Estimates of emissions from hydrogen transportation fueling infrastructure and vehicles

Field: Environmental Science (Physical Sciences)

Losses of hydrogen used as a transportation fuel could exceed 10% from vehicles and their refueling stations. These emissions erode greenhouse gas benefits because atmospheric hydrogen itself contributes to climate change. Losses at this level are also an economic concern and exacerbate upstream environmental impacts from hydrogen production. We reviewed the literature for estimates of hydrogen emissions within the "pump to wheels" (PTW) boundaries for present day technology. Additional insight was gained from methane emissions from natural gas transportation, a similar but more mature topic. Our effort addressed both fueling stations with on-site production and those employing liquid hydrogen storage with delivery from off-site plants. We considered that liquid hydrogen can be converted to high pressure gaseous hydrogen for vehicle fueling either through use of compressors or cryogenic pumps. Vehicles considered included cars, buses, and trucks employing fuel cells, and trucks employing hydrogen internal combustion engines. Reported losses were scarce and highly variable across studies, due in part to varied technologies and practices and the degree of utilization of the refueling stations. We therefore adopted specific scenarios employing current technology and practices for utilization and refueling of light-duty passenger, heavy-duty truck and transit bus fleets to calculate and estimate percentage losses from individual segments of the PTW chain. The largest potential losses were associated with delivery, storage and handling of liquid hydrogen while emissions from compressed hydrogen storage tanks and delivery, vehicle fittings, and refueling nozzle venting were small. Our review will serve to inform subsequent studies of benefits and impacts of hydrogen adoption in the transportation arena.Implications: Hydrogen is being presented and adopted as a carbon-free alternative to traditional fossil fuels in the transportation sector. Carbon emissions from hydrogen production have been well characterized, but hydrogen emissions themselves interfere with chemical processes that serve to check concentrations of atmospheric greenhouse gases. Recent published analyses suggest a 100-year global warming potential of approximately 10 for hydrogen, so that direct hydrogen emissions are of concern in quantifying climate change benefits. This is similar to concerns that methane emissions diminish the benefits of natural gas as a low carbon fuel. Our review and study have gathered the scattered literature and data relevant to hydrogen emissions, employed analogies from natural gas deployment, and derived realistic estimates of hydrogen emissions for the pump-to-wheels (PTW) transportation sector. Our results demonstrate that losses depend on the type and scale technology in place and can be substantial as a percentage. The results should be combined with existing upstream climate change emissions for hydrogen production and upstream distribution to improve quantitative assessment of the net environmental benefit offered by hydrogen in the transportation sector. This in turn should guide future investment and policy decisions. A direct implication of the results is that effort should be made to abate pump-to-wheels hydrogen emissions through adoption of best technology and practice, through improvement of hydrogen recovery, and through sizing of fueling infrastructure that is appropriate for vehicle fleet size.