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Transportation Sector

Transportation accounts for 27% of energy use in the U.S. (EIA, 2019) and is the largest contributor of greenhouse gases, accounting for 28% of U.S. emissions (EPA, 2019). Globally, transportation accounts for 19% of end-use energy consumption and 14% of global emissions (EPA, 2019). Historically, transportation has relied heavily on fossil fuels. However, the transportation sector is ripe for rapid electrification and reliance on alternate fuels.


Biodiesel can be made from any fat or oil. Current U.S. biodiesel production is primarily from oil from soybeans (such as the ones in the picture) or from recycled restaurant cooking oil. Cleaner burning and renewable biodiesel is most often blended at 20% with petroleum diesel. Source: National Renewable Energy Laboratory

Biofuels are appealing alternatives to fossil fuels given that they are bio-derived and renewable, many of which have lower life-cycle emissions compared to fossil fuels  (EPA, 2020). Examples of biofuels include ethanol (derived from crops like switchgrass, corn, sugarcane, sorghum, and others) and biodiesel (derived from algae, sorghum, canola, and others). Most combustion engine technology does not allow for the sole combustion of bio-derived fuels but they can be mixed with traditional fossil fuels. Examples include E85 which is a 85:15 blend of ethanol and gasoline and B20, which is a 20:80 blend of biodiesel and fossil diesel.

Biofuels involve the cultivation of large amounts of bio-crops which have drawbacks such as:

Policies to promote biofuels should prevent biofuel crop production on land that would naturally regenerate as forests; opt for best practices accounting instead of treating all biomass as a zero-emissions feedstock; and curtail expansion of biofuels produced from food crops or from converting viable cropland into biofuel production (Reid et al. 2020).

Related AGCI Resource:


Light-duty electric trucks. Source: National Renewable Energy Laboratory.

Electric light duty vehicles are powered through an electric powertrain. EVs can have significant emission savings when electricity is sourced from renewable or nuclear energy sources. There are multiple kinds of EVs. Non plug-in hybrid EVs are powered by an internal combustion engine and an electric motor, which uses energy stored in batteries (which are only charged through regenerative braking and by the internal combustion engine), therefore resulting in better fuel economy than standard gas-powered vehicles. Plug-in hybrid EVs have an electric powertrain with a plug-in rechargeable battery and a standard internal combustion engine. The electric powertrain usually has smaller mile ranges between 20-30 miles/charge, sufficient for many trips within city limits. Battery electric vehicles (BEVs) have only an electric powertrain and have a large battery on-board, allowing for longer ranges often in the hundreds of miles/charge.

There are several barriers to the adoption of EVs including cost and range anxiety. The Union of Concerned Scientists published a report summarizing the barriers to adoption and identifies market action by key automobile manufacturers. Pre-emptive deployment of charging infrastructure helps alleviate range anxiety that customers face, and could be achieved through robust public-private partnerships.

It is important to consider that currently in the U.S. alternative vehicles and especially electric vehicles are priced higher than traditional internal combustion engine vehicles and are not accessible across U.S. socio-economic strata. A report by Puget Sound Energy elucidates the barriers to EV adoption and success stories are highlighted such as the BlueLA EV car sharing program for underserved communities. The International Council on Clean Transportation (ICCT) estimates that in the U.S. electric vehicle initial cost parity is coming within 5-10 years (ICCT, 2019). This is mostly driven by continuous reductions in battery costs and scale improvements. 

Comparative ownership costs over time of different vehicles (EVs with a range of  150, 200, and 250 miles, a plug-in hybrid with an electric range of 50 miles, and a conventional vehicle) through three different types of vehicles: car, crossover, and sport utility vehicle (SUV). Cost parity for battery electric vehicles and conventional vehicles is projected within the next 5-10 years. Source ICCT, 2019.


Electric Vehicle Grid-Integration research is done at the Vehicle Testing and Integration facility (VTIF) at NREL in Golden, Colorado, U.S.A. Source: National Renewable Energy Laboratory

In addition to providing a means for carbon-free transportation, EVs also have the potential to fill an energy storage function within the electricity grid. Research has shown that passenger vehicles are parked 95% of the time. When not in use, EVs can act as stationary storage devices supplying electricity to the grid – a process known as vehicle to grid or V2G. There is significant decarbonization potential in doing so, especially when EVs have been charged by renewable electricity and when the energy supplied by EVs helps avoid the dispatch of gas powered peaker plants. 

V2G helps reduce overall grid emissions and lower electricity costs. Thus EV fleet owners can derive revenue through providing storage services to the grid. The Los Angeles Air Force Base carried out a pilot V2G study to estimate the value proposition of providing grid services and they identified earnings equaling $100-$150/vehicle/month. Such revenue can make owning EVs cheaper. However, an important fact to consider is that the increased number of charge-discharge cycles can hamper the longevity of the EV battery life. 


