Energy Excursions

Geothermal Gradient 

Some geothermal energy is available where water reaches the surface at hot springs like this one in Yellowstone National Park in Wyoming. Bodies of hot groundwater can be found in many areas with volcanic or tectonic activity, and these groundwater reservoirs can reach the surface, creating geysers and hot springs.

But because most of Earth’s geothermal energy is dry and at depth, we must know something about how the temperature in the earth increases with depth so that we can drill to reach this heat source.

Geothermally heated water reaches the surface at hot springs like this one in Yellowstone National Park.
In the high mountains surrounding the Yellowstone Plateau, water falls as snow or rain and slowly percolates through layers of porous rock, finding its way through cracks and fissures in the earth’s crust created by the ring fracturing and collapse of the caldera. With the rise of superheated water through this complex plumbing system, the immense pressure exerted over the water drops as it nears the surface. The heat energy, if released in a slow steady manner, gives rise to a hot spring, the most abundant and colorful thermal feature in the park.1U.S. EPA. (2022, September 26). Geothermal Heating and Cooling Technologies.

Geothermal Gradient

The rise of temperature vs. the increase in depth within the earth is called the geothermal gradient. This gradient can vary in different spots across the planet; a typical value for a geothermal gradient is 25 ˚Celsius per 1 kilometer (25 ˚C/km) or 2 °Fahrenheit (°F/100 ft).2National Geographic. (n.d.). Geothermal Energy. National Geographic Education.

Steam and hot water pockets can serve as geothermal sources for generating electricity; however, most geothermal power plants must enhance dry geothermal through drilling and injection of water. Typically, the water injected into the ground through one well is heated at depth and that thermal energy is returned to the surface through another well as hot water or steam. Geothermal drilling can range from a few feet (low temperature geothermal) to 2–3 miles deep.3Office of Energy Efficiency & Renewable Energy. (n.d.) Geothermal Basics.

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