For a long time, people have known about the spontaneous manifestations of gigantic energy lurking in the depths the globe... The memory of mankind keeps legends about catastrophic volcanic eruptions that claimed millions of human lives, unrecognizably changed the appearance of many places on Earth. The power of the eruption of even a relatively small volcano is colossal, it is many times higher than the power of the largest power plants created by human hands. True, there is no need to talk about the direct use of the energy of volcanic eruptions: so far people do not have the opportunity to curb this rebellious element, and, fortunately, eruptions are quite rare events. But these are manifestations of energy lurking in the bowels of the earth, when only a tiny fraction of this inexhaustible energy finds an outlet through the fire-breathing vents of volcanoes.
Small European country Iceland (literally translated "land of ice") is fully self-sufficient in tomatoes, apples and even bananas! Numerous Icelandic greenhouses get their energy from the heat of the earth; there are practically no other local sources of energy in Iceland. But this country is very rich hot springs and famous geysers - hot water fountains, with the precision of a chronometer bursting out of the ground. And although non-Icelanders have priority in using the heat of underground sources (even the ancient Romans to famous baths- the thermal baths of Caracalla - they brought water from the ground), the inhabitants of this small northern country operate an underground boiler house very intensively... The capital city of Reykjavik, which is home to half of the country's population, is heated only by underground sources. Reykjavik is an ideal starting point for exploring Iceland: from here you can go on the most interesting and varied excursions to any corner of this unique country: geysers, volcanoes, waterfalls, rhyolite mountains, fjords ... Everywhere in Reykjavik you will experience CLEAN ENERGY - the thermal energy of geysers , emanating from the ground, the energy of cleanliness and space of an ideally green city, the energy of a cheerful and incendiary nightlife Reykjavik all year round.
But not only for heating people draw energy from the depths of the earth. Power plants using hot underground springs have been operating for a long time. The first such power plant, still very low-power, was built in 1904 in the small Italian town of Larderello, named after the French engineer Larderelli, who back in 1827 drew up a project to use the numerous hot springs in the area. Gradually, the capacity of the power plant increased, more and more units were put into operation, new sources of hot water were used, and today the power of the station has already reached an impressive value - 360 thousand kilowatts. In New Zealand, there is such a power plant in the Wairakei region, with a capacity of 160 thousand kilowatts. At 120 km from San Francisco in the United States, a geothermal station with a capacity of 500 thousand kilowatts produces electricity.
Geothermal energy
For a long time, people have known about the spontaneous manifestations of gigantic energy lurking in the bowels of the globe. The memory of mankind keeps legends about catastrophic volcanic eruptions that claimed millions of human lives, unrecognizably changed the appearance of many places on Earth. The power of the eruption of even a relatively small volcano is colossal, it is many times higher than the power of the largest power plants created by human hands. True, there is no need to talk about the direct use of the energy of volcanic eruptions - so far people do not have the opportunity to curb this rebellious element, and, fortunately, eruptions are quite rare events. But these are manifestations of energy lurking in the bowels of the earth, when only a tiny fraction of this inexhaustible energy finds an outlet through the fire-breathing vents of volcanoes.
Geyser is hot spring which spews its water at a regular or irregular height, like a fountain. The name comes from the Icelandic word "pours". The appearance of geysers requires a certain favorable environment, which is created only in a few places on earth, which determines their rather rare presence. Almost 50% of geysers are located in National park Yellowstone (USA). The activity of the geyser may stop due to changes in the bowels, earthquakes and other factors. The action of the geyser is caused by the contact of water with magma, after which the water quickly heats up and, under the action of geothermal energy, is violently thrown upwards. After the eruption, the water in the geyser gradually cools down, seeps out to magma again, and gushes again. The frequency of eruptions of various geysers varies from several minutes to several hours. The need for a large energy for the operation of a geyser - main reason their rarity. Volcanic areas can have hot springs, mud volcanoes, fumaroles, but there are very few places where geysers are located. The fact is that even if a geyser was formed in the place of activity of a volcano, subsequent eruptions will destroy the surface of the earth and change its state, which will lead to the disappearance of the geyser.
Energy of the earth ( geothermal energy) is based on the use of the natural heat of the Earth. The bowels of the Earth are fraught with a colossal, practically inexhaustible source of energy. The annual radiation of internal heat on our planet is 2.8 * 1014 billion kWh. It is constantly compensated by the radioactive decay of some isotopes in earth crust.
Sources of geothermal energy can be of two types. The first type is underground pools of natural heat carriers - hot water (hydrothermal springs), or steam (steam thermal springs), or a steam-water mixture. In essence, these are "underground boilers" directly ready to use, from where water or steam can be produced using conventional boreholes. The second type is the warmth of the hot rocks... By pumping water into such horizons, you can also get steam or superheated water for further use for energy purposes.
But in both use cases main drawback is, perhaps, in a very weak concentration of geothermal energy. However, in places where peculiar geothermal anomalies are formed, where hot springs or rocks come relatively close to the surface and where, when immersed in depth, the temperature rises by 30-40 ° C for every 100 m, the concentration of geothermal energy can create conditions for its economic use. Depending on the temperature of water, steam or steam-water mixture, geothermal sources are subdivided into low and medium-temperature (with temperatures up to 130 - 150 ° C) and high-temperature (over 150 °). The nature of their use largely depends on the temperature.
It can be argued that geothermal energy has four advantageous distinctive features.
First, its reserves are practically inexhaustible. According to the estimates of the late 70s, to a depth of 10 km, they are such a value that is 3.5 thousand times higher than reserves traditional species mineral fuel.
