Researchers at the National Renewable Energy Laboratory set out to test whether summer heat could be stored underground and reused to warm buildings in winter.
The team, which published its study in Energy and Buildings, focused on one of the coldest places in the United States: Fairbanks, Alaska.
Borehole thermal energy storage collects waste heat, like from a nearby coal plant, during summer and stores it underground in a field of deep, vertical shafts. When winter hits, that heat is pulled back and distributed through geothermal heat pumps to warm buildings.
In Fairbanks, where heating demand is over five times higher than cooling demand, this kind of system could cut dependence on dirty energy sources.
To test its potential, geothermal research engineer Hyunjun Oh and his team, with assistance from the NREL Alaska Campus and Army Corps of Engineers, modeled a 20-year BTES setup for two Department of Defense buildings in Fairbanks.
With 40 vertical boreholes drilled 91 meters deep and spaced about 100 meters from the structures, the modeling suggested the system met heating needs for two decades — even without preheating.
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When the boreholes were preheated by injecting hot water for five years, thermal output rose, especially in the early years. The team also found that boreholes at the center of the design held better than those at the edges, highlighting opportunities to reduce energy loss in future designs.
Most geothermal systems need naturally hot reservoirs deep underground. BTES works differently. Builders can install BTES in cold regions and charge it with waste heat from nearby sources. That setup could work well in remote towns with access to big utility networks.
Seasonal thermal energy storage in Finland shows this setup has the potential to be scaled and cut pollution from heating. A soil-based storage project from Kaunas University of Technology in Lithuania tested a similar system, backing up BTES' potential in colder regions.
While thermal storage targets winter heating, solar, including community solar, offers year-round savings on power bills. In many homes, rooftop panels can push monthly costs close to zero, depending on where you live and how much power you use.
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Oh said the Fairbanks study is one of the first in the U.S. to explore how BTES could work with geothermal heat pumps. Since the study looked at just one site, the team recommends tailoring future systems to match local heat sources and energy needs.
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