Is Geothermal Energy Renewable? Geothermal Sustainability

Is Geothermal Energy Renewable_featured image

Geothermal energy is a renewable, sustainable resource with many advantages and few disadvantages. Geothermal energy comes from inside the earth and the higher temperatures are found at lower depths, due to the temperature gradient between different levels of rock strata.

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What is geothermal energy in simple words?

The word geothermal is derived from two Greek words:

  1. Geo (Earth) and
  2. Therme (Heat)

It is the natural sustained heat that is always present inside the Earth. It can be considered renewable for the foreseeable future, so is a very good resource for mankind.

How is geothermal energy generated?

The center of the earth is mostly iron which is at a very high temperature  (5,430°C or 9,806°F) due to gravity pressing down on it. There is a substantial temperature gradient between the lower and upper depths and it’s the uppermost layers of rock that we can use to extract useful heat.

The heat energy appears in the upper layers of the Earth’s crust in the form fumaroles, hot springs, and rock strata heated by lava underneath it. Between 1 and 2 km down, the temperature can reach 200 to 250 degrees C and can be extracted quite efficiently.

Video –  Geothermal energy is renewable

 

Where does geothermal energy come from?

Geothermal is always there, underneath the ground we walk on this is why it’s consider to be renewable. Most of the time we can’t see it or feel it because the upper layers of soil are at a temperature very close to the air temperature above it.

However, just a few meters down there comes a point where the temperature is constant and is unaffected by the climate above.

In some places on Earth the heat comes to the surface in the form of hot mud or water. Deeper underground reservoirs of hot water heated by the rock strata around them find their way to the surface under pressure, using natural fissures as a pipeline.

Types of geothermal energy

Strictly speaking, there’s only one geothermal heat, but this manifests in 3 different types that we can use to convert into useful energy:

  1. Dry steam
  2. Super-heated water (flash-steam)
  3. Binary cycle – hot water

1. Dry steam geothermal energy comes in two kinds:

  • Direct dry steam – this steam is already available underground as heated water under pressure. It vaporizes and turns to steam with less than 10% liquid water and used to turn turbines (electricity generator).
  • Hot dry rock (HDR), or enhanced geothermal system (EGS) injects water into dry rock and brings it back to the surface as steam. In this system fissures are created artificially so that the rock strata acts as a more efficient heat exchanger.
3 types of renewavle geothermal energy

The 3 types of geothermal sustainable energy are hot dry rock, dry steam and hot water (binary cycle)

2. Flash-steam

In this type hot water under pressure is brought up to the surface under it’s own pressure, which is higher than atmospheric pressure at the surface. The water should be steam, but the high pressure keeps it in liquid form.

However, as the pressure reduces nearer the surface the hot water is ‘flashed’ into steam, which is used to drive turbines to generate electricity. The water that doesn’t convert to steam is injected back underground to be heat up again in the hot water reservoir deep underground.

3. Binary cycle

Hot water (binary cycle) – in this type, hot water not much hotter than boiling point is brought to the surface and the heat is exchanged with another, more volatile liquid before being injected back underground to continue the cycle. The liquid is then vaporized and used to drive a turbine.

How does geothermal energy work?

This inexhaustible source of renewable energy can be converted into useful energy in various ways. In most cases a complex conversion plant is needed, but in some cases steam or hot water can be used directly for space heating in buildings, or for agricultural uses, such as drying vegetables and fruits.

The type of geothermal plant needed depends on the nature or type of underground heat energy available. In general it is more sustainable than any other energy source. What form does geothermal energy take?

Types of geothermal power plant

Geothermal technologies offer various methods of extracting useful heat from underground in a much cleaner fashion than with fossil fuels. The type of plant depend on the type of geothermal resource available and at what depth. The 3 common types of geothermal power plant are dry steam, binary cycle and flash steam.

 

Dry steam geothermal plant operation

This type uses dry steam directly to drive a turbine

Dry steam plants draw the steam form bore-holes that are 7000 to 10,000 feet deep. The steam is simply piped directly into the
turbine generator electricity generation.

It’s a bonus that the condensed water residue from the turbine is often used to cool down machinery.

Dry steam plants represent a very efficient means of producing electrical power – unfortunately it’s not often used, as there are not many dry steam source outside of California.

It’s a shame because this type of plant can be very efficient, because the dry steam can be used to drive a turbine directly without needing to exchange heat with another liquid.

Flash Steam Power Plants

Flash steam plant converts geothermakl energy into electrical powerIn a flash steam power plant, extremely hot water flows up through the wells via the pressure gradient created by the temperature difference between the earth’s surface and the underground reservoir.

As it travels through the wells, the some of the water turns to steam that can be separated from the liquid water and used to power a steam turbine. This is the most common type of geothermal power plant.

