Geothermal energy: What you need to know

Renewable energy sources are found all around us – even beneath our feet. Deep within the earth, the temperature can be thousands of degrees above what we feel at the surface, and that energy can be harnessed and used in a number of different ways.

Geothermal energy comes from the heat beneath the Earth’s surface that comes from the original formation of the planet as well as the continuous radioactive decay of materials in rocks. This heat causes volcanic activity and molten rock to flow, tectonic plate boundaries to shift, and even the Old Faithful geyser at Yellowstone National Park to erupt. Geothermal energy is a renewable source of energy because heat is constantly being produced deep within Earth.

Geothermal energy produces very few harmful gaseous emissions, unlike burning fossil fuels which releases carbon dioxide. Geothermal energy is also “baseload”, meaning it works year-round and at all times of the day. In terms of energy efficiency, geothermal energy projects are able to produce gigawatts of electricity without much loss. In the past year, geothermal energy produced 3.673 GW of electricity according to NREL.

There are several types of geothermal reservoirs below our feet, from high temperature to very low temperature, all with unique applications:

High-temperature reservoirs

Steam can be harvested directly from high-temperature reservoirs to power sizeable geothermal power plants. In general, a high-temperature geothermal reservoir is generally more than 150°C and is found near areas of active volcanism.

Middle-temperature reservoirs

Middle-temperature geothermal reservoirs can generate steam to drive turbines, albeit with lower performance than high-temperature reservoirs. These reservoirs are usually between 100°C and 150°C.

Low temperature reservoirs

Geothermal reservoirs between 30°C and 100°C are classified as low-temperature and are often used for industrial processes and direct heating systems.

Very low-temperature reservoirs

These reservoirs are found just about everywhere on Earth and are used for small-scale geothermal heat pump systems.

Geothermal energy can be generated across the world with different degrees of ease. In countries like the United States, drilling must be used to reach geothermal heat. In Iceland, however, geothermal heat sources are abundant and easy to access. The U.S. generates the most amount of geothermal energy of any other country according to the U.S. Department of Energy and its national laboratories.

The basic principle of any geothermal energy system is that thermal energy from the Earth’s core can be captured and used at the surface. Via a heat exchanger, water or steam can carry this heat energy to the earth's surface. Once it reaches the surface, the water or heat can be used for many different purposes including heating and cooling buildings.

Several geothermal technologies have been developed that use the earth's heat for various end cases. Most commonly, geothermal energy is used for heat or electrical generation at one of many possible scales.

There are three main ways we use geothermal energy:

Direct heating

Direct heating (also known as direct use) geothermal systems involve using the heat from the Earth’s crust directly to heat and cool water and air. Ancient cultures in Rome and China used natural hot springs for cooking, heating, bathing, and more. Some places today still use hot springs for bathing. Nowadays, natural hot springs are used to heat buildings directly. Commercial buildings and even entire communities can rely on district heating systems which pipe naturally heated water through buildings to generate warm air. 

Furthermore, direct geothermal heating has applications in some industrial processes. For example, the process of milk pasteurization often uses direct heat from geothermal sources.

Electricity generation

Geothermal resources  can also be used for electrical power generation. Closed loop G geothermal power plants use high-temperature water or steam from deep within the earth to spin turbines that generate electricity. These plants need to be built where high and middle geothermal reservoirs exist; in the United States, those reservoirs are most often found in western states such as California, Idaho, Nevada, Utah, and Arizona.

Heat pumps

In contrast to direct heating, we can also use geothermal energy to indirectly heat (and cool) buildings. take advantage of very low temperature geothermal reservoirs to heat or cool buildings. Heat pumps rely on the fact that the temperature of the ground a few feet below the surface is more or less at a constant temperature year-round, meaning it can be used to heat or cool air.

All geothermal power plants operate on the same basic principle: they use steam to spin turbines, which generate electricity. There are three main types of geothermal power plants: dry steam, flash steam, and binary cycle.

Dry steam geothermal power plants

A dry steam geothermal power plant uses steam from underground reservoirs via a production well to spin turbines. After spinning turbines and generating electricity, the steam condenses into water and is pumped back into the earth through an injection well. This type of geothermal plant is known as a “dry” steam power plant because the water extracted from within the earth is in gas form, and no liquid water comes directly out of the production well. The first geothermal plant was a dry steam facility in Larderello, Italy.

Flash steam geothermal power plants

A flash steam geothermal plant doesn’t use steam directly from the earth to generate electricity. Rather, it pumps hot water at high pressure underground to the surface via its production well into a flash tank that is kept at a much lower pressure. Because of the flash tank’s low pressure, the hot, high pressure rapidly turns to steam in the tank, known as “flashing.” This steam is then used to spin turbines and generate electricity. Like a dry steam plant, the steam condenses into water and is returned to the Earth via an injection well once used to spin turbines.

Binary cycle geothermal power plants

A binary cycle geothermal power plant uses water at lower temperatures than flash steam plants, usually between 107°C and 182°C. Instead of using the water and steam directly, binary cycle plants use the heat from underground reservoirs to heat a second fluid with a low boiling point, known as a working fluid. When the working fluid vaporizes, it is used to spin a turbine and generate electricity.

Like dry steam and flash steam power plants, the water is returned to the earth via an injection well once used to heat the working fluid.

Enhanced Geothermal Systems

This system (also known as EGS) is a type of geothermal project that uses drilling and fracturing with geothermal fluid to provide liquidity and permeability in areas with existing dry underground rock. Water is then pumped through an injection well from which heat can be absorbed and converted into steam which powers a turbine. Unlike fuels like natural gas, this process produces no greenhouse gases or harmful emissions. It can, however, lead to increased seismic activity.