background information2
geothermal ENERGY

What is a geothermal power station?

Geothermal energy is the name given to the naturally occurring heat energy that is available below the Earth’s surface. (This name comes from geo, meaning earth, and thermal, meaning heat.) The total amount of available heat energy below our feet is enormous! The story behind all this heat energy is told later.

Geothermal power stations are power stations that generate electricity using accessible geothermal energy – that is, heat energy that can be extracted from underground and used by humans.

The first geothermal power stations tapped into natural sources of hot steam and hot water, such as the underground sources of geysers. Geysers are a kind of natural spring on the surface of the Earth, from which boiling water and steam gush high into the air. An example is shown in Figure 1.

Figure 1 A natural geyser in Iceland. The word geyser comes from the Icelandic word geysa, meaning to gush.

Source: http://upload.wikimedia.org/wilipedia/commons/8/8e/GeysirEruptionNear/jpg Accessed: 6 August, 2010

The very first geothermal plant was built in the 1920s at Lardello in Italy. Its turbines were turned by steam that came from a natural geothermal reservoir (an underground source of heat energy). This reservoir consisted entirely of steam. Most natural geothermal reservoirs around the world contain either a mixture of steam and water, or just water.

You can learn more about how electricity is generated, and the role of turbines and generators in this process, in the STELR Student Resources.

Figure 2 shows an example of a geothermal power station.

Figure 2 A geothermal power station at Innamincka in central Australia.
Image courtesy of Geodynamics

The Geodynamics power station at Innamincka is an example of an Engance Geothermal System or EGS. It uses the heat from hot rocks, generally granite bodies, located between 3 and 5 km below the surface for power generation.

Take a virtual tour of this power station by clicking on this link http://vimeo.com/69616371

The heat energy of the Earth

Have you ever gone down into a mine deep under the Earth’s surface, or seen films of miners working in them? One thing is clear – the deeper you go down below the surface, the hotter it gets. Fresh cooled air must be pumped all the time down into deep mines, or it would be too hot for the miners to work. They would soon collapse from heat exhaustion.

Why does it get hotter as you go further down?

Scientists have worked out the Earth’s structure by piecing together a huge amount of evidence. They concluded that the Earth is built in layers. The main layers are shown schematically in Figure 3.

Figure 3 The structure of the Earth. CREDIT: The Smithsonian Institute

The Earth’s crust is not the same thickness everywhere, as implied by the diagram in Figure 3. Its thickness varies from about 5 km to 70 km. Moreover, it is ‘broken’ into several giant-sized sections called tectonic plates. The edges of the plates, known as plate boundaries, are very important regions.

The temperature of the mantle, the layer beneath the crust, ranges from about 1000 ⁰C at the top of the layer to about 4000 ⁰C in the deepest part of the lower mantle. As a result of its high temperature, the rocky material in the mantle is in what is described as a semi-molten state, which means it flows, though only very slowly. Convection currents slowly move material around within the layer. The tectonic plates virtually float on top of the mantle and move about due to these convection currents. However, the plates move extremely slowly – just a few centimetres per year.

One piece of evidence that there are molten rocks below the Earth’s surface is the material ejected when volcanoes erupt. This molten material, which is called magma, is ejected out of reservoirs known as magma chambers.

Most volcanoes occur at plate boundaries. So do most earthquakes. (It was the evidence of all this geological activity concentrated in certain regions that made geologists realise that the crust is broken into plates.)

Figure 4  A volcano that erupted in Indonesia in 1994, at Mount Rinjani.
Source: http://upload.wikimedia.org/wikipedia/commons/f/f2/Rinjani_1994.jpg   Accessed: 4 August, 2010

Find out more!
Visit the website http://www.volcanolive.com/john/ to follow the adventures of the Australian scientist, volcano adventurer and documentary film maker, John Seach, as he travels to different volcanoes across the world.

