The geothermics , of the Greek Géo (ground) and megacal (heat), is the science which studies the internal thermal phenomena of the terrestrial sphere and technology which aims to exploit it. By extension, geothermics indicates also geothermal energy resulting from the energy of the Earth which is converted into heat and/or electricity.

One distinguishes three types of geothermics:

  • geothermics with high energy (electrical production),
  • geothermics with low energy (production of heat),
  • geothermics with very low energy (geothermic heat pump taking the heat contained in the ground)

Geothermal energy is exploited in networks of heating and warm water since thousands of years in China, in ancient Rome and the Mediterranean basin.

The raising of prices of energy and the need to emit less gas with greenhouse effect make it more attractive. In 2007, in France BRGM has with ADEME, created a Département Geothermics to promote it, after being themselves associated with various research programs, of work of public service. Two of its subsidiary companies CFG Services (services and specialized engineering) and Ebullient Géothermie (which exploits the powerplant of Ebullient in Guadeloupe) are implied in geothermics.

Principle

It is a question of extracting the heat contained in the basement to use it like Chauffage or contrary to him to restore heat and to use the cold air obtained for the Climatisation. One can also use geothermal energy to transform it into electricity. There exists a Flux geothermic naturalness on the surface of the sphere, but it is so weak that it cannot be directly collected. Actually one exploits the Chaleur accumulated, stored in certain parts of the basement (sheets of water) by making one or more drillings, more or less deep (S) according to the desired temperature or the local heat gradient.

Energy is potentially considerable, because one km ² of rock, on a 10 km depth, contains on average a quantity of energy being equivalent to 15 million Mtoe.

Heat gradient

The more one drills deep in the earth's crust, the more the temperature increases. On average, the increase in temperature reaches 20 to 30 degrees per kilometer. This heat gradient depends much on the area of the sphere considered. It can vary 3°C/100 m (sedimentary areas) until 1000°C/100 m (volcanic areas, zones of rift as in Iceland or New Zealand). Most of the heat of the Earth is produced by the natural Radioactivité rocks which constitute the earth's crust: it is the nuclear energy produced by the disintegration of the Uranium, the Thorium and the Potassium.

See geothermic article Gradient

Types of geothermics

One classically distinguishes three types of geothermics according to the level of temperature available to the exploitation:
  • geothermics with high energy or privileged geothermics exploits very hot hydrothermal sources, or very major drillings where water is injected under pressure in the rock. This geothermics is especially used to produce electricity. It is sometimes subdivided in two subcategories:
    • average geothermics energy (at the temperatures ranging between 100 and 150°C) by which the electrical production requires a technology using an intermediate fluid
    • geothermics high energy (with the higher temperatures with 150°C) which allows the electrical production thanks to the vapor which spouts out with enough pressure to supply a turbine.
  • basic geothermics energy: geothermics of the deep tablecloths (between a few hundred and several thousands meters) at the temperatures located between 30 and 100°C. Principal use: networks of district heating.
  • the geothermics of very low energy: geothermics low depths on the levels of temperature ranging between 10 and 30°C. Principal uses: heating and individual air-conditioning by thermodynamic devices generally functioning with electricity, from where the term barbarian electro-thermodynamics, more commonly called “aerothermic heat pumps” (drawing from the surrounding air) and “geothermic heat pump” (drawing from the ground or water at a shallow depth) much more powerful than the first.

Compared to others renewable energies, the geothermics of depth (high and low energy), has the advantage of not depending on the atmospheric conditions (sun, rain, wind). It is thus a quasi-continuous energy source bus it is stopped only by maintenance actions on the geothermal power station or the distribution network of energy. The geothermal fields have one lifespan of several tens of years (30 to 50 years on average).

Since 1973, B. Lindal had synthesized in a table the possible applications of geothermics.

“Geothermic Doublet”

Several diagrams of installation exist:
  • single drilling: one or more drillings of pumping without drilling of re-injection
  • doublet: one or more drillings of pumping and one or more drillings of nonreversible re-injection
    • doublet: each drilling always functions in pumping or reversible injection
    • doublet: each drilling functions alternatively in pumping and injection

In general the principle of the “geothermic doublet” is retained to increase the lifespan of the exploitation of the ground water from which one draws warm water. The principle is to make two drillings: the first to draw water, the second to reinject it in the tablecloth. The drillings can be distant one from the other (at each end of the tablecloth to induce a movement of water circulation in the tablecloth, but it is not practical from a point of view of maintenance) or close to a few meters but with drillings on the rake (always with an aim of moving away the points from puncture and re-injection of water).

