In this insight article, we delve into the complexities of hydrogen-powered vehicles and their emissions beyond the tailpipe. While hydrogen fuel cells offer near-zero tailpipe emissions, the true environmental impact hinges on factors like vehicle design, mileage, and, crucially, the carbon intensity of hydrogen production.
As we continue the discussion from our previous article on fuel cell sizing and cost, we must recognize that hydrogen-powered vehicles aren’t inherently decarbonized. Yes, hydrogen fuel cells produce virtually zero tailpipe emissions, but the full environmental picture is more complex. The total emissions footprint of hydrogen vehicles depends on several critical factors, including vehicle design, miles traveled, and—most importantly—the carbon intensity of hydrogen production itself.
For businesses and policymakers aiming to decarbonize their fleets and meet sustainability goals, understanding the complexities of these emissions are crucial. Simply switching to hydrogen vehicles doesn’t guarantee a low-emission outcome. The entire lifecycle of the vehicle, from fuel production to mileage patterns, needs to be optimized to deliver true environmental benefits.
For a hydrogen truck, lifecycle operating emissions can be approximated as the sum of (mileage x fuel economy x H2 emission intensity) over the life time of the vehicle.
Addressing these variables is critical for any organization looking to assess emissions. Overlooking these elements could mean falling short of your sustainability targets, even with the latest hydrogen technologies. To truly reduce fleet emissions, stakeholders need to look beyond tailpipe emissions and consider the full supply chain and operational lifecycle.
In this case study, we examine the well-to-wheel operating emissions of a Class 8 long-haul hydrogen truck using a hydrogen fuel cell hybrid electric powertrain. With an average annual mileage of 87,200 miles over a ten-year lifetime, we explore how variations in fuel cell size and hydrogen emission intensity impact the truck’s emissions profile. The size of the fuel cell plays a significant role in determining fuel economy. Larger fuel cells operate more efficiently under lower loads, which improves fuel economy, but this increased efficiency comes at the cost of added weight.
The hydrogen emission intensity (the amount of CO2 emissions generated through hydrogen production and supply chain processes) is another key factor. Using the Sesame tool, we examined three different hydrogen production scenarios with pipeline transmission and distribution to understand their respective emission intensities: high-intensity hydrogen produced through Steam Methane Reforming (SMR) (gray hydrogen), medium-intensity hydrogen produced via SMR with Carbon Capture and Storage (CCS) (blue hydrogen), and low-intensity hydrogen produced through renewable electrolysis (green hydrogen). For the CCS scenario, additional steps such as CO2 liquefaction, storage, and pipeline transmission to a saline aquifer for long-term storage were considered. These pathways represent a range of carbon intensities from high to low, showing how critical it is to consider the hydrogen supply chain in assessing lifetime emissions. The emission intensities for gray, blue, and green hydrogen are given in Table 1.
Table 1: Emission intensity of fuels considered in this article. Scenario analysis was performed using Sesame Sustainability.
| Fuel | Emission Intensity |
| Diesel | 11.3 kgCO2e/gal |
| Gray H2 | 12.4 kgCO2e/kgH2 |
| Blue H2 | 3.6 kgCO2e/kgH2 |
| Green H2 | 0.95 kgCO2e/kgH2 |
A diesel truck was included in the analysis to serve as a baseline for comparison. The diesel truck was modeled with a fuel economy of 7.51 miles per gallon and an emission intensity of 11.3 kgCO2 per gallon. It is represented in Figure 1 as a black, dashed line.
The contour plot generated from the above considerations provides insightful trends into the hydrogen truck’s lifetime emissions (Figure 1). At zero hydrogen emission intensity, the contour lines are vertical, which indicates that fuel cell size has no effect on total emissions, as the emissions would be zero regardless of fuel economy. However, as hydrogen emission intensity increases, the relationship between fuel cell size and total emissions becomes more complex. The curved contours reflect the non-linear relationship between fuel economy and fuel cell size and the increasingly slanted lines demonstrate emissions become increasingly sensitive to this parameter. Interestingly, the diesel contour intersects the SMR contour at about 600 kW, meaning that a hydrogen truck using SMR hydrogen will have higher lifetime emissions than a diesel truck if the fuel cell size is less than 600 kW. This case study underscores the need for a detailed understanding of both powertrain design and hydrogen production methods when assessing the environmental impact of hydrogen-powered trucks.
Figure 1: Contour plot for the lifetime emissions of a hydrogen heavy-duty truck considering variations in hydrogen emission intensity and fuel cell size. Hydrogen production pathways considered are shown as vertical lines and include renewable electrolysis (green), SMR with CCS (blue), and SMR (gray). An average diesel truck’s emissions serve as a comparison and is shown as a black dashed contour.
Hydrogen-powered trucks face complex, open-ended challenges, and our analysis is just a snapshot. Key parameters like fuel economy, duty cycle, and hydrogen emission intensity each require extensive modeling to fully understand their impact. We showcased a limited number of scenarios using Sesame, but the range of possibilities is vast. For instance, hydrogen emission intensity varies widely depending on production and distribution methods. Not explored in this study was variations in transmission and distribution. Hydrogen can be moved in various phases, be it liquid, gas, or embedded in another molecule. Transport options like truck, tanker, or pipeline further complicate transmission and distribution. Each decision point adds layers to a single scenario, illustrating the intricacies of hydrogen’s supply chain emissions.
Sesame Sustainability, with its robust modeling capabilities, empowers users to tailor scenario analyses across hundreds of variables and technologies. This enables businesses, policymakers, and researchers to explore diverse configurations and outputs for hydrogen’s lifecycle emissions. By integrating techno-economic modeling with emissions analysis, Sesame provides a comprehensive platform for evaluating sustainability strategies, whether assessing hydrogen production pathways or comparing powertrain performances. Designed to handle the nuanced complexities of technoeconomic and life cycle assessments, Sesame offers the precision and flexibility needed to support informed, data-driven decisions as the hydrogen industry evolves, ensuring that decarbonization efforts are effective and feasible in real-world applications.
Tune in next time for an in-depth analysis on hydrogen supply chain emissions and cost!
Dr. Rob Jones is an experienced business development and strategy leader with a strong background in chemical engineering and sustainability. He holds an MBA from MIT Sloan School of Management and a PhD in Chemical Engineering from MIT. Rob has worked across various sectors, including energy, venture capital, and software development, where he has applied his expertise in business strategy, market analysis, and technology assessment.
