The default GREET model Version 1.8b was revised and modified by Life Cycle Associates, LLC for the California Air Resources Board (ARB) to become CA-GREET to perform the life cycle analyses of fuels used in developing California’s Low Carbon Fuel Standard (LCFS) regulations. CA-GREET differs from the default GREET model in three main ways:

1. CA-GREET has a regional lookup table with regional inputs that allows a user to select from eight regions in a pull-down menu, representing feedstock and fuel regions involved in the production of California fuels, and California-specific input parameters rather than U.S. average parameters.
2. Calculation errors in GREET1.8b have been corrected (GREET1.8c.0 corrects most of these calculation errors as well).
3. Calculation components have been added; new fuel pathways have also been added using the existing GREET structure.  The model is configured for CNG and LNG from dairy digester gas and landfill gas, respectively.  The model is also configured for waste cooking oil and tallow pathways.  The model is not populated with all of the inputs for numerous sub-pathways such as corn and sugar cane ethanol.  Most sub-pathway inputs still require user modifications.

CA-GREET calculations are contained in a spreadsheet, which is publicly available and not user-protected. The user can download the spreadsheet (link above), modify any of the inputs or formulate, and observe the effects on the results and intermediate calculations. However, the CA-GREET fuel sheets are not easy to follow, equations embedded in each cell can be very long (3-4 lines), and the dependent cells are often located in different worksheets. The spreadsheet does not show many intermediate calculations or results. This said, all the calculations are contained in the CA-GREET spreadsheet, and can be followed and understood, albeit with considerable effort.

The inventory data contained in the GREET model is derived from the U.S. Department of Energy (DOE) Energy Information Administration (EIA) and the U.S. Environmental Protection Agency (EPA), with simulated fuel economy results from Argonne’s Powertrain System Analysis Toolkit (PSAT) model. There is no text documentation for the EPA GREET file, but the spreadsheet files are available in the RFS2 docket online. ARB has released fuel pathway reports for the LCFS that document fuel pathway inputs, calculations, and results from the CA-GREET model in some detail, and are available on the ARB website. However, the fuel pathway documents (one per pathway) often show only general results and do not explain or document input parameters. If a user needs to understand the calculations underlying a result, he/she must refer to the individual pathway CA-GREET spreadsheet. An instructions sheet near the front of the model provides directions for calculating results for each fuel type. No published user-manual is available.

Life Cycle Associates developed CA-GREET as noted above. Its user (un-)friendliness results from the characteristics of default GREET, combined with pressure to complete fuel pathway calculations for the LCFS. But, having developed the model modifications, we can certainly navigate through the model to extract calculation results of interest in any application.