Two types of flash plants exist – single and dual flash. The difference is that two separators are used and they more efficient. However, they are more expensive as more piping and equipment is used.

Binary Cycle Power Plants

Binary cycle plants use intermediate fluid to transfer heat

Binary geothermal plants direct the hot water from the underground reservoir to a heat exchanger, also called a vaporizer, where another fluid with a lower boiling pump circulates on the secondary side.

The secondary fluid vaporizes after absorbing the heat and used to drive the turbine.

Binary cycle geothermal plants produce almost zero emissions, which is much less than the flash stem plants.

All plants take up very little land compared to most other forms of energy generation or conversion.

Images Source: U.S. Department of Energy, Energy Efficiency & Renewable Energy (public domain)

Source: Compare geothermal plant type efficiencies PDF

How does geothermal energy work in a house?

Using geothermal power to heat buildings, or even cool buildings, is one of the best ways a home-power can gain massively from this technology.

As an energy source, geothermal can’t be beaten for it’s low carbon emissions and renewability, but the way the energy is extracted from underground is completely different from electricity generation.

What is a Heat Pump and how does it work?

A heat pump transfers thermal energy from one place to another. It acts like a refrigerator in reverse. The beauty of a heat pump is that they don’t need much electrical power to do their job. They can transfer up to 400% more thermal energy than the amount of electrical energy needed to run them.

Mostly used in for heating houses and other buildings, heat pumps use refrigeration cycles powered by electric or gas compressors. The electric power required compared to other forms of heating is much less, although initial installation costs can be high.

Ground-source heat pumps can either extract heat from a network of piping coils laid over a large area about 12 feet underground, or from pipes dropped into twin boreholes drilled up to 100 yards deep. The temperature at 12 feet depth is a constant 55 degrees F, while for the vertical boreholes temperature increases by 1 degree F every 40 yards.

piping coils for ground source heat pump laid underground

Piping coils extract geothermal heat laid in 12 feet deep trenches

What is geothermal energy used for?

Any type of energy is useless unless mankind can turn it into something we can use. Geothermal isn’t as flexible as other energy sources in that it’s difficult to transport in it’s primary form.

Fluids lose their thermal energy very quickly if stored or piped over any distance – heat energy from under the ground has to be converted or used as soon as it reaches the surface.

5 uses of geothermal energy

  1. The primary use of geothermal heat on the large scale is for generating electrical power
  2. Space heating – heating and cooling buildings directly or using heat pumps
  3. Health and well-being – natural spas, hot springs and mud baths
  4. Agricultural use – drying fruits and vegetables
  5. Cooking

What are the 3 main uses of geothermal energy?

Generating electrical power is by far the biggest way geothermal power is used, closely followed by heating and cooling buildings, and then health. The 3rd main usage, in the form of hot springs or health spas may come as a surprise to many, but there are hundreds of such spas across the globe. Taking hot baths for health has been common since ancient Greek and Roman times.

Where is geothermal energy used in the world?

Geothermal as an energy resource is more prevalent in some countries than others simply because it’s more easily accessible.

List – Top global geothermal usage by country capacity

  1. United States – 3,714 MW
  2. Indonesia – 2,133 MW
  3. Philippines – 1,918 MW
  4. Turkey – 1,526 MW
  5. New Zealand – 1,005 MW
  6. Mexico – 962.7 MW
  7. Italy – 944 MW
  8. Kenya – 861 MW
  9. Iceland – 755 MW
  10. Japan – 603 MW

Some countries, such as Turkey, has seen a lot of growth, while most haven’t increased output significantly.

Source: ThinkGeoEnergy.com

Is geothermal energy renewable or nonrenewable?

First of all we need to be sure what we mean by ‘renewable’. Most things are renewable given enough time, but mankind’s energy needs are immediate. This means that any energy source needs to be renewable within our lifetimes,  in fact less than that – a resource should be available constantly if it’s to be sustainable.

Fossil fuels are obviously not classed as renewable, as they take millions of years to form deep underground. Geothermal is considered renewable because it’s always there due to the heat inside the planet

What is a renewable resource?

I define renewable resource as something that can be used in great quantities but always replenishes itself by natural processes. Some resources could be considered renewable because human beings replenish them, like wheat for example.

However, things that we grow are not really renewable naturally. If the weather is bad, crops don’t grow. If we take all the minerals and metal ores out of the ground, they will eventually run out – they are not renewable.

Other resources, such as geothermal energy, wind, sunlight and wave power can be considered to be truly renewable.

Table – Which resources are renewable or non-renewable?

 Resource

Renewable?

