Geothermal energy: how the heat of the Earth turned into an effective energy resource

It is given: there is a hot core inside the Earth, with its help it is necessary to generate electricity.
Question: how to do it?
Answer: to build a geothermal power plant.
We understand exactly how, from where underground steam and how much benefit from such a power plant.

The oldest and most popular method of producing electricity on an industrial scale today is the rotation of a generator turbine by a powerful stream of hot steam from boiling water due to forced heating. If you think about it, then in the coal-fired thermal power plant and in modern NPP the essence of work is boiling water with the only difference that coal is burned for this in TPPs, and TVELs are heated in the reactor at the NPP.

But why heat the water, if in some places it comes from the ground is already hot? Can you use it directly? You can: in 1904, Italian Piero Ginori Conti launched the first generator, working from a pair of natural geothermal sources, abundantly present in Italy. Thus, the world's first geothermal power plant appeared, which is still in operation.
')
However, to ensure geothermal power plants acceptable efficiency and cost, you need water of a certain temperature, which is not deeper than a certain level. If you want to build a geothermal power station (say, at your dacha plot), you will first have to drill wells to aquifers, where water under enormous pressure warms up to 150–200 ° C and is ready to come to the surface as superheated boiling water or steam. And then, like fossil fuel power plants, the incoming steam will rotate the turbine, which will drive the generator, which produces electricity. To use the planet’s natural heat to produce steam is geothermal energy. Now for the details.

A little about the warmth of the Earth

The surface temperature of the solid core of the Earth at a depth of about 5,100 km is approximately 6,000 ° C. When approaching the crust, the temperature gradually decreases.


An understandable graph of rock temperature changes as you move to the center of the Earth. Source: Wikimedia / Bkilli1

The so-called geothermal gradient — a change in temperature over a certain part of the earth’s formation — is on average 3 ° C for every 100 meters. That is, in the mine at a depth of 1 km there will be a thirty-degree heat — who has been in such a mine will confirm this. But depending on the region, the temperature gradient changes - for example, in the Kola ultradeep well, at a horizon of 12 km, a temperature of 220 ° C was recorded, and in some places of the planet, at tectonic faults and zones of volcanic activity, to achieve similar temperatures, it is enough to drill from several hundred meters to several kilometers, usually from 0.5 to 3 km. In the US state of Oregon, the geothermal gradient is 150 ° C per 1 km, and in South Africa it is only 6 ° C per 1 km. Hence the conclusion: you cannot build a good geothermal station anywhere (before starting work, make sure that your holiday plot is in the right place). As a rule, suitable places are those where there is a strong geological activity - earthquakes often occur and active volcanoes are present.

Types of geothermal power plants

Depending on which source of geothermal energy is available (say, in your DSC), you will choose the type of power station. Let's figure out what they are.

Hydrothermal station

The simplified scheme of a direct cycle hydrothermal power plant will be understandable even to a child: hot steam rises from the ground through the pipe, which spins the turbine of the generator, and then rushes into the atmosphere. Everything is really that simple if we are lucky to find a suitable source of steam.


Geothermal power plants direct cycle. Source: Save On Energy

If the available well does not hit steam, but steam-water mixtures with temperatures above 150 ° C, then a combined-cycle station will be required. Before the turbine, the separator will separate the steam from the water - the steam will go to the turbine, and the hot water will either be released into the well or go to the expander, where under low pressure conditions it will give off additional steam for the turbine.

If your holiday village is not lucky with hot springs - for example, if the water temperature from the ground is less than 100 ° C at an economically acceptable depth - and you really want to have a Geothermal Power Plant , then you will need to build a complex binary geothermal station whose cycle was invented in the USSR . In it, the fluid from the well is not supplied to the turbine in any form at all. Instead, in the heat exchanger, it warms the other working fluid with a lower boiling point, which, turning into steam, spins the turbine, condenses and returns to the heat exchange chamber. In the role of such working fluids can be, for example, freon, one of the types of which (fluorodichlorobromomethane) boils already at 51.9 ° C. The binary cycle can be combined with a combined one, when steam will be supplied to one turbine, and separated water will be directed to another circuit to heat up the heat carrier with a low boiling point.


Geo TPP binary cycle. Source: Save On Energy

Petrothermal station

Heated underground sources are a very rare phenomenon on a global scale, as you may have noticed, which sharply limits the potential area for introducing geothermal energy, so an alternative approach was developed: if there is no water in the hot depths of the earth's crust, it means that you need to upload it. The petrothermal principle involves pumping water into a deep well with heated rock, where the fluid turns into steam and returns to the power plant turbine.


