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A Look at Geothermal

We've all heard about geothermal having the potential to trump all other forms of renewable energy. So I wanted to take a more in depth look at it.

Introduction to Geothermal

No article like this would be complete without the obligatory cheesy video!

Geothermal energy is tapped from the ground. When underground water comes in contact with the heat of the Earth's magma, it can get to temperatures of up to 370ºC. Holes can be drilled into the ground and this water or steam comes to the surface, either by its own steam bubbles, or it can be pumped. This steam can then be used to power turbines in the same way as conventional fossil fuel burning power plants.

Today only 10GW is utilised from geothermal for electricity, around 28GW is used directly for heating. Because of the ease of distribution of water, it is far more efficient to use geothermal just for heat than for generating electricity.

With regards to generating electricity, the thermal efficiency of geothermal electricity plants is low due to the lower temperature of the liquids compared to steam. This does not affect operational costs like in a coal burning plant, but it does factor into the viability of the plant. A geothermal plant needs to produce more heat than its pumps consume. In order to do this, plants use one of 3 different types of specialised heat cycles. The most recent development, binary cycle, allows for lower temperature water to be used, so can be far more efficient.

Dry steam

Flash steam

Binary cycle

Click any of the images to see a larger version. Source: NREL

Advantages of Geothermal

Of course, there are no fossil fuels burnt.
No reliance on sun or wind.
Geothermal plants take up little land.
Lower maintenance costs than conventional systems
Geothermal systems are flexible; they can be easily expanded or subdivided.

Disadvantages of Geothermal

Only suitable in areas with tectonic plate activity, or where there have been volcanoes, although this is improving with technology.
Large costs for land surveys required to find suitable sites.
Large costs for initial infrastructure.
A large percentage of the electricity created must go back into running its own pumps if the system is to be green.
In some cases, a site that has happily been extracting steam and turning it into power for many years, may suddenly stop producing steam.
When this happens, harmful gases may escape from the holes drilled. These must be captured by the site developers.
Closed loop systems use antifreeze which can produce CFCs.

Ground Source Heat

The ground is sometimes hot not from the heat of the Earth below but from solar energy held by the ground. This heat can be harnessed with heat pumps almost anywhere to heat and cool buildings. Of course these heat pumps would need to run on renewable energy. Ground source heat systems can easily be installed for individual houses. There are many companies that will install a heat pump for you.

Other Uses for Geothermal

It can also be used for desalination of salt water (requiring less maintenance than reverse osmosis techniques).
Speeding up crop growth.
Speeding up the growth of prawns, alligators and more for fish farms (currently utilised in the US).
Allows growing of flowers in the summer.

Conclusion

So while geothermal may not be the holy grail it's made out to be, there are many areas where it can be improved if effort is put into it. Although it is currently limited to where it can be used, it is still possible today to supply 100% of the electrical needs of 39 countries in Africa, South America and the Pacific. This can only improve with technology. Binary cycle systems already have huge potential.

In addition, Ground source heat pumps have the potential to reduce the energy usage of individual houses, and geothermal's ability to improve crop growth and perform desalination could be extremely vital for providing a sustainable society.