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Old Tech, New Tricks

Grad students are adapting centuries-old engine designs to capitalize on abundant geothermal energy

May 03, 2019 •

The Challenge: Is it possible to harness low-temperature geothermal energy?

The Research: Adapt a traditional Stirling engine so that it can be powered by temperatures not much hotter than a cup of tea

The Players: Graduate students David Miller, '16 BSc(MechEng), Jason Michaud, '16 BSc(MechEng), and Calynn Stumpf, '16 BSc(MechEng), working under David Nobes, professor of mechanical engineering

The potential for geothermal energy surrounds us. "Geophysicists tell us there's more geothermal energy under the ground than there is hydrocarbon energy in Alberta," says grad student Calynn Stumpf, '16 BSc(MechEng). But it turns out that not all geothermal energy is equal. "The problem is, here it's all below 100 degrees Celsius," he says. It's difficult to harness such low temperatures to create electricity, but now a team is adapting a two-century-old engine design to help solve a modern challenge.

Stumpf is part of a team of grad students - which includes David Miller, '16 BSc(MechEng), and Jason Michaud, '16 BSc(MechEng) - working to adapt a traditional Stirling engine to harness energy at these less-than-100 C temperatures. The Stirling engine is not the same as the internal combustion engine in your car, but it is similar in some key ways.

"Engines turn over because of a pressure difference," says Miller. "In a Stirling engine, it's hotter on one side of the engine and colder on the other side." As gas inside the engine moves from one side to the other, it is heated or cooled. This leads to pressure cycles that force the piston to move, he explains. The motion of the piston turns a shaft, and it's the power from this rotation that can be harnessed for things like locomotion or electricity.

In your car, the heat comes from burning fuel and the temperatures can reach several thousand degrees.

But, unlike the ones in our cars, Stirling engines can convert heat from any source into electricity. Since the heat doesn't have to come from the burning of hydrocarbon fuels, the engines can generate electricity without carbon dioxide emissions.

The team's Stirling engine taps into an "ultra‑low" temperature source. In other words, these engines can run when the hot side of the cylinder is 95 degrees Celsius - hot enough to make some ramen or a decent cup of tea - and the cold side just above freezing. Suddenly, the low-temperature geothermal energy that is abundant in Alberta is a viable source of power. But a larger temperature differential generates even more power and that's where Alberta's bone-chilling winters can be a plus, explains Michaud.

This group of Alberta-born students - Miller is from St. Paul, Stumpf from Lacombe and Michaud from Sherwood Park -- can easily picture massive Stirling engines dotting the landscape, taking advantage of the province's low-grade geothermal heat.

They aren't there yet, though. They've built lab-bench-scale Stirling engines, but you'd need more than 10 of them to power a single 60‑watt light bulb. The team is currently reworking designs to maximize the power output. They will be using the data from this work to design a bigger engine - and one that's cost-effective. "It becomes more expensive to build a bigger engine," says Michaud. "It's important that we are able to predict the power output of these engines before we start committing those resources." The three students graduate this year, so a new team of students will carry their work forward. 

-with files from Kenneth Tam

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