Rochester, Minnesota, is a growth market. It’s the state’s third-largest municipality, behind only the Twin Cities of Minneapolis and St. Paul, and home to the Mayo Clinic, a name synonymous with world-class healthcare and instrumental in forging the city’s nickname of Med City. Rochester has also never experienced a population dip since railroad tracks were first laid and residents and businesses began settling in the 1860s. The city’s population, according to the latest census, sits north of 121,000. The 20-year, $5.6 billion economic development initiative known as Destination Medical Center is in full swing, fulfilling the long-time vision of transforming Rochester into an innovation hub, with a growing Mayo Clinic at its center. And just last March, Geothermal Rising held its first Thermal Energy Network Symposium in Rochester.

That last one isn’t a coincidence. Med City is going all in on geothermal.

Looping into TENs

The city was selected as the symposium’s inaugural host, in part, because Rochester Public Utilities, the city’s electrical services provider, has stated its intent to provide the city with 100% renewable energy by 2030, a goal that has overwhelming support from elected officials, businesses, and residents.

Largely aiding in this effort is the development of Thermal Energy Networks (TENs), which comprise ambient pipe loops connecting multiple buildings and delivering thermal heating and cooling energy via, in Rochester’s case, water-source heat pumps. Waste heat can also be recovered from buildings through the city’s sewer system. (Not to be confused with District Energy Systems, which can draw energy from fossil fuel–burning cogeneration plants, TENs are decentralized and demand each building in its network have its own ground- or water-source heat pump.)

Currently, Rochester’s City Hall has been connected to the city’s first TEN via two wells, is now fully electrified, and is on pace to run exclusively on renewables by 2030. Scot Ramsey, Rochester’s facilities and property manager, calls this their “pilot project,” with efforts underway to expand the networks to interconnect four other public buildings (the library, art center, civic center, and a theater) and up to six privately owned multifamily buildings. “We’re doing analysis of the [energy] modeling, to see what the rates would be and if we can give [building owners] parity,” Ramsey says. “When we get more people on the network, everyone’s rates go down. Thermal Energy Networks are capital-intensive at the beginning, but once that capital is taken care of for the infrastructure, it’s very inexpensive for the utilities.”

Once scaled up, such systems have established their ROI for building owners. According to Zeyneb Magavi, co-executive director of the non-profit Home Energy Efficiency Team (HEET), in speaking with MIT Technology Review last year, “every time a ‘loop’ of thermal energy network is added to the grid, its ability to predict and manage power flow becomes more accurate. This interconnectedness helps the system become more resilient in high-stress situations.”

Don’t waste a good crisis

In 2021, a decades-old steam line that delivered heating and cooling to Rochester city buildings via a county-run waste-to-energy facility was decommissioned, and the projected cost of replacing it was deemed a non-starter. “Rochester Public Utilities had already made the commitment to be renewable by 2030. There were long-term goals for electrification and reducing greenhouse gases. That was all in place,” Ramsey says. “Combine that with the fact that we lost our steam in 710,000 square feet of building, that was the catalyst, the crisis. And you should never waste a good crisis.”

As Ramsey tells it, city leaders began by looking at centralized district energy system models. “It was originally envisioned as a four-pipe system, with natural gas serving the connected facilities. We started down that path.”

Several factors prompted the city to change course. Other networked geothermal systems in the state had already been installed (or were in the process of being developed) and proven themselves viable. Minnesota is also rich in groundwater sources, particularly in the southeastern portion. And Darcy Solutions, a local outfit with a proprietary and game-changing geothermal technology, made its presence known.

Geothermal in the city

In addition to being capital-intensive, traditional geothermal systems can be cumbersome. Tapping into geothermal energy, while renewable, also typically requires the drilling and boring of dozens upon dozens of wells to draw enough heating and cooling capacity for the buildings in question. Couple that reality with the prospect of heating and cooling the building stock of a mid-size city, and managing the logistics of such an endeavor becomes too much to bear. “Geothermal just isn’t practical where land is scarce,” says Robert Ed, director of marketing and strategy for Darcy Solutions, whose systems are currently being deployed for the TENs currently or imminently serving Rochester’s public buildings.

Indeed, Minnesota-based Darcy Solutions does things a little differently. The company has developed a groundwater-based system that installs a heat exchanger into the aquifer, where water temperatures remain pretty much constant. That exchanger feeds into a closed loop that connects directly to buildings’ HVAC systems; groundwater from compliant water-supply wells is drawn from and returned to the same source, never depleting the aquifer. And perhaps best of all, each Darcy well can generate between 20-50 times the amount of energy compared to a traditional borehole. In other words, Darcy Solutions technology is designed to work on a tight urban site.

“Because you’re using groundwater as the conduit for thermal capacity in an aquifer, which is much more than just water, it’s all the surrounding rock of that aquifer, you’re able to generate significantly more thermal capacity on a much, much smaller footprint. That allows you to have one or two water wells outside your building versus needing 40 or 50 borings that have to be spaced apart so that they’re not competing with each other for that thermal energy,” Ed says.

By the numbers, Ed estimates that optimal productivity for a Darcy well in Minnesota is “roughly 45 tons of heating,” which translates to 540 MBH or 540,000 BTU per hour. “And then we can [generate between] 130 to 150 tons of cooling. We get more cooling out of the wells than heating … we’re limited on the temperatures we can send down the well so that water doesn’t freeze, because it’s just potable water we’re using as a conduit.”

Coupled with this consolidated efficiency and the networks’ capacity to enable buildings to share and exchange thermal energy, resulting in very little waste heat, Ramsey confirms that retrofitted buildings are converting their heating water temperatures from 180° F to 130° F. He concedes this raised some concerns among his engineer colleagues, but those were quickly put to bed, provided said buildings are engineered to run efficiently in cold weather, geothermal or not. Those buildings will do fine, and many of them are over-engineered as it is, he says. “If you’re converting buildings to 130° F and interconnecting them, and then optimizing their performance, those things can probably save you 10 to 15% of what you’re currently spending.”

Legislation matters

Ramsey, a mechanical engineer and healthcare administrator by training, admits that when it comes transitioning buildings to renewables, it can “hard to quantify the intangibles.” It’s not always an easy sell, he says. Luckily, the tangibles at his disposal more than make up the difference. “We have businesses that waste a lot of heat. A lot of heat goes down the drain or out the roof, which is just a total waste of money.” Or if one is inclined to calculate loss in carbon versus dollars, Ramsey says, either way “we should be harnessing” the capacity of the network’s ambient loop to share energy and minimize waste. “Recycle it so someone else can use it.”

However you crunch the numbers, civic leaders and the going public are on board with the city’s plan, and state legislators seem intent on replicating Rochester’s example wherever possible. Currently, four proposed laws are up for debate in the State House, each designed to provide grants, rebates, or similar guidance in a statewide effort to leverage geothermal energy and “strategic electrification” for widespread building decarbonization. One of those proposed laws would grant the Commerce Department authority to study the potential for TEN pilot projects throughout the state.

Thermal Energy Networks are not easy to pull off. They require, at least in Rochester’s case, integration with renewable electricity, data smart systems for energy sharing, ambient energy transportation loops interconnected to multiple building heat pumps, and much more. Given the mechanical demands, upfront costs, and environmental concerns of any given city, TENs are clearly not for everyone. But there is little doubt—after performing extensive evaluation of the city’s energy needs, the quality of its building stock, and its groundwater conditions—that they’re an ideal match for Rochester.

“The most efficient system matches the supply with demand,” Ramsey says. “And water is the best way to move energy around.”

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