basement, several types of geothermal energy are currently being developed worldwide.

There are three main types of geothermal energy, which can be differentiated according to depth, temperature, or the use of the heat resource: very low to low energy geothermal energy (temperature below 90 ° C), medium energy geothermal energy (temperature above 90 ° C) and high energy geothermal energy (temperature above 120 ° C).

The depth of these operations varies depending on the geology of the subsoil. Indeed, on Earth, the temperature increases on average by 30 ° C every kilometer (this is called the “geothermal gradient”), but this gradient is not identical at all points of the globe and can vary. Locally very strongly, sometimes reaching 100 ° C per kilometer. The higher the rise, the less you will have to dig to find high temperatures.

Therefore, the three major types of geothermal energy differ in several aspects, particularly in the risks associated with them.

Low energy geothermal energy

Low-energy geothermal energy concerns the domestic or industrial sector and applies to using the heat or the softness of the basement to heat, cool, and produce hot water for single dwellings, buildings, or even tertiary buildings.

At very shallow depths (less than 200 meters and around 20 ° C), geothermal heat pumps are the best way to recover this heat and produce energy. In this case, the heat pumps are connected to vertical “probes” or to horizontal “exchangers,” which circulate a heat transfer liquid in-depth: it heats up in contact with the heat from the basement and cools in the heat pump by a compressor/expansion valve system, to transfer its calories to the medium to be heated, for example, domestic hot water or a heating circuit.

This near-surface geothermal energy is the most developed in France because it is the least expensive and the easiest to set up. It is also the least risky because geothermal probes and horizontal exchangers operate in closed loops: the heat transfer fluid never comes into contact with the external environment and always carries out the same circuit. This is a heat exchange by thermal diffusion only.

When an aquifer is present in the subsoil, that is to say, a rock whose porosity and permeability allow water to circulate freely, “geothermal doublets,” composed of a production borehole and injection drilling, can be set up. In this case, the rising fluid heat is used to heat the urban networks; then it is reinjected into its original environment at a lower temperature.

In Île-de-France, many geothermal doublets are installed to supply district heating networks using a deep aquifer located between 1.5 and 2 km deep, at an average temperature of between 60 and 85 ° vs. The geological history of the Paris basin allowed a “reservoir” rock to be deposited over 160 million years ago. The presence of these deep aquifers depends entirely on past geological history.

In operation by geothermal doublet, the hot water from the aquifer in production is reintroduced into its original environment so that the heat transfer fluid does not come into contact with the external environment (nor with a potential surface water table) so as not to involve contamination and physicochemical imbalance between the different settings.

Medium and high energy geothermal energy

Medium-energy geothermal energy concerns deeper projects and temperatures generally above 90 ° C. The purpose of this geothermal energy is to use the high temperature of the depths to produce heat, or electricity (to a lesser extent), or even both at the same time. The main uses of this type of geothermal energy are industrial and include the extraction of chemicals, the drying of industrial products, or the recovery of metals.

High-energy geothermal energy seeks to capture water at temperatures above 120 ° C, in the form of steam, which will generate electricity using turbines. This type of geothermal energy is developed in specific geological contexts, involving the presence of hot bodies providing the heat source: it may be the proximity of the earth’s mantle or magmatic bodies, as can be found in Guadeloupe, on the Bouillante site, or in Tuscany.

Drilling several kilometers deep is necessary to produce this heat, given the average geothermal gradient. In the absence of a deep aquifer, freshwater must be injected at depth, where it heats up, then pumped to the surface, generally through two production wells. Therefore, for this method, it is necessary to find, in-depth, a warm and naturally fractured environment in which water can circulate and store heat.

In this context, the use of “hydraulic stimulation” increases the permeability of reservoirs fractured at depth. This technique, derived from the petroleum world, aims to inject fresh water under pressure to open pre-existing fractures. We then speak of “EGS” for Enhanced Geothermal System. Unlike hydraulic fracturing, which seeks through the injection of water and chemicals to create new fractures over a large area, hydraulic stimulation is much less risky because the injection pressures are four to five times lower than those of hydraulic fracturing. This hydraulic stimulation step frequently induces earthquakes, most being of such low magnitudes that the injection site population does not feel them.

On the other hand, resorting to hydraulic stimulation requires having a perfect knowledge of the reservoir in depth: its nature, geometry, the orientation of the fractures, or the quantity of tectonic tension previously accumulated. What happened recently at the Vendenheim site in the Bas-Rhin shows the lack of knowledge of the deep reservoir properties. Indeed, most of the data used by geoscientists are indirect data resulting from geophysical observations. A borehole diameter represents only a few square centimeters of surface for a reservoir that can extend over several square kilometers.

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