Geothermal Power Plants

“Going beyond the conventional to provide energy security without burning fossil fuels”

When a geothermal resource is available at temperatures above 120oC (give or take), electricity can be generated in a geothermal power plant.

The exact specification of the power plant will be tailored to the geothermal environment, but broadly speaking there are three key types of geothermal power plant:

1. Binary Power Plant

2. Flash Steam Power Plant

3. Dry Steam Power Plant

Every type of plant uses hot water to generate steam, which turns a turbine to generate electricity. However, depending on the temperature of the water, different methods are required to maximise the efficiency of the energy conversion process.

Below you can discover the key differences between each type of power plant, and how this leads to key variations in the surface infrastructure required to generate electricity in different parts of the world.

worker-03
Aerial photograph of the Insheim 5MW power plant in Germany

Binary Power Plants

A binary power plant uses hot geothermal fluid to heat a secondary working fluid in a heat exchanger. This working fluid turns to steam at a lower temperature than pure water, and it’s vapour can then be used to turn a turbine to produce electricity.

The geothermal fluid is kept within a closed loop and reinjected through an injection well to recharge the geothermal reservoir. The secondary fluid is also within its own closed loop, meaning nothing ever leaves the power plant. No steam. No gases. No smell.

A binary plant is used for lower temperature reservoirs, with fluid less than 200oC. It is therefore the best type of plant for the resource in Cornwall, where temperatures of 180oC are found at around 5km depth.

Image taken from saveonenergy.com

Want to see a 1.7MW Binary Power Plant in Action?

Click the link below and on the new website scroll to the left and click on the target. You will see the Soultz-sous-Forets geothermal power plant in France, nestled quietly amongst the villages and countryside.

Flash Steam Power Plants

Image taken from saveonenergy.com

Flash steam plants are utilised when a geothermal reservoir is more than 200oC. They are therefore often associated with volcanic environments where shallow magma chambers generate significant amounts of heat near the surface.

At such high temperatures, fluid may rise up through a well under its own pressure. As it rises, the pressure drops and some of it will boil and “flash” to steam. This steam can be used directly to power a turbine to generate electricity.

As the fluid will be in direct contact with the power plant, the chemistry of the fluid is critical, and the infrastructure must be built to withstand potentially corrosive volcanic fluids.

If the reservoir is associated with volcanic activity, there may be  gases such a Hydrogen Sulphide (H2S) released when the geothermal fluid reaches the surface. This is what commonly gives flash steam power plants the distinctive rotten egg smell! 

In addition, a number of flash plants are not required to recharge the reservoir by reinjection, so they may have cooling towers which instead emit the fluid as large plumes of steam. The higher temperatures also offers the potential for generating more electricity, but with more electricity comes significantly more cooling fans and pipework.

Warakei Geothermal Power Plant, New Zealand's oldest and largest geothermal plant (image from thinkgeoenergy)

Dry Steam Power Plants

If a hot steam reservoir is found at depth, a dry steam plant may be installed. These take steam directly from the Earth and use it to turn the turbine to generate electricity.

As with flash steam plants, these are only suitable for the hottest geothermal reservoirs (generally associated with volcanoes) and the gas is in direct contact with the power plant so it’s chemistry is critical. 

Image taken from saveonenergy.com
The cooling towers of the Larderello dry steam power plant sit within the beautiful Tuscan countryside, heating and powering local farmhouses, businesses and transport links. Image from swisseduc.ch

The world’s first geothermal power plant in Larderello, Italy, is a dry steam plant. It was built in 1913 and used to power the Italian railway system and the nearby villages of Larderello and Volterra. The plant has expanded over the years and now produces around 800MW of electricity!

As at flash steam plants, large cooling towers are used by dry steam plants to release excess heat to the atmosphere. 

In Cornwall, the steam generated is contained in a closed loop within the binary plant so no cooling towers will ever be required, and no steam, gases or smells emitted!