Secondly, geothermal energy is quite widespread. Its concentration is associated mainly with the belts of active seismic and volcanic activity, which occupy 1/10 of the Earth's area. Within these belts, some of the most promising "geothermal regions" can be identified, examples of which are California in the USA, New Zealand, Japan, Iceland, Kamchatka, North Caucasus in Russia. In the former USSR alone, by the beginning of the 90s, about 50 underground hot water and steam basins were opened.
Thirdly, the use of geothermal energy does not require high costs, because in this case it comes about “ready-to-use” energy sources created by nature itself.
Finally, fourthly, geothermal energy is completely harmless from the ecological point of view and does not pollute the environment.
Man has long been using the energy of the Earth's internal heat (remember, at least the famous Roman Baths), but its commercial use began only in the 1920s with the construction of the first geo-power plants in Italy, and then in other countries. By the beginning of the 1980s, about 20 such stations with a total capacity of 1.5 million kW were operating in the world. The largest of them is the Geysers station in the USA (500 thousand kW).
Geothermal energy is used to generate electricity, heat homes, greenhouses, etc. Dry steam, superheated water or some kind of coolant with a low boiling point (ammonia, freon, etc.) is used as a heat carrier.
In our country rich in hydrocarbons, geothermal energy is an exotic resource that, given 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 is quite efficient.
Geothermal energy is the warmth of the earth's interior. It is produced in the depths and comes to the surface of the Earth in different forms and with varying intensity.
The temperature of the upper layers of the soil depends mainly on external (exogenous) factors - sunlight 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 a change in air temperature and with some delay, increasing with depth. The influence of daily fluctuations in air temperature ends at depths from a few to several tens of centimeters. Seasonal fluctuations cover deeper layers of soil - up to tens of meters.
At a certain depth - from tens to hundreds of meters - the soil temperature is kept constant, equal to the average annual air temperature at the Earth's surface. It is easy to be convinced of this by going down into a sufficiently deep cave.
When the average annual air temperature 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 reaches 200-300 m in places.
From a certain depth (its own for each point on the map), the action of the Sun and the atmosphere weakens so much that endogenous (internal) factors come out on top 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 mainly associated with the decay of the radioactive elements located there, although other sources of heat 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 grows with depth. Miners are faced with this phenomenon - it is always hot in deep mines. 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 surface of the Earth, is small - on average, its power is 0.03–0.05 W / m 2, or about 350 W · h / m 2 per year. Against the background of the heat flow from the Sun and the air heated by it, this is an imperceptible value: the Sun gives everyone square meter the earth's surface is about 4,000 kWh annually, that is, 10,000 times more (of course, this is on average, with a huge variation between polar and equatorial latitudes and depending on other climatic and weather factors).
The insignificance of the heat flow from the depths to the surface on most of the planet is associated with the low thermal conductivity of rocks and features geological structure... But there are exceptions - places where the heat flow is high. These are, first of all, zones tectonic faults, increased seismic activity and volcanism, where the energy of the earth's interior finds a way out. Such zones are characterized by thermal anomalies of the lithosphere, here the heat flux reaching the Earth's surface can be several times and even orders of magnitude more powerful than the "usual" one. Volcanic eruptions and hot water springs carry a huge amount of heat to the surface in these zones.
It is these areas that are most favorable for the development of geothermal energy. On the territory of Russia, this is, first of all, Kamchatka, Kurile 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 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 ° C for every 100 m. The ratio of the temperature difference between two points at different depths to the difference in depth between them is called the geothermal gradient.
The reciprocal is the geothermal step, or depth interval, at which the temperature rises by 1 ° C.
The higher the gradient and, accordingly, the lower the step, the closer the warmth of the depths of the Earth 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 rise with depth can vary dramatically. On the Earth's scale, fluctuations in the magnitudes of geothermal gradients and steps reach 25 times. For example, in Oregon (USA) the gradient is 150 ° C per km, and in South Africa it is 6 ° C per km.
The question is, what is the temperature at great depths - 5, 10 km or more? If the trend continues, temperatures at 10 km depth should average around 250–300 ° C. This is more or less confirmed by direct observations in superdeep wells, although the picture is much more complicated than a linear increase in temperature.
For example, in Kola superdeep 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 ° C was already recorded, at a depth of 10 km - 180 ° C, and at 12 km - 220 ° C.
Another example is a well drilled in the Northern Caspian region, where a temperature of 42 ° C was recorded at a depth of 500 m, 70 ° C at 1.5 km, 80 ° C at 2 km, and 108 ° C at 3 km.
It is assumed that the geothermal gradient decreases starting from a depth of 20-30 km: at a depth of 100 km, the assumed temperatures are about 1300-1500 ° C, at a depth of 400 km - 1600 ° C, in the Earth's core (depths over 6000 km) - 4000-5000 ° C.
At depths of up to 10–12 km, the temperature is measured through drilled wells; where they are absent, 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 outflowing 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 this problem is solved for us by nature itself with the help of a natural heat carrier - heated thermal waters that emerge 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 term "thermal waters". As a rule, they mean hot groundwater in a liquid state or in the form of steam, including those coming out 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 groundwater, steam, steam-water mixtures is hydrothermal energy. Accordingly, the 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 rather high temperatures, as a rule, start 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 - 3500 and 35 trillion tons of fuel equivalent, respectively. This is quite natural - the warmth of the depths of the Earth is everywhere, and thermal waters are found locally. However, due to obvious technical difficulties for generating heat and electricity, they are currently used for the most part thermal waters.
Waters with temperatures between 20-30 ° C and 100 ° C are suitable for heating, temperatures between 150 ° C and above - and for generating electricity in geothermal power plants.
