Inside a Borehole Heat Store

What is borehole thermal energy storage?

A BTES field looks like a ground-source heat pump loop turned into a heat bank. Holes are drilled in a regular pattern, each holding a plastic U-shaped pipe, and the holes are sealed with grout. Warm fluid runs through the pipes and the surrounding ground slowly warms. Later the flow is reversed and the ground gives its heat back.

It is one of the main ways of doing seasonal thermal energy storage. Unlike a water tank, nothing is pumped into the ground itself, so the field is a closed circuit and the surface can be covered with a lawn or a park.

How is the field laid out?

Designers pick hole depth, spacing and the number of holes from the heat they need to store and the thermal properties of the ground. Holes are usually placed on a tight grid so the heated zones merge into one body. Pipes are connected at the surface in groups, which lets operators isolate a section if something goes wrong.

Drake Landing used 144 boreholes about 35 to 37 m deep, spaced 2.25 m apart, in a roughly circular field about 35 m across. Details of that design are on the borehole storage page. Compare this with a ground-source heat pump, where spacing is wider because the aim is to avoid heat build-up; see geothermal drilling depth and spacing.

Why does the hot zone sit in the middle?

When the store is charged, hot water enters at the centre and flows outward. It cools as it gives up heat, so the centre becomes the hottest region and the edge the coolest. When heat is taken back, water enters at the edge and moves inward, picking up heat as it goes and leaving at the centre at its warmest. This keeps the best heat where it is most useful and reduces losses at the boundary.

Drake Landing's core was designed to approach 80 C by late summer, and monitoring recorded an average core temperature of about 42 C in October 2007 and about 54 C in May 2012.

What goes wrong with heat losses?

The ground never holds heat perfectly. Some warmth leaks sideways into undisturbed soil, some rises through the surface, and some is carried off by moving groundwater. Insulation across the top and a compact, round shape reduce the first two. A site with little groundwater flow reduces the third.

The first years are the costly ones. The ground around the field starts cold and has to be warmed before the store can deliver a high share of its heat. That is the subject of the ramp-up guide.

Where does BTES fit best?

It suits places with drillable, low-flow ground and a group of buildings to serve, ideally with a district heating loop. Compare it with pit storage, which needs no drilling but needs a large liner and cover, and aquifer storage, which needs a suitable water layer. The Drake Landing field was wound down as the community was decommissioned from 2024. For the current state of the ground, the Town of Okotoks and ATCO are the places to ask.

How is the ground charged and discharged in a year?

A simple yearly rhythm drives the field. From spring to autumn, spare solar heat goes in whenever collectors are producing more than the homes need. From late autumn through early spring, the flow reverses and the field supplies the loop. In a mild spell the store may be left alone, and in the shoulder months both directions can happen within a few weeks. Monitoring at Drake Landing showed heat flowing into the field in spring and summer, with nothing drawn out in some May months at all. Operators watch the temperature of the core and the return water to decide when to switch modes. A well-run field is quiet: pumps, valves and sensors do the work, and the park above it looks like any other lawn.

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