Simplified diagram of the petrothermal power plant

It is necessary to drill at least two wells: water will be supplied to one from the surface in order to turn into steam from the heat of rocks and exit through the other well. And then the process of generating electricity will be completely analogous to the hydrothermal station.

Naturally, to connect two wells under the ground at a depth of several kilometers is unrealistic - the water between them communicates due to faults resulting from the injection of fluid under enormous pressure (hydraulic fracturing). To prevent crevices and voids from closing over time, granules, such as sand, are added to the water.

On average, one well for the petrothermal process produces a stream of steam-water mixture sufficient to generate 3-5 MW of energy. So far, such systems have not been implemented at an industrial level, but work is underway, in particular, in Japan and Australia.

Benefits of geothermal energy

It follows from the above that the use of Earth’s heat to produce electricity on an industrial scale is not cheap. But very beneficial for several reasons.

Inexhaustibility. Fossil-fired power stations - natural gas, coal, fuel oil - are highly dependent on the supply of this very fuel. Moreover, the danger lies not only in the cessation of supply due to disasters or a change in the political situation, but also in an unplanned sudden increase in prices for raw materials. In the early 1970s, a fuel crisis broke out due to political turbulence in the Middle East, which led to a fourfold increase in oil prices. The crisis has given new impetus to the development of electric transport and alternative types of energy. One of the advantages of the use of terrestrial heat is its practical inexhaustibility (as a result of human actions, at least). The annual heat flux of the Earth to the surface is about 400,000 TWh per year, which is 17 times more than all the power plants of the planet produce over the same period. The core temperature of the Earth is 6000 ° C, and the cooling rate is estimated at 300-500 ° C for 1 billion years. You should not worry about the fact that humankind is able to accelerate this process by drilling wells and pumping water there - the core temperature drops by 1 degree releases 2 · 1020 kWh of energy, which is millions of times more than the annual electricity consumption of all humanity.

Stability. Wind and solar power plants are extremely sensitive to weather and time of day. No sunlight - no production, the station gives a stock of batteries. The wind has weakened - again there is no development, again batteries come in with not infinite capacity. At observance of technical processes on the return return of water to the well, the hydrothermal power station will continuously function 24/7.

Compactness and convenience for difficult areas. Power supply to remote areas with isolated infrastructure is not an easy task. It is complicated even more if the area has poor transport accessibility, and the terrain is not suitable for the construction of traditional power plants. One of the important advantages of geothermal power plants has become their compactness: since the coolant is literally taken from the ground, a turbine hall and a generator room and a cooling tower are built on the surface, which together take up very little space.

A geothermal station with an output of 1 GWh / year will occupy an area of ​​400 m2 - even in the mountainous terrain of a geothermal power station, a very small area and a road will be required. For the solar station with the same output, 3240 m2 will be required, for the wind station - 1340 m2.

Environmental friendliness. By itself, the operation of a geothermal station is almost harmless: its carbon dioxide emission into the atmosphere is estimated at 45 kg CO2 per 1 kWh of generated energy. For comparison: the same kilowatt-hour for coal stations accounts for 1000 kg of CO2, for oil stations - 840 kg, gas - 469 kg. However, nuclear power plants account for only 16 kg — something of which, and they produce a minimum of carbon dioxide.

The possibility of parallel mining. Surprisingly, but a fact: in some units of the geothermal power station, in addition to electricity, gases and metals are dissolved, dissolved in the steam-water mixture coming from the ground. They could simply be put together with the spent condensed steam back into the well, but considering what amounts of useful elements pass through the geothermal power station, it is wiser to adjust their production. In some areas of Italy, the steam from wells contains 150-700 mg of boric acid per kilogram of steam. One of the local hydrothermal power plants at 4 MW consumes 20 kg of steam per second, so the production of boric acid there is put on an industrial basis.

The disadvantages of geothermal energy

Working fluid is dangerous. As noted above, geothermal power plants do not produce additional toxic emissions, only a small amount of carbon dioxide, an order of magnitude smaller than that of gas-fueled thermal power plants. Which, however, does not mean that groundwater and steam are always pure substances, akin to mineral drinking water. The steam-water mixture from the depths of the earth is saturated with gases and heavy metals, which are characteristic of a particular part of the earth's crust: lead, cadmium, arsenic, zinc, sulfur, boron, ammonia, phenol, and so on. In some cases, such an impressive cocktail flows through the pipes to the geothermal power plant, and its discharge into the atmosphere or water bodies will immediately cause a local ecological catastrophe.


