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.
Carleton’s 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 expected to deliver 70% 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
Fiscal Year 2023 Data
Greenhouse Gas Emissions
Greenhouse Gas Emissions are down 63% from our baseline reporting year of fiscal year 2008. This is thanks to two commercial-scale wind turbines, the geothermal system, and the “greening” of Xcel’s electricity grid. There has been a slight uptick as air travel comes back post-COVID, and small variations happen from year to year related to temperature conditions.
Energy Consumption
Carleton’s district energy system has reduced its energy consumption by 46% compared to our average use for the five years before we began construction on the Utility Master Plan. This change in total energy use includes a 70% 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
Ultimately, Carleton will see a 10-15% reduction in utility cost per square foot compared to before the hot water / geothermal utility transition. But Carleton’s utility prices are currently 4% higher per square foot of space, a temporary increase while we transition the campus from firm gas supply back to our interruptible supply contract with Xcel Energy.
For one year, we needed to buy firm gas (which is much more expensive than interruptible) to avoid any gas service disruptions while we installed and tested our backup fuel system. An interruptible gas contract means that Carleton can switch to an alternate fuel source and jump off the public gas supply when notified by Xcel Energy that more capacity is needed to serve intense winter heating loads.
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. As of Spring 2022, our propane backup fuel system is now fully tested and ready, so we will transition back to interruptible gas rates for the coming fiscal year.
Campus Carbon Emissions: 2008–2023
