The heat energy of the earth as a source of heating. Used ground heat exchangers and heat supply schemes

Geothermal energy- this is the energy of heat that is released from the internal zones of the Earth over hundreds of millions of years. According to geological and geophysical studies, the temperature in the Earth's core reaches 3,000-6,000 °C, gradually decreasing in the direction from the center of the planet to its surface. The eruption of thousands of volcanoes, the movement of blocks of the earth's crust, and earthquakes indicate the action of the powerful internal energy of the Earth. Scientists believe that the thermal field of our planet is due to radioactive decay in its depths, as well as gravitational separation of core matter.
The main sources of heating the planet's interior are uranium, thorium and radioactive potassium. Processes radioactive decay on continents they occur mainly in the granite layer of the earth's crust at a depth of 20-30 km or more, in the oceans - in the upper mantle. It is assumed that at the base of the earth's crust at a depth of 10-15 km, the probable temperature value on the continents is 600-800 ° C, and in the oceans - 150-200 ° C.
A person can use geothermal energy only where it manifests itself close to the Earth’s surface, i.e. in areas of volcanic and seismic activity. Now geothermal energy is effectively used by such countries as the USA, Italy, Iceland, Mexico, Japan, New Zealand, Russia, the Philippines, Hungary, and El Salvador. Here, the internal heat of the earth rises to the very surface in the form of hot water and steam with temperatures up to 300 ° C and often breaks out as the heat of gushing sources (geysers), for example, the famous geysers of Yellowstone Park in the USA, geysers of Kamchatka and Iceland.
Geothermal energy sources divided into dry hot steam, wet hot steam and hot water. The well, which is an important source of energy for the electric railway in Italy (near Larderello), has been powered by dry hot steam since 1904. Two other famous hot dry steam sites in the world are the Matsukawa Field in Japan and the Geyser Field near San Francisco, which also have a long and effective use of geothermal energy. The most moist hot steam in the world is found in New Zealand (Wairakei), geothermal fields of slightly less power are in Mexico, Japan, El Salvador, Nicaragua, and Russia.
Thus, four main types of geothermal energy resources can be distinguished:
ground surface heat used by heat pumps;
energy resources of steam, hot and warm water at the surface of the earth, which are now used in the production of electrical energy;
heat concentrated deep below the surface of the earth (possibly in the absence of water);
magma energy and heat that accumulates under volcanoes.

Reserves geothermal heat(~8 * 1030J) is 35 billion times the annual global energy consumption. Just 1% of the geothermal energy in the earth's crust (10 km depth) can provide an amount of energy that is 500 times greater than all the world's oil and gas reserves. However, today only a small part of these resources can be used, and this is due, first of all, to economic reasons. The industrial development of geothermal resources (the energy of hot deep waters and steam) began in 1916, when the first geothermal power plant with a capacity of 7.5 MW was commissioned in Italy. Over the past time, considerable experience has been accumulated in the field of practical development of geothermal energy resources. The total installed capacity of existing geothermal power plants (GeoTES) was: 1975 - 1,278 MW, in 1990 - 7,300 MW. The greatest progress in this matter has been achieved by the USA, Philippines, Mexico, Italy, and Japan.
The technical and economic parameters of geothermal power plants vary over a fairly wide range and depend on the geological characteristics of the area (depth of occurrence, parameters of the working fluid, its composition, etc.). For the majority of geothermal power plants put into operation, the cost of electricity is similar to the cost of electricity produced at coal-fired power plants and amounts to 1200 ... 2000 US dollars / MW.
In Iceland, 80% of homes are heated using hot water extracted from geothermal wells near the city of Reykjavik. In the western United States, about 180 homes and farms are heated using geothermal hot water. According to experts, between 1993 and 2000, global electricity generation from geothermal energy more than doubled. Inventories geothermal heat There are so many in the US that they could, theoretically, provide 30 times more energy than the government currently consumes.
In the future, it is possible to use the heat of magma in those areas where it is located close to the Earth's surface, as well as the dry heat of heated crystalline rocks. In the latter case, wells are drilled over several kilometers, cold water is pumped down, and hot water is received back.

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

Geothermal energy is heat earth's bowels. It is produced in the depths and reaches the surface of the Earth in different forms and with different intensities.

The temperature of the upper layers of the soil depends mainly on external (exogenous) factors - solar illumination and air temperature. In summer and during the day, the soil warms up to certain depths, and in winter and at night it cools down following changes in air temperature and with some delay that increases with depth. The influence of daily fluctuations in air temperature ends at depths from a few to several tens of centimeters. Seasonal fluctuations affect deeper layers of soil - up to tens of meters.

At some depth - from tens to hundreds of meters - the soil temperature remains constant, equal to the average annual air temperature at the Earth's surface. You can easily verify this by going down into a fairly deep cave.

When average annual temperature air in a given area is below zero, this manifests itself as permafrost (more precisely, permafrost). IN Eastern Siberia The thickness, that is, the thickness, of year-round frozen soils in some places reaches 200–300 m.

From a certain depth (different for each point on the map), the action of the Sun and the atmosphere weakens so much that endogenous (internal) factors come first and the earth’s interior heats up from the inside, so that the temperature begins to rise with depth.

The heating of the deep layers of the Earth is associated mainly with the decay of radioactive elements located there, although other heat sources are also called, for example, physicochemical, tectonic processes in the deep layers of the earth's crust and mantle. But whatever the reason, the temperature of rocks and associated liquid and gaseous substances increases with depth. Miners face this phenomenon - in deep mines It's always hot. At a depth of 1 km, thirty-degree heat is normal, and deeper the temperature is even higher.

The heat flow of the earth's interior reaching the Earth's surface is small - on average its power is 0.03–0.05 W/m2, or approximately 350 Wh/m2 per year. On the background heat flow from the Sun and the air heated by it is an imperceptible quantity: the Sun gives everyone square meter the earth's surface is about 4000 kWh annually, that is, 10,000 times more (of course, this is on average, with a huge spread between polar and equatorial latitudes and depending on other climatic and weather factors).

