PRIMARY ENERGIES

Water and fire

Although wind energy is subject to the vagaries of the wind and solar energy to those of the Sun, the Earth has always offered two blue-chip energy sources: the water that flows continuously on its surface and the ‘fire’ in its belly. Although the exploitation of water for hydroelectric power is starting to reach its limit, fire still offers huge potential.

Le barrage d’Itaipú – le plus puissant au monde – fournit près du quart de l’énergie électrique au Brésil. Ces solutions gigantesques ne sont pas envisageables en Europe où l’on tendrait à créer des centrales dites de «très petite hydraulique». © CNRS Photothèque Hervé Thery
The Itaipu Dam – the world’s most powerful dam – supplies nearly one quarter of Brazil’s electric power. Such massive solutions cannot be envisaged in Europe, where the trend is to create mini-hydropower plants. © CNRS Photothèque Hervé Thery
Adduction d’eau chaude à travers des terres volcaniques, en Islande. © Shutterstock
Hot water being piped through a volcanic area in Iceland. © Shutterstock

Humans have been exploiting the power of water for thousands of years, in the form of tides or water courses. The use of dams to convert water into electricity, which began in the 19th century, offers a renewable and controlled form of energy. What is more, with hydroelectricity it is possible to gear electricity production to demand. For instance, the Grand’Maison Dam in the French Alps can deliver a whopping 1 800 MW of power in only two minutes. According to the International Energy Agency (IEA), dams supply approximately 16% of the world’s electricity production, placing hydroelectric power in top position among the renewables.

However hydroelectric plants require a specific type of terrain and large expanses of land, which is problematic in environmental, economic and social terms, not to say alarming in the case of China’s planned Three Gorges Dam. In addition to the indirect impact on the ecosystem of slowing down water courses, the performance of dams in terms of the greenhouse- gas effect is undermined by the large amounts of methane generated by plant decomposition in the flooded areas.

It is highly unlikely that Europe will embark on large-scale hydro projects in the future, as research will focus instead on minihydropower plants. These are turbines that generate less than 1 MW, situated along rivers and their tributaries, which require only a twometre difference in height. Well suited to decentralised energy production, mini-hydro - power plants will have only a limited impact on Europe’s electricity production because they have a maximum potential of only around 1.5 GW.

Prospects for geothermal energy

The reverse is true of geothermal energy, as there is plenty of potential for exploiting it. Not only on the Earth’s surface, where heat pumps can be used to recover a portion of the solar radiation absorbed by the ground, but more important, at depth. In fact, under our very feet there is a real boiler fuelled by the natural disintegration of the radioactive elements in rocks (uranium, thorium, potassium) and, to a lesser extent, by the primitive heat accumulated during the Earth’s accretion phase.

More often than not, surface geothermal energy is used as additional heating for domestic circuits or heating and hot-water units. The heat is simply recovered by a fluid contained in a heat exchange system buried a few metres deep.

To channel enough power to produce electricity, we need to turn to deep geothermal energy. The further we go beneath the Earth’s crust, the higher the temperature. In fact it increases by an average of 3 °C every hectometre, with wide local variations. The techniques used depend on both the depth of the bore hole, which can descend as much as five kilometres below the surface, and the type of geology. Sometimes hot springs can be used, otherwise cold water is injected into cracks in rocks. Once heated, pressure causes the water to rise and it is used to drive turbines (1).

Matthieu Lethé

  1. Geothermal energy will be the subject of an upcoming research*eu special issue on earth science.

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Three Gorges Dam

Under construction since 1994, this grandiose piece of civil engineering spanning the Yangtze River, with its power station exceeding 22 000 MW in capacity, is expected to supply China with nearly 90 Tera - watt/h of electricity as from 2009. However, the project has exacted a price: at least 1.2 million displaced people and 600 km2 of farming land and forest flooded by the artificial lake (the surface area of the retaining reservoir spanning more than 600 km of river could total 58 000 km2). According to the Ecological Society of America, Chinese biologists estimate that the reservoir area presently harbours 6 400 plant species, 3 500 insect species, 350 fish species and 500 species of terrestrial vertebrate, one-fifth of which are mammals. A large number of the threatened species are endemic, such as the Chinese sturgeon and the Yangtze dolphin.



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