Argonne Study Recognizes Benefits of Charge-Depletion Hybrids

Study finds that use of the fuel to serve a PHEV in CD mode would provide more kilometers of service than competing options evaluated.

Published: 02-Jan-2008

Researchers at the US Department of Energy’s Argonne National Laboratory, which has the lead role in DOE efforts to evaluate plug-in hybrid electric vehicles (PHEVs) and PHEV technology, recently compared US near term (to ~ 2015) alternative pathways for converting energy to light-duty vehicle kilometers of travel (VKT) in plug-in hybrids (PHEVs), hybrids (HEVs), a simulated fuel cell HEV and PHEV, and conventional vehicles (CVs).

The study was focused on identifying the pathway that provided the most vehicle kilometers from each of the five main feedstocks—oil, natural gas, coal, farmed trees and wind/solar renewable energy—assessed. The study, presented in a paper at EVS-23, calculated values for total energy, energy by fuel type, total greenhouse gases (GHGs), volatile organic compounds (VOC), carbon monoxide (CO), nitrogen oxides (NOx), fine particulate (PM 2.5) and sulfur oxides (SOx).

Since the focus of the study was to determine what could be accomplished by offering a PHEV option as an addition to an HEV, the team highlighted the effects of CD [charge depleting] operation in this study.

This allows us to think in terms of the potential per kilometer effects of choosing batteries for PHEVs, in lieu of continued use of conventional petrol or natural-gas-fueled powertrains, petrol HEVs, E85 FFVs, or future use of E85 FFV HEVs, emerging clean diesel (CIDI) engines or fuel cell (FC) powertrains.
The team assumed a US compact-sized car, and used the GREET 1.7 fuel cycle model and the new GREET 2.7 vehicle cycle model as the foundation for the study. It also isolated the PHEV emissions contribution from varying kWh storage capability of battery packs in HEVs and PHEVs from ~16 to 64 km of charge depleting distance.

Among the findings of the study were:

More kilometers of service from coal are obtained by the use of coal-generated power to support PHEV CD mode than by converting coal to synthetic diesel (CTL) for use in diesel engines.

More kilometers of service from farmed trees are obtained by the conversion of the biomass to power to support the PHEV CD mode than by converting trees to ethanol for use in HEVs or PHEVs in charge sustaining mode.

For wind and solar, PHEV CD mode provides far more kilometers of service than the use of the renewable electricity in electrolysis to create hydrogen.

Biofuels do not look significantly better than coal-based options on a total energy basis, but do for greenhouse has emissions. Renewable energy options reduce GHG emissions by a factor of three or more, compared to the fossil-based options.

Thus, regardless of which abundant domestic fuel one would wish to use, use of the fuel to serve a PHEV in CD mode would provide more kilometers of service than competing options evaluated.

While we have here highlighted and isolated the effects of CD operation of a PHEV, we recognize that evaluations of the aggregate annual impacts, including CS [charge sustaining] operation, should be considered. Future research will endeavor to explore CD, CS, and annual average operations separately and jointly, and in more detail than discussed here. Finally, we concede that this study is limited, in terms of HEV and PHEV powertrain types investigated, and in the time interval considered. It also largely ignores the potential implications of regulations or taxes on carbon emissions. Nevertheless, it verifies the promise of PHEVs in the near-term, helping assure that research and development dedicated to the introduction and implementation of this technology is well founded. Resources
L. Gaines, A. Burnham, A. Rousseau, D. Santini (2007) Sorting Through the Many Total-Energy-Cycle Pathways Possible with Early Plug-In Hybrids

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