In general, geothermal resources on the territory of Russia in terms of tons of equivalent fuel or any other unit of energy measurement are about 10 times higher than the reserves of fossil fuel.
Theoretically, only geothermal energy could fully satisfy energy needs 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 Eyjafjallajokull volcano ( Eyjafjallajökull) in 2010 year.
It is thanks to this geological specificity that Iceland has enormous reserves of geothermal energy, including hot springs that come out to the surface of the Earth and even gush out in the form of geysers.
In Iceland, more than 60% of all energy consumed is currently taken from the Earth. Including geothermal sources provide 90% of heating and 30% of electricity generation. We add that the rest of the country's electricity is produced at hydroelectric power plants, that is, also using a renewable energy source, thanks to which Iceland looks like a kind of global environmental standard.
The domestication of geothermal energy in the 20th century helped Iceland noticeably 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 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 clean sources energy: the needs for it are generally small.
In addition to Iceland, a high share of geothermal energy in the total balance of electricity generation is provided in New Zealand and the island states. 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, given 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 known examples is Italy, a place in the province of Tuscany, now called Larderello, where as early as the beginning of the 19th century, the local hot thermal waters, poured out naturally or extracted from shallow wells, were used for energy purposes.
Boron-rich underground water was used here to obtain boric acid. Initially, this acid was obtained by evaporation in iron boilers, and ordinary firewood from the 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 for the operation of drilling rigs, and at the beginning of the 20th century - for heating local houses and greenhouses. In the same place, in Larderello, in 1904, thermal water vapor became an energy source for generating electricity.
Some other countries followed the example of Italy in the late 19th and early 20th centuries. For example, in 1892, thermal waters were first used for local heating in the United States (Boise, Idaho), in 1919 in Japan, and in 1928 in Iceland.
In the United States, the first hydrothermal power plant appeared in California in the early 1930s, in New Zealand in 1958, in Mexico in 1959, in Russia (the world's first binary geothermal power plant) in 1965 ...
Old principle on a new source
Electricity generation requires a higher temperature of the hydro source than for heating - more than 150 ° C. The principle of operation of a geothermal power plant (GeoPP) is similar to the principle of operation of a conventional thermal power plant (TPP). In fact, a geothermal power plant is a kind of thermal power plant.
At TPPs, as a rule, coal, gas or fuel oil act as the primary source of energy, and water vapor serves as the working fluid. Fuel, burning, heats water to the state of steam, which rotates the steam turbine, and it generates electricity.
The difference between GeoPPs 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 an electric generator in a "ready-made" form directly from the production well.
There are three main schemes of GeoPP operation: direct, using dry (geothermal) steam; indirect, based on hydrothermal water, and mixed, or binary.
The application of this or that scheme depends on the state of aggregation and the temperature of the energy carrier.
The simplest and therefore the first of the mastered schemes is the straight line, in which the steam coming from the well is passed directly through the turbine. The world's first GeoPP in Larderello also operated on dry steam in 1904.
GeoPPs with an indirect scheme of work are the most common in our time. They use hot underground water, which is pumped into the evaporator under high pressure, where part of it is evaporated, and the resulting steam rotates the turbine. In some cases, additional devices and circuits are required to purify geothermal water and steam from aggressive compounds.
Waste steam enters the injection well or is used for space heating - in this case, the principle is the same as in the operation of a CHP.
At binary GeoPPs, hot thermal water interacts with another liquid that acts as a working fluid with a lower boiling point. Both fluids are passed through a heat exchanger, where thermal water evaporates the working fluid, the vapor of which rotates 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 schematic diagram in this case is also quite simple. It is necessary to drill two interconnected wells - injection and production wells. Water is pumped into the injection well. At depth, it heats up, then heated water or steam formed as a result of strong heating is supplied to the surface through the production well. Further, it all depends on how petrothermal energy is used - for heating or for generating electricity. A closed cycle is possible with the injection of waste steam and water back into the injection well or another way of disposal.
The disadvantage of such a system is obvious: to obtain a sufficiently high temperature of the working fluid, it is necessary to drill wells on great depth... And these are serious costs and the risk of significant heat loss when the fluid moves upward. Therefore, petrothermal systems are still less widespread than hydrothermal ones, although the potential of petrothermal energy is orders of magnitude higher.
Currently, Australia is the leader in the creation of the so-called petrothermal circulation systems (PCS). In addition, this direction of geothermal energy is actively developing in the USA, Switzerland, Great Britain, and Japan.
Lord Kelvin's gift
The invention of a heat pump by physicist William Thompson (aka Lord Kelvin) in 1852 provided mankind with a real opportunity to use the low-potential heat of the upper soil layers. The heat pump system, or, as Thompson called it, the heat multiplier, is based on physical process heat transfer from environment to the refrigerant. In fact, it uses the same principle as in petrothermal systems. The difference is in the heat source, in connection with which a terminological question may arise: 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 contained in them are heated not by the deep heat of the earth, but by the sun. Thus, it is the sun in this case that is the primary source of heat, although it is taken, as in geothermal systems, from the earth.
The operation of a heat pump is based on a delay in the heating and cooling of the soil compared to the atmosphere, as a result of which a temperature gradient is formed between the surface and deeper layers, which retain heat even in winter, similar to what happens in water bodies. The main purpose of heat pumps is space heating. In fact, it is a “reverse refrigerator”. Both the heat pump and the refrigerator interact with three components: the internal environment (in the first case - the heated room, in the second - the cooled refrigerator chamber), the external environment - the energy source and the refrigerant (refrigerant), it is also the heat carrier that provides heat transfer 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, the liquid refrigerant enters the evaporator through a throttle (pressure regulator), where, due to a sharp decrease in pressure, the liquid evaporates. Evaporation is an endothermic process that requires external heat absorption. As a result, heat is taken from the inner walls of the evaporator, which provides a cooling effect in the refrigerator chamber. Further, from the evaporator, the refrigerant is sucked into the compressor, where it returns to the liquid state of aggregation. This is a reverse process leading to the release of the removed heat into the external environment. As a rule, it is thrown into the room and the back of the refrigerator is relatively warm.