Geothermics high energy

Geothermics high energy, or major geothermics , more rarely called geothermics high temperature, or geothermics high enthalpy, is an energy source contained in tanks generally located with more than 1500 meters of depth and whose temperature is higher than 80°C. Thanks to the high temperatures, it is possible to produce electricity and to make Cogénération (joint production of electricity thanks to steam turbines and of heat with the recovery of the condensates of the vapor).

The more one drills deep in the earth's crust, the more the temperature increases. On average, the increase in temperature reaches 20 to 30 degrees per kilometer. This heat gradient depends much on the area of the sphere considered. The zones where the temperatures are much stronger, called anomalies of temperature, can reach several hundreds of degrees for low depths. These anomalies are generally observed in the volcanic areas. In geothermics, they are indicated like layers of high Enthalpie, and are used to provide energy, the high temperature of the layer (between 80°C and 300°C) allowing the electrical production.

The exploitation of heat coming from geothermics high energy is old. The baths in hot sources were already practiced in the Antiquité in many areas of the world. It is at the beginning of the 20th century that a geothermal power station of electrical production was for the first time carried out at Larderello (Italy). Geothermics high temperature currently knows an important revival, in particular because protection against the corrosion and the techniques of drilling strongly improved.

New technological applications are possible to recover the heat of the Earth. The Cogénération already makes it possible to combine the production of heat and electricity on the same unit, and thus increases the output of the installation. An European project of major geothermics to Soultz-under-Forests aims at producing electricity thanks to the natural energy of the fissured hot rocks (in English Hot Dry Rock ).

Installations in the world

Geothermics is the principal energy source of the Iceland. There exist three important powerplants which provide approximately 17% (2004) of the electrical production of the country. Moreover, geothermic heat provides the heating and warm water from approximately 87% of the inhabitants of the island.

One of the geothermic sources most important is located at the the United States. The Geysers, to approximately 145 km in the north of San Francisco, started the production in 1960 and has a power of 2000 megawatts electric. In the south of the California, close to Niland and Calipatria, about fifteen powerplants produce approximately 570 megawatts electric.

Geothermics is particularly profitable in the zone of the Rift in Africa. Three power stations were recently built with the Kenya, respectively of 45  MW, 65  MW and 48  MW. Planning envisages to increase the production of 576  MW in 2017, covering 25% of the needs for Kenya, and thus reducing the dependence of the country to the oil importations.

In Guadeloupe, the only French reference as regards geothermics high temperature is at Bouillante, not far from the volcano inhabitant of Guadeloupe of the Soufrière. It was carried out in 1984 the first drilling a depth of 300  m on the basis of which the installation of a power station of 5  MW was decided. Very close to this site, three new deeper wells of production (1 km on average) were brought into service in 2001 and one power station, built in 2003 (Ebullient 2), made it possible to put in production, at end 2004,11  Additional MW. This new contribution of energy covers approximately 10% of the annual requirements in electricity for the island.

In metropolitan France, one currently drills with great depth (about 5  000  m with Soultz-under-Forests) in “dry hot rocks” where water is injected; one creates a heat exchanger thus.

In Germany, a power station using the geothermics of 3,4 megawatts, should function in Unterhaching close to Munich as from 2007, and produce in Cogénération heat and electricity. Drilling reached 3350 meters of depth, and 150 liters of water spout out a second at a temperature of 122°C.

Electricity is produced starting from geothermics in more than 20 countries in the world: the China, the Iceland, the the United States, the Italy, the France, the Germany, the New Zealand, the Mexico, the Nicaragua, the Costa Rica, the Russia, Filipino, the Indonesia, the Japan and the Canada.

Geothermics low energy

One speaks about “geothermics low energy” when drilling makes it possible to reach a temperature of water between 30°C and 100°C in layers located between 1500 and 2500  m of depth. This technology is used mainly for the District heating collective by heating network, and certain industrial applications.

