Carleton’s new geothermal energy system is a multitasking wonder, heating and cooling campus, cutting carbon emissions, and serving as a living laboratory.
No one can accuse Carleton of not being frugal. The college used its original 1910 steam plant to heat the campus for more than 100 years. But, as it became clear that parts of the aging infrastructure had reached their limits, Carleton’s facilities team faced a fork in the road.
Martha Larson, manager of campus energy and sustainability, recalls asking: “Do we replace the existing steam equipment, or do we look to new technologies?”
After working with a team of outside consultants to evaluate options, Carleton decided to build a plant for the future, making the college one of the first in the nation to use geothermal energy on a campus scale. By saving money and significantly cutting carbon emissions, the new system is vital to Carleton reaching its goal to become carbon neutral by 2050.
Cowling’s Centralized System
In the late 1800s/early 1900s, a Carleton student job might have been to shovel coal into the furnace of one of the college’s four buildings: Willis Hall, Goodsell Observatory, Scoville Library, or Laird Hall. In cold-weather months, each building used a coal furnace and fireplaces for heat. In the summer, Carls relied on paper fans or even a dip in the Cannon River to cool off.
During his tenure from 1909 to 1945, Donald Cowling, Carleton’s third president, greatly expanded the campus and elevated its stature as a premier liberal arts institution. His administration oversaw the construction of Skinner Memorial Chapel, Sayles-Hill Gymnasium, Laird Stadium, and several residence halls.
But among his first and most visionary efforts was to centralize Carleton’s utilities system by building in 1910 a heating plant (now the facilities building) and the legendary Carleton tunnels. The plant housed coal-fueled boilers, which heated water to generate steam. Under high pressure, the steam propelled itself through pipes in the tunnels into radiators and other heating equipment in buildings throughout campus.
“Carleton’s steam plant was revolutionary for that time,” says Larson. “It centralized the maintenance staff, mess, and coal deliveries and paved the way for expanding the campus. And that technological approach lasted for more than a century.”
Natural gas ultimately replaced coal as the fuel for the boilers, and the college upgraded and replaced the original equipment along the way, but steam boilers remained in use at Carleton until 2021.
Free Heat
Utilities infrastructure is still a long-lasting investment. Carleton’s facilities team kept this in mind as it weighed options for a new heating and cooling system.
“We knew that steam-plant technology would eventually become obsolete,” says Larson. “Steam plants are antiquated and inefficient, and skilled operators are getting harder to find. And so, to take Carleton into the 21st century and to move closer to our goal of being carbon-neutral by 2050, we transitioned to hot water heating accompanied by a geothermal heat pump heating and cooling system.”
Geothermal technology, or ground source heat exchange, exchanges heat with the earth to deliver or remove heat in buildings. “Earth maintains a near-constant temperature close to 50 degrees in all seasons,” says Larson. “Think of a basement. It feels cool in the summer when the air temperature is hot and warm in the winter when the air outside is cold.”
A geothermal energy system pumps water through piping circuits that extend far beneath Earth’s surface. The circuits go down 520 feet deep on campus — the height of three Carleton smokestacks. This underground network of piping, called borefields, exchanges energy with Carleton’s heating and cooling loops, which control temperatures in the buildings.
The heating, cooling, and geothermal circuits connect in a heat pump to exchange energy. Depending on the season, the geothermal loop evens out the temperature imbalance between campus heating and cooling loops, depositing excess heat into the ground in summer or extracting it in winter.
“Whenever you have an overlap where the campus needs both heating and cooling at the same time, you get free heat,” says Larson. “The chilled water loop absorbs extra heat from the buildings. This heat comes from people’s bodies, the lights, solar heat captured through the windows, and heat from computers and other technology. It is all free heat created without extra cost or carbon emissions.”
That extra heat used to be wasted through evaporation in the cooling tower. But the geothermal system transfers it directly into the hot water loop to be used where heating is needed or to the geothermal loop, which deposits it into the ground, where the system can retrieve it for later use.
Applied Learning
Workers placed the borefields in the Bald Spot, the Mini Bald Spot (on the east side of campus between Nourse and Myers), and Bell Field — a major component of the utility construction project that took nearly two years to complete. Professors took note, eyeing a unique learning opportunity for students.
Dan Maxbauer, assistant professor of geology, and his Introduction to Geology students observed the construction of the Bald Spot borefields. Students worked with the drilling contractors who logged sediments and rock coming up from the drilling process. Maxbauer later developed several labs in which students examined those specimens.
“We spend much of the first half of the term in Introduction to Geology driving around the Northfield area looking at exposed bedrock,” says Maxbauer. “There’s very little visible bedrock on campus. By seeing the sediment in the borefields, the students connected the regional geology to Carleton. They also saw why geology is important to realworld needs like utilities.”