Visit http://news.bbc.co.uk/2/hi/science/nature/8284372.stm to find out about the ‘Ring of Fire’ in the Pacific. Notice how this links to the plate boundaries in this region. Why is it called the ‘Ring of Fire’? What fraction of the world’s active volcanoes that occur above sea level is located in this ring? How is the speed at which tectonic plates move related to your fingernails?

What is the main source of heat from within the Earth?

The temperature of the core ranges from about 4000 ⁰C in the outer core to about 5000 ⁰C in the inner core. There are a number of reasons for these hotter temperatures deep within the Earth. One is that nuclear reactions are occurring within the core. These reactions produce a lot of heat energy. If it was not for these nuclear reactions, it is believed that the Earth would have cooled into a solid ball long ago.

Did you know?

In chemical reactions, the atoms present at the start are simply rearranged. Their nucleus (core) is not changed. Ever since the Earth was formed, about 4.5 billion years ago, chemical reactions have occurred within all the layers of the Earth, as well as in the atmosphere and in the oceans when they formed. All the minerals present in rocks, for example, were produced in chemical reactions. (See the giant mineral crystals in Figures 6 and 7!)

In nuclear reactions, the nucleus (core’) of each of the atoms involved is changed in some way. This means that the elements present at the start can be changed into new elements.

Some nuclear reactions can produce vastly more energy than can be produced in chemical reactions. One example of this is the nuclear reactions that occur on the Sun, which are the source of all the energy it radiates out into Space.

From piecing together a huge amount of evidence, scientists have concluded that all the atoms of the different elements present on Earth were produced in nuclear reactions in other stars much larger than the Sun, billions of years ago.

The Earth’s natural temperature gradient

The increase of temperature the further you go down below the Earth’s surface is termed the Earth’s natural temperature gradient, or the geothermal gradient.

The Earth’s natural temperature gradient is, on average, about 3 ⁰C per 100 metres down. Table 1 shows how this works.

Table 1  The Earth’s natural temperature gradient

Depth below the surface 

Approximate temperature

1-5 m

16 ⁰C

100 m

19 ⁰C

200 m

22 ⁰C

300 m

25 ⁰C

400 m

28 ⁰C

 . . . 

. . . 

1000 m (1 km)

46 ⁰C

 . . . 

 . . .


This means that down in some of our deeper underground mines, the temperature at the lowest levels might be around 50 ⁰C or more. No wonder cool fresh air must be constantly pumped into these mines!

Is the temperature gradient the same everywhere on Earth? 

In fact, the answer to this question is – no.  The temperature gradient varies considerably from place to place and also can vary with time and season and depth in a given location. In some places in New Zealand, for example, the temperature gradient is so high that the temperature is already greater than 200 ⁰C at a depth of less than 400 m!

What are the best places for geothermal power stations?

Geothermal power stations are best located where you don’t have to drill down very far to reach very hot rocks and reservoirs of hot water and/or steam.

That means they need to be located where the temperature gradient is as high as possible. This includes places where:
  • The Earth’s crust is thinner.
  • Active volcanoes are close by.
  • There is a magma chamber close to the surface. 
  • There are radioactive minerals, such as uranium ore, present in the rocks, which give out heat energy. (Radioactive ‘decay’ is one kind of nuclear reaction.) 
Regions that are suitable for developing geothermal power stations are called geothermal fields.

Most natural geothermal fields are located at the plate boundaries, because this is where volcanoes are concentrated and hot magma is close to the surface. New Zealand, for example, is located on a plate boundary and has a lot of natural hot springs and geysers. It has several geothermal power stations, including one at Waireki. This was the second geothermal power station ever built.

Figure 5 shows regions where geothermal fields are located. Notice that the North Island of New Zealand and Italy (where the world’s first geothermal power station was built) are in these zones. However, Australia is not on a plate boundary and is not within these zones.

Figure 5  A world map showing the major tectonic plates and location of the world’s geothermal provinces (circled in red).