GreenPower Bus, an all-electric double decker bus. Electric buses are more expensive than traditional ones, but several cities around the world are adopting them since they emit zero emissions at the tailpipe, require less maintenance than traditional buses, and are a lot quieter. Source: National Renewable Energy Laboratory.

Electrification of public transportation, and especially buses, would come with local air pollution and noise abatement benefits. The market for electric buses globally is quite nascent at present.  China is the most developed electric bus market with a global market share of 99% (Greenbiz, 2019).

The biggest hurdle to widespread adoption of electric buses is currently the underlying economics, represented by the Total Cost of Ownership (TCO). The TCO of electric buses are 50-160% more than ICE buses depending on the modality of charging. Much of this is due to initial capital cost, infrastructure costs (to set up robust charging networks to ensure seamless operation of electric buses), and maintenance costs (including battery replacements). 

Overnight charging results in long periods of idle time resulting in a larger TCO, but opportunity charging – which involves charging strategically for short intervals along the bus route – can enable 24/7 operations and reduces the TCO (Mahmoud et al., 2016). Electric buses can also serve as an important V2G energy storage resource. Electric school buses are likely to be operated as part of a fleet and would have defined routes for daily travel thus increasing the predictability of being available to provide grid storage services while idle (especially during off-peak summer months when grid peak demand is typically highest (GTM Research, 2019). 

Like buses, semi-trucks present both challenges and opportunities to power through alternative means, including electrification via batteries, overhead power lines, as well as hydrogen and other renewable energy carrier fuels. Some of these technologies are not far off, while others (such as direct electrification, with the greatest potential to mitigate GHGs), will require substantial infrastructure investments before they can become mainstreamed. Researchers have been exploring the most promising low carbon technologies.

Battery technology is critical for electrification of buses, and especially long-range heavy duty trucking. Batteries are currently too expensive and too large to compete in the market. In cases such as these, hydrogen can play a massive role in decarbonizing such transport. Hydrogen-powered fuel cells on board vehicles can help generate electricity to power the drivetrain, with relatively harmless tailpipe emissions. Emissions can be further curbed by using “blue hydrogen” or “green hydrogen” produced using renewable energy.

Levelized Cost of Driving comparison between BEV medium/heavy duty and bus. The three projected scenarios reflect futures with qualitatively different investment in and improvement of electric technologies cost and performance. The Slow Advancement cases represent futures where electrification follows current trends without major advances, whereas the Rapid Advancement projections are consistent with futures in which public and private research and development (R&D) investment in electric technologies spurs technology innovations, manufacturing scale-up increases production efficiencies, and consumer demand and public policy yields technology learning. The Moderate Advancement projections fall between the Slow and Rapid projections, reflecting electric technology progress beyond current trends. Source: Jadun, et al. 2017. Electrification Futures Study: End-Use Electric Technology Cost and Performance Projections through 2050. Golden, CO: National Renewable Energy Laboratory. NREL/TP-6A20-70485.


NASA’s all-electric X-57 Maxwell plane being delivered in two parts, with the wings separated from the fuselage. This crewed X-plane seeks to further advance the design and airworthiness process for distributed electric propulsion technology for general aviation aircraft. Source: National Aeronautics and Space Administration (NASA)

Aviation and shipping, like heavy ground transport, require high energy density fuel to allow for long-distance travel. Alternative low-carbon fuels have been seen as the most promising avenue to lower emissions associated with air travel (as opposed to electrification with batteries). One alternative outlined by the Kleinman Center for Energy Policy at University of Pennsylvania is a carbon-neutral hydrogen jet fuel, which relies on hydrogen produced using carbon capture and storage or electrification in fuel production/electrolysis processes, and negative emissions technologies to offset airline flying emissions. Making low-carbon fuels economical remains an obstacle. Design, operations, infrastructure, socio-economic and policy measures will be key factors in decreasing aviation emissions (Singh et al. 2017).

Just as biofuels have been explored for ground transport, they are also being considered for aviation and maritime shipping. High energy and non-edible crops like algae, camelina, and jatropha are all being experimented with to make jet fuel, either as direct biofuel, or as bio-synthetic paraffinic kerosene. To date, progress has been made in developing these fuels as additives to jet fuel blends, but they have not reached a point where they can altogether replace jet fuel (Yilmaz and Atmanli, 2017). 

The promise of alternative fuels ultimately lies in if fuel quality can improve, while keeping costs down and maintaining sustainable feedstocks, conversion technologies, and co-product allocation. Biofuels coupled with energy efficiency improvements in maritime shipping design and operation, could account for an 85% drop in global shipping emissions by 2050, assuming carbon-neutral feedstock sourcing (Bouman et al. 2017).