MIT Study Compares 2030 Electric Propulsion Systems; Conventional Hybrids, Fuel Cell, Battery and Plug-in Hybrid Electric Vehicles Offer Comparable GHG Reductions
|Overview of projected greenhouse gas and fuel consumption outcomes for different propulsion systems in 2030 vehicles. Click to enlarge.|
Electric propulsion systems, including gasoline hybrid electric vehicles (HEV), plug-in hybrid vehicles (PHEV), fuel cell vehicles (FCV) and battery vehicles (BEV), can, with projected evolution of the supporting technologies, “reduce or eliminate the transport sector’s reliance on petroleum,” according to a study done at MIT by Matthew Kromer and Professor John Heywood.
However, the study also concludes that continued use of fossil fuels without effective carbon capture and sequestration for producing electricity and hydrogen constrain the greenhouse gas (GHG) and energy reductions of all the different forms of electric propulsion to about 60% below that of present day technology. In other words, without cleaner power pathways, PHEVs, BEVs and FCVs are not projected to offer much of a greater reduction in GHG than an HEV, and in many cases, the GHG profile is worse.
The study, presented at the SAE 2008 World Congress in Detroit, quantifies the potential of electric propulsion systems to reduce petroleum use and greenhouse gas emissions in 2030 US light duty vehicles. The paper is a follow-on to a study presented last year that assessed the potential improvement of more conventional automotive powertrain technologies 25 years into the future. (Earlier post.)
For the purpose of the study, the authors used a 2006 2.5L Toyota Camry as the basis for the future propulsion systems. For consistency across vehicle platforms, they held vehicle performance and size constant at present-day levels. Holding the parameters constant, they noted, is at odds with historical trends, which have shown a steady increase in size and performance over the last 25 years. However, by keeping the characteristics at current levels, they could quantify efficiency gains enabled by the technological progress.
The future vehicles do include a number of evolutionary, non-powertrain improvements which are applied consistently across different vehicle technologies, including improved aerodynamics, reduced rolling resistances and some weight reduction.
|Projected Characteristics for PHEV Battery Packs|
They assumed the use of lithium-ion battery packs with several adjustments to present-day performance characteristics: the ability to maintain rate capability at high depth-of-discharge and evolutionary improvements in battery specific power and specific energy.
They assumed that specific energy improves by a factor of 1.5 over current Li-ion packs (a rate of about 2% per year) for both high-power and high-energy batteries.
To characterize the impact of the uncertainty over future power generation pathways, the authors considered three different generation scenarios: US national average grid mix, 100% coal, and 100% natural gas. Calculations were based on the projections from the EIA Long-Term Energy Outlook for 2030 and include 9% transmission and distribution losses, and 10% charging losses.
The simulation results showed that advanced technology vehicles offer a number of paths to reduce petroleum consumption: the hybrid electric vehicle (HEV) in 2030 offers a 63% reduction over the 2006 baseline vehicle and a 43% reduction compared to a 2030 gasoline vehicle.
The plug-in hybrids offer even greater reductions, with the magnitude dependent on the all-electric range of the PHEV. A 2030 PHEV-30 (30 mile electric range) offers an 81% reduction in petroleum consumption compared to the 2006 baseline vehicle, and a 71% reduction compared to the 2030 conventional gasoline vehicle. The PHEV-60 takes those percentages up to 88% and 81%, respectively.
However, they noted:
Reducing vehicle energy use and GHG-emissions beyond the level offered by the gasoline hybrid presents a much greater challenge. Under the assumed fuel production pathways, the hybrid, plug-in hybrid, and fuel-cell vehicle each offer a 40-45% [GHG figure] lower than the 2030 NA-SI baseline. These results suggest that without an effective effort to develop cleaner fuel pathways, a transition to alternative fuel powertrains does not deliver a significant CO2 benefit beyond that offered by the gasoline hybrid vehicle.