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In this chapter:

  • Geothermal energy is heat energy from inside the Earth
  • Geothermal energy is often carried by water that arises to the Earth`s surface
  • Water can be heated by pumping it through hot dry rocks under the earth
  • Geothermal energy allows less burning of fossil fuels, but it could cause other environmental problems

What is geothermal energy?

Geothermal energy is heat energy from inside the Earth. Geo means earth and thermal refers to heat. The inside of the Earth is still extremely hot from when the planet was formed billions of years ago.

Scientists speak of four main layers of the Earth, including its inner core. The inner core, made of solid metal, is extremely hot. Around it is a layer called the outer core, which is made up of liquid metal - also extremely hot. Around that is a deep layer called the mantle, which is made of slow-moving molten rock. The thinnest and coolest layer is called the crust. On the outside of the crust are the oceans, the continents and all living things.

In places where the crust is thin, some of the heat from the lower layers breaks through to the surface of the planet. The most dramatic example of this is a volcanic eruption, where the hot molten rock shoots up and spills out onto the land or sea around it.

Sometimes water or steam shoots out of the Earth from hot underground reservoirs. These are formed over many years. Water from the surface of the Earth seeps down through cracks and is absorbed into porous layers of rock deep under ground. The water builds up and is heated by the rocks. When the water becomes very hot, it shoots upwards again, sometimes many metres into the air. The places where these jets of hot water or steam occur are called geysers. In other places there may be less pressure, so the water comes to the surface more gently as hot springs or seepages.

There are quite a few places around the world where geysers and hot springs are found. There are famous ones in the United States of America, Iceland, New Zealand and Japan. A town in England is called Bath because the ancient Romans built a public bath there over its natural hot spring, which is still running today.

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How is geothermal energy used?

Geothermal energy is used in various ways. Sometimes the hot water that comes to the surface is pumped directly through pipes to heat homes, fish farms and greenhouses. It is also piped under some roads and pavements in cold climates to prevent them from icing over. In some places people even swim in the ponds and lakes formed by the warm water. Much of the water that is used is pumped back into the ground to refill the reservoirs.

In some places, power stations are built which convert this natural heat energy into electrical energy. If the water or steam that comes from the ground is over 175 degrees, it is hot enough for it steam to turn turbines directly. In some places where the water is over 100 degrees but less than 175 degrees, it is used differently. In this case the water is used to heat another liquid which boils at a lower temperature than water does. The second liquid then produces steam which is used to turn the turbines and therefore generate electricity.

The countries mentioned above, particularly Iceland, use geothermal energy a lot. They are able to do so because there are easily accessible sources of that energy occurring naturally. One problem with geothermal energy is that it is much more difficult to use in places that are far from hot springs or geysers.

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Even though geothermal energy does not always appear on the surface, it does exist deep under ground beneath all parts of the Earth. Scientists have started to experiment with accessing this heat energy in other places. These methods are called hot dry rock schemes. Water is pumped deep into the ground to where the rocks are hot. The rocks heat the water which is then pumped out of the ground again and used to generate electricity. Sometimes small explosives are used under the ground to create more gaps for the water to flow through.
 
To get the water down to the hot rocks and back to the surface, very deep wells need to be drilled - sometimes up to five kilometres into the Earth. For now, the technology required to do this is not very well developed.

Benefits and drawbacks of geothermal energy

Geothermal energy does not require the use of fossil fuels, which means it does not create the same kind of pollution released by these fuels. Some people say that it is not a renewable form of energy because once the heat inside the Earth is used up it will not be able to be replaced. There is, however, such a huge amount of heat energy in the Earth that others consider it to be renewable because they believe human beings will never run out of it. Some people believe this source could provide enormous amounts of energy.

Because geothermal energy is already in the form of heat, it allows people to use less electricity in their homes because they do not need electric heaters or hot water systems. Geothermal power stations use up less space than other types of stations and some consider them to be less ugly.

Aas mentioned above, however, there are problems with accessing geothermal energy in many parts of the world. Drilling into the deep rock is very expensive. Where water comes up naturally from the Earth, it can sometimes carry dangerous minerals such as arsenic or mercury. It could be difficult to prevent these minerals from causing harm to the air or to surface water systems. There are also worries that if underground reservoirs dry up from overuse, the Earth around them could shrink and sink in.

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Some people do not believe that geothermal energy could provide enough power to meet the Earth's needs. Others believe it is not renewable and that geysers could stop operating naturally.

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Question 1/5

1. What do black surfaces do to heat?

They reflect the heat

They have no effect on heat

They reduce heat

They absorb heat

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