From a design perspective, the most challenging decisions revolved around how pedestrians enter and exit the underground garage, given its location beneath a central green. (Cars will come and go via one of the circle’s original access routes, sloped for the change in level.) With so many significant heritage buildings framing the lawn, the architect located a glazed entrance pavilion in front of a Brutalist medical-sciences building set back from the circle. Highly transparent, with slender steel columns, the pavilion is designed to preserve views for pedestrians approaching the circle from the building. Similarly, exit stairs have been discreetly located around the periphery. From the stairs of the entry pavilion, large windows into the geo-exchange mechanical room will showcase the equipment it takes to run the system. The space is also designed for teaching and demonstrations.

Geo-exchange is expected to reduce the university’s emissions by the equivalent of 15,000 metric tons of carbon dioxide per year, making it the single biggest contributor to the institution’s annual reduction target of 44,567 metric tons. At the same time, the project is demonstrating the viability of the technology for retrofitting existing and historic buildings at scale in a tight urban context. “That’s the relevance of this,” says Blumberg. “There’s a massive urgency as the climate crisis speeds up, and one hopes that in the not-too-distant future, all buildings will be built with renewable energy.”

The experience of Colorado Mesa Uni­­versity (CMU), an early adopter of geo-exchange, suggests that the hope is not unrealistic. A 125-acre campus in Grand Junction, Colo­rado, CMU completed its first bore field in 2007, in response to a state requirement for all new publicly funded buildings to achieve LEED Gold certification. The subsequently expanded system now provides heating and cooling for 16 academic and auxiliary buildings totaling 1.2 million square feet. It supplies 90 percent of the energy the campus needs to operate, saves the university some $1.5 million in heating and cooling costs each year, and has reduced the carbon footprint of the campus by about 17,750 metric tons of CO₂ annually. Some of the nearly $12 million that CMU has saved since 2008 has been shared with students in the form of reduced tuition increases.

PHOTOGRAPHY: COURTESY COLORADO MESA UNIVERSITY

Colorado Mesa University’s geo-exchange system uses its swimming pool as a heat sink.

“The most basic principle of our system is that it allows us to move heat energy across campus,” says Kent Marsh, CMU’s vice president of capital planning, sustainability, and campus operations. For example, at the beginning of November, single-occupancy offices on the north side of a classroom building will be calling for heat, while a full lecture hall with ample glazing on the south side of the building will still need cooling. “Our system allows us to grab that excess heat from the lecture hall and simply move it to the offices on the north side,” says Marsh. “That’s the basic transfer, what we call ‘least-energy path.’ ”

The system can also transfer surplus heat to another floor, or pipe it to another building altogether via a central loop 5 feet below grade. If heat isn’t needed in any of those places, it’s sent to a heat sink, such as the university’s 800,000 gallon swimming pool or the domestic hot-water preheating system. If there’s energy left over after those uses—and with Grand Junction’s summer temperatures hitting 100 degrees-plus for weeks on end, there often is—it gets stashed in one of eight bore fields for retrieval when the seasons change.

Over the course of the system’s 15 or so years of operation, some lessons have emerged. One is that smaller loops within the larger system help distribute energy more efficiently. That’s because the rate of heat transfer is highest when temperature difference, known as ∆T (delta T), is greatest. If a loop is pulling heat from building after building, there comes a point when there isn’t enough ∆T between the water in the loop and the next overheated building for heat to transfer effectively. Better instead to have a smaller number of buildings on a smaller loop, collect the heat, dump it in a nearby sink or drill field, and send the cooled water around again (or, in winter, the reverse).

The other discovery has been that a thorough understanding of how heat is translated among various uses doesn’t necessarily exist yet, says Marsh. “As I connect a new building, I monitor the temperature of our loop pipe,” he explains. “When the water in that loop gets too warm, then I know that we’ve connected too much load to the system, and I need another drill field or another sink to dump some of that waste heat.” CMU is now collaborating with researchers to make this process operate less through trial and error by developing ways to quantify how much heat from a building of a certain occupancy type, size, and heating or cooling load can be transferred to meet other needs.

Continuing to expand what is already one of the largest geo-exchange systems in North America, CMU has recently secured funding to complete the connection of the campus’s remaining 14 buildings. The drilling and infrastructure are expected to be largely finished next summer, with building connections to be completed in the following two to three years. Beyond that, Marsh wonders what the potential is for expanding the campus system to the broader community—to serve a new high school now being built across the street, for example, or a municipal swimming pool a quarter mile away. “When you think about this in the broader context of decarbonization,” he asks, “could, one day, a municipality that provides sewer and water services, or an energy utility that provides gas and electricity, provide a geo-resource in the same way?”

Taking notice of the potential that CMU’s system represents, the governor of Colorado has become an advocate for what he has tagged “the heat beneath our feet” across Colorado and among the Western states. In Canada, even before its system is complete, the University of Toronto has already exposed thousands of students of multiple disciplines to the potential of geo-exchange—a form of advocacy that may well have greater impact than the system itself. In New Jersey, the architectural treatment and prominence that Princeton has given its system is a similarly powerful form of advocacy. As a growing number of campuses convert their systems, it’s not unreasonable to hope that geo-exchange-based district energy will gather steam—and eventually replace it too.

Supplemental Materials:

“District Energy Systems Overview”, U.S. Department of Energy, September 2020.