How Does Earth Produce Geothermal Energy?
What is Geothermal Energy?
Geothermal energy is heat energy generated and stored in the Earth (Geothermal Energy Technologies Office). It is a renewable energy source that taps into the natural heat inside the earth to produce steam and hot water. “Geo” means earth, and “thermal” means heat, so geothermal energy is literally the heat from the Earth (TWI Global). Humans have used geothermal energy for bathing since Paleolithic times, but the first industrial use of geothermal power was in 1904 in Larderello, Italy to generate electricity (TWI Global).
Geothermal energy originates from the original formation of the planet, from radioactive decay of minerals, and from solar energy absorbed at the surface. The geothermal gradient, which is the difference in temperature between the core of the planet and its surface, drives a continuous conduction of thermal energy from the core to the surface (Geothermal Energy Technologies Office).
How is Earth’s Internal Heat Produced?
Earth’s internal heat comes from three primary sources:
- Radioactive decay of elements – The decay of radioactive isotopes in Earth’s crust and mantle, like uranium, thorium, and potassium, produce heat. This radiogenic heat currently accounts for nearly half of the heat escaping Earth’s interior.[1]
- Leftover heat from earth’s formation – Heat has been retained in Earth’s core since the planet formed about 4.5 billion years ago. The kinetic energy and gravitational energy generated during planet formation was converted into thermal energy that still emanates from Earth’s core today.[2]
- Friction of tectonic plate movement – The movement of tectonic plates generates frictional heating, adding to Earth’s internal heat, especially at plate boundaries.[1]
These three heat sources maintain Earth’s internal temperature and drive geologic activity on the planet’s surface and within its interior today.
How Does the Heat Reach the Surface?
The heat from Earth’s interior reaches the surface primarily through convection currents in the molten magma and conductive heat transfer through the surrounding solid rocks.
Convection currents are generated as mantle magma heats up and rises towards the surface, while cooler magma sinks downwards. This magma movement enables efficient heat transfer from deep within Earth towards the crust. The magma can rise up through cracks and faults, bringing heat closer to the surface.
In addition, heat conducts slowly outwards from the hot magma into the cooler surrounding solid rocks. While rocks are poor heat conductors compared to metals, the temperature gradient enables steady diffusion of heat outwards over geological time. The further the distance from magma, the cooler the rocks become.
So both convection currents in magma and conduction through rocks allow Earth’s internal heat to be transferred and tapped at shallower depths. Convection brings the heat nearer the surface, while conduction spreads the heat through the rocks.
Where is Geothermal Energy Found?
Geothermal energy is found in areas with high underground heat flow, such as along tectonic plate boundaries or in volcanic regions. The earth’s crust is thinner in these locations, allowing heat to rise more easily to the surface.
Some of the most common geothermal sites include:
- Geysers – Geysers occur where superheated water rises to the surface explosively in intermittent jets.
- Hot springs – Heated groundwater flows up through rock fractures to the surface in hot springs.
- Fumaroles – Openings in the earth’s crust that emit steam and hot gases.
The western United States, Alaska, Hawaii, Iceland, New Zealand, Indonesia, the Philippines, Central America, and parts of Africa are some key areas with geothermal activity along plate boundaries. According to the U.S. Energy Information Administration, most geothermal power plants in the U.S. are located in western states and Hawaii, where resources are close to the surface.
Types of Geothermal Power Plants
There are three main types of geothermal power plants: dry steam, flash steam, and binary cycle. Each type harnesses geothermal energy in slightly different ways.
Dry steam power plants use steam directly from a geothermal reservoir to turn the generator turbines. The first geothermal power plant built in the world was a dry steam plant – the Valles Caldera in New Mexico which opened in 1958. Dry steam reservoirs are rare, found in places like The Geysers in California.
Flash steam power plants are the most common. They use water at temperatures over 360°F (182°C) that is pumped under high pressure to the generation equipment at the surface where it is converted to steam to drive the turbines. Flash steam plants account for about 65% of installed geothermal capacity worldwide according to https://www.energy.ca.gov/data-reports/energy-almanac/data-renewable-energy-markets-and-resources/types-geothermal-power.
Binary cycle power plants operate on water at lower temperatures of about 225-360°F (107-182°C). The hot water is passed through heat exchangers, which transfer the heat to a separate fluid with a much lower boiling point. This causes the secondary fluid to flash to vapor, which then drives the turbines. Binary cycle plants are favorable where geothermal reservoirs produce lower temperature water.
Geothermal Power Generation
Geothermal power plants generate electricity by using steam produced from reservoirs of hot water deep underground. There are three main types of geothermal power plants:
In dry steam plants, steam from geothermal reservoirs is piped directly to turn turbines connected to electricity generators. The first geothermal power plant built in 1904 in Tuscany, Italy was a dry steam plant.
