Numerous decommissioned coal-fired power plants have the potential to become reliable backup or emergency energy sources for the grid, eliminating the dependence on fossil fuels. Instead, they can utilize thermal energy trapped in soil.

The idea involves accumulating a large mound of soil near the coal facility and embedding industrial heaters within it. During periods of low electricity demand, these devices transform inexpensive electricity into heat, storing it in the soil at around 600°C. When electricity demand peaks, the heat can be transferred from the soil through heated liquid pipes.

A generator linked to the turbine blades of a coal plant can convert this heat into supplemental energy. The heat transforms water into steam, turning the turbine blades to produce electricity. “Rather than burning coal to heat water for steam, we harness heat from the energy stored within the soil,” explains Ken Caldeira from Stanford University in California.

This type of energy storage is crucial in supporting renewable energy sources like wind and solar, which often generate power intermittently. Soil offers a more affordable, abundant, and accessible resource for long-term energy storage compared to alternatives like lithium batteries and hydrogen fuels.

“The most exciting aspect is the low cost of energy capacity, especially since it is significantly cheaper than other energy technologies,” states Alicia Wongel at Stanford University.

Nonetheless, this approach has its challenges. “In such systems, minimizing plumbing and electrical costs is crucial, yet can be difficult,” notes Andrew Maxson from the Electric Power Research Institute, a non-profit research organization based in California.

Most soil consists of naturally heat-resistant materials like silicon dioxide and aluminum oxide, which makes it “very resilient to heat,” says Austin Vernon from Standard Thermals in Oklahoma. His startup aims to commercialize this “thermal” technology, especially for repurposing retired coal power plants in conjunction with nearby solar and wind energy sources.

There are many retired coal facilities across the United States. Close to 300 coal-fired power plants were shut down between 2010 and 2019, and an additional 50 gigawatts of coal capacity is expected to reach retirement age by 2030. In the late 2000s, cheaper natural gas and renewable energy began to outcompete coal.

Christian Phong from the Rocky Mountain Institute, a research organization in Colorado, views the idea of repurposing defunct coal plants positively. “This provides an opportunity for local communities to engage in the clean energy transition, generating jobs and additional tax revenue while navigating the shift away from coal,” he remarks.