Can Underground Thermal Batteries Warm Northern Cities in Deep Winter?

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Having grown up in Canada’s north and spent far too many winters trudging through snowy downtown streets in Toronto, Ottawa, and Edmonton, I know firsthand just how brutal Canadian winters can be—and how urgently our cities need practical, scalable, low-carbon heating solutions. Even if you haven’t spent months navigating icy sidewalks, you’ve likely heard Canadians joke that there are two seasons: winter and construction. It’s not far off.

Cities like Edmonton face annual heating demands measured by around 5,000 heating degree days—a fancy way of quantifying just how relentlessly cold a place is. A “heating degree day” is simply one degree Celsius below a baseline of 18°C, accumulated over each day throughout the year. So when a city racks up 5,000 heating degree days annually, it means buildings there require huge amounts of energy to stay comfortable, usually from fossil fuels.

As a note, this is one in a series of articles on geothermal. The scope of the series is outlined in the introductory piece. If your interest area or concern isn’t reflected in the introductory piece, please leave a comment.

To decarbonize urban heating at the scale needed, seasonal thermal energy storage (STES) with ground-source geothermal could be pivotal. This technology captures summer heat—whether from solar thermal panels, surplus renewable electricity, or waste industrial heat—and stores it underground, retrieving it months later when temperatures plunge. It sounds ambitious, but it isn’t science fiction: district-scale projects in Canada and Europe already demonstrate impressive results, reducing fossil fuel dependency dramatically. Northern cities facing severe winters and dark, energy-intensive months stand to benefit the most.

And to be honest, I thought it was science fiction. I dismissed the idea out of hand for years based on assumptions of rapid loss of heat to the surrounding ground. That turned out to be true to a lesser extent than I’d assumed and also to be more easily fixed than I had assumed.

One of the best-known examples is the Drake Landing Solar Community in Okotoks, Alberta, just south of Calgary. Established in 2007, Drake Landing used roughly 2,300 square meters of solar collectors mounted atop neighborhood garages to harvest heat during sunny summer months, injecting that energy into a network of 144 underground boreholes. Over several seasons, these boreholes warmed to around 80°C, creating a thermal battery beneath residents’ feet. By year five, this underground heat source met over 90 percent—and in peak years, even 100 percent—of the community’s winter heating needs.

Impressive? Yes. Economically straightforward? Unfortunately, no. After about 17 years, the system faced expensive maintenance that ultimately led the community back to natural gas, the default in Alberta. Still, Drake Landing delivered an invaluable proof-of-concept: ground-source seasonal storage can reliably heat entire neighborhoods even through frigid Alberta winters.

Across the Atlantic, Denmark took another route with district-scale STES. Dronninglund, a town of around 1,350 households, built a thermal storage system centered around a giant, insulated water pit of about 62,000 cubic meters. Paired with nearly 38,000 square meters of solar collectors, the system captures summer heat at about 80°C and stores it efficiently—so efficiently that annual heat losses are kept below 10 percent. Today, Dronninglund’s seasonal thermal storage supplies half the town’s annual heat, delivering around 15,000 MWh per year. The economics also pencil out well: initial costs around €14.6 million were partly subsidized by renewables grants, but long-term operational expenses are minimal, mostly covering pumps and maintenance. Heat costs have stabilized near €50 per MWh—reasonably competitive with conventional heating, especially given rising carbon prices and fuel volatility.

Sweden offers another striking example at Stockholm’s Arlanda Airport, operating the largest aquifer thermal energy storage (ATES) system globally since 2009. Rather than boreholes or insulated pits, Arlanda uses natural groundwater aquifers as giant seasonal energy banks. During the hot months, cool groundwater (~6°C) chills the airport’s ventilation system, then the warmed water (around 20°C) is returned underground. Months later, as winter approaches, that same warmed groundwater is pumped back out to heat airport buildings and even melt snow from aircraft stands. Arlanda’s aquifer storage shifts about 22 GWh of thermal energy annually, equivalent to the needs of a city neighborhood of about 25,000 people. The system cuts external energy use by about 19 GWh per year, slashing emissions significantly—roughly equal to the electricity used by 2,000 typical homes annually. In Europe, aquifer storage has become almost routine in some countries: in the Netherlands, over 1,000 ATES systems are in operation, now a standard option for large buildings to meet seasonal cooling/heating needs​

As exciting as these projects are, seasonal thermal storage isn’t without challenges. First, underground heat storage tends to lose energy to the surrounding earth. Early years at Drake Landing saw losses over 60 percent, though performance improved steadily as the ground warmed up. Designers manage these losses by reducing the temperature difference between storage and surrounding earth, using insulation above the boreholes, or carefully selecting geological sites to minimize groundwater flow. Another practical step is adding heat pumps, allowing stored heat at moderate temperatures—say around 30–40°C—to be boosted efficiently to distribution temperatures near 60°C. While these solutions add complexity, they significantly boost efficiency and reduce operational losses.

Economically, upfront capital costs for seasonal geothermal storage remain high—typically around €30 per cubic meter for large insulated pits and closer to €50 per cubic meter for borehole fields. But scale makes a huge difference: larger district-scale projects achieve far better economics than small installations, benefiting from lower per-unit costs. A bit of government support, smart carbon pricing, or integration with surplus renewable energy—especially excess summer wind or solar electricity—can further tip the scales towards economic viability. In northern cities, where fossil fuels carry heavy long-term environmental and financial costs, seasonal storage can provide stability and resilience against volatile fuel prices.

