Blog

Oct 26, 2024

Carleton College Utility Master Plan

The last time Carleton College made a major shift in its campus utilities was over 100 years ago with construction of the central plant in 1910. Before that, each individual building was heated by a

The last time Carleton College made a major shift in its campus utilities was over 100 years ago with construction of the central plant in 1910. Before that, each individual building was heated by a coal furnace and open fireplaces. In 2021, more than a century after the central plant was constructed, the college completed its utility plan for the next 100 years.

Carleton’s clean energy journey began with construction of our two wind turbines (installed in 2004 and 2011) which put us on a path to consuming carbon-free electricity. But we needed solutions to drive down carbon emissions, and we were particularly interested in finding low carbon ways to serve our intense winter heating loads. Our Utility Master Plan (UMP) made huge gains on that front by transitioning Carleton’s district heating system from steam distribution to low temperature (120 F) hot water tied to a geothermal heat pump and three geothermal bore fields.

All of these efforts align with concepts outlined in our 2011 Climate Action Plan which is the guiding document for Carleton’s goal to be a carbon neutral campus by the year 2050. Our new utility system incorporates four forms of renewable energy — wind, solar photovoltaic, solar thermal, and geothermal — and is flexible enough to take advantage of future advancements in renewable energy technologies.

For more information about the Utility Master Plan, explore this website and watch this webinar from Martha Larson, Manager of Campus Energy and Sustainability.

Greenhouse Gas Emissions are down 56% 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. We have currently exceeded both our 2025 and 2030 interim reduction milestones on the path to carbon neutrality by 2050. Scroll down for a graph of Carleton’s carbon reduction progress.

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.

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.