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# Geothermal Power

Back in the Roman era, hot springs were viewed as sacred sites of healing power. Pliny the Elder, a well-known Roman author, wrote about the benefits of hot mineral baths for musculoskeletal ailments. It wasn't long before these springs were harnessed to create heating systems, warming public baths and houses.

How were these hot springs maintained? Well, the answer is by geothermal energy. The term comes from the Greek words “geo”, meaning Earth, and “thermos”, meaning heat. Put the two together, and what does it mean?

## Renewable Energy: Geothermal Power

One method of generating renewable energy comes from harnessing geothermal energy. What is it?

Geothermal energy is heat that comes from within the Earth.

Geothermal energy comes from radioactive decay of rocks inside the Earth's core. Scientists have estimated that Earth's core is approximately 5500ºC – equivalent to the surface of the Sun. As long as radioactive decay is taking place in the core, geothermal energy is renewable and will never run out.

Some elements have unstable isotopes.

Isotopes are forms of an element with the same number of protons but a different number of neutrons.

Unstable isotopes decay to form new stable isotopes of the same element, or a different element with a lower atomic mass. When this happens, the atom expels subatomic particles and electromagnetic radiation in the form of high-energy gamma rays.

### Tectonic Plate Boundaries

Energy travels from the core into the mantle, the semi-molten rock layer underneath the crust. Heating the mantle drives convection currents (circular movements in the mantle). Above the mantle, the Earth's crust is split into pieces known as tectonic plates. Convection currents drag the plates in different directions.

Due to convection currents, tectonic plates are constantly moving (very slowly), often colliding or getting stuck at the boundaries between plates. When this happens, pressure builds up until it is released in the form of an earthquake or volcanic eruption.

Volcanoes occur at two types of plate margin (boundary).

• Destructive plate margin: a denser plate subducts (sinks) below another plate and melts

• Constructive plate margin: two plates are being pulled apart and magma rises to plug the gap

Figure 1 – Due to its position on a constructive plate margin, Iceland is home to 130 volcanoes. Unsplash

In volcanic areas, magma rises to the surface, transferring thermal energy to underground rocks and water.

### Geothermal Power: Definition

So, what's the definition of geothermal power?

Geothermal power is electricity or heat generated from geothermal energy.

Over 72 countries have utilised geothermal energy in some form, and even more have access to geothermal resources.

## Geothermal Power Plants

Most geothermal power plants are found in volcanic areas, where geothermal energy is most accessible.

Geothermal power stations are found in locations such as Iceland, Italy, Hawaii, and California.

Volcanic activity warms subterranean rocks, which heat underground water. It rises naturally to the surface as hot water and steam. At the surface, the steam can be used to drive turbines that power electricity generators.

In some places, volcanic activity heats rocks, but no hot water or steam rises to the surface. To harness the geothermal energy, deep wells are drilled down to the hot rocks and cold water is pumped down. The water runs through the rocks, absorbing thermal energy. It returns to the surface as steam, where it's used to drive turbines and generators.

## Geothermal Power: Efficiency

Geothermal power plants have a lower conversion efficiency than other types of thermal power plant (such as fossil fuel and nuclear power stations), averaging at 12%.

Conversion efficiency is the useful energy output divided by the input.

However, geothermal energy has a better availability compared to other types of thermal power plants. Geothermal power is available at all hours of the day, regardless of the time of year. Overall, geothermal power plants have an average availability of 90% – greater than the 75% average availability for coal power plants.

Geothermal power is on the rise. In 2019, 27 countries produced approximately 88 billion kWh (kilowatt hours) of electricity from geothermal sources. The US is leading the world in geothermal energy production; many western states are home to volcanoes. The state of California alone produces 70.5% of the country's energy.

## Geothermal Power: Examples

Compared to other energy resources, geothermal power is a relatively new industry, experiencing exciting new developments.

### Low-temperature Fluids

Low-temperature geothermal systems are a cost-effective and carbon-free method of heating homes and generating power. What is low-temperature geothermal energy?

Low-temperature geothermal energy is heat obtained from geothermal fluid in the ground at temperatures of 150ºC or below.

In contrast, traditional high-temperature geothermal energy is obtained from fluid above 150ºC. It's typically obtained in areas of high tectonic activity.

#### Low-temperature Technology

The Organic Rankine Cycle (ORC) is a technology enabling low-temperature geothermal energy. It uses geothermal brine at 93-150ºC. The brine is mixed with a working fluid that has a lower boiling point than water.

The working fluid is usually an oil-based refrigerant or a hydrocarbon.

The fluid is compressed into steam, which creates energy.

Binary geothermal plants use ORC technology in a closed loop. Brine is pulled from the earth into a chamber containing working fluid, which evaporates into steam when the brine enters the chamber. The fluid-brine steam is transported to turbines, which power an electrical generator.

#### Uses of Low-temperature Geothermal Energy

Low-temperature energy is typically used in direct, low-intensity applications, such as:

• District heating

• Spas

• Aquaculture

• Greenhouses

• Fisheries

• Snow melting (especially in Iceland)

### District Heating Systems

One use of low-temperature geothermal energy is district heating.

District heating is the use of geothermal energy to heat individual, commercial, and industrial buildings through a distribution network.

Hot geothermal fluid is transported via a single-pipe distribution line. Consumers tap into the line at their homes, where it's used for heating or domestic hot water. The domestic hot water is heated using a heat exchanger, transferring thermal energy from the geothermal fluid to water from the taps.

The city of Pompeii was heated and cooled using geothermal energy.

District heating is one of the cheapest methods of cutting carbon emissions. Furthermore, it's convenient and easy for consumers.