A heat pump works in much 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, passing along an external circuit, laid in the ground or in a reservoir, and then enters the evaporator.
In the evaporator, heat is transferred to an internal circuit filled with a refrigerant with a low boiling point, which, passing through the evaporator, changes from a liquid to a gaseous state, taking heat away.
Further, the gaseous refrigerant enters the compressor, where it is compressed to high pressure and temperature, and enters the condenser, where heat exchange takes place between the hot gas and the coolant from the heating system.
The compressor requires electricity to operate, however, the transformation ratio (the ratio of consumed and generated energy) in modern systems is high enough to ensure their efficiency.
Currently, heat pumps are widely used for space heating, mainly in economical developed countries.
Eco-correct energy
Geothermal energy is considered environmentally friendly, which is generally true. First of all, it uses a renewable and practically inexhaustible resource. Geothermal energy does not require large areas, unlike large hydroelectric power plants 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 indicator for a coal-fired power plant, for example, is 3600 m 2. The ecological advantages of GeoPPs also include low water consumption - 20 liters fresh water per 1 kW, while TPP and NPP require about 1000 liters. Note that these are environmental indicators of the "average" GeoPP.
But negative side effects are still available. Among them, noise, thermal pollution of the atmosphere and chemical pollution - water and soil, as well as the formation of solid waste are most often distinguished.
The main source of chemical pollution of the environment is the actual thermal water (with high temperature and mineralization), which often contains large amounts of toxic compounds, in connection with which 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. Here, the same dangers arise as when drilling any well: destruction of soil and vegetation cover, soil and groundwater pollution.
At the stage of operation of the GeoPP, the problems of environmental pollution persist. 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 environment, they become sources of its pollution. In addition, an aggressive chemical environment can cause corrosion damage to the structures of the GeoTPP.
At the same time, emissions of pollutants at GeoPPs are on average lower than at TPPs. For example, emissions carbon dioxide for each kilowatt-hour of generated electricity, up to 380 g at GeoPPs, 1,042 g - at coal TPPs, 906 g - at fuel oil and 453 g - at gas TPPs.
The question arises: what to do with the waste water? With low salinity, after cooling, it can be discharged into surface waters... Another way is to pump it back into the aquifer through an injection well, which is preferred and predominantly used today.
Extraction of thermal water from aquifers (as well as pumping out ordinary water) can cause subsidence and movement of the soil, other deformations of geological layers, and micro-earthquakes. The likelihood of such phenomena, as a rule, is small, although individual cases have been recorded (for example, at the GeoPP in Staufen im Breisgau in Germany).
It should be emphasized that most of the 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 a more extensive development of geothermal energy, environmental risks can increase and multiply.
How much is the energy of the Earth?
Investment costs for the construction of geothermal systems vary in a very wide range - from $ 200 to $ 5,000 per 1 kW of installed capacity, that is, the most cheap options comparable to the cost of building 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, the need for water purification can multiply the cost.
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 cost of construction nuclear power plant and is comparable to the cost of building wind and solar power plants.
The obvious economic advantage of GeoTPP is a free energy carrier. For comparison, in the cost structure of an operating TPP or NPP, 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 the external conjuncture of energy prices. In general, the operating costs of the GeoTPP are estimated at 2–10 cents (60 kopecks – 3 rubles) per 1 kWh of produced capacity.
The second largest (after energy) (and very significant) item of expenditure is, as a rule, wage plant personnel, which can differ dramatically by country and region.
On average, the cost of 1 kWh of geothermal energy is comparable to that for TPPs (in Russian conditions - about 1 ruble / 1 kWh) and ten times higher than the cost of generating electricity at hydroelectric power plants (5-10 kopecks / 1 kWh ).
Part of the reason for the high cost lies in the fact that, unlike thermal and hydraulic power plants, GeoTPP has a relatively small capacity. In addition, it is necessary to compare systems located in the same region and in 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 it is necessary to dispose of waste water and in what ways it is done, whether a combined use of the resource is possible. So, chemical elements and compounds extracted from thermal water can provide additional income. Let us recall the example of Larderello: chemical production, and the use of geothermal energy was originally an auxiliary one.
Geothermal energy forwards
Geothermal energy is developing somewhat differently than wind and solar. She is currently in significant to a greater extent depends on the nature of the resource itself, which differs sharply by region, and the highest concentrations are tied to narrow zones of geothermal anomalies, associated, as a rule, with areas of tectonic faults and volcanism.
In addition, geothermal energy is less technologically capacious in comparison with wind, and even more so with solar energy: the systems of geothermal plants are quite simple.
V general structure The geothermal component accounts for less than 1% of world electricity production, but in some regions and countries its share reaches 25-30%. Due to the linkage to geological conditions, a significant part of the geothermal energy capacities is concentrated in the third world countries, where there are three clusters of the industry's greatest development - the islands of Southeast Asia, Central America and East Africa. The first two regions are included in the Pacific "Earth's fire belt", the third is tied to the East African Rift. Most likely, geothermal energy will continue to develop in these belts. More distant perspective- development of petrothermal energy, using the heat of the earth layers lying at a depth of several kilometers. This is an almost ubiquitous resource, but its extraction requires high costs; therefore, petrothermal energy is developing primarily in the most economically and technologically powerful countries.
