In the narrative of low-carbon transformation of public transportation, hydrogen fuel cell buses are often labeled "zero-emission": the main emission is water vapor, with almost no carbon dioxide. However, a recent analysis published by CleanTechnica points out that if the entire process of hydrogen production, purification, compression or liquefaction, transportation, storage, refueling, and potential leaks are included in the "well-to-wheel" (WTW) accounting boundary, the total greenhouse gas emissions of hydrogen-powered buses are not necessarily better than those of diesel buses. In some real-world hydrogen supply and electricity structures, the counterintuitive result of "hydrogen emitting more than diesel" may even occur. Michael Barnard, founder and chief strategy officer of TFIE Strategy Inc. in Canada, emphasizes that the key issue is not vehicle exhaust, but rather the system boundaries and engineering parameters that "take everything into account": where the hydrogen comes from, how it gets onto the vehicle, and the extent of loss and leakage during the process determine the final climate performance.
"Zero emissions" does not equal "low emissions."
In carbon accounting for transportation, two common boundaries exist: one is "vehicle-to-wheel," primarily focusing on exhaust emissions during vehicle operation; the other is "well-to-wheel," which accounts for emissions from upstream fuel or electricity production, processing, transportation, and end-use. Hydrogen fuel cell buses have a significant advantage in the TTW (Total Time-to-Wave) dimension: they emit almost no carbon dioxide during operation, making them easily perceived by the public as "zero emissions." However, a CleanTechnica article warns that emission reduction targets for public transportation systems refer to "total system emissions," not "vehicle exhaust emissions." When policies, subsidies, or procurement evaluations focus solely on TTW indicators, emissions may be "transferred" from the vehicle end to the upstream energy chain, ultimately leading to the opposite of expected results in the WTW (Total Time-to-Wave).
More importantly, buses are typical public systems engineering projects: long vehicle lifespan, high annual mileage, and refueling heavily reliant on fixed stops and a stable supply chain. In such a scenario, efficiency losses and carbon intensity differences in the fuel chain are amplified, significantly impacting life-cycle performance. In other words, whether hydrogen-powered buses are more carbon-efficient is not determined by the word "hydrogen," but by the "actual carbon footprint of the hydrogen supply system."
The carbon intensity of hydrogen production versus electricity generation is the dividing line.
The CleanTechnica article first points to the question of "where does hydrogen come from?" In the real world, hydrogen is not naturally present in the industrial supply chain; it needs to be produced through methods such as water electrolysis or fossil fuel reforming. Taking water electrolysis as an example, "turning electricity into hydrogen, and then turning hydrogen into electricity to power vehicles" involves multiple energy conversions: electrolysis efficiency, hydrogen purification and drying, compression or liquefaction, station storage and refueling, and fuel cell conversion efficiency all incur losses. The article suggests, in an engineering context, that even without considering extreme scenarios, from the perspective of the energy chain of common processes, delivering 1 kg of usable hydrogen to a vehicle through electrolysis often requires far more electricity than "directly charging a battery." If the marginal electricity of the power grid still relies mainly on natural gas or coal power, then carbon emissions from the hydrogen production stage could accumulate rapidly.
This aligns with the assessment of the global hydrogen industry structure by authoritative institutions. The International Energy Agency (IEA) in Paris has repeatedly pointed out in multiple hydrogen energy thematic reports and annual reviews that the current global hydrogen supply is still dominated by fossil fuel-based hydrogen production, with a limited proportion of "low-emission hydrogen" and significant differences in the carbon intensity of the hydrogen supply structure. This background becomes clear when applied to public transportation applications: if a project relies on high-carbon grid electrolysis for hydrogen production, or on fossil fuel-based hydrogen production that has not achieved high capture-rate emission reductions, then the advantage of "zero emissions at the vehicle end" may be offset upstream.
For public transportation operators, this logic can be summarized into an actionable conclusion: to evaluate the emission reduction effect of hydrogen buses, the hydrogen supply source must be traceable and auditable, and the emission intensity of the electricity source and hydrogen production path must be clearly defined. Otherwise, "green hydrogen" may be valid in contracts and promotional materials, but may not be valid in the system's emission accounting.
Getting hydrogen onto the vehicle also requires careful accounting.
