(Bloomberg) -- From Taylor Swift and Kylie Jenner to Elon Musk, celebrities have come under scrutiny for using private jets, which are huge emitters.

Aviation is responsible for more than 2% of the world’s carbon dioxide emissions, and private jets are the most carbon-intensive form of air travel. A 2021 report found that some private jets emit two tons of CO2 per hour — at least five times more pollution per passenger than commercial flights. That’s 50 times worse than trains.

While some private jetsetters have turned to carbon offsets as a means to neutralize their emissions, a handful of startups are promising the world’s 23,000-plus jet owners another option: hydrogen-powered aircraft. The gas can replace fossil fuels and power planes without emitting carbon dioxide. While the technology could be an ideal solution, it also has yet to be proven, raising questions of whether flying privately using hydrogen is a climate hack for the ultra-wealthy or a risky strategy for cleaner aviation.For now, flying privately on a hydrogen-powered jet is more of an idea under development than a reality for any traveler. But Beyond Aero, a French startup founded by three aerospace engineers, aims to commercially launch a six-seat jet by 2030 and has secured letters of intent from prospective individual buyers for 19 hydrogen jets, as well as from a number of airlines.“The high-net-worth individuals were looking to reduce their emissions,” says Eloa Guillotin, Beyond’s co-founder and chief executive officer. “They want to be one of the first to fly a [hydrogen] electric aircraft.”

Read more: Europe’s Spending Billions on Green Hydrogen. It’s a Risky Gamble

Switzerland-based Sirius Aviation AG has also ramped up efforts to develop its version of hydrogen-powered private jets. Both it and Beyond rely on hydrogen to power electric motors. Sirius has already received prepayment from wealthy individuals for three jets that cost millions of dollars each. Those jets have yet to be built, though Sirius wants to schedule its first demonstration flight as soon as next year. Equipped with a small kitchen and toilet, the startup says its two-seater will take off and land like a helicopter while having fixed wings to fly like an airplane. It could help eliminate so-called “flight shame,” the unease associated with air travel among eco-conscious flyers, according to the startup’s founder Alexey Popov.

“Our aircraft does not produce any carbon footprint,” Popov says. “It's completely zero emission.”

That claim depends on a number of factors, though, not least of which is what’s used to make the hydrogen.

“The first question is: which type of hydrogen?” says Thomas Gelin, a climate and transportation specialist at Greenpeace Europe. While using hydrogen as a jet fuel doesn’t release CO2 in itself, Gelin says that there could be a hidden climate cost.

Hydrogen is the universe’s most abundant element, but it must be extracted from other compounds to be used as a fuel. There are different types, known by different colors used to label different ways to produce hydrogen. The vast majority of hydrogen made today is stripped from natural gas in a process that vents carbon dioxide into the atmosphere. Dubbed gray hydrogen, to produce 1 kilogram of it emits anywhere from 10 to 14 kilograms of carbon dioxide, according to the International Energy Agency.

To lessen its environmental impact, the industry is shifting gears to make more “green hydrogen” that relies on machines known as electrolyzers to break down water into hydrogen and oxygen. If the electricity is generated from renewables, you get green hydrogen. More than half of the world’s hydrogen is expected to be produced this way by 2050, but at present, green hydrogen accounts for roughly 1% of the global hydrogen output.

There are many uses for hydrogen and demand will likely increase in the coming years. As electricity can’t provide the intense heat needed for certain industrial processes such as steelmaking and cement production, those sectors will likely count on green hydrogen to clean up their act. While it can play a role in aviation, Gelin questions if private travel is the best use.

“Do we really want to block a portion of that fairly hard-to-produce [green] hydrogen for private jets?” he says.

Airlines are increasingly powering their fleets with biofuel and synthetic fuel, and some are experimenting to fly planes with rechargeable batteries, but neither is anywhere close to making a meaningful impact on aviation’s emissions. Nor are they a perfect solution due to technological limitations and supply chain bottlenecks. 

Hydrogen-powered aircraft could help bridge the gap, serving roughly one-third of all passenger aviation traffic by 2050, according to a 2022 study by the nonprofit International Council on Clean Transportation.

But that potential comes with a lot of big ifs. The first and biggest is if jet makers can perfect hydrogen propulsion technology and redesign aircraft to handle the fuel in the first place. But there are also questions about whether regulators will sign off on hydrogen-powered jets, if green hydrogen supply can grow as well as the infrastructure to transport it, and perhaps most importantly if hydrogen aviation can actually be cheap enough to be feasible.  

Hydrogen currently costs as much as four times more than regular jet fuel. Industry advocates say that the gap would narrow as hydrogen production scales and that maintenance costs would be lower thanks to fewer moving parts in a hydrogen-powered propulsion system compared to a conventional one. In the eyes of some sustainable transportation experts, the introduction of hydrogen private jets will help address some of the unknown.

“This is a climate solution and we have to start somewhere,” says Ethan Elkind, a lead researcher specializing in transportation and climate change at the University of California at Berkeley. “Starting with private jets can make sense for hydrogen because that allows the technology to get operationalized, tested and really just out there started to see if it's viable.”

Elkind, who chairs the aviation working group for California’s hydrogen hub, likens the importance of hydrogen private jets in aviation to the role that the Roadster once played in the early days of Tesla Inc. As the company’s first car, the Roadster cost about $100,000 when it debuted in 2008, pricing most consumers out of the market. But that model also helped the electric car maker scale its technology and lower costs so that Tesla now serves millions of drivers across the world.

“If it can work for smaller business jets, then it can eventually be scaled to meet broader public commercial markets,” Elkind says. On the flip side, he adds, any failures in hydrogen private jets could also give a reality check to the global aviation industry.

Both Sirius Aviation and Beyond Aero plan to expand their offerings to include larger commercial aircraft once the technology matures. Sirius Aviation is also working to deploy its technology in smaller planes used for medical evacuation.

Read more: Few Stations and $200 to Fill Up: Life on California’s ‘Hydrogen Highway’

It remains to be seen whether hydrogen jet makers can deliver on their promise. The sector’s cheerleaders need only look at the bumpy road of hydrogen cars to see how difficult it is to challenge the status quo and compete with other green alternatives. Despite billions of dollars of investment, hydrogen fuel cell cars have struggled to make inroads in the US while EVs have taken off. In California — the only state that sells such cars and that has committed more than $250 million to build a “hydrogen highway” — drivers bought 3,143 hydrogen-powered cars last year. By contrast, statewide EV sales hit 380,000 units.

Even if flying privately on hydrogen materializes, Gelin questions whether the world’s richest should continue their carbon-intensive lifestyle. Banking on a technology that could cut emissions at some point down the road is particularly risky when there are ways to cut emissions today.

Instead of waiting years for hydrogen-powered planes to take off — assuming they ever get off the ground at all — Gelin says jet setters could reduce their carbon footprint today by switching to lower-emissions travel options. In fact, there are some early signs of change. Futbol Club Barcelona, for one, last year for the first time favored a chartered train over a private jet when the football players went to Madrid for a match. Celebrities can also encourage lower carbon behaviors. Billie Eilish, for example, partnered with public transit agency LA Metro to provide free shuttles to her listening party in Los Angeles earlier this month.

With countries racing for global leadership in climate-friendly technology, there is a looming risk that the pursuit of cutting-edge solutions could overshadow behavior change, Gelin says. “That is a major concern,” he adds. 

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