In France, a network of district heating located in Paris region uses geothermics low energy. The installations of heat pumps on tablecloth continue to develop in Paris region because they correspond to techniques of heating and cooling adapted particularly well to the sectors tertiary and residential.

A geothermal power station functioning on the principle of the doublet was brought into service in 1994 at Riehen in Switzerland, for the heating of the local buildings. Since December 2000, part of produced heat is exported in Germany and thus supplies a district of the city close to Lörrach. The enlarging caused minis Earthquake in December 2006.

The production of heat by means of a Heat pump on tablecloth, rests on the taking away and the transfer of the energy contained in subterranean water towards the buildings to heat. In addition, a heat pump can ensure simultaneously and/or successively requirements in heating and/or air-conditioning/cooling. This category is all the same, from a technician point of view and financial investment, more family of geothermics of very low energy.

Geothermics very low energy

Geothermics very low energy is a geothermics on the level of the temperatures ranging between 10 °C and 30 °C. In this case, heat not comes the depths of the earth's crust, but of the sun and the streaming of rainwater, the ground of the ground playing a part of hot source because of its inertia and its bad conductivity thermics.

This technology is applied to:

  • air-conditioning passivates with for example the system of the Puits of Provence,
  • the heating and the Climatisation with the geothermic heat pump

These systems make it possible to make, compared to the single use of a primary energy, saving energy on the Chauffage and the production of Warm water. Nevertheless they require an external energy source, generally the electricity, which must remain available.

The geothermics of heat pump consists in drawing heat present in the ground through vertical or horizontal sensors, according to the configuration of the ground.

A thermodynamic system (or heat pump) has an operation comparable with that of a domestic refrigerator: it ensures the heating of a room starting from an external source of heat (of which the temperature is lower than that of the room to heat). It draws the 2/3 of the energy of heating in the heat produced by the entrails of the ground (géo = ground, megacal = heat) and the other third is an electric contribution for the compressor.

How does that function? All is played thanks to the change of state, when a fluid passes from the liquid state in a gas state, and conversely. It is simple: a long pipe of sheathed polyethylene polyethylene or copper is buried in the garden. One makes circulate inside a liquid, which is heated a little in contact with the ground. Like this liquid with the property to start to boil at very low temperature, it passes then from the liquid state to the state vapor. This vapor is compressed by a compressor located in the house. The simple fact of compressing it causes to increase its temperature. It is then led to a condenser which remakes it to pass in the liquid state. During this change of state it is released again from the heat, which is transmitted to the water of heating (radiator, heating floor,…). The liquid continues its cycle, and after being itself slackened, sets out again in closed circuit to seek heat in the ground of the garden.

There exist three kinds of the horizontal systems:

  • the system glycolée water/water
  • the system water/ground (=fluide refrigerating)
  • the system ground/ground

The operation of the thermodynamic machines (here the CAP) is founded on the capacity of the refrigerants to vaporize and condense with room temperature. The refrigerant more used for geothermics is the R-134a fluid. Its chemical composition: hydrofluorcarbone or H_F_C of formula: CH2F-CF3

Its essential properties are:

  • its boiling point to atmospheric pressure is of -26 °C; what thus enables him to evaporate more quickly at low temperature, therefore better passage of heat.
  • its latent heat of important evaporation. With -26 °C (its boiling point) with atmospheric pressure its latent heat is of 216 kJ/kg. Release much energy.
  • its low mass volume of the vapor in cubic meter which enables him to use a small compressor.

From the point of view of the budget of investment, heat pumps, installed with more than 90% in nine (sources: Ademe, Sofath) do not enter in competition with the electric heating by Joule effect (electrical resistance), but rather with all the other true ecological means (solar credit, wood energy, but before all architectures climatic and Bioclimatique).

The Heat pump would probably gain to transfer towards an operation starting from thermal engine, being able to use fuels resulting from the biomass (biogas for example), and this obviously for reasons of economy of scale, in great units, thus making it possible to locate the production close to the place of use and to increase the potentials of production of local renewable energies while avoiding amplifying the current problems upstream electric meter.

Seisms

The geothermic installations need sometimes to be scaled. For that, one injects water under pressure, which in certain cases can start seisms of magnitude which can go up to 4,6. A means of limiting their intensity is to use Hydrochloric acid in the place of water.

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