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. The 2016 Utility Master Plan (UMP) made huge gains on that front by creating a path to transition 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.
In other words, the UMP was a major strategy for accelerating the advancement of 2011 Climate Action goals. The plan integrated 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. Into the future, we aim to continue our national leadership in higher education emissions reductions and create a new utility plan that continues our strategy of integrating climate goals into practical paths. The expansion of Physical Education, Athletics, and Recreation (PEAR) especially calls for thoughtful strategy for green design and utilities.
Fiscal Year 2025 Data
Greenhouse Gas Emissions
Scope 1 and 2 emissions, or the emissions from natural gas and purchased eletricity are down 70% since our 2008 baseline. 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. Scroll down for a graph of Carleton’s carbon reduction progress.
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
Carleton sees a 10-15% reduction in utility cost per square foot compared to before the hot water / geothermal utility transition. We use an interruptible gas contract, which 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 carbon emissions between 2008 and 2025.
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