Carleton was the first college campus in Minnesota to install a district-energy scale geothermal system, and the first college campus to completely transition off steam heating, a milestone set in May 2019. We have since expanded geothermal capacity using new Darcy technology.
Carleton’s district geothermal system was sized to optimize the highest benefit for least capital cost. On our coldest day, the geothermal system will serve only 20% of Carleton’s heating load, but on an annual basis, it is delivers up to 60% of our total heating and cooling energy.
Bore Fields
- Bell Field: 95 horizontal bores, 510 feet long, completed in Summer/Fall 2017
- Mini Bald Spot: 77 vertical bores, 520 feet deep, completed in Summer/Fall 2017
- Bald Spot: 134 vertical bores, 520 feet deep, completed in Summer/Fall 2018
Expanding Geothermal Capacity
In the summer of 2026, Carleton installed a Darcy Solutions geothermal well in the Parish House yard. The Darcy Well heats buildings in the winter by drawing warmth from an underground aquifer, and cools them in the summer by transferring excess heat back below ground. The process is completely self-contained—no groundwater is ever removed, and no substances are injected. The Darcy Well is connected to heat pumps that provide heating and cooling to Parish, Douglas, Page, and Aslanian Houses. Even with air conditioning newly added to all four houses, overall carbon emissions have dropped. While electricity use has bumped up slightly to run the heat pumps, natural gas is now reserved only for the coldest winter days. Darcy Wells transfer heat more efficiently with aquifers than Carleton’s traditional geothermal system transfers heat with the ground, meaning one Darcy Well can provide as much heating and cooling than a dozen or more traditional geothermal wells. This project will inform how we expand geothermal into other places on campus, especially sites with limited space.
Fiscal Year 2025 Data
Greenhouse Gas Emissions
Scope 1 and 2 emissions (operational and purchased electricity) are down 70% from our baseline reporting year of fiscal year 2008. This is thanks to two commercial-scale wind turbines, the geothermal system, and “greening” of Xcel’s electricity grid. There has been a slight uptick in air travel post-COVID, and small variations happen from year to year related to temperature conditions.
Energy Consumption
The installation of our district geothermal system reduced its total energy consumption by 46% compared to our average use for the five years before we began construction on the Utility Plan. This change includes a 60% reduction in natural gas consumption and a 15% increase in electricity consumption since the geothermal heat pump system electrified a large portion of Carleton’s heating load.
Campus square footage has grown over time, but our energy use per square foot (for buildings on the district energy system) has now plummeted to 58 kBTU / SF. This massive reduction in energy consumption shows how well the geothermal heat pump can capture and “recycle” heat that already exists on campus. Previously, excess heat would have been evaporated through the cooling towers and thrown away as waste. Now we capture heat absorbed by the cooling system and deposit it directly into the heating system, or exchange it with the geothermal bore fields.
By switching from steam to hot water, our campus heating distribution system is also operating much more efficiently as it moves heat from the central plant to all the buildings connected to our district energy system. Hot water conducts heat more efficiently that steam, generating far less heat loss as it travels through the distribution pipes.
Utility Cost and Back-Up Support
Carleton has seen a 10-15% reduction in utility cost per square foot compared to before the hot water / geothermal utility transition. Carleton’s backup fuel system is served by a propane-air mix, allowing Carleton to continue heating the campus with an alternate fuel source if we lose connection to the natural gas supply for either planned curtailment or unexpected interruptions.
