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| Fig. 1 : US transportation emissions. (Image source: A. Schechter, after the EPA. [2]) |
The trucking industry is a vital backbone of the US and global economies. In the US, trucking by far is the most relied on freight transport mode; trucks transported 12.5 billion tons of freight (65% of total weight), valued at more than $13.1 trillion (73% of total value), 10X more than rail. [1] The energy requirements for this sector are substantial, given that trucks need to operate over long distances, often carrying heavy loads.
As shown in Fig. 1, the environmental footprint of the trucking industry is significant, contributing to 6.7% of the U.S. GHG Emissions. [2] The trucking industry's role is so crucial that disruptions in this sector can lead to widespread and immediate consequences. For instance, truck strikes in the US and Europe have historically highlighted the industry's significance. During these strikes, disruptions and shortages in essential commodities national security concerns, underscoring the reliance of modern society on uninterrupted truck transport. [3] Therefore, in order to decarbonize the trucking industry and reduce greenhouse gas emissions, alternative energy sources for trucking must be explored.
It's important to note that the energy required to run trucks is independent of the fuel used. So in order to decarbonize the trucking industry while maintaining its scope and scale, the same amount of energy used today must be sourced from fuel other than petroleum. The issue the industry faces today is that no other energy source today is able to fulfill the energy required in trucking. There are some cleaner energy sources that are gaining ground, such as natural gas (including electric-powered and natural gas powered trucks) and hydrogen, but as explained below, none of these options are viable today.
The energy consumption of trucking is intensive, attributed to both the amount of miles that trucks travel, the weight of the freight, as well as the vehicle efficiency. Table 1 summarizes findings from the EPA, showing a unique breakdown of the energy consumed by vehicle modality type and CO2 produced. [2] The energy and CO2 produced are nearly the same numbers.
As shown in Table 1, the energy consumed by medium and heavy duty trucks exceed passenger cars [2]. For the sake of analyzing the freight market specifically, we will assume that the Light-Duty vehicle category is made up of a mix of passenger and commercial vehicles and not representative of the long-haul trucking industry.
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| Table 1: Energy use breakdown per vehicle type. [2] |
A few takeaways from Table 1 include: Light-duty Vehicles produce the greatest amount of CO2, followed by medium- and heavy-duty trucks, and then passenger cars. Busses and rail constitute a minute portion of the transportation in the US, both in terms of energy consumed and CO2 produced. The efficiency of medium- and heavy-duty vehicles on a per-km basis is significantly less than other modalities, 36% as efficient as light-duty vehicles and 27% as efficient as passenger vehicles. Measuring efficiency on a km basis as opposed to weight basis does skew the results in favor of lighter vehicles, however it's still an important factor as we consider the distance goods are transported across the country and the environmental toll it causes.
Looking forward, the trucking industry is being heavily pressured to use more sustainable energy sources. Medium- and Heavy-Duty Trucks rely primarily on distilled fuel (primarily diesel fuel). Long-haul trucks are designed to cover extensive distances without refueling, requiring a fuel type like diesel that has high energy density that allows for more energy in a tank of a given size. [2]
Technological advancements are slow, but have started to allow the industry to gradually shift towards alternative fuels such as natural gas. There is also a lot of development exploring the use of hydrogen and electric batteries for trucking, however given the long ranges and heavy loads, these sources have large barriers to overcome before they can be mass adopted. [4,5]
Natural gas presents an opportunity to reduce, though not completely eliminate, carbon emissions. Since natural gas contains carbon, it generates approximately 20-30% fewer carbon emissions compared to diesel. The energy content per carbon atom is comparable for both diesel and natural gas. This similarity means that the CO2 emissions are directly proportional to the amount of carbon consumed, regardless of the fuel type. Counting atoms, we find that every kg of diesel burned emits 44/14 kg of CO2, and every every kg of natural gas burned emits 44/16 of CO2. From the energy contents of 4.2 × 107 J kg-1 and 5.1 × 107 J kg-1 for diesel and natural gas, respectively we then have [6]
| kg of CO2 per Joule
produced by burning diesel kg of CO2 per Joule produced by burning natural gas |
= | 5.1 × 107 J kg-1 4.2 × 107 J kg-1 |
× | 16 14 |
= | 1.39 |
Therefore, switching from diesel to natural gas does not significantly reduce carbon emissions because the energy content per carbon atom (and thus CO2 emissions per unit of energy) is nearly the same. Therefore, switching from diesel to natural gas as a decarbonization strategy is not effective in significantly reducing CO2 emissions.
