Our detailed computer simulations estimated the greenhouse gas (GHG) emissions for each of the alternative vehicle scenarios over the 21st century. This chart shows that hydrogen-powered fuel cell electric vehicles will have to be added to plug-in hybrids and biofuels to achieve our goal of cutting GHGs to 80% below 1990 levels.  Plug-in hybrids and biofuels alone will not reach our GHG goal.

To summarize:

• “Business as usual”--100% gasoline cars: if we all keep driving regular (non-hybrid) gasoline-powered internal combustion engine vehicles (ICVs), then GHGs keep rising as more cars are on the road driving more miles each year, as shown by the upper dashed line labeled “100% Gasoline Vehicles” on the chart above
• Gasoline-powered hybrid electric vehicle (HEV) scenario: adding gasoline-powered hybrid electric vehicles (HEVs) helps; GHGs are reduced substantially, although GHGs rise again in the last half of the century as vehicle miles traveled cancels out the improved fuel economy of HEVs
• Gasoline plug-in hybrid scenario: introducing gasoline-powered plug-in HEVs (orange line) also cuts GHGs significantly by mid-century when the model assumes that the electrical utility grids substantially cut their use of coal power, and/or install carbon capture and storage (CCS) technology at most coal plants.  Plug-in hybrids would cut GHGs to approximately 20% below 1990 levels by the 2070 time period.
• Biofuel plug-in hybrid scenario: biofuels such as cellulosic ethanol can also play a significant role, when combined with plug-in hybrids. As shown in the graph, the scenario with biofuel-powered PHEVs are projected to cut GHGs to 43% below 1990 levels by 2070.
• Fuel cell electric vehicle scenario: However, our detailed computer simulations demonstrate that, in order to achieve our goal of reducing greenhouse gases to 80% below 1990 levels (dashed red line at bottom of graph), society will have to begin the process of replacing the internal combustion engines on HEVs and PHEVs with all-electric vehicles.  The fuel cell EV scenario would reach an 80% reduction below 1990 levels by approximately 2070 with the parameters in this model

We also analyzed two other scenarios that have lower probability of being implemented by the auto industry: hydrogen-powered hybrid internal combustion engine cars, and full-function, long-range battery EVs.  The GHGs from these two secondary options are shown by the dashed lines in this chart:

For the full technical details of our computer simulation results, see the paper that was published in the International Journal of Hydrogen Energy in August of 2009.  For a detailed description of the input assumptions for these models, see another article published in Dec

Hydrogen ICE hybrid electric vehicle scenario: replacing gasoline and biofuels with hydrogen on an ICE HEV achieves almost the same GHGs reductions as the hydrogen FCEV (dashed green curve just above the FCEV curve). Battery electric vehicle scenario: the battery-powered electric vehicle scenario (blue dashed curve third from the bottom of the graph) does achieve the 80% reduction below 1990 levels by approximately 2090 in this model; for the next few decades, however, the BEV does not reduce GHGs significantly since most grid electricity in the US comes from burning coal, the most carbon-intensive fuel source. Once the utility grids begin to reduce their carbon footprint by replacing old coal plants and/or adding carbon capture and storage (CCS) to existing or future plants, then the GHGs also decline for the battery EV scenario.  [Note that this BEV scenario assumes that up to 98% of all light duty vehicles (cars and trucks) sold by the end of the century are powered by advanced batteries. This seems very unlikely, given the projected performance of advanced batteries as seen from the known attributes of advanced battery technology in 2009, and the lack of charging access at night for some fraction of vehicle owners. We therefore conclude that this BEV scenario has a low probability of success, at least compared to fuel cell EVs.] Early GHG reductions.  Advocates of PHEVs claim that we need to begin deploying PHEVs now to achieve reasonable GHG reductions after 2020.  Out model does assume tha PHEVs enter the marketplace first, as shown in this graph:There are 10 million PHEVs on the road by 2024, but FCEVs do not reach the 10 million mark until five years later, in 2029: Despite this head-start for PHEVs, it turns out that hydrogen-powered FCEVs cut GHGs more than gasoline-powered PHEVs during the decade from 2020 to 2030 In 2030, the PHEV scenario only cuts GHGS by 2.2% compared to the base case with HEVs, while the FCEV scenario cuts GHGs by 4.4%, or two times greater reductions than the PHEV scenario: The FCEV Scenario cuts GHGs more since FCEVs immediately reduce GHGs by 50% compared to gasoline-powered cars, even if the hydrogen is made from natural gas, while PHEVs still rely on gasoline for part of their travel, and most electricity used to charge PHEV batteries in the US comes from burning coal, the dirtiest (Highest carbon content) fuel.

ember of 2009.

Greenhouse gases (GHGs) with 100% Coal-generated electricity. Some advocates of BEVs and PHEVs have claimed that BEVs and PHEVs will generate lower GHG emissions even if the electricity is generated by burning coal. The highly respected Argonne National Laboratory GREET model [1] refutes this supposition. As shown in this figure, a BEV would generate 11% more GHGs than a conventional (non-hybrid) gasoline car, and plugging in a gasoline HEV will generated more GHGs than running that HEV on gasoline only:

In addition, even with the average US grid mix, plugging in HEVs that are using lower carbon fuels such as hydrogen, E-85 (mixture of 85% ethanol and 15% gasoline) and even diesel fuel always increases GHGs (in other words, the GHGs for AER =0 corresponding to an HEV is lower than any PHEV with finite AER...the lines all rise as you go to higher all-electric range (AER)):

For more details on GHGs from coal-based electricity, see this report.

Proposed EPA/DOT Motor Vehicle Fuel Economy Labels for 2012 to include GHG ratings.  The US Environmental Protection Agency and the US Department of Transportation are proposing to add greenhouse gas (GHG) emissions data to the vehicle fuel economy stickers placed on all new vehicles. This is welcome news, since consumers should be given credible information regarding the climate change impact of new car purchases. Unfortunately, they are proposing to list only the vehicle tailpipe GHG emissions, excluding all upstream emissions from fuel processing and, in the case of electric vehicles, the GHGs that come from electricity generation plants (most of which are fueled by coal in the U.S., the dirtiest (highest carbon content) fuel.  The impact of this proposed GHG rating system is illustrated in these two tables. The first table shows the ratings of a set of alternative vehicles based on the full well-to-wheels (WTW) GHG emission, the only fair measure of the total impact of motor vehicles on climate change gases.

The second table shows these same vehicles sorted according to tailpipe emissions only:

Finally, we plotted the alternative vehicle GHG rankings with tailpipe-only emissions against the GHG rankings for WTW GHGs. The results look like a random scatter diagram, indicating that there is no serious correlation between tailpipe-only GHGs and WTW GHGs, the true measure of climate change impact (If tailpipe GHGs were a good surrogate for WTW GHGs, then the red dots would all end up on the straight line), :

IFor more details on the proposed fuel economy labels and to make comments about the proposed label with only tailpipe GHGs listed, contact the EPA.

For more details on our calculations of GHGs for the 31 alternative vehicles, see this report.

DOE GHG alternative vehicle GHG Estimates.

The U.S. Department of Energy uses this bar chart to compare the estimated WTW GHGs from various mid-sized alternative vehicles and fuels in the 2030 to 2045 time period (except for the top bar for current gasoline ICVs)

 [Source:  2010 DOE Hydrogen and Fuel Cell Program plan, available at: http://www1.eere.energy.gov/hydrogenandfuelcells/mypp/program_plan.html ]

[1]“The Greenhouse gases, regulated emissions and energy use in transportation model”, by the Argonne National laboratory, available at http://greet.es.anl.gov/main