Natural gas

NO

Crude oil

NO

Minerals

NO

Coal

NO

Water

NO

Topsoil

NO

Sand

NO

Biomass

NO

Fish

NO

Sunshine

YES

Wave-power (oceans)

YES

Wind

YES

Geothermal

YES

Gravity

YES - but how use?

Animals

YES - if well-managed

Why geothermal energy is not renewable?

The only way in which geothermal could be considered non renewable is that way, way in the future, planet Earth may not exist. If the sun expands and snuffs out our planet, then nothing will exist.

However, as long as the planet exists, there will be geothermal heat, even if the surface of our planet is lifeless and barren. The heat underground comes from the pressure on trillions of tons of rocks pressing down under gravity, so will exist even if the sun goes out!

Geothermal energy advantages and disadvantages

All energy resources have benefits and drawbacks (pros and cons) and geothermal is no different. The infographic below features the 4 main positive and negative aspects of using geothermal as an energy source:

Infographic – advantages and disadvantages of geothermal energy

Infographic - renewable geothermal pros and cons

Drawbacks and benefits of geothermal energy

Advantages of geothermal energy

Geothermal projects generally operate very cleanly. Clean energy is very important for us at the present time. The use of fossil fuels over the past 50 years have polluted our plant until we now find ourselves at a crisis point.

The carbon footprint of geothermal is low compared to traditional energy sources. Together with wind and solar, underground heat represents a powerful future energy source that doesn’t contribute a lot to the destruction of our environment.

Disadvantages of geothermal energy

The biggest disadvantage is the initial cost of drilling and building the plant. On the plus side, running costs tend top be quite low compared to coal, gas and nuclear. Not all locations have easy access to geothermal and so this is a significant drawback.

The most efficient plants use high temperature steam which is found deep underground. Drilling this far down is obviously a costly business. Drilling and fracking (creating artificial fissures in rock strata) can destabilize the ground and cause minor earthquakes, or crack the foundations of nearby buildings.

Geothermal heat isn’t easily transport, so it has to be used or converted at the location where it is extracted. Finally, greenhouse gases can be released in significant quantities, primarily methane, and also toxic compounds such as hydrogen sulphide.

What are 3 main disadvantages of geothermal energy?

Extracting heat from geothermal sources come with these 3 main disadvantages:

  • Installation cost is high
  • Location specific (it isn’t available everywhere)
  • Deep drilling required for the most efficient plants

Table – Geothermal Pros and Cons

Geothermal Advantages (Pros/benefits)

Geothermal Disdavantages (drawbacks/cons)

Low running costs once established

High installation & surveying costs (investment)

Low carbon footprint (38 gms CO2/kWh)

Machines needed to extract and convert heat

Low ecological impact

Above machines made by fossil fuel energy

Renewable and sustainable

Can cause minor earthquakes

Generate electricity cleanly

Location dependent to some extent

Heat buildings directly

Deep drilling needed for high temperature plant (higher temp. = higher efficiency)

Flexible agricultural use 

Possible release of greenhouse and toxic gases

Is geothermal energy expensive?

On the larger scale a geothermal power plant is expensive to install but running costs are much lower than traditional methods of generating electricity. A 1 megawatt (MW) can cost between 2 to 5 million USD to build and commission.

On the small scale, home heating using heat pump technology is more expensive to install than traditional heating, such as gas/oil furnace forced air and central hot water systems, but offers long term savings due to low running costs.

A heat pump installation for an average US home can cost between $18000 to $30000.

Hydroelectric energy advantages and disadvantages

Hydroelectric power is another renewable energy resource that is largely location dependent i.e. not every country has large rivers that can be dammed to form reservoirs for driving electrical generating turbines.

  • What are 5 advantages of hydropower?

  • 100% renewable natural resource
  • Doesn’t rely on fossil fuels
  • Predictable & reliable
  • Safe to use
  • Large and small scale use
  • Low greenhouse gas emissions
  • What are 5 disadvantages of hydropower?

  • Expensive to build
  • High environmental impact – flooding
  • Impact on local communities
  • Cover natural carbon sinks – vegetation
  • Depends on long term water availability
  • Is location dependent

 

Advantages of hydroelectric power over thermal power

In addition to the obvious advantages listed above there are a couple of secondary benefits not often considered, tourism for one. You would be surprised at how many people visit the Hoover dam in the USA every year – 7 million!

Hoover dam is the most visited dam in the world, but all dams get lots of visitors due to the fact that they are impressive feats of civil engineering. Improved crop irrigation is another plus of hydropower, which also gives rise to better flood control, protecting communities in flood plains.

Other resources relating to renewable geothermal energy

Geothermal explained – U.S. Energy Information

Why is geothermal energy a renewable resource?

What is Geothermal Energy? How Does it Work?

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