Tapping aquifers for heating and cooling

Using the earth to heat and cool buildings through ground-source geothermal systems has been around for decades.

But tapping underground water to do the same thing has been something rarely seen outside northern European countries until now.

Developed at the University of Minnesota, the Twin Cities-based startup Darcy Solutions debuted a creative approach to an aquifer thermal energy system in 2018. After doing a handful of small pilots, the firm has grown significantly as building owners seek to decarbonize heating and cooling and employ more efficient systems than traditional natural gas-based ones.

Darcy Director of Marketing and Strategy, Robert Ed, said the company has been "busy with project work,” including more than 30 installations for schools, government and commercial buildings. Offering a few examples, Ed said Darcy has projects at two public schools in St. Paul, another two at Winona schools, and at Woodbury's Central Park renovation.

The technology works best in buildings of 20,000 square feet or more, he said. Darcy’s primary innovation was to place convection-based heat exchangers directly into aquifers. The heat exchanger captures the warmth of flowing groundwater and transfers heat to a separate closed loop system connected to a building.

Key to Darcy's growth has been the technology's ability to operate in cold climates, incentives in the Inflation Reduction Act, stricter state building guidelines and its small footprint compared to traditional ground-source geothermal systems, Ed said.

Both ground source and geothermal and aquifer-based geoexchange offer an affordable technology to heat and cool larger buildings in northern climates. "Geothermal is the most efficient technology I know of for heating and cooling and buildings," Ed said. "Heating and cooling and buildings are one of the primary drivers of emissions from the whole [building] sector, so if you can solve for that, you're lowering your carbon footprint and improving your project economics."

In an article on the advantages of geothermal energy, the consultancy WIPFLI suggested that geothermal energy saves 67% compared to natural gas heating. Geothermal systems still use electricity, but the amount they must buy is significantly less than natural gas.

"In a northern climate, being able to use geothermal energy and not having to expend a lot of energy to provide thermal capacity [for heating] is a big advantage," Ed said.

Potential geothermal customers in the past have been scared off by cost, but the IRA makes the payoff much quicker. Ed said that clients installing Darcy start with a base tax rate of 6%, which can grow to 30% by paying prevailing wages to workers on geothermal projects. He said that clients can add another 20% or more as a tax credit through depreciation.

The tax credit lowers the first cost of geothermal, making it competitive with traditional natural gas HVAC systems, Ed said, while providing other advantages. "It lowers that first cost to a point where you're beating legacy technologies using fossil fuels," he said. "You're also getting this operational efficiency that you don't necessarily get with those systems."

Darcy's other advantage comes down to its small footprint. Geothermal systems often require dozens and sometimes hundreds of wells for more significant buildings. Rather than use the ground, Darcy installs a heat exchanger in a water well connected to an aquifer, of which Minnesota and the Midwest have plenty.

The heat exchanger captures the water's temperature but does not use any of it. One Darcy well replaces roughly 20 or more geothermal wells, he said. The smaller footprint allows buildings in dense urban neighborhoods with no land for geothermal to tap an aquifer-based geoexchange system.

Saint Paul Public Schools Indoor Air Quality Coordinator Angela Vreeland said Darcy's small footprint was more important than the financial incentives. Many schools, including the two with Darcy systems, have limited land available for a geothermal wellfield of the size needed to provide their buildings with heating and cooling.

One of the region's biggest contractors has worked with Darcy on several projects. Kraus Anderson's Matt Stringfellow, manager of the Mechanical & Electrical Systems Group, said geothermal, in general, remains a costly option. But it's perhaps the best option for government agencies seeking to reduce carbon emissions.

"It's a commitment to sustainability," Stringfellow said. "That usually tends to be more public projects than private these days."

Government clients also like geothermal and aquifer thermal projects because they help meet the Minnesota Sustainable Building Guidelines , or SB 2030. Projects receiving state money must abide by the guidelines, which are strict enough almost to require a geothermal system, Stringfellow said.

While innovative, Stringfellow said that Darcy's technology isn't so out of the box that clients fear being the first ones to try it. If a system fails, a rig can be used to pull a malfunctioning heat exchanger out of a well for repair. "The servicing side of it is pretty straightforward," he said.

As it has grown, Darcy has hit an occasional hiccup. A developer pulled out of a Malcolm Yards project that looked like a go last year and could have included several buildings served by Darcy wells. Geothermal energy projects can take time to get to the finish line. Dan King, Darcy's chief technology officer, said projects "have a long sales cycle" lasting two years from concept to installation.

But Ed said Darcy recently expanded into Wisconsin and plans to add more states soon. Geothermal, in general, and Darcy, in particular, will likely continue to grow as companies and communities look for opportunities to discontinue natural gas.

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