The insignificance of heat flow from the interior to the surface in most of the planet is associated with the low thermal conductivity of rocks and the characteristics of geological structure. But there are exceptions - places where the heat flow is high. These are, first of all, zones of tectonic faults, increased seismic activity and volcanism, where the energy of the earth’s interior finds an outlet. Such zones are characterized by thermal anomalies of the lithosphere; here the heat flow reaching the Earth’s surface can be several times and even orders of magnitude more powerful than “usual”. Volcanic eruptions and hot springs bring enormous amounts of heat to the surface in these zones.

These are the 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 an increase in temperature with depth is a universal phenomenon, and the task is to “extract” heat from the depths, just as mineral raw materials are extracted from there.

On average, temperature increases with depth by 2.5–3°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°C.

The higher the gradient and, accordingly, the lower the stage, the closer the heat of the Earth’s depths comes to 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 an Earth scale, fluctuations in the magnitudes of geothermal gradients and steps reach 25 times. For example, in Oregon (USA) the gradient is 150°C per 1 km, and in South Africa- 6°C per 1 km.

The question is, what is the temperature at great depths - 5, 10 km or more? If the trend continues, temperatures at a depth of 10 km should average approximately 250–300°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 Kola ultra-deep well, drilled in the Baltic crystalline shield, the temperature changes at a rate of 10°C/1 km to a depth of 3 km, and then the geothermal gradient becomes 2–2.5 times greater. At a depth of 7 km, a temperature of 120°C was already recorded, at 10 km - 180°C, and at 12 km - 220°C.

Another example is a well drilled in the Northern Caspian region, where at a depth of 500 m a temperature of 42°C was recorded, at 1.5 km - 70°C, at 2 km - 80°C, at 3 km - 108°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°C, at a depth of 400 km - 1600°C, in the Earth's core (depths more than 6000 km) - 4000–5000° C.

At depths of up to 10–12 km, temperature is measured through drilled wells; where they are not present, 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 pouring 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 underground waters in a liquid state or in the form of steam, including those that come to the surface of the Earth with a temperature above 20°C, that is, as a rule, higher than the air temperature.

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

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

On the territory of Russia, the potential of petrothermal energy is one 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 depths of the Earth is available everywhere, and thermal waters are found locally. However, due to obvious technical difficulties, thermal waters are currently mostly used to generate heat and electricity.

Waters with temperatures from 20–30 to 100°C are suitable for heating, temperatures from 150°C and above are suitable for generating electricity in geothermal power plants.

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

Theoretically, only through geothermal energy could it be possible to fully satisfy energy needs countries. 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 ( Eyjafjallajökull) in 2010 year.

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

In Iceland, over 60% of all energy consumed currently comes from the Earth. Geothermal sources provide 90% of heating and 30% of electricity generation. Let us add that the rest of the country’s electricity is produced by hydroelectric power plants, that is, also using a renewable energy source, making Iceland look like a kind of global environmental standard.

The domestication of geothermal energy in the 20th century significantly helped Iceland in economically. Until the middle of the last century it was very poor country, now ranks first in the world in terms of installed capacity and production of geothermal energy per capita and is in the top ten in absolute value of 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 island countries South-East Asia(Philippines and Indonesia), countries of Central America and East Africa, the territory of which 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.

The use of geothermal energy has a very long history. One of the first famous examples- Italy, a place in the province of Tuscany, now called Larderello, where at the beginning of the 19th century local hot thermal waters, flowing naturally or extracted from shallow wells, were used for energy purposes.

Water from underground springs, rich in boron, was used here to obtain boric acid. Initially, this acid was obtained by evaporation in iron boilers, and ordinary wood from nearby forests was taken as fuel, but in 1827 Francesco Larderel created a system that worked on the heat of the waters themselves. At the same time, the energy of natural water vapor began to be used to operate drilling rigs, and at the beginning of the 20th century - for heating local houses and greenhouses. There, in Larderello, in 1904, thermal water vapor became an energy source for generating electricity.

The example of Italy was followed by several other countries at the end of the 19th and beginning of the 20th centuries. For example, in 1892, thermal waters were first used for local heating in the USA (Boise, Idaho), in 1919 in Japan, and in 1928 in Iceland.

In the USA, the first power plant operating on hydrothermal energy appeared in California in the early 1930s, in New Zealand - in 1958, in Mexico - in 1959, in Russia (the world's first binary GeoPP) - in 1965 .

Old principle on a new source

Electricity generation requires a higher hydrosource temperature than for heating - more than 150°C. The operating principle of a geothermal power plant (GeoPP) is similar to the operating principle of a conventional thermal power plant (CHP). In fact, a geothermal power plant is a type of thermal power plant.

At thermal power plants, the primary energy source is usually coal, gas or fuel oil, and the working fluid is water vapor. Fuel, when burned, heats water into steam, which rotates a steam turbine, which generates electricity.

The difference between a GeoPP is that the primary source of energy here is the heat of the earth’s interior and the working fluid in the form of steam is supplied to the turbine blades of the electric generator in a “ready” form directly from the production well.

There are three main operating schemes for GeoPPs: direct, using dry (geothermal) steam; indirect, based on hydrothermal water, and mixed, or binary.

The use of one or another scheme depends on the state of aggregation and temperature of the energy carrier.

The simplest and therefore the first of the mastered schemes is direct, in which steam coming from the well is passed directly through the turbine. The world's first geoelectric power station in Larderello in 1904 also operated on dry steam.

GeoPPs with an indirect operating scheme are the most common in our time. They use hot underground water, which is pumped under high pressure into an evaporator, where part of it is evaporated, and the resulting steam rotates a turbine. In some cases, additional devices and circuits are required to purify geothermal water and steam from aggressive compounds.

The exhaust steam enters the injection well or is used for heating the premises - in this case the principle is the same as when operating a thermal power plant.

At binary GeoPPs, hot thermal water interacts with another liquid that performs the functions of a working fluid with a lower boiling point. Both fluids are passed through a heat exchanger, where thermal water evaporates the working fluid, the vapors of which rotate the turbine.

