A large-scale development in Minnesota is bringing Aquifer Thermal Energy Storage (ATES)—a form of Seasonal Thermal Energy Storage (STES)—back into the mainstream. By using underground aquifers as seasonal batteries for heat and cold, the system delivers lower-cost, lower-emissions heating and cooling for entire communities.
Forty-Five Years After the City’s First Study
Nearly half a century ago, the U.S. Department of Energy conducted a clean energy experiment beneath the University of Minnesota campus, storing hot water in aquifers more than 100 meters underground for months at a time. Forty-five years later, one of the first large-scale aquifer thermal energy systems in the United States is now under construction less than 10 miles from the original test site.
The Original Concept
The core idea behind seasonal thermal energy storage is straightforward: excess heat is stored underground during the summer and retrieved in winter for space heating. Conversely, cold can be injected during winter and recovered in summer to reduce air-conditioning demand.
The Heights Development
“The Heights” is a mixed-use development located on the site of a former golf course on the eastern side of the city, covering approximately 45 hectares. The project will draw thermal energy from aquifers located 100 to 150 meters below ground to create a community-scale heating and cooling system.
How the System Supplies Energy
Multiple groundwater wells in the northern portion of the site will be operated by high-efficiency electric heat pumps, with part of the electricity supplied by solar panels. The system will provide low-cost, low–greenhouse gas heating and cooling to 850 homes and several light industrial buildings. Michael Ahern, Senior Vice President of System Development at Ever-Green Energy, explained that the groundwater network also functions as a “thermal battery,” storing surplus heat in summer for use during the winter.
Historical Context and Global Adoption
This system will be among the few large-scale aquifer thermal energy storage projects operating in the United States since the Department of Energy’s field tests in the 1980s. According to a 2024 study, more than 3,000 similar systems are currently in operation worldwide, the vast majority of them in the Netherlands.
Forgotten Research, Now Rediscovered
Marc Hoyer, a retired scientist who worked on the original Saint Paul project in the 1980s, recalls that researchers took turns sleeping in trailers next to the wells, waking every hour to record temperature and pressure for round-the-clock monitoring. A 1991 federal report concluded that seasonal thermal energy storage could reduce U.S. primary energy demand, with aquifers identified as the most cost-effective option. Learning that the technology is returning to prominence in the U.S., Hoyer said he was relieved: “I thought no one cared about this anymore.”
“The LED of HVAC”
According to a 2024 study, aquifer thermal energy storage can reduce greenhouse gas emissions by up to 74% compared with conventional heating and cooling systems—an efficiency gain comparable to the transition from incandescent lighting to LEDs. Yu-Feng Lin, Director of the Illinois State Water Survey, described it succinctly: “This is the LED of heating and cooling.”
How It Works: A More Stable Temperature Source
Like air-source heat pumps, aquifer-based heating and cooling systems rely on electrically driven high-efficiency equipment. The key difference lies in the stability of the heat source and sink. Air-source heat pumps lose efficiency during extreme heat or cold. By contrast, the aquifer beneath The Heights maintains a temperature of around 10°C year-round, allowing the system to operate under more favorable conditions even when summer temperatures reach 30°C or winter temperatures fall below −10°C.
Energy Efficiency Advantage: Geothermal Beyond Power Generation
Lin noted that when people think of geothermal energy, they often picture magma or steam-based electricity generation. In reality, geothermal can also serve as a highly efficient solution for heating and cooling. Warming part of an aquifer in summer improves heating efficiency in winter, while injecting cold water in winter can reduce air-conditioning energy demand in summer.
Technology Comparison: Aquifer Geothermal vs. Conventional Geothermal Networks
The system is similar to existing district geothermal heating and cooling networks in the region, both of which use rock formations and sediments as sources and storage media for thermal energy. The key difference is that aquifer geothermal systems require fewer wells, but each well is larger and more expensive to drill, resulting in comparable overall costs. Aquifer geothermal also depends on suitable shallow groundwater resources, meaning it is not viable everywhere. Minnesota’s abundant groundwater makes it particularly well suited. Rob Thornton, CEO of the International District Energy Association (IDEA), commented: “Using local resources effectively just makes sense.”