The principle of geothermal energy is very simple: hot water and steam from deep in the earth’s crust is used to drive turbines. It produces no harmful polluting gases and has the potential to become one of the main sources of renewable energy of the 21st century.
The natural heat energy produced from the earth is called geothermal heat energy. The source of geothermal energy is the continuous heat flux flowing from the interior of the earth towards its surface. Geothermal power plants pipe hot water or steam through wells that sometimes reach deep down to reservoirs underground. The thermal energy is then converted into electricity using different technologies:
- Dry steam power plants extract very hot steam from reservoirs in the earth. The steam activates turbines that generate electricity.
- Geothermal flash steam power plants use water temperatures of a least 182°C and convert it to steam to drive generator turbines. When the steam cools, it condenses water which is injected back into the ground to be used again.
- Geothermal binary cycle power plants can use water temperature as low as 57°C. The thermal energy is used to heat a fluid that turns into steam at low temperatures. This steam is pushed through a turbine to generate electricity. The water never touches the fluid and is re-injected into the well, where it heats up again, closing the cycle.
Pros and cons
The geothermal resources of the earth are vast, clean and plentiful. Unlike most other renewable energy resources, geothermal energy is available throughout the year, has an inherent storage capability and is independent of weather conditions.
Its storage capability makes it an ideal stabilizing energy, which can compensate for the fluctuating nature of other forms of renewable energy, originating from the sun or the wind. Underground thermal energy storage (UTES) systems store energy by pumping heat into an underground space. Thermal energy can be stored in boreholes, aquifers and caverns or pits. The storage medium is water but can also be molten salts, soil and rocks. Boreholes are man-made vertical heat exchangers that work to transfer heat between the energy carrier and the ground layers.
Cost is one of the drawbacks of geothermal energy: plants are expensive to install and they are generally limited to locations where a combination of heat, permeability of the earth and flow make extraction economical for electricity generation. Geothermal energy resources differ from one geographic location to another, depending on depth, temperature and pressure, abundance of ground water and underground chemical composition. Geothermal energy resources typically vary in temperature from about 50 to 350°C. The high temperature geothermal resources (above 200°C) are generally found in volcanic regions and island chains. The medium temperature (between 150 and 200°C) and low temperature geothermal resources (under 150°C) exist in most continental regions and are fairly widespread. Low temperature resources are used directly for heating while the higher temperature ones are used for conversion into electricity.
Another issue is the relative abundance of greenhouse gases (GHGs) below the surface of the earth, which can be released into the atmosphere through geothermal activity. However, since geothermal power plants do not burn fuel to generate electricity, the levels of air pollutants they emit are low compared to fossil fuels, according to the International Energy Agency (IEA). Geothermal power plants emit 97% less acid rain-causing sulfur compounds and about 99% less carbon dioxide than fossil fuel power plants of a similar size. Most geothermal power plants use scrubbers to remove the hydrogen sulfide naturally found in geothermal reservoirs and inject the geothermal steam and water that they use back into the earth. This recycling helps to renew the geothermal resource.
From Germany and Turkey to Kenya
As highlighted in the most recent REN 21 report, Turkey and Indonesia remained in the lead for new geothermal installations in 2019, followed closely by Kenya. Other countries that added new geothermal power plants in 2019 (or added capacity at existing facilities) were Costa Rica, Japan, Mexico, the United States and Germany.
The top 10 countries with the largest stock of geothermal power capacity at the end of 2019 were the United States, Indonesia, the Philippines, Turkey, New Zealand, Mexico, Kenya, Italy, Iceland and Japan. Several amongst them see geothermal electrical energy as one of the ways to meet their renewable energies target, in an attempt to align with the Paris Agreement on Climate Change. For instance, the Indonesian government’s target for 23% renewables in the energy mix by 2025 assumes an installed geothermal power capacity of 7 GW (7% of the energy mix).
Marit Brommer is the Executive Director of the International Geothermal Association (IGA). In an interview she gave to REN 21, she explains that the geothermal industry has a lot in common with the fossil fuel extraction business: the technologies used for extracting energy are similar, even if fossil fuel is extremely polluting and not renewable. As the price of oil has come down to historically low levels during the COVID-19 pandemic, it is no longer covering the costs of drilling, etc…In her mind, it is an opportunity that oil companies, which are already investing in renewable energies, should be taking: use the technology they know and switch to producing clean and renewable geothermal energy.
“The overlap between geothermal and oil and gas is in exploring, drilling and production. With this comes expert understanding of the earth’s sub-surface. It takes expert knowledge to find the right spots to drill, how to drill, what equipment is needed, and how to use it. During the current crisis, many skilled workers in oil and gas drilling companies are on standby. These workers could be re-deployed to the geothermal sector,” she argues.
IEC expertise makes the difference
For the geothermal industry to continue expanding everywhere around the world, the technology used must meet proper safety and performance benchmarks. IEC International Standards ensure that systems and devices employed are tested and meet the appropriate standards of quality and efficiency. IEC Technical Committee 5 develops specifications and standards for the rating and testing of steam turbines. In 2020, it released the second edition of a key standard specifying the requirements for steam turbines: IEC 60045–1, which now includes automation safety specifications. The standard can be used for geothermal steam turbines but also for turbines employed in concentrated solar power plants, another form of renewable energy.
A version of this story was published on the IEC e-tech website earlier this year
