NREL Modeling Shows Geothermal and Borehole Thermal Energy Storage Can Reliably Heat Buildings in Extreme Cold

New Study Demonstrates Efficient Performance—Even on Frozen Alaska Soils

July 10, 2025 | By Hannah Halusker | Contact media relations

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New energy storage research from NREL, a U.S. Department of Energy national laboratory, has demonstrated a way to store and reuse heat underground to meet the heating demands of cold regions like Alaska.

Published on June 17 in the journal Energy & Buildings, the feasibility study examined a 20-year period in which borehole thermal energy storage (BTES)—a system that stores heating or cooling energy underground—could reliably supply heating to two U.S. Department of Defense buildings in Fairbanks, Alaska.

Through building energy usage and system performance modeling, researchers show how waste heat from a nearby coal plant could be captured during summer months, stored underground, and then drawn on in the winter to warm the buildings via geothermal heat pumps (GHPs).

The analysis was led by Hyunjun Oh, a geothermal research engineer in NREL’s thermal energy science and technologies research group, in collaboration with researchers Conor Dennehy, Saqib Javed, and Robbin Garber-Slaght at NREL’s Alaska Campus. NREL’s Applied Research for Communities in Extreme Environments program is a nonprofit, industry-based initiative dedicated to advancing extreme energy efficiency, building science, and socioeconomic research for communities in Alaska and the broader Circumpolar North. The project was also in partnership with the U.S. Army Corps of Engineers’ Cold Regions Research and Engineering Laboratory.

BTES relies on a network of narrow holes drilled vertically underground, known as boreholes, which act as a rechargeable battery for heat. During warmer months, waste heat can be pumped into the boreholes, where it is insulated by surrounding soil and rock until it is needed. In the winter, circulating pumps move a water-antifreeze solution through the boreholes to pick up stored heat and deliver it to the building’s geothermal heat pump. Rather than extracting heat from cold outdoor air, the heat pump uses this warmer fluid to efficiently transfer heat into the building’s heating, ventilation, and air conditioning system.

NREL researchers modeled the heating and cooling demands of the cold-climate buildings using EnergyPlus software and found that the annual heating demand was 5.6 times higher than the cooling demand—an imbalance typical of climates like Alaska’s, where winters are long and cold and summers are short and mild.

To meet this heating load, the team predesigned a system of 40 boreholes at a depth of 91 meters located about 100 meters away from the buildings, in alignment with regulatory guidelines and nearby land availability. They then modeled the 20-year performance of the BTES system, running simulations for two scenarios: one in which the ground subsurface was preheated for five years using a hot water injection before supplying heat to the buildings and one without preheating.

In both scenarios, wells at the center of the borehole field produced about one-third more thermal energy than those on the outer edges, likely because the outer wells lost heat to the surrounding ground. This finding offers insight into how borehole fields can be better designed and insulated for more balanced energy distribution.

Additionally, systems that underwent preheating before regular use showed even better performance, with higher underground temperatures and greater thermal energy production during the first eight years of operation compared to systems without preheating.

Altogether, the results point toward BTES as a reliable heating solution in cold climates, helping communities capture waste heat and use energy more efficiently.

Oh said that, while there have been extensive case studies validating GHP performance in cold regions of Europe, this is one of the first to show the potential of GHPs connected to BTES in the United States.

“This paper demonstrates that even cold subsurface conditions—like those in Alaska, where 50% to 90% of the ground has permafrost—can be used for heating,” Oh said. “A geothermal heat pump system can supply higher efficiency if we consider seasonal or storage-system-integrated operations.”

The study also showed that the local subsurface in Fairbanks is well suited for other kinds of geothermal systems, too. The research team used thermal response tests and previous literature to estimate the geothermal gradient—the rate at which temperature increases with depth—at about 27.9 degrees Celsius per kilometer.

This gradient allows usable heat to be accessed at relatively shallow depths underground, making it a candidate for direct use or a future distributed energy system, Oh said.

As this study was intended to assess the practicality of BTES and GHP at a specific location in Fairbanks, the team recommends comprehensive future analyses that go beyond the scenarios described here to better tailor energy systems to local conditions and available waste heat sources.

The study, “Techno-Economic Feasibility of Borehole Thermal Energy Storage System connected to Geothermal Heat Pumps for Seasonal Heating Load of Two Buildings in Fairbanks, Alaska,” was funded by the U.S. Department of Energy Geothermal Technologies Office and the U.S Department of Defense Environmental Security Technology Certification Program.