Pounds of CO2 Emitted for Each Fleet Mode:
We calculated the emissions for each fleet mode by multiplying the reported quantity of fuel consumed in that mode by the emissions factor for that fuel (lbs of CO2/gallon, 1000 cubic feet or kWh) to obtain the pounds of emissions produced for all the fuel consumed by each mode. The emissions factor for each fuel was acquired and compiled from the Department of Energy's Energy Information Administration (EIA) database (www.eia.doe.gov/oiaf/1606/factors.html) and Argonne National Laboratory's Greenhouse Gases, Regulated Emissions, and Energy Use in Transportation (GREET) model (www.transportation.anl.gov/ttrdc/greet). Below is a table of emissions factors for each of the major fuel types included in the calculator:
Emission factors shaded in green have been taken from EIA's database, all others have been extracted from the GREET model.
* CNG is based on a diesel equivalent gallon of 138,000 BTU as required by Exhibit 17 of the 2000 FTA Reporting Manual (Resource Module).
Pounds of CO2 Equivalents Offset by Increased Ridership:
After automobile drivers have converted to using public transportation, agencies may be able to gain credit from the reduction in auto emissions.
An average automobile emission rate is 1.1 pounds of carbon dioxide per vehicle mile. For the mode where there is increased ridership, we found the average length trip in passenger miles 1. Making the assumption that one mile of travel taken by transit is equivalent to four miles of travel taken by car 2, we multiplied the number of new riders by the average length mode trip to get the total miles traveled that would have been taken by car, and then multiplied this factor by the 1.1 pounds of CO2 per vehicle mile 3 equivalent to determine the total emission reduction.
Electricity Emissions Factors:
When users change their electric generation mix, the following emissions factors for various types of fuel sources were used for our calculations in determining emissions for each fleet mode based on fuel utilization and their associated emissions factors. This data was extracted from 1999 EIA data. Below is a table of emissions factors for each of the major fuel sources, conventional and alternative, included in the calculator:
Comparison of Emission Reduction Costs for Buses:
Calculating the cost of emission reduction for various fuels and technologies requires a wide range of assumptions. Table 1 illustrates the assumptions made in the calculator, and indicates how the user can assume other values if specific information is available about choices.
In the table, a diesel bus is shown as the standard choice. Emissions are calculated for each fuel per mile and compared to those of diesel. (Note that fuel cells using hydrogen fuel use less than half the energy as combustion engines that use the other fuels.) The cost of fuels are estimated using the best available current information. Of course, the availability of the fuels is another major question in many areas, and costs will vary widely as fuels -- including diesel - become more or less available.
Any decision to incorporate alternate technologies or fuels into transit fleets will be heavily influenced by the projected costs of implementation. However, projecting the costs of implementing alternative fuels is very challenging since most of the technologies have not been thoroughly tested under operating conditions and neither the buses nor the fuel infrastructures have been defined by a mass market. Costs are continually changing as companies compete within a currently limited market and as products undergo a refining process. Appendix C of the final report contains a methodology for comparing estimated costs based upon the current costs of alternative fuels.
The emissions per vehicle mile for buses running on alternative fuels are first calculated using data in Table C-1. All of the technologies are compared with the current standard petroleum diesel. Cost of fuels is based on the current costs as reported by government research institutions. Vehicle costs have been chosen to reflect a mature system in which fuels and technology are available at market costs. The costs to reduce emissions are calculated as dollars per ton of equivalent CO2. Overall, the results indicate that for some of the alternative technologies - such as hydrogen fuel cells, B20 and CNG - fuel cost savings can compensate for additional costs that would be incurred from purchasing AFV buses. The estimated savings from the decrease in fuel costs over time could result in cost-effective reductions in emissions by transit agencies.
TABLE 1: COSTS OF REDUCING GHG EMISSIONS IN BUSES WITH ALTERNATE FUELS