What’s the “Best” Green Transpo Technology?
22 September 2004
There are many different technology options and possible combinations available and emerging for sustainable transportation, short and long term. The complexity and number of variables makes it difficult for stakeholders to sit down and to evaluate definitively the environmental and energy impact of these systems. For automakers faced with different types of regulations, policy makers trying to craft those regulations and consumers trying to do the right thing, the problem can become very confusing.
Advances in internal combustion engines and combustion control (both gasoline and diesel, with a gradual transition to some form of HCCI) and hybrids play a fundamental role. Ongoing improvements in transmissions and improvements in the vehicles themselves (shape, weight, electronic systems, APUs) are also key factors. Waste Heat Recovery (recapturing some of the otherwise wasted heat energy of the combustion) will also prove an important avenue.
Fuels also are fundamental. In addition to the enhancements to “standard” petroleum fuels, such as the use of additives, the advent of ultra-low sulfur diesel, and increasing blending with biofuels, there is the potential for large-scale biofuel use and the incorporation of other, clean-burning synthetic or alternative fuels.
Finally, there is the transition to a non-combustion engine regime, such as hydrogen fuel cell vehicles.
How does one decide what is “best” from an energy and emissions point of view?
To work through this, researchers build models making basic assumptions about the development and combination of technologies, and then calculate the environmental and energy impacts of these different automotive systems during three identifiable stages:
- Well to tank (fuel production up to the point of pumping the fuel into the vehicle)
- Tank to wheels (fuel use in the vehicle)
- Cradle to grave (manufacture and destruction of the vehicle)
Like any similar effort, the results are only as good as the model, and the detail that goes into the model. Nevertheless, even simpler models can have directional results.
John Heywood and his colleagues at MIT’s Sloan Automotive Laboratory have done quite a bit of research on this during the last five years. He presented an overview of their findings at the recent Diesel Engine Emissions Reduction Conference (DEER 04).
The chart to the right depicts the energy consumption of a set of different vehicle technologies relative to a baseline the MIT team defined as evolutionary improvements in fuel and vehicles up to 2020—improvements similar in their relative amount to those achieved over the last 25 years. Compared to that baseline, a gasoline vehicle in 2001 consumed 1.37 times the energy in its lifecycle.
You’ll note that a diesel hybrid design has lower impact in the energy lifecycle than a hydrogen fuel cell vehicle. Only combining fuel cells with batteries to create a fuel cell hybrid (rather than a gasoline or diesel hybrid) produces a lower impact. This is due to the energy cost of the production of hydrogen, depicted in the chart below.
|Vehicle||Energy, % of Total||GHG, % of Total|
|Operation||Fuel Cycle||Vehicle Mfg||Operation||Fuel Cycle||Vehicle Mfg.|
|Gasoline ICE Hybrid||69||14||17||67||17||16|
|Diesel ICE Hybrid||70||10||20||70||11||19|
|Hydrogen FC Hybrid||44||35||21||0||79||21|
|Gasoline FC Hybrid||66||14||20||65||16||19|
|Weiss, et. al., 2003|
None of the scenarios above factor in alternative or synthetic fuels. Running biodiesel in lieu of the petroleum diesel projected in the studies would certainly have an impact, although the calculations of the overall energy lifecycle become complicated by the calculations of the energy cost of agriculture. By some calculations (more on this below) biodiesel provides a worse emissions scenario despite being a cleaner-burning fuel because of the energy and emissions cost of agriculture. The ethanol industry has faced a similar situation as proponents and detractors argue back and forth over the “field-to-wheel” benefits.
Another, more granular view of the impact of different technology choices comes from the more detailed scenario work being done as part of the analysis of AB 1493— the California Climate Change bill that will mandate reduction of CO2 as a pollutant.
The combinations are too numerous to reproduce here, but the gist of the research findings is that technology combinations—either on-the-shelf or soon-to-emerge—could cost-effectively reduce greenhouse gas emissions by the amounts targeted in the bill. Some alternative fuels were included as part of the study; biodiesel was not.
The chart to the right is a summary taken from the final staff report (long and detailed) on technologies and the rationale behind the bill. Click to enlarge. (The consideration hearing is tomorrow, 23 Sep.)
If you can pick through the chart without going cross-eyed, you can see some major potential wins in addition to the more minor, but additive, techniques. Hybrid technologies are one. Advanced combustion techniques and intelligent combustion control are another.
One of the most comprehensive models for emissions on the fuel side comes from Mark Delucchi at UC Davis. Not only does his model factor in more types of GHGs (Greenhouse Gases) than most other studies, he also incorporates alternative and synthetic fuels. His model finds that biodiesel and corn ethanol are among the worst in terms of aggregate increases in GHG emissions, along with synthetics (Fischer Tropsch) produced from coal. (Table 58 in his report has a summary of the emission findings.)
His overall conclusion: conventional life cycle models of energy use and emissions may reasonably well represent differences between similar alternatives, but need further development to represent adequately differences between modes of transportation or between dissimilar fuel production pathways (such as biofuels vs. fossil fuels).
That’s an excellent point. Tank to wheels seems a more tractable analysis problem than well (or field) to tank. But rather than wait for a grand answer (42?), stakeholders can take definitive steps now to reduce fuel consumption, reduce emissions, and lay the foundation for sustainability. The scientists and engineers are doing their parts. The two stakeholders that can make the most difference now in the shorter term are the policy-makers and the consumers.
On the Road in 2020: A life-cycle analysis of new automobile technologies, M.A. Weiss, J.B. Heywood, E.M. Drake, A. Schafer, and F. AuYeung, MIT Energy Lab. Report, MIT EL 00-003, October 2000
Comparative Assessment of Fuel Cell Cars, M.A. Weiss, J.B. Heywood, A. Schafer, and V.K. Natarajan, MIT Lab. For Energy and Env. Report, MIT LFEE 2003-001 RP
- Coordinated Policy Measures for Reducing the Fuel Consumption of the U.S. Light-Duty Vehicle Fleet, A.P. Bandivadekar, and J.B. Heywood, MIT LFEE 2004-001 RP
Speaker presentations from CARB workshop on AB 1493
A Lifecycle Emissions Model (LEM): Lifecycle Emissions from Transportation Fuels, Motor Vehicles, Transportation Modes, Electricity Use, Heating and Cooking Fuels, and Materials,Delucchi, Mark A. ITS-Davis December 2003.
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