Published: Aug. 17, 2015

Originally published online: July 22, 2013

Carbon Emissions Inventory 
for the University of Colorado Boulder Campus

click here for: Tulane University Greenhouse gas Inventory


In the United States, 98% of carbon dioxide is emitted as the result of the combustion of fossil fuels. Consequently, carbon dioxide emissions and energy use are highly correlated. At the University of Colorado, electricity and steam consumption have increased substantially between 1990 and 1999. However, the cogeneration plant has also substantially decreased the carbon output per unit of energy consumed. This inventory compares carbon dioxide emissions from 1990 and 1999 to see how the cogeneration plant and increased energy demand has effected emissions.

Our calculation shows emissions going down 5.2 %over the 10 year. For comparison, electricity use has been going up 4-5% every year.


What this inventory includes:

  • CO2 emissions due to heating, cooling, and providing electricity to campus buildings
  • CO2 emissions due campus fleet vehicles
  • Carbon equivalent due to leakage of natural gas in pipelines

What this inventory does not include:

  • Emissions due to trips to and from campus by faculty and students in non university vehicles
  • Emissions due to burning fuels other than natural gas on campus
  • Emissions due to CFC's leaking from cooling systems
  • Emissions due to gases other than CO2(except for natural gas leakage)

A “bubble” was placed around the CU campus. For 1990, this bubble includes the central steam and chiller plant and all the campus buildings. For 1999 the bubble now includes the cogeneration plant in place of the steam plant. Emissions are calculated at each interface of this bubble, whether energy is entering or leaving the campus. For example, in 1999-2000, emissions from natural gas are calculated in two places: 1) from natural gas that the cogeneration facility purchases and 2) natural gas that individual campus buildings purchase. In 1999-2000, emissions from electricity is actually calculated at three different interfaces: 1) electricity purchased form PSCO by the cogeneration facility, 2) electricity purchased by PSCO from the cogeneration facility, and 3) electricity purchased from PSCO from individual campus buildings. The campus is a net exporter of electricity. We value the net export based upon the actual fuel mix for CU power production Public Service's overall fuel mix. The PSCO fuel mix is 91% coal; 8% natural gas; and 1% renewables.

In 1990, this methodology is much simpler, simply because CU did not export any electricity before the cogeneration plant was built. Therefore all emissions are calculated based on energy, either in the form of natural gas or electricity, entering the “bubble” that represents the campus. The emissions coefficients used in this inventory assume, of course, that all natural gas that enters the campus is eventually combusted. In 1990, the PSCO fuel mix was 98% coal and 2% natural gas.

Natural Gas. Natural gas is actually purchased from both the central plant and individual campus buildings. Emissions were calculated by applying emission coefficients (obtained from the Department of Energy) to the amount of gas used at both points.

Electricity. In 1990, emissions from electricity were calculated by multiplying an emission coefficient by the amount of electricity bought from the utility. This is based upon the actual fuel mix of Public service (almost all coal). This methodology was somewhat complicated by the implementation of the cogeneration plant in 1992. Today, the cogeneration plant provides most of the campus with electricity in addition to selling back to the utility. However, the cogeneration plant (as well as several campus buildings) still purchases electricity from the utility. The emissions due to the net export of electricity are calculated and credited (subtracted) from the emissions inventory.

We calculated the emissions credit assuming that the net sales from the cogeneration plant are displacing electricity production that would have been produced at the average fuel mix of PSCO. Under this assumption, total emissions from CU actually drop from 1990-1999 due to the fact that our relatively low emissions cogeneration plant is displacing relatively dirty coal burning.
Another possible approach would be to assume that the net exports are replacing electricity that would have been produced by PSCO in natural gas turbines. The argument for this assumption is that while the bulk of power production by PSCO is coal, the marginal production is in natural gas turbines. That is, the bulk of their new capacity, and the plants whose ouput can rapidly vary in response to load fluctuations are natural gas power plants. Under this assumption, total emissions have gone up over the last decade.

Fleet Vehicles. Emissions from campus fleet vehicles were calculated by multiplying the total number of gallons of gasoline used (obtained from the Transportation Center) by an emissions coefficient for gasoline.