Storing Summer Heat for the Winter

What does seasonal thermal energy storage actually do?

A house in southern Alberta needs most of its heat between October and March. A solar collector makes most of its heat between May and September. Seasonal storage closes that gap by moving heat across half a year instead of half a day.

The store is not a battery and it holds no electricity. It is a very large, well-insulated lump of something that holds warmth: soil, rock, water or a mix. Heat goes in during the warm months, sits there, and is drawn back out when the weather turns. The general idea of thermal energy storage covers day-length stores as well; the seasonal kind simply works on a much longer clock and at a much larger size.

Which types of seasonal heat store exist?

Four designs show up again and again.

• Tank stores: a big insulated water tank, partly or fully buried. Simple, but cost climbs quickly with volume.
• Pit stores: a lined, covered excavation filled with water or gravel and water. See how pit stores work.
• Borehole stores: hundreds of metres of pipe in drilled holes that heat the surrounding ground. The detail is in the guide to borehole thermal energy storage.
• Aquifer stores: warm and cold water pumped into and out of a natural underground water layer, covered in aquifer thermal energy storage.

Which one fits depends on geology. Boreholes need ground that can be drilled and does not move water away. Aquifers need the right water-bearing layer. Pits need space and a watertight liner.

Why is the size so important?

Heat leaks out through the surface of a store, and the amount of surface grows more slowly than the amount of volume. Double the width of a cube and the surface area quadruples, but the volume grows eightfold. Bigger stores therefore lose a smaller share of what they hold. That is why seasonal storage tends to make sense for groups of buildings rather than a single house.

A rough worked example shows the logic. Suppose a store holds 1,000 units of heat and loses 300 over the summer and winter, a 30% loss. If the same design were scaled up tenfold in volume, the surface would grow by about 4.6 times, so the loss would be closer to 14%. The numbers are illustrative, not measured, but the direction is reliable.

How did Drake Landing use it?

Drake Landing Solar Community in Okotoks, Alberta, put the borehole approach to work. In summer, 800 flat-plate collectors on garage roofs fed heat to short-term tanks and then into 144 boreholes about 35 m deep under a park. In winter the flow reversed and the warmed ground supplied a loop serving 52 homes. Monitoring reported a 97% solar share of space heating by 2012 and 100% in the 2015-16 heating season. The borehole field and the full heat cycle are described on their own pages.

The store did not start at full strength. It spent its first years warming the ground it sat in, an effect explained in why seasonal storage takes years to reach full output.

What are the limits?

Seasonal storage is not a household upgrade you order from a catalogue. It needs a heat source, a distribution system that works at modest temperatures, land, and a patient owner. Low-temperature delivery, such as a hydronic system or a warm-air coil, helps because the store never gets as hot as a boiler. Maintenance matters too: Drake Landing was wound down from around 2020 onward after deterioration and upkeep problems, and decommissioning began in 2024. The technology worked; keeping a one-of-a-kind system running over decades proved harder.

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