Operating principle of binary GeoPP. Hot thermal water interacts with another liquid that performs the functions of a working fluid and has a lower boiling point. Both fluids are passed through a heat exchanger, where thermal water evaporates the working fluid, the vapors of which, in turn, rotate the turbine

This system is closed, which solves the problem of emissions into the atmosphere. In addition, working fluids with a relatively low boiling point make it possible to use not very hot thermal waters as a primary source of energy.

All three schemes use a hydrothermal source, but petrothermal energy can also be used to generate electricity.

The circuit diagram in this case is also quite simple. It is necessary to drill two interconnected wells - injection and production. Water is pumped into the injection well. At depth it is heated, then the heated water or steam formed as a result of strong heating is supplied to the surface through the production well. Then it all depends on how petrothermal energy is used - for heating or for generating electricity. A closed cycle is possible with pumping waste steam and water back into the injection well or another disposal method.

Scheme of operation of a petrothermal system. The system is based on the use of a temperature gradient between the surface of the earth and its interior, where the temperature is higher. Water from the surface is pumped into an injection well and heated at depth, then the heated water or the steam generated as a result of heating is supplied to the surface through the production well.

The disadvantage of such a system is obvious: to obtain a sufficiently high temperature of the working fluid, it is necessary to drill wells to great depths. And these are serious costs and the risk of significant heat losses when the fluid moves upward. Therefore, petrothermal systems are still less widespread compared to hydrothermal ones, although the potential of petrothermal energy is orders of magnitude higher.

Currently, the leader in the creation of so-called petrothermal circulation systems (PCS) is Australia. In addition, this area of geothermal energy is actively developing in the USA, Switzerland, Great Britain, and Japan.

Gift from Lord Kelvin

The invention of the heat pump in 1852 by physicist William Thompson (aka Lord Kelvin) provided humanity with a real opportunity to use the low-grade heat of the upper layers of the soil. A heat pump system, or heat multiplier as Thompson called it, is based on the physical process of transferring heat from the environment to a refrigerant. Essentially, it uses the same principle as petrothermal systems. The difference is in the heat source, which may raise a terminological question: to what extent can a heat pump be considered a geothermal system? The fact is that in the upper layers, to depths of tens to hundreds of meters, the rocks and the fluids they contain are heated not by the deep heat of the earth, but by the sun. Thus, it is the sun in in this case- the primary source of heat, although it is taken, as in geothermal systems, from the ground.

The operation of a heat pump is based on the delay in heating and cooling of the soil compared to the atmosphere, resulting in the formation of a temperature gradient between the surface and deeper layers that retain heat even in winter, just as it happens in reservoirs. The main purpose of heat pumps is space heating. In essence, it is a “reverse refrigerator”. Both the heat pump and the refrigerator interact with three components: internal environment(in the first case - a heated room, in the second - a cooled compartment of a refrigerator), the external environment - a source of energy and a refrigerant (coolant), which is also a coolant that ensures the transfer of heat or cold.

A substance with a low boiling point acts as a refrigerant, which allows it to take heat from a source that has even a relatively low temperature.

In the refrigerator, liquid refrigerant flows through a throttle (pressure regulator) into the evaporator, where due to a sharp decrease in pressure, the liquid evaporates. Evaporation is an endothermic process requiring the absorption of heat from outside. As a result, heat is removed from the inner walls of the evaporator, which provides a cooling effect in the refrigerator chamber. Next, the refrigerant is drawn from the evaporator into the compressor, where it is returned to a liquid state. This is a reverse process leading to the release of removed heat into the external environment. As a rule, it is thrown indoors, and the back wall of the refrigerator is relatively warm.

A heat pump works in almost the same way, with the difference that heat is taken from the external environment and through the evaporator enters the internal environment - the room heating system.

In a real heat pump, water is heated by passing through an external circuit placed in the ground or reservoir, and then enters the evaporator.

In the evaporator, heat is transferred to an internal circuit filled with a low-boiling point refrigerant, which, passing through the evaporator, changes from a liquid to a gaseous state, taking away heat.

Next, the gaseous refrigerant enters the compressor, where it is compressed to high pressure and temperature, and enters the condenser, where heat exchange occurs between the hot gas and the coolant from the heating system.

The compressor requires electricity to operate, but the transformation ratio (the ratio of energy consumed to energy produced) in modern systems is high enough to ensure their efficiency.

Currently, heat pumps are quite widely used for space heating, mainly in economically developed countries.

Eco-correct energy

Geothermal energy is considered environmentally friendly, which is generally true. First of all, it uses renewable and practically inexhaustible resource. Geothermal energy does not require large areas, unlike large hydroelectric power stations or wind farms, and does not pollute the atmosphere, unlike hydrocarbon energy. On average, a GeoPP occupies 400 m 2 in terms of 1 GW of generated electricity. The same figure for a coal-fired thermal power plant, for example, is 3600 m2. The environmental advantages of GeoPP also include low water consumption - 20 liters fresh water per 1 kW, while thermal power plants and nuclear power plants require about 1000 liters. Note that these are the environmental indicators of the “average” GeoPP.

But negative side effects still exist. The most common among them is noise, thermal pollution atmosphere and chemical - water and soil, as well as the formation of solid waste.

Main source chemical pollution environment - thermal water itself (with high temperature and mineralization), often containing large quantities of toxic compounds, and therefore there is a problem of disposal of waste water and hazardous substances.

The negative effects of geothermal energy can be traced at several stages, starting with the drilling of wells. The same dangers arise here as when drilling any well: destruction of soil and vegetation cover, contamination of soil and groundwater.

Pollution problems at the GeoPP operation stage environment are saved. Thermal fluids - water and steam - usually contain carbon dioxide (CO 2), sulfur sulfide (H 2 S), ammonia (NH 3), methane (CH 4), table salt (NaCl), boron (B), arsenic (As ), mercury (Hg). When released into the external environment, they become sources of pollution. In addition, an aggressive chemical environment can cause corrosive destruction of geothermal power plant structures.

