Using Groundwater to Store Heat and Cold

What is aquifer thermal energy storage?

An aquifer is a layer of sand, gravel or rock that holds groundwater and lets it move slowly. ATES drills at least two wells into the same layer. In summer, water is pumped from the cold well, warmed by waste or solar heat or by a building's cooling system, and injected into the warm well. In winter, the flow runs the other way. The groundwater itself is the storage medium, and the surrounding sand and rock holds some of the warmth too.

It is one of the standard options in seasonal thermal energy storage, alongside boreholes and pits.

What does a site need?

Not every location can host one. Typical requirements include a layer with enough permeability to take and give up water, groundwater that moves slowly so the stored warmth does not drift away, and water chemistry that will not clog or corrode the wells. Local regulators also control groundwater use, so approvals are part of the plan. These checks are usually done by a hydrogeologist, and results decide whether the project goes ahead.

This site dependence is the main contrast with borehole storage, which circulates fluid in closed pipes and does not move groundwater.

How is ATES used for heating and cooling together?

Because the system keeps both a warm and a cold well, a building can use it twice. In summer it takes cold water from the cold well for cooling and sends the warmed water to the warm well. In winter the warm well supplies water to a heat pump, which lifts it to useful temperature, and the cooled return goes back to the cold well. The cycle recovers energy that would otherwise leave the building.

A heat pump is usually part of the loop. If you are unfamiliar with them, air-source versus ground-source heat pumps sets out the basics.

What are the risks and limits?

• Water chemistry can cause scaling or clogging in the wells and heat exchangers.
• A flowing aquifer can carry stored heat away from the wells.
• Regulation of groundwater varies, so permits and monitoring can be demanding.
• Drilling and well testing add up-front cost before any design is certain.

A small residential project seldom justifies those steps, so ATES is more common for large buildings or district systems.

Why did Drake Landing choose boreholes instead?

Drake Landing stored its heat in 144 closed-loop boreholes under a park, rather than in an aquifer. A closed field avoids groundwater permits for the store itself and keeps the water in the heating loop clean. The design is explained on the borehole storage page, and the broader picture is in how it works. The choice illustrates that the right store is the one that fits the site, not the one that looks best on paper.

What would a project team do first?

The usual first step is a desktop study of the geology, followed by test drilling and pumping tests to learn how the layer behaves. Only then do designers size wells and a heat pump. Because results can end a project, teams budget for the study as a stage of its own. The same stepwise thinking is useful for any large heating project, and geothermal drilling depth and spacing shows how a closed-loop field is tested before it is finalized. Anyone interested should ask the provincial water authority how groundwater use is licensed in their area, since the rules differ across Canada.

The takeaway is practical: ATES is a specialist tool. Where the aquifer exists, it can store a great deal of heat and cold at modest running cost. Where it does not, another store is the answer.

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