The warmth of the earth

Kirill Degtyarev, Research Fellow, Lomonosov Moscow State University M. V. Lomonosov.

In our country, rich in hydrocarbons, geothermal energy is a kind of exotic resource that, in the current state of affairs, is unlikely to compete with oil and gas. Nevertheless, this alternative form of energy can be used almost everywhere and quite efficiently.

Photo by Igor Konstantinov.

Change in soil temperature with depth.

Temperature increase of thermal waters and dry rocks containing them with depth.

Change in temperature with depth in different regions.

The eruption of the Icelandic volcano Eyjafjallajökull is an illustration of violent volcanic processes occurring in active tectonic and volcanic zones with a powerful heat flow from the earth's interior.

Installed capacities of geothermal power plants by countries of the world, MW.

Distribution of geothermal resources on the territory of Russia. The reserves of geothermal energy, according to experts, are several times higher than the energy reserves of organic fossil fuels. According to the Geothermal Energy Society Association.

Geothermal energy is the heat of the earth's interior. It is produced in the depths and comes to the surface of the Earth in different forms and with different intensity.

It is these areas that are most favorable for the development of geothermal energy. On the territory of Russia, these are, first of all, Kamchatka, the Kuril Islands and the Caucasus.

At the same time, the development of geothermal energy is possible almost everywhere, since the increase in temperature with depth is a ubiquitous phenomenon, and the task is to “extract” heat from the bowels, just as mineral raw materials are extracted from there.

On average, the temperature increases with depth by 2.5-3 o C for every 100 m. The ratio of the temperature difference between two points lying at different depths to the difference in depth between them is called the geothermal gradient.

The reciprocal is the geothermal step, or the depth interval at which the temperature rises by 1 o C.

The higher the gradient and, accordingly, the lower the step, the closer the heat of the Earth's depths approaches the surface and the more promising this area is for the development of geothermal energy.

In different areas, depending on the geological structure and other regional and local conditions, the rate of temperature increase with depth can vary dramatically. On the scale of the Earth, fluctuations in the values ​​of geothermal gradients and steps reach 25 times. For example, in the state of Oregon (USA) the gradient is 150 o C per 1 km, and in South Africa - 6 o C per 1 km.

The question is, what is the temperature at great depths - 5, 10 km or more? If the trend continues, the temperature at a depth of 10 km should average about 250-300 o C. This is more or less confirmed by direct observations in ultra-deep wells, although the picture is much more complicated than a linear increase in temperature.

For example, in the Kola super-deep well drilled in the Baltic crystalline shield, the temperature to a depth of 3 km changes at a rate of 10 ° C / 1 km, and then the geothermal gradient becomes 2-2.5 times greater. At a depth of 7 km, a temperature of 120 o C was already recorded, at 10 km - 180 o C, and at 12 km - 220 o C.

Another example is a well laid in the Northern Caspian, where at a depth of 500 m a temperature of 42 o C was recorded, at 1.5 km - 70 o C, at 2 km - 80 o C, at 3 km - 108 o C.

It is assumed that the geothermal gradient decreases starting from a depth of 20-30 km: at a depth of 100 km, the estimated temperatures are about 1300-1500 o C, at a depth of 400 km - 1600 o C, in the Earth's core (depths of more than 6000 km) - 4000-5000 o WITH.

At depths up to 10-12 km, the temperature is measured through drilled wells; where they do not exist, it is determined by indirect signs in the same way as at greater depths. Such indirect signs may be the nature of the passage of seismic waves or the temperature of the erupting lava.

However, for the purposes of geothermal energy, data on temperatures at depths of more than 10 km are not yet of practical interest.

There is a lot of heat at depths of several kilometers, but how to raise it? Sometimes nature itself solves this problem for us with the help of a natural coolant - heated thermal waters that come to the surface or lie at a depth accessible to us. In some cases, the water in the depths is heated to the state of steam.

There is no strict definition of the concept of "thermal waters". As a rule, they mean hot groundwater in a liquid state or in the form of steam, including those that come to the Earth's surface with a temperature above 20 ° C, that is, as a rule, higher than the air temperature.

The heat of groundwater, steam, steam-water mixtures is hydrothermal energy. Accordingly, energy based on its use is called hydrothermal.

The situation is more complicated with the production of heat directly from dry rocks - petrothermal energy, especially since sufficiently high temperatures, as a rule, begin from depths of several kilometers.

On the territory of Russia, the potential of petrothermal energy is a hundred times higher than that of hydrothermal energy - 3,500 and 35 trillion tons of standard fuel, respectively. This is quite natural - the warmth of the Earth's depths is everywhere, and thermal waters are found locally. However, due to obvious technical difficulties, most of the thermal waters are currently used to generate heat and electricity.

Waters with temperatures from 20-30 to 100 o C are suitable for heating, temperatures from 150 o C and above - and for generating electricity at geothermal power plants.

In general, geothermal resources on the territory of Russia, in terms of tons of reference fuel or any other unit of energy measurement, are approximately 10 times higher than fossil fuel reserves.

Theoretically, only geothermal energy could fully meet the energy needs of the country. In practice, at the moment, in most of its territory, this is not feasible for technical and economic reasons.

In the world, the use of geothermal energy is most often associated with Iceland - a country located at the northern end of the Mid-Atlantic Ridge, in an extremely active tectonic and volcanic zone. Probably everyone remembers the powerful eruption of the Eyjafjallajökull volcano in 2010.

It is thanks to this geological specificity that Iceland has huge reserves of geothermal energy, including hot springs that come to the surface of the Earth and even gushing in the form of geysers.

In Iceland, more than 60% of all energy consumed is currently taken from the Earth. Including due to geothermal sources, 90% of heating and 30% of electricity generation are provided. We add that the rest of the electricity in the country is produced by hydroelectric power plants, that is, also using a renewable energy source, thanks to which Iceland looks like a kind of global environmental standard.

The "taming" of geothermal energy in the 20th century helped Iceland significantly economically. Until the middle of the last century, it was a very poor country, now it ranks first in the world in terms of installed capacity and production of geothermal energy per capita, and is in the top ten in terms of absolute installed capacity of geothermal power plants. However, its population is only 300 thousand people, which simplifies the task of switching to environmentally friendly energy sources: the need for it is generally small.

In addition to Iceland, a high share of geothermal energy in the total balance of electricity production is provided in New Zealand and the island states of Southeast Asia (Philippines and Indonesia), the countries of Central America and East Africa, whose territory is also characterized by high seismic and volcanic activity. For these countries, at their current level of development and needs, geothermal energy makes a significant contribution to socio-economic development.

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