Detailed studies underscore the enormous potential for seasonal thermal storage in northern urban contexts. For instance, rigorous modeling for Helsinki—a city hardly known for mild winters—indicates that borehole storage, combined with solar collectors or renewable-driven heat pumps, could cover 90 percent or more of its heating needs under optimal conditions. Similarly, researchers in Oulu, Finland have considered using seasonal storage to bank waste heat from biomass-powered combined heat and power plants, shifting thermal energy from summer surpluses to offset heavy winter demands. In both scenarios, fossil fuel dependence is dramatically reduced, boosting urban sustainability and resilience.

Beyond carbon reductions and energy security, seasonal geothermal storage aligns with broader strategies for urban decarbonization and renewable integration. Not only can cities sharply cut fossil fuel heating demands—potentially 50 percent or more—but they can also smooth renewable electricity deployment by providing summer “batteries” for excess renewable generation. The scale of the potential impact is large. Even the relatively modest Kuujjuaq pilot study projected nearly 20 tonnes of annual CO₂ savings for a small building—scale that up to city districts, and the cumulative impact becomes transformative.

Applying Bent Flyvbjerg’s lens on risk and uncertainty—particularly his emphasis on black swans—seasonal geothermal storage emerges favorably compared to deep or enhanced geothermal systems. Seasonal storage relies largely on mature, proven technologies: borehole drilling techniques, aquifer management, conventional insulation, and solar collectors or heat pumps. While these projects can face cost overruns or performance shortfalls (as seen at Drake Landing, which encountered unexpectedly high long-term maintenance expenses), their risks typically fall into Flyvbjerg’s category of predictable surprises rather than true black swans. Costs and operational issues, though sometimes underestimated, remain within manageable, well-characterized boundaries, with relatively predictable failure modes such as heat loss or groundwater flow problems. In other words, while seasonal storage projects can blow budgets or timelines, these surprises rarely derail projects entirely, nor do they pose existential threats to surrounding infrastructure.

In contrast, deep or enhanced geothermal projects—like those attempting to inject water into hot rock kilometers beneath cities—sit squarely within Flyvbjerg’s black swan territory. Enhanced geothermal systems (EGS) repeatedly confront risks that are structurally unpredictable, notably induced seismicity and subsurface fractures causing unforeseen operational failures. The now-infamous 2009 Basel earthquake triggered by an EGS drilling project vividly illustrates the kind of catastrophic, unforeseen event Flyvbjerg warns against—one that can shut down a multi-million-dollar initiative overnight and permanently sour public acceptance. Deep geothermal’s black swan potential thus carries far greater tail-risk: massive, uncertain liabilities, unforeseen regulatory shutdowns, and public backlash.

Seasonal geothermal storage, while still complex, offers a safer, less volatile path forward—far closer to Flyvbjerg’s ideal of manageable, calculable risk, and certainly without the potential for dramatic, irreversible black swan calamities lurking beneath deep geothermal’s enticing surface.

But let’s ask another question, about what China is doing with this. It has aggressively pursued ground-source geothermal heating, amassing roughly 77 GW of installed district-scale geothermal capacity in recent years—an impressive scale by any measure. But before anyone gets overly excited and assumes this automatically translates into meaningful seasonal thermal energy storage, a cautionary note is in order. Despite all that capacity, China’s implementation of seasonal storage remains minimal. Most of China’s ground-source deployments are straightforward heat pump systems, tapping steady subsurface temperatures for immediate heating or cooling needs. They lack the sophisticated, large-scale underground reservoirs or borehole arrays that truly move thermal energy across seasons.

That’s not to say China hasn’t dabbled in STES. The 2008 Beijing Olympic Village featured an aquifer thermal energy storage system that successfully shifted heat seasonally, cutting annual energy consumption for heating and cooling by nearly half. Other demonstration projects, such as the Sino-Swedish SWECO pilot in Shijiazhuang, used borehole thermal storage combined with solar collectors, achieving about 40% efficiency at modest scale. But these remain rare exceptions rather than the rule.

A study assessing the potential for large-scale underground seasonal thermal energy storage in northern China, including their preeminent winter city Harbin, known for its massive ice buildings and sculptures festival, identified numerous suitable sites for STES implementation. However, this research primarily focuses on the theoretical potential rather than existing installations.

China’s massive geothermal rollout should thus be viewed carefully: while the headline numbers are enormous, almost none of this vast capacity meaningfully leverages seasonal thermal storage. That suggests that they’ve run the numbers and they don’t add up, countering the example of the 1,000 aquifer-based systems in the Netherlands.

Having braved countless freezing winters, the appeal of leveraging summer’s warmth to counter winter’s bite is intuitive to me. Seasonal geothermal storage—despite its upfront complexity and cost—offers northern cities a realistic, proven path away from fossil-fuel dependence. If Canadian and European cities want to truly break free from carbon-intensive winters, turning the earth and water beneath their streets into seasonal thermal batteries may be among their best opportunities yet.