Denver’s Office of Climate Action, Sustainability and Resiliency embarked on a $4.9 million pilot program funded by the Colorado Energy Office to use water instead of electricity to reduce the use of natural gas for heating and cooling in five million square feet of municipal buildings. The effort comes with pursuit of decarbonization goals, and acknowledging that electrification of large buildings in Denver would be “prohibitively expensive,” officials said.

The city will use the award from CEO to study a multisource district thermal water system to provide heating and cooling through a shared water loop for 14 city buildings comprising 5.5 million square feet currently served by Xcel Energy’s downtown steam and chilled water loops.

“Previous studies have explained the major challenges for both the building owners and Public Service – that is, electrification via electric resistance boilers or air-source heat pumps would be prohibitively expensive due to the costs to upgrade the electrical infrastructure within and serving the building,” said Senior Energy Project Manager Drew Halpern in testimony before the Colorado Public Utilities Commission. “Therefore, the city has continued to explore more cost-effective and achievable solutions to this challenge, including scoping a pilot project in our city-owned facilities – a multisource ambient temperature district thermal system.”

By converting the chilled water system to an ambient temperature system that uses both deep geothermal wells and heat from the sewer system, the building’s systems can be converted to water-to-water heat pumps for heating and cooling.

In the winter the loop would operate at higher temperatures, and in the summer lower temps, with the option of adding chilled water to help balance cooling loads.

The hybrid system would use about 500 geothermal wells drilled to a depth of 1,200 feet, distributed across seven locations, sewer heat recovery and supplemental heat pump chillers for temperature regulation. It would include renovation of the old Cherokee Boiler Plant to serve as a central utility plant.

“From our analysis, we project an immediate 83% reduction in GHG (green house gas) emissions after conversion to an ambient loop,” said Halpern.

This will substantially reduce both electrical loads, and natural gas for heating and will avoid the expense and difficulty of installing greater electrical capacity in the downtown core that would be required for full building electrification, city officials said.

“The city sees this approach as the only practical path to achieving full, or even close-to-full, decarbonization of the downtown building stock without a near-complete rebuild of the electric grid alongside corresponding buildout of generation, transmission, and other necessary infrastructure that comes with its own substantial GHG emissions toll,” Halpern added.

The project also estimates saving 80 million gallons of water per year by shutting down Xcel’s downtown steam loop.

“This figure balloons to an incredible estimated 1.4 billion gallons per year when expanded to the entire current steam system,” said Halpern.