If "where the hydrogen comes from" determines the upstream baseline, then "how the hydrogen gets onto the vehicle" determines the additional costs. The physical properties of hydrogen mean that its storage and transportation are not easy: it must be either high-pressure compression or cryogenic liquefaction. Both pathways imply additional energy consumption and equipment investment, introducing operational efficiency variables.
On the compression pathway, publicly available research and institutional data indicate that compressing hydrogen to the pressure required for high-pressure refueling consumes considerable electricity. When hydrogen refueling stations are small-scale and equipment utilization is insufficient, the overall energy consumption per unit of hydrogen will further increase. On the liquid hydrogen pathway, the liquefaction process itself is generally considered a high-energy-consuming stage, while also requiring the maintenance of a cryogenic storage and transportation system. If liquid hydrogen needs to be transported long distances across regions, transportation fuel costs, evaporation losses, and station-end regasification will continuously accumulate.
More importantly, public transport demonstrations often face the structural problem of "insufficient early-stage scale": a small number of vehicles, low station utilization, and the hydrogen supply chain has not yet achieved economies of scale. In this case, the fixed energy consumption and fixed emissions per unit of hydrogen are more likely to be diluted, thus increasing the WTW (Total Weighted Emissions) result. In other words, even if a hydrogen supply pathway has the potential to become more low-carbon when scaled up, it may perform poorly during the demonstration phase due to utilization and logistics radius.
Furthermore, the "real hydrogen consumption" at the vehicle end is also a sensitive parameter emphasized in this article. For example, a third-party assessment by the U.S. National Renewable Energy Laboratory of fuel cell bus demonstrations in North America revealed in a public report that the average hydrogen consumption of fuel cell buses on certain routes could reach approximately 15 kg/100 km, significantly higher than the "single-digit to 10 kg/100 km" empirical range often cited in discussions. Once hydrogen consumption increases, emissions from the upstream hydrogen production and supply chain will also increase proportionally, making the risk of a "system not performing optimally" higher. For media reports, this fact reminds us that discussions on hydrogen bus emission reduction cannot rely solely on "ideal parameters"; they must consider factors such as route gradient, climate, passenger load, start-stop frequency, and air conditioning load, and cite verifiable operational data.
Hydrogen leakage is not a trivial matter.
The CleanTechnica article also specifically emphasizes a variable that has often been overlooked but is now being re-examined by academia and policymakers: hydrogen leakage. Traditional views often consider leakage as "energy loss" or "safety risk," but recent atmospheric chemistry research indicates that while hydrogen is not a direct greenhouse gas like carbon dioxide, it indirectly enhances the greenhouse effect by influencing atmospheric chemical processes. For example, recent studies published in authoritative journals such as *Nature* have provided quantitative estimates of hydrogen's "indirect global warming potential" and warned that if leakage control is inadequate after the expansion of hydrogen energy systems, climate benefits could be significantly eroded. Interpretations from institutions such as Stanford University in the United States also emphasize that establishing a rigorous leakage monitoring, detection, and maintenance system is one of the necessary conditions for the hydrogen economy to fulfill its emission reduction commitments.
Applying this conclusion to the public transportation scenario, it means that the operation and maintenance of hydrogen refueling stations and onboard hydrogen storage systems should not be considered "optional," but rather as part of the emission reduction project: the standards and maintenance frequency of key components such as valves, seals, manifolds, connectors, and refueling nozzles will have a macroscopic impact on whether "hydrogen buses are truly more carbon-efficient." Especially in urban operating environments with frequent refueling and dense stations, even small-scale, continuous leaks can lead to significant climate impacts and economic losses.
For hydrogen fuel cell buses to truly become a low-carbon mode of transportation, at least three engineering prerequisites must be met simultaneously: First, the hydrogen supply must be a traceable, low-emission source, with clear disclosure of the emission intensity from electricity or fossil fuel-based hydrogen production; second, the hydrogen supply chain must be as short and efficient as possible, with energy consumption, utilization rates, and economies of scale in the compression/liquefaction and transportation stages incorporated into the business model; and third, a verifiable leak control and operation and maintenance system must be established to ensure that the indirect climate effects of hydrogen do not negate the benefits of emission reduction. For long-term assets like public transportation, only by incorporating these "invisible upstream and operation and maintenance stages" into procurement and regulatory indicators can "zero emissions" more likely approach "low emissions" in a comprehensive, end-to-end sense.