At the same time, emissions of pollutants from GeoPPs are on average lower than from thermal power plants. For example, carbon dioxide emissions for every kilowatt-hour of electricity generated are up to 380 g at GeoPPs, 1042 g at coal-fired thermal power plants, 906 g at oil-fired power plants and 453 g at gas-fired thermal power plants.

The question arises: what to do with waste water? If the mineralization is low, it can be discharged into surface waters after cooling. Another way is to pump it back into the aquifer through an injection well, which is preferably and predominantly used at present.

Extraction of thermal water from aquifers (as well as pumping out ordinary water) can cause subsidence and soil movements, other deformations of geological layers, and micro-earthquakes. The probability of such phenomena is, as a rule, low, although isolated cases have been recorded (for example, at the GeoPP in Staufen im Breisgau in Germany).

It should be emphasized that most of GeoPPs are located in relatively sparsely populated areas and in third world countries, where environmental requirements are less stringent than in developed countries. In addition, at the moment the number of GeoPPs and their capacities are relatively small. With larger-scale development of geothermal energy, environmental risks may increase and multiply.

How much is the Earth's energy?

Investment costs for the construction of geothermal systems vary in a very wide range - from 200 to 5000 dollars per 1 kW of installed capacity, that is, the cheapest options are comparable to the cost of constructing a thermal power plant. They depend, first of all, on the conditions of occurrence of thermal waters, their composition, and the design of the system. Drilling to great depths, creating a closed system with two wells, and the need to purify water can increase the cost many times over.

For example, investments in the creation of a petrothermal circulation system (PCS) are estimated at 1.6–4 thousand dollars per 1 kW of installed capacity, which exceeds the costs of constructing a nuclear power plant and is comparable to the costs of constructing wind and solar power plants.

The obvious economic advantage of GeoTES is free energy. For comparison, in the cost structure of an operating thermal power plant or nuclear power plant, fuel accounts for 50–80% or even more, depending on current energy prices. Hence another advantage of the geothermal system: operating costs are more stable and predictable, since they do not depend on external energy price conditions. In general, the operating costs of geothermal power plants are estimated at 2–10 cents (60 kopecks–3 rubles) per 1 kWh of power produced.

The second largest expense item after energy (and a very significant one) is, as a rule, the wages of plant personnel, which can vary dramatically across countries and regions.

On average, the cost of 1 kWh of geothermal energy is comparable to that for thermal power plants (in Russian conditions- about 1 rub./1 kWh) and ten times higher than the cost of generating electricity at hydroelectric power stations (5–10 kopecks/1 kWh).

Part of the reason for the high cost is that, unlike thermal and hydraulic power plants, geothermal power plants have a relatively small capacity. In addition, it is necessary to compare systems located in the same region and under similar conditions. For example, in Kamchatka, according to experts, 1 kWh of geothermal electricity costs 2–3 times less than electricity produced at local thermal power plants.

Indicators economic efficiency the operation of a geothermal system depends, for example, on whether waste water needs to be disposed of and in what ways this is done, and whether combined use of the resource is possible. Thus, chemical elements and compounds extracted from thermal water can provide additional income. Let us recall the example of Larderello: chemical production was primary there, and the use of geothermal energy was initially of an auxiliary nature.

Geothermal energy forwards

Geothermal energy is developing somewhat differently than wind and solar. Currently, it depends to a much greater extent on the nature of the resource itself, which varies sharply by region, and the highest concentrations are associated with narrow zones of geothermal anomalies, usually associated with areas of tectonic faults and volcanism.

In addition, geothermal energy is less technologically intensive compared to wind and, especially, solar energy: geothermal station systems are quite simple.

IN general structure The geothermal component accounts for less than 1% of global electricity production, but in some regions and countries its share reaches 25–30%. Due to the connection to geological conditions, a significant part of geothermal energy capacity is concentrated in third world countries, where there are three clusters of the greatest development of the industry - the islands of Southeast Asia, Central America and East Africa. The first two regions are included in the Pacific “belt of fire of the Earth”, the third is tied to the East African Rift. It is most likely that geothermal energy will continue to develop in these belts. A more distant prospect is the development of petrothermal energy, using the heat of the layers of the earth lying at a depth of several kilometers. This is an almost ubiquitous resource, but its extraction requires high costs, so petrothermal energy is developing primarily in the most economically and technologically powerful countries.

In general, given the widespread distribution of geothermal resources and an acceptable level of environmental safety, there is reason to believe that geothermal energy has good development prospects. Especially with the growing threat of a shortage of traditional energy resources and rising prices for them.

From Kamchatka to the Caucasus

In Russia, the development of geothermal energy has a fairly long history, and in a number of positions we are among the world leaders, although the share of geothermal energy in the overall energy balance of the huge country is still negligible.

Two regions became pioneers and centers for the development of geothermal energy in Russia - Kamchatka and North Caucasus, and if in the first case we are talking primarily about electric power, then in the second - about the use of thermal energy of thermal water.

In the North Caucasus - in Krasnodar region, Chechnya, Dagestan - the heat of thermal waters was used for energy purposes even before the Great Patriotic War. In the 1980–1990s, the development of geothermal energy in the region, for obvious reasons, stalled and has not yet emerged from the state of stagnation. Nevertheless, geothermal water supply in the North Caucasus provides heat to about 500 thousand people, and, for example, the city of Labinsk in the Krasnodar Territory with a population of 60 thousand people is completely heated by geothermal waters.

In Kamchatka, the history of geothermal energy is connected, first of all, with the construction of GeoPPs. The first of them, the still operating Pauzhetskaya and Paratunka stations, were built back in 1965–1967, while the Paratunka GeoPP with a capacity of 600 kW became the first station in the world with a binary cycle. This was the development of Soviet scientists S.S. Kutateladze and A.M. Rosenfeld from the Institute of Thermophysics SB RAS, who in 1965 received an author's certificate for the extraction of electricity from water with a temperature of 70°C. This technology subsequently became the prototype for more than 400 binary GeoPPs in the world.

