Geothermal energy: what you need to know
Last updated 12/23/2020
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.
What is geothermal energy?
Geothermal energy comes from heat within the earth that comes from the original formation of the planet as well as the continuous radioactive decay of materials in rocks. This heat drives volcanoes to explode, tectonic plates to move, 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.
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 large geothermal power plants. In general, a high temperature geothermal reservoir is generally more than 150°C and are found near areas of active volcanism.
Middle temperature reservoirs
Middle temperature geothermal reservoirs can be used to 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.
How does geothermal energy work? How do we use it?
The basic principle of any geothermal energy system is that energy from within the earth in the form of heat can be captured and used at the surface. Several technologies have been developed that use the earth's heat for a variety of 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 geothermal systems involve using the heat from the earth 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.
Geothermal energy can also be used for electrical generation. 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, Nevada, Utah, and Arizona.
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.
What are the different types of geothermal power plants?
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 extracted 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 that is extracted from within the earth is in gas form, and no liquid water comes directly out of the production well.
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 then is 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 it is 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 degrees C and 182 degrees 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 it is used to heat the working fluid.