In general, given the ubiquitous distribution of geothermal resources and an acceptable level of environmental safety, there is reason to believe that geothermal energy has good prospects development. Especially with the growing threat of a shortage of traditional energy sources 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 total energy balance of a huge country is still negligible.
Two regions - Kamchatka and the North Caucasus - have become pioneers and centers for the development of geothermal energy in Russia, and if in the first case we are talking primarily about the electric power industry, then in the second - about the use of thermal energy of thermal water.
In the North Caucasus - in Krasnodar Territory, Chechnya, Dagestan - the heat of thermal waters for energy purposes was used even before the Great Patriotic War... In the 1980s and 1990s, the development of geothermal energy in the region for obvious reasons stalled and has not yet emerged from a 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 associated primarily with the construction of GeoPPs. The first of them, still operating Pauzhetskaya and Paratunskaya stations, were built back in 1965-1967, while the Paratunskaya GeoPP with a capacity of 600 kW became the first station in the world with a binary cycle. It was the development of Soviet scientists S.S.Kutateladze and A.M. Rosenfeld from the Institute of Thermophysics of the Siberian Branch of the Russian Academy of Sciences, who in 1965 received an author's certificate for the extraction of electricity from water with a temperature of 70 ° C. This technology later became a prototype for more than 400 binary GeoPPs in the world.
The capacity of the Pauzhetskaya GeoPP, commissioned in 1966, was initially 5 MW and subsequently increased to 12 MW. Currently, a binary block is under construction 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 12 MW power units, 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 it is winter 9-10 months a year. The equipment of the Mutnovsky GeoPPs, currently one of the most modern in the world, is completely created at domestic enterprises of power engineering.
At present, the share of Mutnovskie plants in the total structure of energy consumption of the Central Kamchatka power unit is 40%. An increase in capacity is planned in the coming years.
Separately, it should be said about Russian petrothermal developments. We do not have large DSPs yet, but there are advanced technologies for drilling to great depths (about 10 km), which also have no analogues in the world. Their further development will allow to drastically reduce the cost of creating petrothermal systems. The developers of these technologies and projects are N. A. Gnatus, M. D. Khutorskoy (Geological Institute, RAS), A. S. Nekrasov (Institute of Economic Forecasting, RAS) and specialists from the Kaluga Turbine Works. The project for a petrothermal circulation system in Russia is currently at an experimental stage.
There are prospects for geothermal energy in Russia, albeit relatively distant: at the moment, the potential is quite large and the positions of traditional energy are strong. At the same time, in a number of remote regions of the country, the use of geothermal energy is economically profitable and is in demand now. These are territories with high geoenergetic potential (Chukotka, Kamchatka, Kuriles - Russian part Pacific "Earth's fire belt", the mountains of South Siberia and the Caucasus) and at the same time remote and cut off from the centralized power supply.
Probably, in the coming decades, geothermal energy in our country will develop precisely in such regions.
2. Thermal regime of the Earth
The earth is a cold space body. Surface temperature depends mainly on external heat. 95% of the heat of the Earth's upper layer is external (solar) warm, and only 5% warm internal , which comes from the bowels of the Earth and includes several sources of energy. In the interior of the Earth, the temperature increases with depth from 1300 ° C (in the upper mantle) to 3700 ° C (in the center of the core).
External heat... Heat comes to the surface of the Earth mainly from the Sun. Each square centimeter of surface receives about 2 calories of heat in one minute. This quantity is called solar constant and determines the total amount of heat supplied to the Earth from the Sun. For a year, it amounts to 2.26 · 10 21 calories. The depth of penetration of solar heat into the bowels of the Earth depends mainly on the amount of heat that falls per unit surface area, and on the thermal conductivity of rocks. The maximum depth to which external heat penetrates is 200 m in the oceans, and about 40 m on land.
Internal warmth... With depth, there is an increase in temperature, which occurs very unevenly in different territories. The increase in temperature follows the adiabatic law and depends on the compression of the substance under pressure when heat exchange with the environment is impossible.
The main sources of heat inside the Earth:
The heat released during the radioactive decay of elements.
Residual heat, preserved from the time of the formation of the Earth.
Gravitational heat released during the compression of the Earth and the distribution of matter in terms of density.
Heat generated by chemical reactions taking place in the depths of the earth's crust.
Heat released by tidal friction of the Earth.
There are 3 temperature zones:
I - variable temperature zone ... The change in temperature is determined by the local climate. Daily fluctuations practically attenuate at a depth of about 1.5 m, and annual fluctuations at depths of 20 ... 30 m. Iа - freezing zone.
II - constant temperature zone located at depths of 15 ... 40 m, depending on the region.
III - temperature rise zone .
The temperature regime of rocks in the bowels of the earth's crust is usually expressed by a geothermal gradient and a geothermal step.
The amount of temperature rise for every 100 m depth is called geothermal gradient... In Africa, at the Witwatersrand field, it is 1.5 ° С, in Japan (Echigo) - 2.9 ° С, in South Australia - 10.9 ° С, in Kazakhstan (Samarinda) - 6.3 ° С, on the Kola Peninsula - 0.65 ° C.
Rice. 3. Zones of temperatures in the earth's crust: I - zone of variable temperatures, Iа - freezing zone; II - zone of constant temperatures; III - zone of temperature rise.
The depth at which the temperature rises by 1 degree is called geothermal step. The numerical values of the geothermal step are not constant not only at different latitudes, but also at different depths of the same point in the region. The magnitude of the geothermal step varies from 1.5 to 250 m.In Arkhangelsk it is 10 m, in Moscow - 38.4 m, and in Pyatigorsk - 1.5 m.The theoretically average value of this step is 33 m.