Flash steam plants take higher temperature water at temperatures over 300°F (150°C) from geothermal reservoirs and allow it to flash into steam to turn turbine generators. Flash steam plants are the most common geothermal power plant technology today.
Binary cycle power plants differ because they do not use steam directly from geothermal reservoirs. Instead, these plants pass moderately hot geothermal water through heat exchangers to heat a secondary fluid with a low boiling point. This causes the secondary fluid to flash to vapor which then drives turbine generators. Binary cycle plants allow electricity generation from lower temperature reservoirs down to 225–360°F (107–182°C).
In all geothermal power plants, the steam rotates turbines connected to electricity generators to produce power. The geothermal steam eliminates the need to burn fossil fuels to boil water for steam.
Direct Uses of Geothermal Energy
Geothermal energy can be used directly for a variety of applications without needing to convert it into electricity first. Some common direct uses of geothermal energy include:
Heating buildings – Geothermal heat pumps can tap into underground reservoirs of hot water to heat buildings. This takes advantage of the Earth’s constant temperatures below the surface. According to the Whole Building Design Guide, geothermal heat pumps are among the most energy efficient and environmentally friendly systems available for heating and cooling buildings.
Heating greenhouses – Geothermal hot water can be piped directly into greenhouses to provide heat needed to grow plants and flowers year-round, even in colder climates. This provides an energy efficient heating solution.
Fish farming – Geothermal fluids provide a stable warm water environment for raising tropical fish, even in cooler regions. The heat exchangers keep water temperatures constant.
Bathing & cooking – Geothermal hot springs have been used for bathing, cooking, and therapeutic purposes throughout human history. Natural hot springs provide a renewable source of heated water.
Global Geothermal Power Capacity
Geothermal power capacity has been gradually increasing worldwide over the past decade. According to Statista, the total installed global geothermal energy capacity reached approximately 14.9 gigawatts in 2022, up from 10.7 gigawatts in 2016[1]. While geothermal energy accounts for only around 0.5% of total global renewable energy capacity, certain countries utilize it much more extensively.
The countries with the highest geothermal power generation capacity in 2022 were[2]:
- United States – 4,000 MW
- Indonesia – 2,300 MW
- Philippines – 2,000 MW
- New Zealand – 1,100 MW
- Iceland – 765 MW
Other leading countries include Turkey, Kenya, Mexico, Italy and Japan. The widespread adoption of geothermal in some countries demonstrates its viability as a renewable baseload power source. However, there is still ample room for growth worldwide as technology improves and geothermal resources are further developed.
[1] https://www.statista.com/statistics/476281/global-capacity-of-geothermal-energy
[2] https://www.thinkgeoenergy.com/thinkgeoenergys-top-10-geothermal-countries-2022-power-generation-capacity-mw/
Benefits of Geothermal Energy
Geothermal energy provides several key benefits that make it an attractive renewable energy source:
Renewable and sustainable. Geothermal energy is considered renewable, as heat is continuously produced inside the earth (https://www.energy.gov/eere/geothermal/geothermal-faqs). Unlike fossil fuels which can be depleted, geothermal energy is virtually limitless.
Reduces greenhouse gases. Geothermal power plants emit little to no greenhouse gases, as no fuels are combusted. The US Department of Energy states geothermal electricity generation emits just 6% of the carbon dioxide of a fossil fuel plant (https://www.energy.gov/eere/geothermal/geothermal-faqs).
Reliable baseload power. Geothermal plants provide constant baseload power 24/7, unaffected by weather conditions. This makes geothermal a reliable and consistent source of electricity.
Challenges of Geothermal Energy
While geothermal energy has many benefits, it also comes with some challenges that limit its widespread adoption. Some of the main challenges facing geothermal energy include:
High upfront costs – Constructing a geothermal power plant requires significant upfront capital investment. Drilling wells thousands of feet into the earth and installing the power generation equipment can cost millions of dollars before any electricity is even produced [1]. The development risk and long payback period makes financing difficult.
Potential emissions – Geothermal sites can release hydrogen sulfide, carbon dioxide, ammonia, methane and other gases. Proper site selection and emission control systems can minimize this, but potential emissions can still be a concern [2].
Land subsidence – Extracting large amounts of geothermal fluid can cause the ground surface to sink over time. This needs to be carefully managed at geothermal sites [2].
Limited to certain locations – Geothermal energy is location dependent, with the best sites located near tectonic plate boundaries, hot spots and other geologically active areas. This restricts where geothermal plants can be built [3].
