In the summer of 2007, workers drilled 144 holes into the ground beneath a suburban Calgary neighborhood, each boring 35 meters into the earth. By the following winter, those holes were radiating stored sunlight back into 52 homes above, keeping families warm through -30°C nights without burning natural gas. The homes at Drake Landing Solar Community now get 97% of their heat from summer sunshine captured six months earlier.

The Physics of Patience

Thermal batteries work on a simple principle: some materials hold onto heat for a very long time if you insulate them properly. Water excels at this. It can store 4.2 kilojoules of energy for every kilogram you heat by one degree Celsius—one of the highest thermal capacities of any common substance. The challenge isn't storing heat. It's storing enough of it, in the right place, without losing it before you need it again.

Underground storage solves the insulation problem. Dig deep enough and the earth itself becomes your thermos. Seasonal Thermal Energy Storage (STES) systems pump hot water or antifreeze into the ground during summer, heating the surrounding soil and rock to temperatures between 27°C and 80°C. Come winter, that heat flows back out through the same pipes, warming buildings above.

The technology comes in two main forms. Aquifer Thermal Energy Storage (ATES) uses naturally occurring underground water reservoirs trapped between impermeable rock layers. Borehole Thermal Energy Storage (BTES) creates its own storage volume by drilling vertical shafts—typically 155mm wide and 50 to 300 meters deep—spaced 3 to 8 meters apart. Pipes run down each borehole, transferring heat to and from the surrounding earth.

Why Winter Heat Survives Until Summer

The counterintuitive part: these systems lose remarkably little energy over months. At Richard Stockton College in New Jersey, 400 boreholes drilled 130 meters beneath a parking lot have been operating since 1995. The system loses only 2% of stored heat over six months.

The secret is volume. Heat escapes through surfaces, but accumulates in volumes. A small cup of coffee cools in minutes. A massive underground heat store, insulated by surrounding earth, bleeds warmth so slowly that most remains available months later. The boreholes at Drake Landing heat a cylinder of earth roughly 35 meters deep and 35 meters across. By summer's end, temperatures at the center reach 80°C—hot enough to scald skin—while the surface above shows no sign of the furnace below.

Wells at the center of borehole fields produce about one-third more thermal energy than those at the edges, where heat dissipates faster into surrounding soil. System designers account for this by oversizing the storage volume and accepting modest losses as the cost of doing business.

The Drake Landing Blueprint

The Alberta project remains the most successful residential implementation. Eight hundred solar panels mounted on garage roofs generate 1.5 megawatts of thermal power on summer days. That heat flows into two 120-cubic-meter short-term storage tanks at a central Energy Centre, then gets pumped underground into the borehole field.

During winter, the process reverses. Hot fluid circulates up from the boreholes, through a heat exchanger, and into the district heating network serving all 52 homes. By the fifth year of operation, the system achieved a 90% solar fraction—meaning only 10% of heating came from the backup natural gas boiler.

Homeowners pay about $60 monthly for heating, comparable to conventional systems. But each house produces 5 tonnes fewer greenhouse gas emissions annually and uses 110.8 gigajoules less energy than typical Canadian homes. The $7 million construction cost included substantial government subsidies, raising questions about economic viability without public support.

Where Permafrost Meets Solar Storage

A 2025 study by the National Renewable Energy Laboratory tested whether BTES could work in Fairbanks, Alaska, where annual heating demand exceeds cooling demand by a factor of 5.6 and much of the ground stays frozen year-round. The modeling examined 40 boreholes at 91 meters depth over a 20-year period.

The system functioned even with 50% to 90% of surrounding ground frozen. Permafrost, it turns out, can actually help. Frozen soil conducts heat differently than thawed soil, but it still conducts. The ice acts as additional thermal mass, stabilizing temperatures and reducing unwanted heat migration. The study suggests BTES could work almost anywhere humans need heating—even in climates where it seems absurd to talk about storing summer warmth.

The Netherlands Standardizes Underground Heat

While North American projects remain experimental showcases, the Netherlands has installed over 1,000 ATES systems. They're now a standard construction option for commercial buildings. Germany's Reichstag has used ATES since 1999, drawing from two aquifers at different depths to heat and cool the seat of federal parliament.

Europe's adoption reflects different economics and geography. Dense cities with limited surface area favor underground solutions. Higher energy costs make capital-intensive storage more competitive. Favorable geology—particularly the abundant aquifers beneath Northern Europe—provides ready-made storage volumes without drilling.

The technology faces real barriers. BTES boreholes cost thousands of dollars each. Drake Landing's 144 boreholes represent a substantial fraction of the $7 million budget. Drilling permits can be difficult in areas with groundwater concerns. The systems require district heating networks or large single buildings to justify the infrastructure investment. A single home can't economically drill its own borehole field.

When Summer Becomes a Fuel Source

The deeper implication: seasonal thermal storage decouples energy supply from energy demand. Solar panels produce maximum output when heating needs are minimum. Wind patterns, hydroelectric flows, and industrial waste heat all vary seasonally. STES systems let us harvest energy when it's abundant and cheap, then deploy it when it's scarce and expensive.

This matters more as electricity grids add intermittent renewables. Battery storage helps balance daily fluctuations, but seasonal storage addresses a different problem. Heating represents roughly half of global energy consumption. If even a fraction of that demand could shift to stored summer heat, the impact on winter peak loads would be substantial.

Drake Landing's boreholes have a life expectancy exceeding 100 years. The pipes will need replacement, but the holes themselves become permanent infrastructure. We're not just storing heat. We're building thermal reservoirs that could serve buildings for generations, turning summer sunshine into a fuel source that never depletes and never emits carbon.