The capacity of the Pauzhetskaya GeoPP, commissioned in 1966, was initially 5 MW and was subsequently increased to 12 MW. Currently, a binary unit is being built at the station, which will increase its capacity by another 2.5 MW.

The development of geothermal energy in the USSR and Russia was hampered by the availability of traditional energy sources - oil, gas, coal, but never stopped. The largest geothermal energy facilities at the moment are the Verkhne-Mutnovskaya GeoPP with a total capacity of power units of 12 MW, commissioned in 1999, and the Mutnovskaya GeoPP with a capacity of 50 MW (2002).

Mutnovskaya and Verkhne-Mutnovskaya GeoPPs are unique objects not only for Russia, but also on a global scale. The stations are located at the foot of the Mutnovsky volcano, at an altitude of 800 meters above sea level, and operate in extreme climatic conditions, where there is winter for 9–10 months of the year. The equipment of the Mutnovsky GeoPPs, currently one of the most modern in the world, was entirely created at domestic power engineering enterprises.

Currently, the share of Mutnovsky stations in the overall energy consumption structure of the Central Kamchatka energy hub is 40%. There are plans to increase capacity in the coming years.

Special mention should be made about Russian petrothermal developments. We don’t have large drilling centers yet, but we have advanced technologies for drilling to great depths (about 10 km), which also have no analogues in the world. Their further development will radically reduce the costs of creating petrothermal systems. The developers of these technologies and projects are N. A. Gnatus, M. D. Khutorskoy (Geological Institute of the Russian Academy of Sciences), A. S. Nekrasov (Institute of National Economic Forecasting of the Russian Academy of Sciences) and specialists from the Kaluga Turbine Plant. Currently, the petrothermal circulation system project in Russia is at the experimental stage.

Geothermal energy has prospects in Russia, although they are relatively distant: at the moment the potential is quite large and the position of traditional energy is strong. At the same time, in a number of remote areas of the country, the use of geothermal energy is economically profitable and is already in demand. These are territories with high geoenergy potential (Chukotka, Kamchatka, the Kuril Islands - the Russian part of the Pacific “fire belt of the Earth”, the mountains of Southern Siberia and the Caucasus) and at the same time remote and cut off from centralized energy supplies.

Probably, in the coming decades, geothermal energy in our country will develop precisely in such regions.

Kirill Degtyarev,
Researcher, Moscow State University M. V. Lomonosova
“Science and Life” No. 9, No. 10 2013

As society developed and became established, humanity began to look for more and more modern and at the same time economical ways to obtain energy. For this purpose, various stations are being built today, but at the same time, the energy contained in the bowels of the earth is widely used. What is it like? Let's try to figure it out.

Geothermal energy

Already from the name it is clear that it represents the heat of the earth’s interior. Under the earth's crust there is a layer of magma, which is a fiery liquid silicate melt. According to research data, the energy potential of this heat is much higher than the energy of world reserves natural gas, as well as oil. Magma - lava - comes to the surface. Moreover, the greatest activity is observed in those layers of the earth on which the boundaries of tectonic plates are located, as well as where the earth’s crust is characterized by thinness. Geothermal energy from the earth is obtained as follows: lava and water resources The planets touch, causing the water to heat up sharply. This leads to the eruption of the geyser, the formation of so-called hot lakes and underwater currents. That is, precisely those natural phenomena whose properties are actively used as energy.

Artificial geothermal springs

The energy contained in the bowels of the earth must be used wisely. For example, there is an idea to create underground boilers. To do this, you need to drill two wells of sufficient depth, which will be connected at the bottom. That is, it turns out that geothermal energy can be obtained in almost any corner of the land industrially: through one well, cold water will be pumped into the formation, and through the second, hot water or steam will be extracted. Artificial heat sources will be profitable and rational if the resulting heat produces more energy. The steam can be sent to turbine generators, which will generate electricity.

Of course, the heat removed is only a fraction of what is available general reserves. But it should be remembered that the deep heat will be constantly replenished due to the processes of compression of rocks and stratification of the subsoil. As experts say, the earth's crust accumulates heat, the total amount of which is 5000 times greater than the calorific value of all the fossil subsoil of the earth as a whole. It turns out that the operating time of such artificially created geothermal stations can be unlimited.

Features of sources

The sources that make it possible to obtain geothermal energy are almost impossible to fully utilize. They exist in more than 60 countries around the world, with the largest number of terrestrial volcanoes on the territory of the Pacific volcanic ring of fire. But in practice it turns out that geothermal sources in different regions worlds are completely different in their properties, namely average temperature, mineralization, gas composition, acidity and so on.

Geysers are sources of energy on Earth, the peculiarity of which is that they spew boiling water at certain intervals. After the eruption has occurred, the pool becomes free of water; at its bottom you can see a channel that goes deep into the ground. Geysers as energy sources are used in regions such as Kamchatka, Iceland, New Zealand and North America, and single geysers are found in some other areas.

Where does the energy come from?

Uncooled magma is located very close to the earth's surface. Gases and vapors are released from it, which rise and pass through the cracks. Mixing with groundwater, they cause them to heat up and themselves turn into hot water in which many substances are dissolved. Such water is released to the surface of the earth in the form of various geothermal sources: hot springs, mineral springs, geysers, and so on. According to scientists, the hot bowels of the earth are caves or chambers connected by passages, cracks and channels. They are just being filled with underground waters, and very close to them there are pockets of magma. This is how the earth's thermal energy is naturally generated.

Earth's electric field

There is another alternative source of energy in nature, which is renewable, environmentally friendly, and easy to use. True, this source is still only being studied and not used in practice. So, potential energy The earth lies in its electric field. Energy can be obtained in this way by studying the basic laws of electrostatics and the characteristics of the Earth's electric field. In essence, our planet, from an electrical point of view, is a spherical capacitor charged up to 300,000 volts. Its inner sphere has negative charge, and the external one - the ionosphere - is positive. is an insulator. Through it there is a constant flow of ionic and convective currents, which reach a force of many thousands of amperes. However, the potential difference between the plates does not decrease.

