The Greenhouse gases, Regulated Emissions and Energy in Transportation (GREET) model has become the standard for use is performing life cycle assessments of transportation fuels. GREET is a publicly available spreadsheet model developed at the Argonne National Laboratory (ANL) that can be downloaded and run from a user’s computer. The model is a spreadsheet workbook with several macros that can be used directly or manipulated with a graphical user interface (GUI) packaged with the model download, although the GUI is considered unhelpful by many because it obscures access to the inputs and facilitates input to only a limited set of key assumptions.

Life Cycle Associates, LLC has extensive experience in using and modifying GREET. We specifically modified the model for use by the California Air Resources Board (ARB) in establishing the low carbon fuel standard (LCFS). And our proprietary software products (PEEK/POKE and MOUSE) are designed to work with GREET. This experience has given us intimate knowledge of the inner workings of GREET, and experience in navigating the many vagaries of the model.

GREET models emissions of the three traditional greenhouse gases (CO₂, CH₄ and N₂O) and the criteria pollutants. Global warming potential values are used to aggregate the three GHG species emissions into a single carbon dioxide equivalent result. Volatile organic compounds (VOC) and carbon monoxide (CO) are counted in their fully oxidized forms as CO₂.

The main sheets in the GREET workbook include a summary of the main input parameters for all fuel pathways, a fuel production time series sheet, which contains input parameters varying by target year, fuel pathway sheets that calculate results, and a results sheet summarizing the well-to-wheels (WTW) results in g/mi for most fuel pathways. Inputs to the model are executed in several input sheets. In addition, data are introduced on a time series sheet, on worksheets for each fuel pathway, and inside calculation cells. The variety of input locations is confusing to users without extensive GREET experience.

The “Vehicle” sheet organizes vehicle fuel efficiency and emission factors. But, the model does not show the fuel carbon separately from combustion methane and nitrous oxide for tailpipe emissions. Most analysts want to understand these separately and compare WTW emissions on a g CO₂e/MJ fuel basis. The model does not present results by fuel pathway component, nor separate well-to-tank (WTT) and tank-to-wheels (TTW) emissions in g CO₂e/MJ in one convenient place. The model does present WTT calculations disaggregated into “feedstock” and “fuel” results, but further disaggregation requires considerable effort.

The GREET model includes provision for a wide range of feedstocks, fuels, and vehicles. GREET models more than 100 fuel production pathways and more than 70 vehicle/fuel combinations for the time period 1990 – 2020 in five-year increments. GREET models light- and heavy-duty conventional spark ignition vehicles, direct injection spark and compression ignition vehicles, grid-independent hybrid electric vehicles, grid-connected hybrid vehicles (PHEVs), battery-powered electric vehicles (BEVs), and fuel cell vehicles (FCVs).

Several updates of GREET have been released in recent years. Version 1.8c is used by the EPA for the RFS2 analysis. Life Cycle Associates modified Version 1.8b for the California Low Carbon Fuel Standard (LCFS) with many of the changes implemented in version 1.8c . Version 1.8d.1 is the most recent version.

The documentation for GREET is extensive and is accessible through the ANL website. Nonetheless, there is not one all-encompassing, up-to-date report on the most recent version of GREET. GREET documentation spans over 10 years, with initial reports and subsequent update reports and user manuals. Therefore, finding the answer to a specific question about an assumption or calculation in the model is not straightforward. Documentation to GREET 1.8d.1 is provided through a brief memo that describes the references for 11 updates to the model. The result of the current approach is a patchwork of inputs and analysis that is not comprehensive.