Reykjavík, the capital of Iceland, first implemented district heating in 1930. Now, 95% of the city's buildings are heated using geothermal district heating systems.

Figure 3 – Geothermal energy heats over 90% of Iceland's homes and generates 25% of the country's electricity. Unsplash

What are the advantages of geothermal power?

• Renewable: the Earth's core is constantly generating thermal energy through radioactive decay, so geothermal energy will not run out.

• Reliable: geothermal energy availability doesn't fluctuate depending on weather or season, unlike other renewable energy sources such as solar power and wind power.
• No Fuel Requirements: unlike other forms of energy generation, geothermal power requires no fuel – and thus, has no fuel costs.
• Clean Energy Source: harnessing geothermal power does not emit greenhouse gases or other harmful pollutants.

• Excellent Potential: although not all geothermal energy can be harnessed, it's estimated that geothermal power plants could provide up to 2 terawatts of power – 13% of the world's energy consumption.

What are the disadvantages of geothermal power?

• Location-specific: the main disadvantage of geothermal power is that not all regions can exploit geothermal energy. Far from tectonic plate boundaries or volcanic hotspots, geothermal energy can't be utilised.

• Earthquake Risks: digging deep wells and pumping water underground near plate boundaries can alter Earth's structure and run the risk of triggering earthquakes. However, most geothermal power plants are distant from densely populated areas, the effects of the earthquakes are minor.

• Expensive: geothermal energy is an expensive energy resource. Building a plant with a capacity of 1 MW (megawatt) can cost up to \$7 million.

• Management: in order to ensure that geothermal power remains a sustainable energy source, geothermal fluid must be pumped back into underground reservoirs faster than it is depleted. As a result, geothermal energy must be properly managed to maintain its sustainability.

I hope that this article has explained geothermal power for you. Geothermal power is electricity generated from geothermal energy. Volcanic hotspots heat underground rocks, which heat water until it rises to the surface as steam. Above ground, the steam is used to drive turbines and electrical generators.

## Geothermal Power - Key takeaways

• Geothermal energy is heat that comes from within the Earth. When this energy is harnessed to provide electricity and heat, it's known as geothermal power.
• Most geothermal power plants are found in volcanic areas. Hot rocks heat up water, so it rises to the surface as steam. At the surface, it's used to power turbines and generators.
• Geothermal power plants are less efficient than other forms of thermal power plants, but they have a higher availability.
• ORC technology allows us to harness geothermal energy at lower temperatures than traditional geothermal power plants. A common use of low-temperature geothermal energy is district heating – common in Iceland.
• Geothermal power is reliable and renewable. It doesn't require any fuel and doesn't emit any greenhouse gases or pollutants. However, the main disadvantage of geothermal power is that it's location-specific. Not all nations can access geothermal energy.

1. Charlotte Helston, Low-Temperature Geothermal Power, Energy British Columbia, 2012

2. Green City Times, Geothermal district heating in Iceland, 2022

3. Meg Cichon, Low-Temperature Geothermal: Digging for Its Vast Opportunity, Renewable Energy World, 2012

4. Nanna Gunnarsdóttir, Iceland's Volcanoes: The Complete Guide, Guide to Iceland, 2022

5. Office of Energy Efficiency and Renewable Energy, Geothermal Technologies Office, 2022

6. Sadiq J. Zarrouk, Efficiency of geothermal power plants: A worldwide review, Geothermics, 2014

7. U.S. Energy Information Administration, Use of Geothermal Energy, 2022

8. The Welding Institute, What are the Advantages and Disadvantages of Geothermal Energy?, 2022

Geothermal power is electricity or heat generated from geothermal energy.

Geothermal energy heats water underground, so it rises to the surface as steam. Once at the surface, the steam drives turbines and electrical generators.

Radioactive decay in the Earth's core releases energy. The energy travels from the core to the mantle, driving convection currents and volcanic activity. Geothermal power plants harness energy in volcanic areas, using it for heating and electricity generation.

In 2019, geothermal energy produced approximately 88 billion kWh of electricity.

Three disadvantages of geothermal power are location-specificity, costs, and the risk of triggering earthquakes.

## Final Geothermal Power Quiz

Question

Define geothermal energy.

Geothermal energy is heat that comes from within the Earth.

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Question

How hot is the Earth's core?

The Earth's core is 5500ºC.

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Question

Define geothermal power.

Geothermal power is electricity or heat generated from geothermal energy.

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Question

How do geothermal power plants work?

Volcanic activity heats rocks, which heat underground water. It rises to the surface as steam, where it drives turbines and generators.

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Question

Define conversion efficiency.

Conversion efficiency is the useful energy output divided by the input.

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Question

What is the average conversion efficiency of geothermal power plants?

12%

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Question

What is the average availability of geothermal power plants?

90%

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Question

Define low-temperature geothermal energy.

Low-temperature geothermal energy is heat obtained from geothermal fluid in the ground at temperatures of 150ºC or below.

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Question

What two substances are used in ORC technology?

Geothermal brine and working fluid are used in ORC technology.

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Question

Define district heating.

District heating is the use of geothermal energy to heat individual, commercial, and industrial buildings through a distribution network.

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Question

What happens when an unstable isotope decays?

When an unstable isotope decays, the atom expels subatomic particles and electromagnetic radiation in the form of high-energy gamma rays

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Question

How many of Reykjavík's buildings are heated using geothermal district heating?

95%

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Question

Geothermal power plants emit greenhouse gases and pollutants.

False

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Question

Digging deep wells for geothermal power plants can trigger earthquakes.

True

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Question

Geothermal energy doesn't fluctuate according to weather or time of year.

True

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