generated from the Earth’s core, is a key renewable energy resource. While other renewable energy sources such as wind or solar rely on specific weather conditions, geothermal energy is not dependent on that. 2,900 kilometres below the Earth’s surface is the hottest part of our planet, the Earth’s core. Heat is constantly radiating outward and warming rocks, water, gas, and other geological material.
Some natural sources of geothermal energy are hot springs, steam vents, underwater hydrothermal vents, and mud pots . Their geothermal heat can be captured and used directly for heating, or the steam is used to generate electricity. Almost anywhere in the world, geothermal heat can be accessed and used as a source of heat. Converting geothermal energy into electricity is a different process, employing a different technology.
The US generates the most amount of geothermal energy of any other country, generating 15 billion kilowatt-hours which is equivalent to burning about 25 million barrels of oil [2,4].
Local conditions will largely determine the technology needed to extract geothermal energy and convert it into electricity. In some places, heat can naturally exist underground in pockets of steam or hot water. In most places, however, this heat needs to be “enhanced” by injecting water which creates the steam needed.
There are a number of ways to make electricity out of geothermal energy, as explained below.
How does geothermal energy generate electricity?
Dry-steam power plants
Dry-steam power plants rely on natural underground sources of steam. This steam is piped directly to a power plant and used to power turbines and generate electricity. This is the oldest type of power plant to generate electricity from geothermal energy, it was first constructed in Italy in 1911 [2,3].
While these power plants have a great potential, the use of this technology is limited by the presence of the sources of steam. For instance, in the US are only two known sources of underground steam. One is The Greysers in California which provides about a fifth of all renewable energy in California [2,3].
Flash-steam power plants
One of the most common technologies, flash steam power plants use geothermal reservoirs of water with temperatures greater than 182°C (360°F). This very hot water flows up through wells in the ground under its own pressure. As a result, the pressure decreases and some of the water rapidly evaporates into steam, or “flashes.” The steam is then separated from the water and funnelled to power a turbine or generator. The remaining water is fed back into the Earth to extract more of its energy [2,4].
This technology is used particularly in volcanic areas such as Iceland or the Philippines. Iceland for example, covers large part of its electricity demand using flash-steam geothermal power plants. The islands of the Philippines have also invested in similar power plants. The largest single geothermal power plant in Philippines is a flash-steam facility in Malitbog.
Binary cycle power plants
This type of power plant uses a unique process to conserve water and generate heat. Water is heated underground to about 107°-182° C (225°-360° F). The hot water is in a pipe and in this way, it heats a liquid organic compound, such as a refrigerant, which has a lower boiling point than water. The fluid is vaporized in a heat exchanger and used to turn a turbine. This powers a generator to create electricity. The only emission in this process is steam. The water in the pipe is recycled back to the ground, to be re-heated by the Earth and reused for generating energy.
There are two types of geothermal resources that can be used in binary cycle power plants to generate electricity: Enhanced Geothermal Systems (EGS) and Low-Temperature or Co-Produced Resources.
Enhanced Geothermal Systems use drilling, fracturing and injection of water to help unlock geothermal energy that is trapped underground. In a lot of areas, it is not possible to tap into sources of geothermal energy because they do not contain liquid or they are not permeable.
EGS creates a vertical “injection well” between 1 and 4.5 kilometres deep, into where cold water is injected. This creates a reservoir of an underground fluid; the fluid absorbs the heat of the earth and it is then piped up to the surface where it warms another fluid which has a low or at least lower boiling point. This makes the fluid evaporate and power a turbine.
The U.S. Geological Survey estimates that potentially 500,000 megawatts of EGS resources are available in the western United States, however, EGS has also been known to cause seismic activity, or small earthquakes.
The second binary technology is called low-temperature and co-produced geothermal resources. This is typically found at low temperatures of around 150°C (300°F) or less.
Co-produced hot water is a by-product of oil and gas wells. It is being examined whether this hot water can be used for producing electricity, helping to lower greenhouse gas emissions and extend the life of oil and gas fields [2,3,4].