Electric Vehicles and Low-Carbon Fuels: How Much Do They Reduce Emissions?

Use of gasoline, diesel and jet fuel – the most common transportation fuels – is one of the largest sources of greenhouse gas (GHG) emissions. Use of these fuels is impacting the climate in significant and long-term ways.

Decarbonizing Transportation

The Biden-Harris Administration has announced a plan to decarbonize transportation in the United States, which means finding replacements for gasoline and diesel and jet fuel. How can we do that? Electric vehicles are good options, especially for cars and small trucks. But electricity is still made with a lot of fossil fuels. If we transition to electric vehicles, how will emissions change? Biofuel, hydrogen or synthetic fuels made from captured carbon dioxide may be good options for aviation and heavy-duty freight. But all of these fuels require energy in the production process, typically using fossil fuels, and may result in shifts in supply chains, affecting other markets and resulting in more emissions. A phase-down of petroleum use and a phase-up of other kinds of transportation fuels and vehicles will have many impacts.

As we launch forward to “decarbonize” transportation, we should be confident that the adoption of the new fuels will indeed have less climate impact than the old ones.

A newly released report from the National Academies of Sciences, Engineering, and Medicine (NASEM), titled “Current Methods for Life-Cycle Analyses of Low-Carbon Transportation Fuels in the United States,” assesses the methods for evaluating low-carbon transportation fuels [1, 2]. I chaired this ad hoc committee.

Chairing a National Academies Study

The U.S. National Academies work to provide independent, objective analysis and advice to the nation. There are strict protocols to guide the committee work. The statement of task is key, and the report must meet that statement. Committees are built to reflect scientific and engineering expertise across the topic.

NASEM reports are tightly structured, composed of findings, conclusions and recommendations (which must be supported by the findings). Each committee member must sign off on each finding, each conclusion and each recommendation. Seeking consensus on each of these, working through many revisions to reach consensus and then more revisions in response to the peer reviews, is challenging. Our committee did this during the COVID-19 pandemic, so we never met in person – but we got to know each other quite well! The result is a carefully developed, well-supported statement of consensus on the status and research needs of life cycle analysis (LCA). We have given a series of briefings to congressional staff, the U.S. Department of Energy (DOE), DOE National Laboratories, U.S. Environmental Protection Agency and the public.

The report has 70 recommendations and is available free online [2].

The main message is about the overall modeling approach. There are two broad approaches to life cycle assessment. Attributional life cycle assessment evaluates the emissions that can be attributed to a given fuel. Consequential life cycle assessment evaluates how emissions would change if a given policy or set of actions are followed.

Our committee concluded that different types of LCA are better suited for answering different questions or achieving different objectives. To understand the consequences of a proposed decision or action on net GHG emissions, we recommended consequential LCA. To support decision making, the committee also made recommendations for research. Overall, we recommended further development of robust methods to evaluate the GHG emissions from development and adoption of low-carbon transportation fuels.

As an example of consequential life cycle assessment, in previous work, my collaborators and I evaluated how the electricity system and its emissions would change with the adoption of electric vehicles [3]. We used a unit commitment optimization model for short-term operation of the power grid, and a production planning optimization model to evaluate the development of the power grid over time as electricity demands increase. We were able to show the value of managing how and when electric vehicles are charged. By charging at off-peak times, existing capacity is used and electricity-generating costs can decrease for all customers.

As this example suggests, optimization models of the energy system and other methods can provide scenarios for how the energy system might develop, as well as ways to better utilize resources at lower cost. One important research area to investigate is how the electricity system will develop and function if large portions of light- and medium-duty vehicles transition from petroleum fuels to electricity. Another important research area includes the supply chains for hydrogen fuel production and biofuel production. Finally, model quality and validation is another important research area. There are many opportunities for the operations research community to contribute to life cycle assessment of new energy systems.

References

1. National Academies of Sciences, Engineering, and Medicine, 2022, “Current Methods for Life Cycle Analyses of Low-Carbon Transportation Fuels in the United States,” Washington, D.C.: The National Academies Press, https://doi.org/10.17226/26402.
2. https://www.nationalacademies.org/our-work/current-methods-for-life-cycle-analyses-of-low-carbon-transportation-fuels-in-the-united-states
3. Choi, D.-G., Kreikebaum, F., Thomas, V. M. and Divan, D., 2013, “Coordinated EV adoption: Double digit reductions in emissions and fuel use for $40/vehicle-year,” Environmental Science and Technology, Vol. 47, No. 18, pp. 10703-10707, http://dx.doi.org/10.1021/es4016926.