Geothermal energy explained

The word “geothermal” comes from the Greek words geo (earth) and therme (heat). The heat beneath our feet is residual energy from the formation of the planet as well as the decay of radioactive elements in the Earth’s crust. This heat continuously flows from the Earth’s core to the surface and will remain available for billions of years.

Geothermal energy harnesses this heat from the Earth’s interior. To obtain geothermal energy, geologists look for underground resources of hot water, or aquifers. Once a resource has been identified, it is then accessed by drilling wells. The depths of these wells range from a few hundred meters to several kilometers.

Geothermal energy is not new. People have used naturally occurring hot springs for heating, cooking and bathing for thousands of years. But the first time this heat was used in a geothermal power plant to generate electricity was in Larderello, Italy, in 1904.

One of the characteristics of geothermal energy is its wide diversity of applications, including the following.

Heating and cooling

Heating and cooling buildings is the most common use of geothermal energy. This can occur directly, when underground temperatures are high enough, or with the help of geothermal heat pumps, which make use of the stable ground temperatures to provide heating during winter and cooling during summer. With geothermal district heating and cooling systems, whole blocks or communities can use this underground, renewable heat to adjust indoor temperatures.

Electricity generation

Geothermal heat is drawn to the surface to produce steam that drives turbines which produce electricity. In dry steam plants, the fluids are hot enough that they are already close to steam, while flash steam plants pump the hot fluids from underground and use the change in pressure to “flash” it into vapor. In both instances, this steam then drives a turbine.

Binary-cycle power plants are used for lower-temperature resources and typically employ a process called the organic Rankine cycle (ORC). In this case, the geothermal fluids pass through a heat exchanger with a secondary fluid that has a lower boiling point. The heat from the underground fluid causes this working fluid to flash to vapor that drives the turbines. The working fluid is then air-cooled or condensed with water and injected back into the ground.

Combined heat and power plants (CHP) produce both electricity and hot water for district heating.

From food industry to mineral extraction

The heat from geothermal wells is also used for industrial processes in greenhouses and fish farms and to dry agricultural products as well as paper, timber and cement. It is an excellent source of clean energy for future production of green hydrogen and green ammonia, and there are opportunities to integrate geothermal wells with carbon capture and storage. Geothermal energy has also become sought after to supply data centers with clean energy and for extraction of minerals such as lithium and silica. The diversity of uses for geothermal energy is wide and growing.

Next-gen geothermal technologies

To generate energy from geothermal systems, three elements are needed:

• Heat in rocks under the surface of the Earth;
• Fluid to carry the heat to the surface; and
• Permeability or porosity, so that the fluid can pass through the hot rocks.

Since the natural occurrence of heat, fluid and permeability is not found everywhere, there is extensive, ongoing research and innovation focused on making geothermal possible anywhere on the planet. Several of these projects have been done in collaboration with oil and gas companies and/or utilized technologies stemming from that industry. They are often called next-gen geothermal technologies. The three most prominent types of next-gen geothermal are outlined below.

• Enhanced Geothermal Systems (EGS) uses thermal, hydraulic or chemical stimulation to create man-made permeability, or fractures, in the underground rocks.
• Advanced Geothermal Systems (AGS), also called Closed-Loop Geothermal, are closed-loop systems that circulate man-made fluids through a sealed piping system that heats up the fluids.
• Supercritical Hot Rock (SHR) are even deeper systems with temperatures over 400 degrees Celsius where water is injected and circulated. The water is heated up and pressurized to a point where it becomes “supercritical,” meaning it can travel faster through the system and hold more energy per unit mass, thus becoming several times more efficient than conventional geothermal systems.

Five main benefits of geothermal energy

The ongoing climate crisis and the need to transition to clean energy have increased attention to, and investment in, geothermal energy. Some of the greatest benefits of geothermal are summarized below.

Geothermal:

• Is a renewable source of energy.
• Is “always on” (often referred to as baseload, stable, or firm), supplying continuous production of energy, 24 hours a day, seven days a week, 365 days a year, no matter the weather or season.
• Requires less space. A geothermal plant uses 12 percent of the space of a solar farm to produce one GW of electricity.
• Is a domestic source of reliable energy that reduces dependence on imported foreign energy and/or imported minerals for energy production.
• Has a wide diversity of applications, including electricity, heating and cooling and industrial processes for manufacturing and the food industry, as well as synergies with other technologies like powering and cooling large data centers, and production of green hydrogen.