John Berini, a visiting assistant professor of geology, featured the geothermal project in the sustainability course he teaches for high school students who participate in Carleton’s Summer Liberal Arts Institute. Berini’s goal was to expose students to multidisciplinary ways of thinking about sustainability.
“I wanted to show students real-world applications,” says Berini. “That’s where the geothermal energy system came into play. Students toured the new geothermal plant, located in the sub-basement of Anderson Hall. They learned about its ability to offset carbon emissions by recycling heat. They also saw the link between the utilities system and geology and how the energy industry is developing concrete sustainability solutions.”
A New Equation
Maintenance manager Mitch Miller, who operates the campus utility system, can attest to its efficacy. “It is wildly more efficient than the old steam plant,” says Miller. “The steam plant was about 75 percent efficient, meaning that 25 percent of the energy it produced was wasted through boiler inefficiencies and heat loss through the piping infrastructure. The new plant is 500 to 600 percent efficient thanks to its ability to capture and repurpose what we once thought of as ‘waste heat.’”
The geothermal pumps run on electricity, which is more expensive than the gas used to fuel the former steam-plant boilers. However, the new system is less expensive overall since it uses less energy and saves operating costs. Additionally, thanks to rapid decarbonization of the electric grid, it will eventually be a carbon-free technology.
“Everything used to be about saving money,” says Larson. “Now we are concerned with saving cost, energy, and carbon. Money is not the only bottom line. We have to think about the future of the planet, too.”
A significant portion of the cost savings is in staffing. “High-pressure steam is extremely dangerous,” says Miller. “Therefore, we had to staff the old steam plant 24 hours a day, 365 days a year. We monitor this system to ensure it operates within the correct parameters, but everything is computerized. The system will notify us automatically via email and text messages if anything is off.”
Mark Evans, who had been the former steam plant’s chief operating engineer, retired in spring 2021. Finding someone who had the skills required to replace him would have been extremely difficult.
“Most steam plant operators came from the Navy because the Navy had high-pressure steam boilers on their ships,” says Miller. “The Navy phased out those boilers so it is now challenging to find skilled operators. Certification is arduous and takes longer than getting a law degree. Sheer lack of steam-plant operators is another reason why we made a move to geothermal.”
Underground Accolades
While Carleton is best known for what happens within its classrooms, it is quickly achieving an equal level of notoriety for what is taking place beneath its campus. Larson and Miller frequently field calls from other colleges that are considering making the switch from steam to geothermal energy.
When Miller began working at Carleton in 2012, he never imagined the college would transition from steam. “But everyone will likely move toward geothermal eventually,” he says. “That’s where the industry is headed.”
Carleton just got there first.
Of Comps and Community
Carleton’s new geothermal system led three students to consider what might make the ultra-efficient system even more efficient. The answer lies in groundwater: water found in the soil and spaces between rocks.
Natasha Dietz ’19, Taiyi Wang ’19, and Jake Gallant ’21 sought to understand the different rock formations that surround the piping in the borefields. Dietz spent the summer of 2018 working alongside borefield drilling contractors, collecting and identifying samples of rock cuttings by shaking sediment through a sieve at every 20 feet of drilling.
“From those samples, I reconstructed the stratigraphic column—the vertical sequence of the rock formations extending from the ground surface, 520 feet down to the very bottom of the borefield,” she says.
For her comps project, Dietz conducted a literature review to determine how groundwater flows through and across those different rock formations found on campus.
In a separate project for his comps, Wang used computer modeling to predict how the speed of groundwater flows might affect the efficiency of the geothermal system. His preliminary research found that faster groundwater flow in the summer could make the system more efficient by carrying heat away from the borefield. In contrast, slower groundwater flow warming the field in the winter is less efficient.
“When engineers design borefields, their main concern is how heat transfers between the closed pipes to the surrounding rock formations,” says Wang. “The engineers for Carleton’s project hadn’t considered the effect of groundwater flow, and so they were interested in my research. It was a fantastic experience for me to work on something that could have a real-world impact.”
Two years later, Gallant built upon Dietz’s and Wang’s projects for his own comps. Five of the Bald Spot wells contain fiber-optic cables that collect temperature data every 30 minutes along the entire 520-foot boring length. Gallant studied the temperature data over time and confirmed Wang’s prediction that faster groundwater flow in the summer made the system more efficient.
“The students who came before me advised me via email, and I spoke with the engineers who designed the system,” says Gallant. “People from all different backgrounds came together to theorize and share knowledge about the project. That collaboration was the best part.”