This suggests that in nature there is a generator, the role of which is to constantly replenish the leakage of charges from the capacitor plates. The role of such a generator is the Earth’s magnetic field, rotating together with our planet in the flow solar wind. The energy of the Earth's magnetic field can be obtained precisely by connecting an energy consumer to this generator. To do this, you need to install reliable grounding.

Renewable sources

As our planet's population grows steadily, we need more and more energy to power our population. The energy contained in the bowels of the earth can be very different. For example, there are renewable sources: wind, solar and water energy. They are environmentally friendly, and therefore can be used without fear of harming the environment.

Water energy

This method has been used for many centuries. Built today great amount dams and reservoirs in which water is used to generate electrical energy. The essence of the operation of this mechanism is simple: under the influence of the river flow, the wheels of the turbines rotate, and accordingly, the water energy is converted into electricity.

Today there are a large number of hydroelectric power plants that convert the energy of water flow into electricity. The peculiarity of this method is that they are renewed, and, accordingly, such structures have a low cost. That is why, despite the fact that the construction of hydroelectric power stations takes quite a long time, and the process itself is very expensive, these structures still have a significant advantage over electricity-intensive industries.

Solar energy: modern and promising

Solar energy is obtained using solar panels, but modern technologies allow the use of new methods for this. The largest system in the world is built in the California desert. It fully supplies energy to 2,000 homes. The design works as follows: mirrors reflect Sun rays, which are sent to the central water boiler. It boils and turns into steam, which rotates the turbine. It, in turn, is connected to an electric generator. Wind can also be used as energy that the Earth gives us. The wind inflates the sails and turns the mills. And now, with its help, you can create devices that will generate electrical energy. By rotating the windmill blades, it drives the turbine shaft, which in turn is connected to an electric generator.

Internal energy of the Earth

It appeared as a result of several processes, the main ones being accretion and radioactivity. According to scientists, the formation of the Earth and its mass occurred over several million years, and this happened due to the formation of planetesimals. They stuck together, and accordingly, the mass of the Earth became more and more. After our planet began to have its modern mass, but was still devoid of an atmosphere, meteoroid and asteroid bodies fell unhindered on it. This process is precisely called accretion, and it led to the release of significant gravitational energy. And the larger the bodies that hit the planet, the greater the volume of energy contained in the bowels of the Earth.

This gravitational differentiation led to the fact that substances began to stratify: heavy substances simply sank, while light and volatile ones floated up. Differentiation also affected the additional release of gravitational energy.

Atomic Energy

Using the earth's energy can happen in different ways. For example, through the construction of nuclear power plants, when thermal energy is released due to the decay of the smallest particles of atomic matter. The main fuel is uranium, which is contained in earth's crust. Many believe that this particular method of generating energy is the most promising, but its use is associated with a number of problems. First, uranium emits radiation that kills all living organisms. In addition, if this substance gets into the soil or atmosphere, then a real technological disaster. The sad consequences of the accident Chernobyl nuclear power plant we experience it to this day. The danger lies in the fact that radioactive waste can threaten all living things for a very, very long time, for millennia.

New time - new ideas

Of course, people do not stop there, and every year more and more attempts are made to find new ways to obtain energy. If the earth's heat energy is obtained quite simply, then some methods are not so simple. For example, it is quite possible to use biological gas, which is obtained by rotting waste, as an energy source. It can be used for heating houses and heating water.

Increasingly, they are being built when dams and turbines are installed across the mouths of reservoirs, which are driven by the ebb and flow of tides, respectively, generating electricity.

By burning garbage we get energy

Another method, which is already used in Japan, is the creation of waste incineration plants. Today they are built in England, Italy, Denmark, Germany, France, the Netherlands and the USA, but only in Japan did these enterprises begin to be used not only for their intended purpose, but also to generate electricity. Local factories burn 2/3 of all waste, and the factories are equipped with steam turbines. Accordingly, they supply heat and electricity to nearby areas. Moreover, in terms of costs, building such an enterprise is much more profitable than building a thermal power plant.

The prospect of using the Earth's heat where volcanoes are concentrated looks more tempting. In this case, there will be no need to drill the Earth too deeply, since already at a depth of 300-500 meters the temperature will be at least twice as high as the boiling point of water.

There is also such a way to generate electricity as Hydrogen - the simplest and easiest chemical element- can be considered an ideal fuel, because it exists where there is water. If you burn hydrogen, you can get water, which decomposes into oxygen and hydrogen. The hydrogen flame itself is harmless, that is, it will not cause harm to the environment. The peculiarity of this element is that it has a high calorific value.

What's next?

Of course the energy magnetic field The earth or the one obtained from nuclear power plants cannot fully satisfy all the needs of humanity, which are growing every year. However, experts say that there is no reason to worry, since the planet’s fuel resources are still sufficient. Moreover, more and more new sources, environmentally friendly and renewable, are being used.

The problem of environmental pollution remains, and it is growing catastrophically quickly. Quantity harmful emissions goes off scale, accordingly, the air we breathe is harmful, the water has dangerous impurities, and the soil is gradually depleted. That is why it is so important to promptly study such a phenomenon as energy in the bowels of the Earth in order to look for ways to reduce the need for fossil fuels and more actively use non-traditional energy sources.

This energy belongs to alternative sources. Nowadays, they increasingly mention the possibilities of obtaining the resources that the planet gives us. We can say that we live in an era of fashion for renewable energy. Many technical solutions, plans, and theories are being created in this area.

It is located deep in the depths of the earth and has the properties of renewal, in other words, it is endless. Classic resources, according to scientists, are beginning to run out, oil, coal, and gas will dry up.

Nesjavellir Geothermal Power Plant, Iceland

Therefore, we can gradually prepare to adopt new alternative methods of energy production. Under the earth's crust there is a powerful core. Its temperature ranges from 3000 to 6000 degrees. The movement of lithospheric plates demonstrates its enormous power. It manifests itself in the form of a volcanic outburst of magma. Radioactive decay occurs in the depths, sometimes prompting such natural disasters.