In a well drilled in Moscow to a depth of 1630 m, the bottomhole temperature was 41 ° C, and in a mine drilled in the Donbass to a depth of 1545 m, the temperature was 56.3 ° C. The highest temperature was recorded in the USA in a borehole with a depth of 7136 m, where it is equal to 224 ° C. The increase in temperature with depth should be taken into account when designing deep structures. According to calculations, at a depth of 400 km, the temperature should reach 1400 ... 1700 ° C. The highest temperatures (about 5000 ° C) were obtained for the Earth's core.
The term geothermal energy comes from the Greek word for earth (geo) and thermal (thermal). In fact, geothermal energy comes from the earth itself... Heat from the earth's core, which averages 3,600 degrees Celsius, radiates towards the planet's surface.
The heating of springs and geysers underground at a depth of several kilometers can be carried out using special wells through which hot water (or steam from it) flows to the surface, where it can be used directly as heat or indirectly to generate electricity by turning on rotating turbines.
Since the water below the surface of the earth is constantly replenishing, and the core of the earth will continue to generate heat relatively human life endlessly, geothermal energy ultimately clean and renewable.
Methods of collecting energy resources of the Earth
Today there are three main methods of harvesting geothermal energy: dry steam, hot water, and binary cycle. The dry steam process directly rotates the turbine drives of the power generators. Hot water enters from the bottom up, then sprayed into the tank to create steam to drive the turbines. These two methods are the most common, generating hundreds of megawatts of electricity in the United States, Iceland, Europe, Russia, and other countries. But the location is limited, as these factories only operate in tectonic regions where it is easier to access heated water.
With the binary cycle technology, warm (not necessarily hot) water is extracted to the surface and combined with butane or pentane, which has a low boiling point. This liquid is pumped through a heat exchanger where it is vaporized and sent through a turbine before being recirculated back to the system. Binary cycle technology provides tens of megawatts of electricity in the United States: California, Nevada and the Hawaiian Islands.
The principle of obtaining energy
Disadvantages of getting geothermal energy
On a utility level, geothermal power plants are expensive to build and operate. Finding a suitable location requires costly well surveys with no guarantee of hitting a productive underground hot spot. However, analysts expect this capacity to nearly double over the next six years.
In addition, areas with a high temperature of the underground source are located in areas with active geological volcanoes. These "hot spots" have formed at the boundaries of tectonic plates in places where the crust is quite thin. Pacific region, often referred to as a ring of fire for many volcanoes with many hotspots, including Alaska, California, and Oregon. Nevada has hundreds of hotspots covering most of the northern United States.
There are also other seismically active regions. Earthquakes and magma movement allow water to circulate. In some places, water rises to the surface and natural hot springs and geysers occur, such as in Kamchatka. The water in the geysers of Kamchatka reaches 95 ° C. 
One of the problems open system geysers is the release of some air pollutants. Hydrogen sulfide is a toxic gas with a very recognizable "rotten egg" smell - a small amount of arsenic and minerals released with steam. Salt can also pose an environmental problem.
Offshore geothermal power plants significant amount interfering salt accumulates in the pipes. In closed systems, there are no emissions and all the liquid brought to the surface is returned.
The economic potential of the energy resource
Hot spots are not the only places where geothermal energy can be found. There is a constant supply of usable heat for direct heating purposes anywhere from 4 meters to several kilometers below the surface of virtually anywhere on earth. Even land in your own backyard or local school has the economic potential in the form of heat to be pumped out into your home or other buildings.
In addition, there is great amount thermal energy in dry rock formations very deep below the surface (4 - 10 km).
The use of new technology could expand geothermal systems, where humans can use this heat to generate electricity on a much larger scale than conventional technologies. The first demonstration projects of this principle of generating electricity were shown in the United States and Australia back in 2013.
If the full economic potential of geothermal resources can be realized, then this will represent a huge source of electricity for production facilities. Scientists suggest that conventional geothermal sources have a potential of 38,000 MW, which can generate 380 million MW of electricity per year.
Hot dry rocks occur at depths of 5 to 8 km everywhere underground and at shallower depths in certain places. Access to these resources involves the introduction of cold water circulating through the hot rocks and the removal of heated water. Currently no commercial use this technology. Existing technologies do not yet allow to restore thermal energy directly from magma, very deep, but this is the most powerful resource of geothermal energy.
With the combination of energy resources and their consistency, geothermal energy can play an irreplaceable role as a cleaner, more sustainable energy system.
Geothermal power plant structures
Geothermal energy is clean and sustainable heat from the Earth. Large resources are found in the range of several kilometers below the surface of the earth, and even deeper, to the high temperature of molten rock called magma. But as described above, people have not yet reached the magma.
Three designs of geothermal power plants
The application technology is determined by the resource. If the water comes from the well as steam, it can be used directly. If the hot water is hot enough, it must pass through a heat exchanger.
The first well for power generation was drilled before 1924. Deeper wells were drilled in the 1950s, but real development takes place in the 1970s and 1980s.
Direct use of geothermal heat
Geothermal sources can also be used directly for heating purposes. Hot water is used to heat buildings, grow plants in greenhouses, dry fish and crops, improve oil recovery, aid industrial processes as milk pasteurizers, and heat water in fish farms. In the US, Klamath Falls, Oregon and Boise, Idaho, geothermal water has been used to heat homes and buildings for over a century. On the east coast, Warm Springs, Virginia draws heat directly from spring water using local heat sources.