Source:  http://www.geothermal.org/GeoEnergy.pdf   Accessed: 12 August, 2010

Iceland – a very special case
Hot springs and geysers occur in Iceland, even though it is located close to the North Pole, for two main reasons:
  1. It sits above a plate boundary where the plates are moving further apart. (They are pulled apart by enormous forces.) At such boundaries hot magma forces its way up into the gap being created.
  2. It has a ‘hot spot’, below which there is a ‘pipe’ of magma that comes from very deep within the Earth (close to the core). 
Hot spots are very rare. Iceland is the only country in the world where both features are present.

In Figure 5, Iceland is enclosed within the red circle to the above left of England, on the boundary between the Eurasian plate and the North American plate. Iceland’s volcano, which recently erupted in spectacular fashion, also formed as result of the enormous upwelling of magma in this location, as did Iceland itself!

Did you know?

The largest natural crystals that have ever been found are located in a cavern in Mexico about 300 m under below the Earth’s surface. Some crystals are more than 11 m long! They formed in this very stable, sealed environment from hot, mineral-rich water which circulated into the cave from chambers below.

The temperature in the cave is a steady 58 ⁰C, which is much hotter than is usually observed at this depth (see Table 1). Clearly the temperature gradient here is much higher than normal. Because of the very hot temperature, those who visit the cave must wear protective gear, including insulated boots and breathing apparatus, as shown in the photographs.

Figure 6 The cave of crystals, Mexico  Source: http://www.greenpacks.org/wp-content/uploads/2008/10/the-cave-of-crystals-1.jpg Accessed: 20 July, 2010

Figure 7 Protection from the heat source: http://www.greenpacks.org/wp-content/uploads/2008/10/the-cave-of-crystals-2.jpg
Accessed: 20 July, 2010

The deepest mine in the world at present is the TauTona gold mine in South Africa. (TauTona means ‘great lion’ in the Setswana language.) It is now 3.9 km deep, has 800 km of tunnels and employs almost 6000 miners. At the lowest level, the rock face has a natural temperature of 60 ⁰C. This is only 2 ⁰C higher than the temperature in the cave of crystals, although it is over 3 km deeper. This shows how the temperature gradient varies across the world.

Air conditioning is used to cool the air in this gold mine to 28 ⁰C. But, as you might imagine, because of this problem, apart from any other risks, there is a limit to how deep mines can go.

‘Tapping’ into heat energy stored deep underground

Most geothermal power stations that are currently operating use heat energy that is stored in one of the following:
  • Reservoirs of hot water and/or steam, known as hydrothermal reservoirs, located just a few hundred metres below the surface; 
  • Hot rocks that are above a magma chamber, which may be thousands of metres below the surface;
  • Hot rocks that are close to an active volcano. 

How is heat energy accessed?

The stored heat energy is accessed by using a carrier to bring it up to the surface. The carrier used presently is water. This works in one of two ways:
  • Drilling wells down into deep reservoirs of hot water. The hot water then naturally rises to the surface. (See Figure 8.)  or 
  • Pumping cold water through a well that reaches down into hot rocks, which heat the water. The hot water is then pumped back up to the surface through separate wells. (See Figure 9.) 
These are discussed in more detail next.

Figure 8  Generating electricity in a geothermal power station using hot water from a hot water reservoir. The hot water rises up a well and its heat energy is used. The now cool water is returned to the reservoir via another well, to be re-heated.
Source: http://hotrockenergy/images/hsa-img.jpg  Accessed: 20 July, 2010

Figure 9  Generating electricity in a geothermal power station using water heated by hot rocks. Cold water is pumped down to hot rocks, which heat the water. The hot water then rises up separate wells and is used. The cooled water is returned to the rocks, to be heated again.
Source: http://www.rise.org.au/info/Res/geothermal/image007.jpg   Accessed 2- July 2010.

