

How Geothermal Systems Work and Where They Fit in the Energy Industry

Emily Folk | Conservation Folks


Under the crust of the earth, there's a layer of magma, which is hot and molten rock. Due primarily to the decay of naturally radioactive substances like uranium and potassium, this layer has a consistent source of heat.

Regions that have active or relatively young volcanoes have the highest subsurface temperatures. These high-heat areas are typically at tectonic plate boundaries or places where the crust is thin enough to allow heat to pass through. These aren't the only places that have geothermal energy available though. At depths between 10 and several hundred feet under the earth's surface nearly anywhere on the planet, there's a steady supply of heat that can be used to heat homes and buildings directly.

Geothermal heat can be used in several ways. You can harness it to produce renewable energy at complex power stations. Small systems can also provide direct heating to individual buildings by circulating heat through a pumping system. Let's take a closer look at these various uses of geothermal energy and how they fit into the energy industry.


Ground Source Heating and Cooling

One of the most common uses for geothermal energy is using the consistent temperature of the earth to heat and cool a building. These kinds of geothermal systems use what's known as a ground source heat pump circulate a liquid through pipes that travel underground and then back into the house. When this liquid is underground, its temperature changes. During the winter, the liquid heats up while underground. A heat pump then extracts the heat and distributes it throughout the home. In the summer, the heat pump takes the heat from the home and sends it into the ground, while the liquid transfer cooler temperatures from underground into the house.

Ground source heating and cooling systems don't require the burning of fossil fuels, so it's considered an environmentally friendly alternative to traditional HVAC systems. The typical cost of one of these systems is around $30,000, but they can save their owners' up to 80 percent off of their heating and cooling costs, meaning you can recoup the costs of the system in about 10 years.


Direct Use of Geothermal Energy

The heat generated underground increases the temperature of water in geothermal reservoirs, producing steam. This heat and steam can be used directly. In the past, many people used natural hot springs for bathing and cleaning. Whether these people realized it or not, they were using geothermal resources. Today, we use wells, piping and heat exchangers to capture this hot water and use it for purposes such as heating spas, melting ice on roadways and heating water on fish farms.


Geothermal for Power Generation

Geothermal energy can also be used for producing power. Geothermal power plants produce renewable energy by using steam or hot water created by natural sub-surface heat to drive turbines that generate electricity.

There are several types of geothermal power plants:

  • Dry steam power plants use steam from underground to turn a turbine which generates power. This steam can be released into the atmosphere or pumped back underground.

  • Flash steam power plants are slightly more complex. They pump pressurized water, which is between 300°F and 700°F, and then reduce the pressure, which immediately turns the water to steam. This sudden creation of steam turns the turbine. The system then cools the water and pumps it back underground.

  • Binary power plants pump pressurized hot water like flash steam plants do. However, the tube that pumps the pressurized water is connected to another tube that contains low-pressure water. When the hot, pressurized water flows into the second tube, it heats the water in the second tube, boiling it and driving the turbine. The water from the first tube then gets pumped back underground.

Only 13 states currently have known geothermal resources that can be used for energy generation, but with further exploration, we may find more. Today, geothermal power plants produce 0.4 percent of total U.S. utility-scale electricity production. As we continue to look for more renewable energy resources though, the importance of geothermal energy may increase, especially as more homeowners realize the savings and environmental benefits that ground source heating and cooling can provide.

Emily Folk - Contributing Author

Emily Folk - Contributing Author


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