In Iceland, almost every building in the country is heated by hot spring water. In fact, Iceland gets over 50 percent of its primary energy from geothermal sources. In Reykjavik, for example, (population 118 thousand), hot water is conveyed over 25 kilometers, and residents use it for heating and natural needs.
New Zealand gets an additional 10% of its electricity. is underdeveloped, despite the presence of thermal waters.
THEM. Kapitonov
Nuclear heat of the Earth
Earthly warmth
The earth is a fairly hot body and is a source of heat. It heats up, first of all, due to the absorbed solar radiation. But the Earth also has its own heat resource comparable to the heat received from the Sun. It is believed that this self-energy of the Earth has the following origin. The Earth emerged about 4.5 billion years ago following the formation of the Sun from a protoplanetary gas-dust disk revolving around it and condensing. At the early stage of its formation, the heating of the earth's substance took place due to the relatively slow gravitational compression. An important role in the thermal balance of the Earth was also played by the energy released when small cosmic bodies fell on it. Therefore, the young Earth was molten. Cooling down, it gradually came to its current state with a solid surface, a significant part of which is covered with oceanic and sea waters... This hard outer layer is called crust and on average on land areas its thickness is about 40 km, and under ocean waters- 5-10 km. The deeper layer of the Earth, called mantle, also consists of a solid. It extends to a depth of almost 3000 km and contains the bulk of the Earth's material. Finally, the innermost part of the Earth is her core... It consists of two layers - external and internal. Outer core it is a layer of molten iron and nickel at a temperature of 4500-6500 K and a thickness of 2000-2500 km. Inner core with a radius of 1000-1500 km is a hard iron-nickel alloy heated to a temperature of 4000-5000 K with a density of about 14 g / cm 3, which arose at a huge (almost 4 million bar) pressure.
In addition to the Earth's internal heat, inherited from the earliest hot stage of its formation, and the amount of which should decrease over time, there is another, long-term, associated with radioactive decay of nuclei with a long half-life - primarily 232 Th, 235 U , 238 U and 40 K. The energy released in these decays - they account for almost 99% of the earth's radioactive energy - constantly replenishes the thermal reserves of the Earth. The aforementioned cores are found in the crust and mantle. Their decay leads to heating of both the outer and inner layers of the Earth.
Part of the enormous heat contained within the Earth constantly comes out to its surface, often in very large-scale volcanic processes. The heat flux flowing from the depths of the Earth through its surface is known. It is (47 ± 2) · 10 12 watts, which is equivalent to the heat that 50 thousand nuclear power plants can generate (the average power of one nuclear power plant is about 10 9 watts). The question arises whether radioactive energy plays any significant role in the total thermal budget of the Earth, and if it does, then what role? The answer to these questions long time remained unknown. Opportunities have now emerged to answer these questions. The key role here belongs to neutrinos (antineutrinos), which are born in the processes of radioactive decay of nuclei that make up the Earth's substance and which are called geo-neutrino.
Geo-neutrino
Geo-neutrino- This is the collective name for neutrinos or antineutrinos, which are emitted as a result of beta decay of nuclei located below the earth's surface. Obviously, due to their unprecedented penetrating ability, the registration of them (and only them) by ground-based neutrino detectors can provide objective information about the processes of radioactive decay taking place deep inside the Earth. An example of such a decay is the β - -decay of the 228 Ra nucleus, which is a product of the α-decay of the long-lived 232 Th nucleus (see table):
The half-life (T 1/2) of the 228 Ra nucleus is 5.75 years, the released energy is about 46 keV. The energy spectrum of antineutrino is continuous with the upper limit close to the released energy.
Decays of 232 Th, 235 U, 238 U nuclei are chains of successive decays that form the so-called radioactive ranks... In such chains, α-decays are interspersed with β - -decays, since during α-decays the final nuclei are shifted from the β-stability line to the region of nuclei overloaded with neutrons. After a chain of successive decays at the end of each row, stable nuclei are formed with a close or equal magic number of protons and neutrons (Z
=
82,N= 126). Such final nuclei are stable isotopes of lead or bismuth. Thus, the decay of T 1/2 ends with the formation of a doubly magic 208 Pb nucleus, and on the path 232 Th → 208 Pb, six α-decays occur, alternating with four β - -decays (in the chain 238 U → 206 Pb, eight α- and six β - - decays; in the 235 U → 207 Pb chain, there are seven α and four β - decays). Thus, the energy spectrum of antineutrinos from each radioactive series is a superposition of partial spectra from individual β - decays that make up this series. The spectra of antineutrinos formed in the decays 232 Th, 235 U, 238 U, 40 K are shown in Fig. 1. The decay of 40 K is a single β - decay (see table). Antineutrinos reach the highest energy (up to 3.26 MeV) in the decay
214 Bi → 214 Po, which is a link in the 238 U radioactive series. The total energy released during the passage of all decay links of the 232 Th → 208 Pb series is 42.65 MeV. For the radioactive series 235 U and 238 U, these energies are 46.39 and 51.69 MeV, respectively. Energy released in decay
40 K → 40 Ca, is 1.31 MeV.
Characteristics of 232 Th, 235 U, 238 U, 40 K cores
| Core | Share in% in the mix isotopes |
Number of cores relates. cores Si |
T 1/2, billion years |
First links decay |
|---|---|---|---|---|
| 232 Th | 100 | 0.0335 | 14.0 | |
| 235 U | 0.7204 | 6.48 · 10 -5 | 0.704 | |
| 238 U | 99.2742 | 0.00893 | 4.47 | |
| 40 K | 0.0117 | 0.440 | 1.25 |
 |
An estimate of the geo-neutrino flux made on the basis of the decay of 232 Th, 235 U, 238 U, 40 K nuclei contained in the Earth's matter leads to a value of the order of 10 6 cm -2 sec -1. By registering these geo-neutrinos, one can obtain information about the role of radioactive heat in the total heat balance of the Earth and check our ideas about the content of long-lived radioisotopes in the composition of the earth's matter.
