Research Publications

The introduction of plug-in hybrid vehicles (PHEVs) to the consumer market has raised questions about how fuel economy should be calculated. The reason fuel economy is difficult to determine for a PHEV is that this type of vehicle can travel on both gasoline and battery electricity. Correlating electricity to a fuel economy equivalent is difficult because consumers are not accustomed to thinking of fuel economy in its energy equivalent form. But, more important than fuel economy, from an environmental assessment perspective, are the life cycle emissions associated with PHEV energy consumption. The life cycle impacts of gasoline use can be determined through established methods. However, the life cycle impacts of the electricity used in a PHEV are more challenging, primarily because no agreed upon accounting methodology has yet been established. This research explores the differences between several methodologies for determining the life cycle impacts of electricity consumption. Two methods are based on historical average grid emissions information applied to electricity consumption, while two other methods are based on electrical grid dispatch models. The grid average emissions methods examine two characteristics: the total grid average and marginal grid average. The dispatch methods examine an hourly moving total grid average, and an hourly moving marginal average. ‘Marginal’ refers to electrical generators in a grid operating above an anticipated base load generation. Applying emissions rates from marginal electrical generators to PHEVs implies that PHEV load is unexpected and it will only be met by these marginal facilities. Using a grid average approach implies that utility planners have anticipated PHEV demand, thus emissions can be assumed as shared between all generating facilities. We find significant differences (up to an order of magnitude in some case) in emissions predictions using these different methods. This has important implications for the life cycle assessment of electricity demand.