In the depths of Minnesota's winter, temperatures can plunge to a bone-chilling -10C or lower. Yet, beneath the surface, a constant thermal resource lies waiting to be harnessed. Nearly half a century after initial experiments, a pioneering clean energy project is set to transform how buildings are heated and cooled in the state.
From 1970s Experiment to Modern Development
Forty-five years ago, the US Department of Energy embarked on an innovative clean energy experiment beneath the University of Minnesota. The objective was straightforward yet ambitious: to store hot water for extended periods within an aquifer situated more than 100 metres underground. This concept of seasonal thermal energy storage aimed to capture excess summer heat and repurpose it for winter warmth.
Now, decades later, one of the nation's first large-scale aquifer thermal energy systems is being constructed less than ten miles from that original test site on the university's St Paul campus. The project, named The Heights, is rising from a former golf course on the city's Greater East Side as a mixed-use development.
How the Aquifer System Works
The system will tap into thermal energy from an aquifer located 100 to 150 metres below ground. Groundwater from wells distributed across the northern half of the 45-hectare development will be drawn using high-efficiency electric heat pumps. These pumps will be partially powered by solar panels, providing low-cost heating and cooling for approximately 850 homes and several light-industrial buildings while generating minimal greenhouse gas emissions.
Michael Ahern, senior vice-president for system development at Ever-Green Energy, the firm designing the system, explained that the groundwater could function as a thermal battery. "We can store excess heat during summer months for use in winter," he stated, highlighting the system's innovative approach to energy management.
A Technology Rediscovered
This project represents one of the first large-scale aquifer thermal energy initiatives to operate in the United States since Department of Energy field tests in the 1980s. Interestingly, while such systems remain relatively rare in the US, over 3,000 similar installations exist worldwide, with the Netherlands hosting the vast majority according to a 2024 study.
Marc Hoyer, an 82-year-old retired scientist who worked for the Minnesota Geological Survey and the University of Minnesota, contributed to the original St Paul project in the 1980s. He recalls team members taking turns sleeping in a trailer beside the wells, waking hourly to record temperature and pressure readings during round-the-clock monitoring. "I figured nobody cared about it," Hoyer admitted from his Maryland home, expressing satisfaction that the technology is finally receiving renewed attention.
Environmental and Efficiency Benefits
The environmental advantages are substantial. Compared to conventional methods, aquifer thermal energy storage can reduce greenhouse gas emissions by up to 74%, as noted in the 2024 study. The Department of Energy likens this efficiency improvement to switching from traditional incandescent bulbs to LED lighting.
"This is the LED version of heating and cooling," remarked Yu-Feng Lin, director of the Illinois Water Resources Center. The system operates similarly to air-source heat pumps but with a crucial advantage: while Minnesota's air temperature fluctuates from summer highs above 30C to winter lows below -10C, the aquifer beneath The Heights maintains a relatively constant temperature of approximately 10C year-round.
Lin, who leads an international consortium developing standards for thermal energy storage, emphasised the energy savings potential. "Think about how much energy you are saving there," he said, noting that charging parts of the aquifer with warm summer water can raise temperatures several degrees, enhancing winter heating efficiency. Conversely, cooling sections with cold winter water reduces summer air conditioning demands.
Practical Implementation and Costs
The aquifer geothermal approach differs from traditional geothermal systems that use rock and sediment layers. While costs are comparable, aquifer systems require less drilling but need access to shallow underground aquifers. Minnesota's abundant subsurface water makes it an ideal location for this technology.
"It kind of makes sense to utilise that local resource," observed Rob Thornton, president and CEO of the International District Energy Association.
The total construction cost for The Heights' aquifer thermal energy system—including wells, heat exchangers, and piping—is approximately £8.9 million ($12 million). This excludes heat pumps and building-specific equipment. Federal tax credits from the Inflation Reduction Act are expected to cover about 50% of the system's total cost, according to Ahern.
Community Impact and Future Potential
For local residents, the projected cost savings could be transformative. Cheniqua Johnson, a St Paul city council member representing Ward 7, highlighted the potential difference between paying $200-$300 monthly bills and reducing costs to under $100. "That is what we're striving for," said Johnson, who also serves as treasurer of the Saint Paul Port Authority. She noted that many community members have faced utility disconnections due to payment difficulties.
Darcy Solutions, a St Paul-based aquifer geothermal startup, is overseeing well drilling for a separate system that will heat and cool a new service centre for Xcel Energy, the site's first major employer.
Kristine Williams, Saint Paul Port Authority's chief development officer, sees this project as a harbinger of future industrial development. "I think that this type of industrial development, while it's unique today, is what we will see going forward," she predicted.
As Minnesota embraces this revived technology, the project stands as a testament to both historical innovation and contemporary environmental commitment, potentially setting a new standard for sustainable urban development across colder climates.
