Ann Arbor Times

Green hydrogen is emerging as a potential solution for decarbonizing transportation, but new energy efficiency findings indicate that it should be used strategically in heavy-duty road, rail, aviation, and marine transportation, according to a University of Michigan study.

Green hydrogen is produced by electrolysis with renewable energy to split water into hydrogen and oxygen. It can be used directly or in synthetic fuels, also known as e-fuels, to decarbonize road, rail, marine, and air transportation. The transportation sector is responsible for about 22% of global and 37% of U.S. fossil fuel carbon dioxide emissions.

To mitigate climate change, it’s crucial to decarbonize both passenger and freight transportation, according to the researchers. They computed the total system energy efficiency of using green hydrogen directly or indirectly in e-fuels to power planes, trains, automobiles, and ships. The system energy efficiency measures the energy used to drive the wheels for ground modes and thrust for planes and ships relative to the total renewable electrical energy invested.

The U-M researchers considered both the direct use of hydrogen in engines or fuel cells as well as indirect use of hydrogen in the form of e-fuels such as e-gasoline, e-diesel, e-jet fuel, e-methanol, and e-ammonia. Comparing these uses with battery electric options, they found that the system inefficiencies during hydrogen or e-fuel production lead to about 80%-90% energy loss of the initial electrical input.

By contrast, electric-powered transport is about three to eight times more efficient than using hydrogen directly or in e-fuels. Their results will appear online Aug. 7 and in print Aug. 21 in the journal Joule.

“We have an urgency to decarbonize transportation given the adverse impacts we are seeing from climate change,” said Greg Keoleian, a senior author of the paper and co-director of U-M’s MI Hydrogen initiative. “We examine where hydrogen can play a role by looking at energetics to help guide deployment along with other factors such as cost, fueling time, range and safety.”

The study was conducted as part of U-M’s MI Hydrogen initiative which aims to foster collaboration among U-M researchers, community groups, government and industry partners to create hydrogen solutions that accelerate clean energy transitions. The research team included scientists from the Center for Sustainable Systems; Michigan Engineering’s Department of Aerospace Engineering; and Department of Naval Architecture and Marine Engineering.

“We found that renewable electricity sources in the U.S. are insufficient to support hydrogen production for light-duty vehicles,” said Tim Wallington first author on the report and a research specialist in the Center for Sustainable Systems at the U-M School for Environment and Sustainability. “Green hydrogen should be used strategically in heavy-duty road rail aviation and marine transportation where electric alternatives are constrained by load and range.”

However Wallington says batteries would not work for heavy transport vehicles that need to cover long distances. Batteries are too heavy and too large to power flight more than 200 miles send a shipping freighter across an ocean or power a train across a continent.

Hydrogen or e-fuels make more sense as a fuel for these heavy transport applications according to the researchers. Using hydrogen as a direct fuel source would require massive changes in fueling infrastructure while using hydrogen-based e-fuels would avoid these changes but are approximately 20%-50% less energy efficient than direct uses of green hydrogen.

To characterize system efficiency and visualize energy inputs/losses for each pathway researchers developed 25 Sankey diagrams starting with renewable electricity inputs tracking flows across production/delivery ending use in fuel cell internal combustion engine/turbofan aircraft compared with another set six diagrams all-electric options.

Researchers also measured energy intensity how much renewable energy takes move freight ton-miles people passenger-miles major modes transportation.

“What we found is trends follow petroleum-based transportation: with hydrogen rail/shipping most efficient aircraft least efficient because you have hold weight up sky,” said Keoleian professor sustainable systems “From sustainable perspective you’ll want use most efficient modes least intensive mode Renewable electricity scarce resource must use wisely”

Study co-authors include doctoral student Maxwell Woody research specialist Geoffrey Lewis Center Sustainable Systems doctoral student Eytan Adler professor Joaquim Martins Department Aerospace Engineering Matthew Collette professor naval architecture/marine engineering.

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