Note: In Figure 9, ‘low thermal conductivity sediments’ are rocks that are good heat insulators. That is, they act rather like a blanket, holding the heat in the layer of hard granite. Otherwise the granite would be much cooler.

Power generation from hot sedimentary aquifers

What is a hot sedimentary aquifer? 

An aquifer is a natural underground reservoir of water. It has formed as water has seeped down through the ground over time, through natural spaces.

Sedimentary rock is rock that has been formed when clays, stones and sands from weathered (worn-down) older rocks have been carried along by water and deposited in various places over time. Over a period of millions of years, in these places these materials have turned into rock due to the weight of the other sediments pressing down on top of them.

One type of sedimentary rock is mostly made from sand and is called sandstone. Sandstone is quite porous – that is, water can seep through it and collect in it.

When water is trapped in porous sedimentary rock and sealed in by less porous rock, it forms a reservoir of water. The layer of non-porous rock above it is called a cap rock. When the trapped water is deep below the surface where the surrounding rock is hot, the water becomes just as hot. This reservoir is known as a hot sedimentary aquifer, or HSA.

How is the hot water used to generate electricity? 

Wells are drilled down into the reservoir of hot water and/or steam, as shown in Figure 8. With an ‘escape route’ now present, the very hot water or steam pushes its way up the well.

There are two main ways in which the hot water or steam can be used at a geothermal plant. Some plants use the first way. The rest use the second way.
  1. Steam from the reservoir, or steam formed when the hot water from the reservoir is under less pressure at the surface, is used directly to turn the turbines and generate electricity. The steam then is condensed back into liquid water. 
  2. The heat energy of the hot water is used to boil an oily fluid that has a low boiling point. The hot gas produced is used to turn turbines and generate electricity. The cooled gas is then condensed back into its liquid state and recycled.
The water from the hydrothermal reservoir, which is now much cooler than when it rose to the surface, is piped back into the reservoir, well away from the well used to obtain the hot water, as shown in Figure 8.  (Why do you suppose this is done?) It then is re-heated by the surrounding hot rock.

This forms what is called a closed loop and ensures that a continual supply of hot water is available for a number of years.

Power generation from hot fractured rocks

What are hot fractured rocks? 

Hot fractured rocks are usually created from granite rocks that are about 4 km to 5 km below the surface. Rocks at this depth are only suitable if they are much hotter than the average temperature at this depth. They need to be at a temperature of at least 200 ⁰C.

Water can only seep through granite at an extraordinarily slow rate, so there is no natural reservoir of water in the granite. To use this resource a ‘reservoir’ must be created – that is, a region through which the water can seep and collect. This is achieved by drilling into the hot rock and pumping cold water under very high pressure into it. This causes the rock to fracture (crack), just like hot glass cracks if you poured cold water over it. The rock is now called hot fractured rock (HFR).

How is hot fractured rock used to generate electricity? 

Cold water is pumped under pressure into the hot fractured rock, through what is known as an injection well. The water seeps through the cracks and is heated by the hot rock. The now hot water, which is under high pressure, is then returned to the surface through other pipes, known as production wells. There it is used to generate electricity. The cooled water is then returned underground, where it is re-heated. This process is shown in Figure 9 and in the video listed next.

Watch this!
For an animation that shows in simple terms how geothermal power stations that use hot fractured rocks work, visit: http://www1.eere.energy.gov/geothermal/gpp_animation.html
Click on each arrow to see how each stage works.

What are the advantages of geothermal power stations?

