ST. PAUL, Minn.—Nearly half a century ago, the U.S. Department of Energy launched a clean energy experiment beneath the University of Minnesota with a simple goal: storing hot water for months at a time in an aquifer several hundred feet below ground.
The idea of the so-called seasonal thermal energy storage was to tuck away excess heat produced in summer, then use it in the winter to warm buildings.
Now, 45 years after the first test wells were drilled under the university’s St. Paul campus, one of the first large-scale aquifer thermal energy systems in the nation is being built less than 10 miles from the original test site.
The Heights, a mixed-use development rising from a former golf course on the city’s Greater East Side, will tap thermal energy from an aquifer 350 to 500 feet below ground. Groundwater drawn from wells spread across the northern half of the 112-acre development, coupled with high-efficiency electric heat pumps powered in part by solar panels, will provide low-cost heating and cooling with little to no greenhouse gas emissions for 850 homes and several light-industrial buildings.
The LED of Heating and Cooling
Compared to conventional heating and cooling methods, such as gas boilers and air conditioners, aquifer thermal energy storage can significantly reduce greenhouse gas emissions. According to the 2024 study, it can cut emissions by up to 74 percent. That’s an efficiency increase similar to switching from conventional incandescent lighting to light-emitting diodes, or LEDs, according to the Department of Energy.
“This is the LED version of heating and cooling,” said Yu-Feng Lin, director of the Illinois Water Resources Center at the University of Illinois Urbana-Champaign, who did not participate in the 2024 study.
Aquifer-based heating and cooling works similarly to air-source heat pumps, an increasingly common electric technology that relies on outdoor air to warm and cool buildings. The devices are highly efficient. However, when temperatures spike in the summer or plummet in the winter, the fans and compressors used by air-source heat pumps have to work harder, reducing their efficiency when they are needed most.
While the air temperature in Minnesota can swing from the 90s in the summer to single digits in the winter, the aquifer beneath The Heights remains approximately 50 degrees Fahrenheit throughout the year.
“Think about how much energy you are saving there,” said Lin, who is a principal investigator for an international consortium led by the U.S. Geological Survey that develops standards and best practices for thermal energy storage. “A lot of people think about geothermal as just hot lava [and] hot steam that pushes a turbine to generate electricity. That’s not all of geothermal.”