The Rise of Geothermal

A vast ocean of subterranean heat lies untapped by conventional geothermal energy development. Reaching it could be a game changer in the race for clean energy.

Several kilometers below where you’re sitting right now, the rock is hot enough to boil water. Brimming just below the surface of the Earth is a virtually inexhaustible supply of clean energy, produced by the decay of radioactive elements in the crust. Why is such an obvious source of energy still untapped? Why is geothermal energy so underground it’s been called "the forgotten renewable"?

Historically, one of the greatest factors limiting geothermal development has been constrained site viability. Conventional geothermal energy production requires the geographic convergence of three elements: heat, fluid, and permeability at depth. Since the concurrence of these ingredients is relatively rare, development has been limited to isolated hotspots.

Near the surface, the temperature of the Earth's crust increases about 25°C per kilometer, but in some places this heat gradient is much steeper. A number of hotspots have been developed for geothermal production in the western United States. The following map shows estimates of temperature at 500 meters below the surface.

Estimated Temperature at Depth



EGS Resource Potential



Cap MW

Though the western states heat up faster, temperature rapidly increases with depth across the whole country.

At only three kilometers below the surface, many locations in the West are hotter than 200°C. Existing geothermal energy development in the U.S. is concentrated in five western states.

Our conventional geothermal plants are built where heat coincides with naturally occuring fluid and permeability. Steam or hot water is piped to the surface through drilled wells, then used to power turbines that generate electricity. Each circle on the map represents an area of geothermal development, sized by its installed capacity in megawatts(MW).

The Geysers area, located north of San Francisco, is the world's largest dry-steam geothermal field. In 2014, the total installed capacity of all operating geothermal plants listed by the National Renewable Energy Lab(NREL) was over 2,300MW, the most of any country worldwide.

In addition, NREL catalogued a large number of projects representing nearly 4,000MW of capacity under development in 2014. The advent of a new approach called enhanced geothermal systems(EGS) means that many more sites may become viable in the near future.

EGS overcomes some of the limitations of conventional geothermal by creating man-made reservoirs, injecting fluid into the subsurface via wells in order to increase permeability. Research and development of EGS systems is ongoing in several locations, including the U.S. Department of Energy's FORGE site in Utah.

Despite the promise of EGS and the leadership of the United States in research and development, geothermal has lagged behind wind and solar in both production and publicity. One reason for the relatively slow growth of geothermal is the large amount of capital needed for development. According to the International Renewable Energy Agency(IRENA), the cost of installation for geothermal remains about double that of wind, solar, and hydropower. The following chart illustrates how the cost of installation for renewables flucuated from 2010-2018, in US dollars per kilowatt.

Installed Cost, US Dollars/Kilowatt 2018

Though the upfront cost of installation remains high, geothermal is extremely efficient once it's installed. If we take into consideration costs over the entire lifespan of a project, geothermal is extremely competetive with the other renewables. The next chart outlines the levelized cost of energy(LCOE) for each renewable from 2010-2018, again with data from IRENA.

Levelized Cost of Energy, US Dollars/Kilowatt Hour 2018

Though geothermal has lagged behind wind and solar in installed capacity, it's superior in several ways. Geothermal power plants have the capacity to operate around the clock, providing a reliable baseline source of clean energy. In addition, geothermal doesn't need sprawling tracts of land like wind and solar farms do, since most development occurs underground.

Right now, geothermal produces only 0.4% of total energy production in the United States. But researchers estimate EGS technology could expand our economically viable resources to 100GW, ballooning current production by a factor of 40. If we're able to better tap into our vast expanses of underground energy, geothermal may play a critical role in our transition to clean energy sources.

Data Sources