Geothermal power stations have many advantages, which include the following:
  1. They are classified as a renewable energy resource because there is virtually no limit to the amount of heat energy that flows towards the Earth’s surface from the interior of the Earth. As a result, heat energy will continue to flow naturally into the hot water reservoirs or hot rocks from their surroundings, for thousands of years to come. 
  2. They are a ‘clean’ energy resource, because research shows that emissions of carbon dioxide and sulfur dioxide from geothermal power stations are less than 10 % of those emitted by coal-fired power stations. (In some geothermal plants, emissions are essentially zero.) 
  3. Geothermal power stations are not tall structures like coal-fired power stations. Being ‘low-rise’, they can be blended into the surroundings and designed so they are not an ‘eyesore’.  In Japan, they are built into a forest and designed to look like a spa complex. 
  4. Some of the heat energy can be used for heating as well. This heat energy can be used for very different purposes, from growing foods in hothouses to carrying out manufacturing processes. In Iceland, for example, the heat energy is not only used to heat buildings but also to keep roads free from ice. In most regions of the world where the problem of roads being covered in snow and ice occurs, salt is tipped over the roads to soften the ice, and then heavy machinery is used to push the slush away. However, salt kills most plants and causes metal structures to corrode far more rapidly. Moreover, the use of heavy vehicles only adds more greenhouse gases and other pollutants to the environment. This is a much less expensive and more environmentally friendly solution to the problem!  
  5. The energy source for geothermal power stations is steady, which means that the plants are able to produce the same amount of electrical power 24 hours a day, 7 days a week. This means they can be relied upon to steadily supply basic daily energy requirements (known as the base load).This differs from other renewable sources of energy. For example, the power solar panels deliver depends on how much sunlight energy is available and the power wind turbines deliver depends on how strong the wind is. 
  6. The total amount of heat energy stored in the rock beneath us is huge! It is believed that, with technological advances, geothermal energy resources have the capacity to provide 50 000 times more energy than can be obtained from crude oil and natural gas reserves. 

What are some disadvantages of geothermal power stations?

Some of the disadvantages of geothermal power stations include the following.

  1. 1 The biggest disadvantage probably is that the costs of exploration and developing a geothermal power station are quite large. Drilling exploration wells in particular is a very risky business - there is no guarantee that a suitable resource will be found. It is quite common to drill wells that cannot be used for injection or production. Yet drilling a geothermal well can cost up to $US 10 million or even more!
  2. 2 Another significant problem is that the fracturing of hot rocks and injection of geothermal fluids causes small earthquakes to occur. Some of these can be felt on the surface. However, the vast majority of them cannot. In fact, they are a very useful source of data. By measuring the tremors, geophysicists can keep track of the movement of geothermal fluids through the reservoirs.
    There is only one case in which a geothermal power station has had to be shut down because of this problem, and that was at Basel in Switzerland. Basel is on a large active fault line. This city was wiped out by an earthquake that occurred naturally long ago, and then rebuilt. The company involved made a very big mistake – to fracture the hot rock, they injected water into the fault! This caused a big earthquake.
  3. The energy efficiency of geothermal power stations depends largely on how hot the fluid is and how fast it can be flowed through the power plant. If the fluid is not very hot, such as only 200 ⁰C, the efficiency is quite low. 
  4. Companies that drill wells down to a depth of 5 km often encounter problems such as broken drill tips and corroded pipes. These problems slow down the drilling projects and increase costs.
  5. When wells are drilled, sometimes hazardous minerals and gases can come up, which can be difficult to dispose of. 

Establishing a new geothermal power station

No matter what kind of power station a company is trying to establish, there are many challenges.

First, there are processes that must be followed. Community consultation is essential. Local residents and businesses need to be informed of the plans and of the basic science and arguments for establishing the project in their region. Their points of view should be taken into consideration and suitable compromises reached where there are differing points of view on aspects such as protecting the local environment, the landscape and people’s health and well-being.

Moreover, in Australia, any local Aboriginal or Torres Strait Islander groups need to be consulted to ensure that their sacred sites are fully respected and protected.

This process takes time, which of course adds to the cost of the project. It also requires great sensitivity and excellent communication skills.

However, it should be said that often communities, or at least the majority of the members of the communities, welcome these developments because they bring employment and other advantages to the region.

