Editor's note: The following article is based on a presentation given by the author during HPAC Engineering's 2009 Engineering Green Buildings (EGB) Conference and Expo, held Sept. 24 and 25 in Nashville, Tenn., part of HVACR Week. HPAC Engineering is accepting proposals for sessions for EGB 2010, to be held Sept. 23 and 24 in Baltimore. For more information, contact Executive Editor Scott Arnold at 216-931-9980 or scott.arnold@penton.com.

Through 2009, 663 college and university presidents had signed the American College & University Presidents' Climate Commitment (ACUPCC) (www.presidentsclimatecommitment.org), pledging "to eliminate net greenhouse-gas emissions from specified campus operations and ...promote ... research and educational efforts ... to equip society to re-stabilize the earth's climate." In part, the ACUPCC states the signatories' agreement to "initiate the development of a comprehensive plan to achieve climate neutrality as soon as possible."

Comparing Carbon Footprints

With such a daunting goal, discussion of the engineering fundamentals of the production of greenhouse gases is in order. This article will discuss the production of the most prevalent greenhouse gas, carbon dioxide (CO₂), by an energy-consuming system found on many college and university campuses: the chilled-water system. Specifically, it will focus on the central energy consumer of that system, the water chiller.

In the United States, more than 83 percent of utilized energy originates from a combustion process.¹ This includes fuels burned to create steam and hot water for heating and cooling and fuels burned to create steam to turn turbines/generators to produce electricity. Although this article will focus on the combustion of coal and natural gas, the logic applies for all combustible hydrocarbon fuels, including propane, No. 2 fuel oil, No. 6 fuel oil, and gasoline.

Determining emission factors is a site-specific task because energy sources vary by region, as do methods of generating and transporting electric energy to electric infrastructure. Many colleges and universities determine their emissions using Campus Carbon Calculator, available from Clean Air-Cool Planet (www.cleanair-coolplanet.org). Campus Carbon Calculator divides the nation into sub-regions and uses U.S. Environmental Protection Agency data to determine regional emissions. Additional supporting information can be obtained from the U.S. Energy Information Administration (www.eia.doe.gov). Utilizing Campus Carbon Calculator Version 6.4, CO₂-emission factors were determined for a chiller project in St. Louis:

• Coal combustion

  The average to be used for St. Louis is 207.9 lb of CO₂ per million British thermal units of coal burned, although the Wyoming Powder Valley coal largely burned by the local electric utility indicates a slightly higher value (see sidebar).

• Natural-gas combustion

  The average to be used for St. Louis is 116.3 lb of CO₂ per million British thermal units of natural gas burned, although the fuel furnished by the local gas utility indicates a slightly higher value (see sidebar).

• Electricity generation

  The average to be used for St. Louis is 1.844 lb of CO₂ per kilowatt-hour of electricity delivered. It is based on a mix of energy sources, including coal, natural gas, nuclear, hydroelectric, and wind. For reference, the use of coal alone would produce 2.19 lb of CO₂ per kilowatt-hour of electricity delivered, while the use of natural gas alone would produce 1.21 lb of CO₂ per kilowatt-hour of electricity delivered (see sidebar). Obviously, electricity in St. Louis is generated predominately through the combustion of coal.

The carbon footprints of eight types of chillers common to college and university campuses were determined. The chillers are:

• High-efficiency electric centrifugal ("Option 1" in Table 1).

• Standard-efficiency electric centrifugal ("Option 2").

• Single-stage steam absorption ("Option 3").

• Two-stage steam absorption ("Option 4").

• Gas-fired two-stage absorption ("Option 5").

• Natural-gas-engine/generator-powered electric without heat recovery ("Option 6").

• Natural-gas-engine/generator-powered electric with heat recovery ("Option 7").

• Electric centrifugal heat recovery ("Option 8").