Battery electric trucks currently face challenges similar to those of natural gas-powered trucks. The primary issue is that most electricity is still generated from fossil fuels. [7] Therefore, switching from diesel to electric vehicles may not significantly reduce CO2 emissions until electricity generation becomes more sustainable. For electric trucks to be a viable solution in decarbonization, there needs to be widespread adoption and scaling of alternative, sustainable energy sources. Even if the energy source was to be rectified, another hurdle in battery electric trucks is the technological limitation associated with batteries. Long-haul electric trucks require large and heavy batteries, which could hinder their efficiency and functionality in freight transport. Additionally, the toll of electrification of the trucking industry imposes significant pressure on grid capacity, especially during peak hours when the grid is most congested presenting a substantial infrastructure challenge. [8]
With more technology development, hydrogen fuel cells show potential future promise in replacing diesel for long-haul trucking because of the high energy density of compressed and liquid hydrogen and hydrogen tanks are lighter and smaller than batteries. [7] Hydrogen is very light and has a much higher energy content per unit mass than diesel. However, because hydrogen is so light it takes up more volume, making storage innovation critical and stringent in the transportation sector. The technology to meet all the storage requirements for hydrogen is extremely advanced requiring extensive research and development. The technology barriers indicate that hydrogen is not a viable decarbonization option for trucking at this time. [9,10]
The decarbonization of the trucking industry is critical because of the significant environmental footprint of the trucking industry and the US continued reliance on trucking to transport goods. While hydrogen fuel cells and electrification present promising avenues for medium and heavy-duty trucks, as well as light-duty vehicles, these solutions come with their own set of challenges, particularly in terms of energy storage and grid capacity. The successful implementation of these technologies will depend on continuous advancements in fuel and battery technology, infrastructure development, and strategic policy initiatives.
© Aline Schechter. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] "Transportation Statistics Annual Report 2022," U.S. Department of Transportation, 2022.
[2] "U.S. Transportation Sector Greenhouse Gas Emissions 1990 -2021," U.S. Environmental Protection Agency, EPA-420-F-23-016, June 2023.
[3] J. L. McGrady, "The Impact of Organized Labor on the Defense Trucking and Railroad Industries," National Defense University, 1993.
[4] B. Borlaug et al., "Heavy-Duty Truck Electrification and the Impacts of Depot Charging on Electricity Distribution Systems," Nat. Energy 6, 673 (2021).
[5] M. Woody et al., "The Role of Pickup Truck Electrification in the Decarbonization of Light-Duty Vehicles," Environ. Res. Lett. 17, 034031 (2022).
[6] D. Englert et al., "The Role of LNG in the Transition Toward Low and Zero Carbon Shipping," World Bank, April 2021.
[7] M. de las Nieves Camacho, D. Jurburg, and M. Tanco, "Hydrogen Fuel Cell Heavy-Duty Trucks: Review of Main Research Topics," Int. J. Hydrog. Energy 48, 29505 (2022).
[8] M. R. Bernard et al., "Charging Solutions for Battery-Electric Trucks," International Council on Clean Transportation, December 2022.
[9] P. P. Edwards, V. I. Kuznetsov, and W. I. F. David, "Hydrogen Energy," Phil. Trans. R. Soc. A 365, 1043 (2007).
[10] E. Rivard, M. Trudeau, and K. Zaghib, "Hydrogen Storage for Mobility: A Review," Materials 12, 1973 (2019).