Moreover, one of the greatest challenges for all new developments is obtaining grants and other investments. These are needed to cover the substantial amount of money required to carry out all the initial research and exploration and then establish the energy resource.

Geothermal power stations in Australia

Australia is not located right on top of a plate boundary, unlike other countries that have developed geothermal power stations. (See Figure 5.)

Despite this, we are very fortunate. Australia is not located on top of any major fault lines either, and is therefore geologically stable. That is, although we have many small earthquakes, there is little likelihood of a major earthquake that might destroy a power station. And out of all the geologically stable regions in the world, we have the hottest rocks closest to the surface. This is shown in Figure 10. The main source of this heat energy is the radioactive decay occurring in granite rocks. We have extensive deposits of uranium ore and other radioactive minerals.

Figure 10 This is a map of Australia that is coloured to show the temperature of the crust at a depth of 5 km.
Source:  http://www.agea.org.au/geothermal-energy-facts/australian-projects-overview/     Accessed: 29 June 2010

Because of these natural geothermal energy resources and our geological stability, an increasing number of geothermal power station projects are under development in Australia. Most are using hot fractured rocks technology. Needless to say, these are located in the red regions of the map where the rocks are hottest. Several are in South Australia.

Despite this, currently there is only one fully operational geothermal station in Australia. This is a small 80 kW plant in Birdsville in south-west Queensland.

Hot rocks under coal - what an idea!

Scientists at the Melbourne Energy Institute (MEI) are investigating the possibility of using hot rocks under Victoria’s vast brown coal deposits in the La Trobe Valley. These are the largest deposits of brown coal in the world. The rocks beneath giant coal beds are hotter than normal because of the insulating properties of the coal.

Figure 11 The brown coal mine at the Loy Yang Power Station. These coal dredges are the size of a building that is several storeys high! The largest open-cut coal mine in the world is nearby. And these are both dwarfed by the brown coal beds that have not yet been tapped into!   CREDIT: Loy Yang Power Station

This is not the only fascinating idea these scientists are exploring. They also are thinking of using a different carrier to bring the heat energy up to the surface, because there is not a lot of water available in the region, and what little there is would be expensive and could corrode the pipes. One possible carrier is liquid carbon dioxide, from carbon dioxide ‘captured’ from the coal-fired power stations in the region. They also are investigating how they can improve its ability to ‘carry’ the heat energy using certain kinds of nanoparticles (particles about the size of molecules). Research on this new technology has already been carried out in places like the United States.

However, this project is in its early stages. The rocks under the coal beds still must be mapped by various imaging techniques, and test wells must be drilled down 5 km into the rock and they need to measure its temperature, ability to let water seep through and other properties. But it is anticipated the results will be similar to those obtained by energy companies in South Australia.

One advantage of this location is that Victoria already has an electricity grid in place in this region, which transmits the electricity across the state from all the coal-fired power stations located there. The project leaders hope that by 2050, most of Victoria’s power will be supplied by geothermal power stations.

Geothermal power stations in the world today

Geothermal energy resources have the potential to meet the world’s huge energy demands. There has been a rapid increase in interest in this energy resource across the world, especially as it can help to significantly reduce carbon dioxide emissions and it is renewable.

So far the US, Philippines, New Zealand and Iceland generate the most electricity using geothermal power stations.

Find out more!

See the image of the Earth from NASA, which shows the location of fully developed geothermal power stations across the world as small ‘beacons’ of light, at: http://geothermal.marin.org/geopresentation/sld069.htm. Click on the forward and backwards arrows to learn more about geothermal power stations.

Also see the map at http://thinkgeoenergy.com/map.This map is being continually updated. Click on each spot to find out more about that power station.

What a career!

Go to the careers page on the STELR website to see the career profile of Australian geophysicist Steve Sewell, who now works in New Zealand at a geothermal power station.

Acknowledgement: We thank Steve for contributing valuable advice and information for this article, as well as information about his career.