Geothermal energy is usually produced by tapping into the Earth’s natural heating and cooling power in one of two ways: with power plants or with ground-source heat pumps, according to Chuck Kutscher, fellow and senior research associate at the University of Colorado’s Renewable and Sustainable Energy Institute. Geothermal plants power turbines to produce electricity, but instead of coal or natural gas, they use underground hot water (think geysers in Yellowstone National Park) or steam.
About five dozen geothermal plants produce just a small fraction of the nation’s electricity, just over a third of a percent, an output that’s been pretty steady over the last decade. Several of the Geysers units in Sonoma County, California, have been generating electricity for nearly a half century, while new plants are coming online in California to help that state meet its carbon-free, renewable energy goals.
On a smaller scale, ground-source heat pumps can be used anywhere to keep indoor spaces like homes and office buildings comfortable by carrying up cool air from underground during summer and warming them with underground heat during the winter. Kutscher said ground-source heat pumps are most cost-effective when they’re part of new building construction and for whole neighborhoods.
“It’s something that makes more sense for a new building than to retrofit an existing building,” he said, pointing to a development called the Whisper Valley Community in Texas where drilling, trenching and piping was done for many houses at once. “It becomes an economy of scale [with] all the houses heating and cooling more efficiently.”
These large- and small-scale technologies can both be used to facilitate the transition from coal and natural gas to emissions-free energy, he said. “As we get more solar and wind on the grid, and these are variable sources, the idea of having a constant source can be attractive.”