Typically, magma heats the surface without going beyond it. This creates geysers or warm pools of water. So you can use physical processes V for the right purposes for humanity.

Types of geothermal energy sources

It is usually divided into two types: hydrothermal and petrothermal energy. The first is formed due to warm springs, and the second type is the difference in temperatures on the surface and deep in the earth. Explaining in your own words, a hydrothermal source consists of steam and hot water, while a petrothermal source is hidden deep under the ground.

Map of geothermal energy development potential in the world

For petrothermal energy, it is necessary to drill two wells, fill one with water, after which a steaming process will occur, which will come to the surface. There are three classes of geothermal areas:

Geothermal – located near continental plates. Temperature gradient more than 80C/km. As an example, the Italian commune of Larderello. There is a power plant there
Semi-thermal – temperature 40 – 80 C/km. These are natural aquifers consisting of fragmented rocks. In some places in France, buildings are heated this way.
Normal – gradient less than 40 C/km. Representation of such areas is most common

They are an excellent source for consumption. They are in rock, at a certain depth. Let's look at the classification in more detail:

Epithermal - temperature from 50 to 90 C
Mesothermal – 100 – 120 s
Hypothermal – more than 200 s

These types consist of different chemical composition. Depending on it, water can be used for various purposes. For example, in the production of electricity, heat supply (heat routes), raw material base.

Video: Geothermal Energy

Heating process

The water temperature is 50 -60 degrees, which is optimal for heating and hot supply of residential areas. The need for heating systems depends on the geographical location and climatic conditions. And people constantly need hot water supply. For this process, GTS (geothermal thermal stations) are constructed.

If for the classic production of thermal energy a boiler house is used that consumes solid or gas fuel, then in this production a geyser source is used. The technical process is very simple, the same communications, thermal routes and equipment. It is enough to drill a well, clean it of gases, then send it with pumps to the boiler room, where the temperature schedule will be maintained, and then it will enter the heating main.

The main difference is that there is no need to use a fuel boiler. This significantly reduces the cost of thermal energy. In winter, subscribers receive heat and hot water supply, and in summer only hot water supply.

Power generation

Hot springs and geysers serve as the main components in the production of electricity. For this purpose, several schemes are used, and special power plants are built. GTS device:

DHW tank
Pump
Gas separator
Steam separator
Generating turbine
Capacitor
Boost pump
Tank-cooler

As we can see, the main element of the circuit is the steam converter. This allows you to obtain purified steam, since it contains acids that destroy turbine equipment. It is possible to use a mixed scheme in the technological cycle, that is, water and steam are involved in the process. The liquid goes through the entire stage of purification from gases, just like steam.

Binary source circuit

The working component is a liquid with a low boiling point. Thermal water also participates in the production of electricity and serves as a secondary raw material.

With its help, steam from a low-boiling source is formed. GTS with such an operating cycle can be fully automated and do not require maintenance personnel. More powerful stations use a dual-circuit circuit. This type of power plant allows reaching a capacity of 10 MW. Double circuit structure:

Steam generator
Turbine
Capacitor
Ejector
Feed pump
Economizer
Evaporator

Practical use

The huge reserves of the sources are many times greater than the annual energy consumption. But only a small fraction is used by humanity. The construction of the stations dates back to 1916. The first geothermal power plant with a capacity of 7.5 MW was created in Italy. The industry is actively developing in countries such as the USA, Iceland, Japan, the Philippines, and Italy.

Active exploration of potential locations and more convenient extraction methods are underway. Production capacity is growing from year to year. If we take into account the economic indicator, then the cost of such an industry is equal to coal-fired thermal power plants. Iceland almost completely covers its housing stock with a GT source. 80% of houses use hot water from wells for heating. Experts from the USA claim that with proper development, geothermal power plants can produce 30 times more annual consumption. If we talk about potential, 39 countries of the world will be able to fully provide themselves with electricity if they use 100 percent of the earth’s subsoil.

The main sources of thermal energy of the Earth are [, ]:

heat of gravitational differentiation;

radiogenic heat;

tidal friction heat;

accretion heat;

frictional heat released due to the differential rotation of the inner core relative to the outer one, outer core relative to the mantle and individual layers within the outer core.

To date, only the first four sources have been quantified. In our country, the main credit for this goes to O.G. Sorokhtin And S.A. Ushakov. The data below is mainly based on the calculations of these scientists.

Heat of Earth's gravitational differentiation

One of the most important patterns in the development of the Earth is differentiation its substance, which continues to this day. Due to this differentiation, the formation occurred core and crust, change in the composition of the primary mantle, while the division is initially homogeneous substance into fractions of different densities is accompanied by the release thermal energy, and the maximum heat release occurs when the earth's matter is divided into dense and heavy core and residual lighter silicate shell - earth's mantle. Currently, the bulk of this heat is released at the boundary mantle - core.

Energy of gravitational differentiation of the Earth over the entire period of its existence, it stood out - 1.46*10 38 erg (1.46*10 31 J). This energy for the most part first goes into kinetic energy convective currents of mantle matter, and then in warm; the other part of it is spent on additional compression of the earth's interior, arising due to the concentration of dense phases in the central part of the Earth. From 1.46*10 38 erg the energy of the Earth's gravitational differentiation went into its additional compression 0.23*10 38 erg (0.23*10 31 J), and was released in the form of heat 1.23*10 38 erg (1.23*10 31 J). The magnitude of this thermal component significantly exceeds the total release of all other types of energy in the Earth. Time distribution total value and the rate of release of the thermal component of gravitational energy is shown in Fig. 3.6 .

Rice. 3.6.

Modern level heat generation during gravitational differentiation of the Earth - 3*10 20 erg/s (3*10 13 W), which depends on the magnitude of the modern heat flow passing through the surface of the planet in ( 4.2-4.3)*10 20 erg/s ((4.2-4.3)*10 13 W), is ~ 70% .