The result of the impact of geothermal water on metals.

If all safety requirements are met, the vapor sent to the atmosphere is carefully filtered from metals and gases, and the condensate is pumped back into the well. But in case of emergency situations or deliberate violation of technical regulations, the geothermal station may cause some damage to the environment.

High cost per kilowatt. Despite the relative simplicity of the construction of the geothermal power station, the initial investment in their construction is considerable. A lot of money is spent on exploration and analysis, as a result of which the cost of geothermal stations fluctuates at the level of $ 2,800 / kW of installed capacity. For comparison: thermal power plants - $ 1000 / kW, wind turbines - $ 1600 / kW, solar power station - $ 1800-2000 / kW, nuclear power plants - about $ 6000 / kW. Moreover, for the geothermal power plant, the average cost is given, which can vary greatly depending on the country, topography, chemical composition of steam and drilling depth.

Relatively low power. Geothermal power plants, in principle, cannot yet compare in terms of power generation with hydro power plants, nuclear power plants and thermal power plants. Even when drilling a large number of wells, the steam flow will still be small, and the electricity produced will be enough only for small settlements.

The most powerful geothermal complex for the year 2019, The Geysers, is spread over an area of ​​78 km2 in California, USA. It consists of 22 hydrothermal stations and 350 wells with a total installed capacity of 1,517 MW (actual output of 955 MW), which cover up to 60% of the energy needs of the north coast of the state. The power of all The Geysers is comparable to the Soviet RBMK-1500 reactor, which once worked at the Ignalina NPP, where there were two, and the NPP itself was located on an area of ​​0.75 km2. Geothermal power plants with a production of 200-300 MW are considered very powerful, most of the stations around the world operate on double-digit numbers.


Hydrothermal Combined Station of The Geysers Complex in California. And there are 22. Source: Wikimedia / Stepheng3

Where does it all work and how promising is it?

As of 2018, geothermal power plants worldwide generate more than 14.3 GW of energy, while in 2007 they produced only 9.7 GW. Yes, not a geothermal revolution, but growth is evident.

The leader in geothermal development is the United States with its 3,591 MW. An impressive value, which, however, is only 0.3% of the total output of the country. Next comes Indonesia from 1948 MW and 3.7%. But in the third place, the interesting begins: in the Philippines, geothermal power plants have an installed capacity of 1,868 MW, while they account for 27% of the country's electricity. And in Kenya, 51% altogether! Japan is also among the ten leaders in terms of the number of kilowatts produced by the geothermal power plant.

The first geothermal power station, the Matsukawa, opened in Japan in 1966. She produced 23.5 MW, and Toshiba produced the turbine and the generator for it. In the 2010s, geothermal energy became most in demand in African countries, where active contracting and construction of a geothermal power plant began. In 2015, the Olkaria IV station was opened in Kenya, one of the four located in the Olkaria zone 120 km from Nairobi, with a capacity of 140 MW. With its help, the government reduces dependence on hydroelectric power plants, the discharge of water from which often leads to devastating floods.


Geothermal Power Plant Olkaria IV in Kenya. Olkaria V and Olkaria VI plan to be commissioned in 2021. Source: Toshiba

Geothermal power plants are also actively being built in Uganda, Tanzania, Ethiopia and Djibouti.

In Russia, the development of geothermal energy is proceeding at a very slow pace, as there is no particular need for the construction of additional power plants. In 2015, the share of such stations accounted for only 82 MW.

Pauzhetskaya geothermal station, built on Kamchatka in 1966, was the first in the USSR. Its initial installed capacity was only 5 MW, now it has reached 12 MW. After it appeared Paratunskaya station with a capacity of only 600 kW - the first binary geothermal power station in the world.

Now there are only four stations in Russia, three of them feed Kamchatka, another one, Mendeleevskaya Geothermal Power Plant of 3.6 MW, supplies the island of Kunashir with the Kuril chain.

On our planet there are many ways to extract electricity without the help of fossil fuels. Some of them, for example, solar and wind energy, are successfully used now. Some, like hydrogen fuel cells, are still at the initial stage of adaptation. Geothermal energy is our foundation for the future, to unleash the full potential of which we are yet to come.