New research from the National Renewable Energy Laboratory (NREL), a U.S. Department of Energy national laboratory, has demonstrated a groundbreaking method to reliably heat buildings in extreme cold regions like Alaska by storing and reusing heat underground. Published on June 17 in the journal Energy & Buildings, the feasibility study examined a 20-year period, confirming that borehole thermal energy storage (BTES) systems, integrated with geothermal heat pumps (GHPs), could consistently meet the heating demands of two U.S. Department of Defense buildings in Fairbanks, Alaska.

The analysis, 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, modeled how waste heat from a nearby coal plant could be captured during summer months. This heat would then be stored underground within the BTES system and subsequently drawn upon in the winter to warm the buildings via GHPs. This project was supported by NREL's Applied Research for Communities in Extreme Environments program and partnered with the U.S. Army Corps of Engineers' Cold Regions Research and Engineering Laboratory.

BTES operates on the principle of a rechargeable thermal battery, utilizing a network of narrow, vertically drilled boreholes. During warmer periods, excess heat is injected into these boreholes, where it is insulated by the surrounding soil and rock. In colder months, a circulating water-antifreeze solution moves through the boreholes, collecting the stored heat and delivering it to the building's GHP system. This method allows the heat pump to efficiently transfer heat into the building's HVAC system using warmer fluid from the ground, rather than extracting it from frigid outdoor air.

NREL researchers utilized EnergyPlus software to model the heating and cooling demands of the cold-climate buildings, revealing an annual heating demand 5.6 times higher than cooling demand—a typical imbalance for Alaskan climates. To address this, the team designed a system of 40 boreholes, each 91 meters deep, situated approximately 100 meters from the buildings. Their 20-year performance modeling included two scenarios: one with a five-year preheating phase of the ground subsurface via hot water injection before regular operation, and one without preheating.

Results indicated that wells at the center of the borehole field consistently produced about one-third more thermal energy than those on the outer edges, suggesting heat loss to the surrounding ground from peripheral wells. Furthermore, systems that underwent preheating demonstrated superior performance, achieving higher underground temperatures and greater thermal energy production during the initial eight years of operation. Hyunjun Oh emphasized the breakthrough, stating, "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. A geothermal heat pump system can supply higher efficiency if we consider seasonal or storage-system-integrated operations." The study also confirmed Fairbanks' local subsurface is suitable for other geothermal systems, with a geothermal gradient of approximately 27.9 degrees Celsius per kilometer, enabling access to usable heat at relatively shallow depths. This research marks a significant step towards more efficient and sustainable heating solutions in challenging cold climates.