Radiogenic heat

Caused by the radioactive decay of unstable isotopes. The most energy-intensive and long-lived ( with half-life, commensurate with the age of the Earth) are isotopes 238 U, 235 U, 232 Th And 40 K. Their main volume is concentrated in continental crust. Current level of generation radiogenic heat:

by American geophysicist V. Vaquier - 1.14*10 20 erg/s (1.14*10 13 W) ,
by Russian geophysicists O.G. Sorokhtin And S.A. Ushakov - 1.26*10 20 erg/s(1.26*10 13 W) .

This is ~ 27-30% of the current heat flow.

From the total amount of radioactive decay heat in 1.26*10 20 erg/s (1.26*10 13 W) in the earth's crust stands out - 0.91*10 20 erg/s, and in the mantle - 0.35*10 20 erg/s. It follows that the share of mantle radiogenic heat does not exceed 10% of the total modern heat losses of the Earth, and it cannot be the main source of energy for active tectono-magmatic processes, the depth of which can reach 2900 km; and radiogenic heat released in the crust is lost relatively quickly through earth's surface and practically does not participate in heating the deep interior of the planet.

In past geological epochs, the amount of radiogenic heat released in the mantle must have been higher. Its estimates at the time of the formation of the Earth ( 4.6 billion years ago) give - 6.95*10 20 erg/s. Since this time, there has been a steady decrease in the rate of release of radiogenic energy (Fig. 3.7 ).

Over all the time in the Earth, it has been released ~4.27*10 37 erg(4.27*10 30 J) thermal energy of radioactive decay, which is almost three times lower than the total heat of gravitational differentiation.

Tidal Friction Heat

It stands out during the gravitational interaction of the Earth primarily with the Moon, as the nearest large cosmic body. Due to mutual gravitational attraction, tidal deformations arise in their bodies - swelling or humps. The tidal humps of the planets, with their additional attraction, influence their movement. Thus, the attraction of both tidal humps of the Earth creates a pair of forces acting both on the Earth itself and on the Moon. However, the influence of the near swelling, facing the Moon, is somewhat stronger than that of the distant one. Due to angular velocity rotation of the modern Earth ( 7.27*10 -5 s -1) exceeds the orbital speed of the Moon ( 2.66*10 -6 s -1), and the substance of the planets is not ideally elastic, then the tidal humps of the Earth seem to be carried away by its forward rotation and noticeably advance the movement of the Moon. This leads to maximum tides Lands always arrive on its surface a little later than the moment climax Moon, and an additional moment of force acts on the Earth and Moon (Fig. 3.8 ) .

Absolute values tidal interaction forces in the Earth-Moon system are now relatively small and the tidal deformations of the lithosphere caused by them can reach only a few tens of centimeters, but they lead to a gradual deceleration of the Earth’s rotation and, conversely, to acceleration orbital movement The Moon and its distance from the Earth. The kinetic energy of the movement of the earth's tidal humps turns into thermal energy due to the internal friction of the substance in the tidal humps.

Currently, the rate of tidal energy release is G. Macdonald amounts to ~0.25*10 20 erg/s (0.25*10 13 W), while its main part (about 2/3) is presumably dissipates(dissipates) in the hydrosphere. Consequently, the fraction of tidal energy caused by the interaction of the Earth with the Moon and dissipated in the solid Earth (primarily in the asthenosphere) does not exceed 2 % total thermal energy generated in its depths; and the share of solar tides does not exceed 20 % from the effects of lunar tides. Therefore, solid tides now play virtually no role in feeding tectonic processes with energy, but in in some cases can act as "triggers", such as earthquakes.

The amount of tidal energy is directly related to the distance between space objects. And if the distance between the Earth and the Sun does not assume any significant changes on the geological time scale, then in the Earth-Moon system this parameter is a variable value. Regardless of ideas about it, almost all researchers admit that in the early stages of the Earth’s development, the distance to the Moon was significantly less than today, but in the process of planetary development, according to most scientists, it gradually increases, and Yu.N. Avsyuku this distance experiences long-term changes in the form of cycles "coming and going" of the Moon. It follows from this that in past geological epochs the role of tidal heat in the overall heat balance of the Earth was more significant. In general, over the entire period of the Earth’s development, it has evolved ~3.3*10 37 erg (3.3*10 30 J) tidal heat energy (this is subject to the successive removal of the Moon from the Earth). The change in the rate of release of this heat over time is shown in Fig. 3.10 .

More than half of the total tidal energy was released in catarchaea (shit)) - 4.6-4.0 billion years ago, and at that time only due to this energy the Earth could additionally warm up by ~500 0 C. Starting from the late Archean, lunar tides had only a negligible influence on the development energy-intensive endogenous processes .

Accretion heat

This is the heat retained by the Earth since its formation. In progress accretion, which lasted for several tens of millions of years, thanks to the collision planetesimals The Earth experienced significant heating. However, there is no consensus on the magnitude of this heating. Currently, researchers are inclined to believe that during the process of accretion the Earth experienced, if not complete, then significant partial melting, which led to the initial differentiation of the Proto-Earth into a heavy iron core and a light silicate mantle, and to the formation "magma ocean" on its surface or at shallow depths. Although even before the 1990s, the model of a relatively cold primary Earth, which gradually warmed up due to the above processes, accompanied by the release of a significant amount of thermal energy, was considered almost universally accepted.

An accurate assessment of the primary accretion heat and its fraction preserved to the present day is associated with significant difficulties. By O.G. Sorokhtin And S.A. Ushakov, who are supporters of the relatively cold primary Earth, the amount of accretion energy converted into heat is - 20.13*10 38 erg (20.13*10 31 J). This energy, in the absence of heat loss, would be enough for complete evaporation earthly matter, because the temperature could rise to 30 000 0 С. But the accretion process was relatively long, and the energy of planetesimal impacts was released only in the near-surface layers of the growing Earth and was quickly lost with thermal radiation, so the initial heating of the planet was not great. The magnitude of this thermal radiation, going in parallel with the formation (accretion) of the Earth, these authors estimate at 19.4*10 38 erg (19.4*10 31 J) .

In the modern energy balance of the Earth, accretion heat most likely plays a minor role.