Rice. 1. Energy spectra of antineutrinos from nuclear decay
232 Th, 235 U, 238 U, 40 K, normalized to one decay of the parent nucleus
To register electronic antineutrinos, the reaction is used
P → e + + n, (1)
in which this particle was actually discovered. The threshold for this reaction is 1.8 MeV. Therefore, only geo-neutrinos formed in decay chains starting from 232 Th and 238 U nuclei can be registered in the above reaction. The effective cross section of the discussed reaction is extremely small: σ ≈ 10 -43 cm 2. Hence, it follows that a neutrino detector with a sensitive volume of 1 m 3 will register no more than a few events per year. Obviously, for the reliable fixation of geo-neutrino fluxes, large-volume neutrino detectors are needed, located in underground laboratories for maximum protection from the background. The idea to register geo-neutrinos with detectors designed to study solar and reactor neutrinos arose in 1998. Currently, there are two large-volume neutrino detectors using a liquid scintillator and suitable for solving this problem. These are neutrino detectors of the KamLAND (Japan) and Borexino (Italy) experiments. Below we consider the device of the Borexino detector and the results obtained on this detector on the registration of geo-neutrinos.
Borexino detector and geo-neutrino registration
The Borexino neutrino detector is located in central Italy in an underground laboratory under the Gran Sasso mountain range, the height of the mountain peaks of which reaches 2.9 km (Fig. 2).
Rice. 2. Layout of the neutrino laboratory under the Gran Sasso mountain range (central Italy)
Borexino is an unsegmented massive detector, the active medium of which is
280 tons of organic liquid scintillator. It filled a nylon spherical vessel 8.5 m in diameter (Fig. 3). The scintillator is pseudocumene (C 9 H 12) with a spectrum-shifting additive PPO (1.5 g / L). The light from the scintillator is collected by 2,212 eight-inch photomultiplier tubes (PMTs) mounted on a stainless steel sphere (SNS).
Rice. 3. Diagram of the Borexino detector device
A nylon vessel with pseudocumene is an internal detector, the task of which is to register neutrinos (antineutrinos). The internal detector is surrounded by two concentric buffer zones that protect it from external gamma rays and neutrons. The inner zone is filled with a non-scintillating medium consisting of 900 tons of pseudocumene with scintillation quenching dimethyl phthalate additives. The outer zone is located on top of the SNS and is a water Cherenkov detector containing 2000 tons of ultrapure water and cuts off signals from muons entering the setup from outside. For each interaction that takes place in the internal detector, energy and time are determined. Calibration of the detector using various radioactive sources made it possible to very accurately determine its energy scale and the degree of reproducibility of the light signal.
Borexino is a detector of very high radiation purity. All materials have been rigorously selected and the scintillator has been purified to minimize internal background. Due to its high radiation purity, Borexino is an excellent detector for detecting antineutrinos.
In reaction (1), the positron gives an instantaneous signal, followed after a while by the capture of a neutron by a hydrogen nucleus, which leads to the appearance of a γ-quantum with an energy of 2.22 MeV, which creates a signal delayed relative to the first. At Borexino, the neutron capture time is about 260 μs. Instantaneous and delayed signals are correlated in space and time, providing accurate recognition of the event caused by e.
The threshold for reaction (1) is 1.806 MeV and, as can be seen from Fig. 1, all geo-neutrinos from the decays of 40 K and 235 U turn out to be below this threshold, and only a part of the geo-neutrinos produced in the decays of 232 Th and 238 U can be detected.
The Borexino detector first detected signals from geo-neutrinos in 2010, and new results were recently published based on observations over 2056 days from December 2007 to March 2015. Below we present the data obtained and the results of their discussion, based on article.
As a result of the analysis of experimental data, 77 candidates for electron antineutrinos were identified that passed all the selection criteria. The background from events imitating e was estimated by the value. Thus, the signal-to-background ratio was ≈100.
Reactor antineutrinos were the main source of background. For Borexino, the situation was quite favorable, since there are no nuclear reactors near the Gran Sasso laboratory. In addition, reactor antineutrinos are more energetic than geo-neutrinos, which made it possible to separate these antineutrinos from the positron in signal magnitude. The results of the analysis of the contributions of geo-neutrinos and reactor antineutrinos to the total number of registered events from e are shown in Fig. 4. The number of detected geo-neutrinos given by this analysis (in Fig. 4 they correspond to the darkened region) is equal to 
... In the geo-neutrino spectrum extracted as a result of the analysis, two groups are visible - less energetic, more intense and more energetic, less intense. The authors of the described study associate these groups with the decays of thorium and uranium, respectively.
The discussed analysis used the ratio of the masses of thorium and uranium in the material of the Earth
m (Th) / m (U) = 3.9 (in the table, this value is ≈3.8). This figure reflects the relative content of these chemical elements in chondrites - the most common group of meteorites (more than 90% of meteorites that fell to Earth belong to this group). It is believed that the composition of chondrites, with the exception of light gases (hydrogen and helium), repeats the composition of the solar system and the protoplanetary disk from which the Earth was formed.
Rice. 4. Spectrum of light output from positrons in units of the number of photoelectrons for candidate antineutrino events (experimental points). The shaded area is the contribution of geo-neutrinos. The solid line is the contribution of reactor antineutrinos.
