It is increasingly clear that breaking aviation’s dependence on fossil fuels to achieve the ambitious objective now being set by some governments for achieving carbon-free flying as early as 2040 will involve several technological stepping stones. In the short term, one of these is sustainable aviation fuel (SAF) in which, to varying degrees, the oil content is reduced in a switch to so-called biofuels. Electric propulsion—either based on a hybrid system that still includes some kind of turbine engine in the powertrain or all-electric using just batteries—is another. But few experts would claim that any of these approaches are some kind of Holy Grail of sustainable flight.

Let’s start with batteries. Although they offer an excellent way to reduce emissions, the energy cost is not the same as that associated with traditional jet fuel. Furthermore, because of energy density challenges, the range just isn’t there for most commercial air transport operations.

“Batteries have far less specific energy—the ratio of energy to mass—than jet fuel and will not achieve the improvements in specific energy needed to fly large aircraft long distances in time to address today’s emissions crisis,” said John-Paul Clarke, co-founder and CTO of Universal Hydrogen. “As such, at least for the foreseeable future, battery-electric aircraft will be relegated to intra-city trips via small aircraft.”

California-based Universal Hydrogen is one of several companies seeking to convert existing airliners, such as ATRs and Dash 8s, to hydrogen power. HyPoint and ZeroAvia are also pioneering these efforts to get beyond batteries, with each taking a different approach.

“Due to their low power density, li-ion [lithium-ion] batteries have limited usefulness for high-frequency vehicles, such as air taxis, that simply cannot spend hours per day charging,” said Alex Ivanenko, HyPoint’s CEO and founder. “That being said, li-ion batteries do make sense for short-distance urban air mobility or private use cases that don’t need to travel long distances or make multiple trips per day.”

SAFs, on the other hand, give you the range but not the emission-free flight. “Whether it be biofuels or e-fuels, SAFs are not true zero-carbon fuels,” said Clarke. “While they are produced with carbon that is captured at the ground level, they re-emit this carbon at altitude.”

According to Clarke, the best SAF implementations abate less than 40 percent of net emissions, with the new fuels having demonstrated at best 80 percent offsetting of in-flight emissions in a 50 percent blend with traditional fuels. Furthermore, both options are expensive and, in the case of biofuel, land-use intensive, which prompts wider environmental concerns.

Between a Battery and an SAF

Filling the gap between electric batteries’ lack of range and SAFs’ lack of emission reduction is hydrogen, say the H₂ evangelists.

“Whether used directly in a combustion engine or in a fuel cell electric configuration, hydrogen benefits from having a high energy-to-mass ratio as well as a mature set of technologies,” Clarke maintained. “That’s why I believe hydrogen is the best solution for decarbonizing aviation.”

Universal Hydrogen’s hydrogen-electric conversion kits for turboprop aircraft replace kerosene-burning turboprop engines with a fuel cell and electric motor in the nacelle and hydrogen modules in the rear of the aircraft. Hydrogen then flows from these modules to the fuel cells, where it reacts with oxygen from the environment to form water (H₂O) and electricity, which powers the motor and turns the propeller.

The company’s first conversion kit is set to achieve approval under an FAA supplemental type certificate as early as 2025. That’s a good 10 years ahead of when Airbus expects to be able to bring a new clean-sheet hydrogen airliner to market under its Zero E program.

Taking a different approach, HyPoint’s system utilizes compressed air for both cooling and oxygen supply to deliver a high-temperature (HTPEM) fuel cell system that the company says is three times lighter than comparable liquid-cooled low-temperature (LTPEM) fuel cell systems. The start-up also leverages several technical innovations, including lightweight bipolar plates and a highly conductive, corrosion-resistant coating, which the company claims allows its solution to outperform existing systems.

Based on early test results, HyPoint's turbo air-cooled hydrogen fuel cell system will be able to achieve up to 2,000 watts per kilogram of specific power, which is more than three times the power-to-weight ratio of traditional hydrogen fuel cell systems. It will also offer up to 1,500 watt-hours per kilogram of energy density, enabling longer-distance journeys. As a result, HyPoint says it can deliver up to a 50 percent reduction in the total cost of ownership for aircraft makers and empower them to create practical, cost-effective zero-emission vehicles.

“Our hydrogen fuel cells can deliver significantly greater specific power and energy density, which means it can support larger vehicles across longer distances,” Ivanenko told FutureFlight. “This will be critical for both hydrogen airplanes and eVTOLs.”

By addressing these core technological barriers, HyPoint says it will cut years off commercial delivery timelines for hydrogen aircraft and unlock the emerging hydrogen aviation market which, according to Allied Market Research, is expected to be valued at more than $27 billion in 2030 and at least $174 billion by 2040.

The Storage Conundrum

Thanks in part to such initiatives as the United Nations’ Green Hydrogen Catapult and the Biden administration’s infrastructure plans in the U.S., the cost of green hydrogen—that made from renewable resources—is plummeting, setting it up to be cost-competitive with jet fuel by mid-decade, far sooner than other alternative fuels. However, before hydrogen can become a game-changer for advanced air mobility, it must address the challenges of storage, distribution, and weight.

Because of hydrogen’s very low density, to store and transport it in bulk, it must be either compressed at high pressure or liquefied. Here, liquid hydrogen has the advantage due to its higher density. However, it too must be stored and transported at cryogenic temperatures, which requires unique solutions.

To address this need, Universal Hydrogen has developed specialized modules for storing both high-pressure gaseous hydrogen and liquid hydrogen. The high-pressure gaseous storage solution is comprised of three distinct layers, with each layer performing a single function.

The innermost layer is an extremely low permeability liner that prevents the hydrogen molecules from escaping. The second is a dry carbon fiber weave that bears the pressure loads, while the third layer is an aramid or similar material that provides abrasion protection. One or more capsules are placed in a lightweight carbon fiber composite frame that bears the crash loads.

The company’s liquid storage solution uses a similar multilayered setup. Because there is no active cooling of the module, the innermost layer of the capsule is designed to bear the increase in pressure that occurs during transportation as the liquid hydrogen begins to boil off. The second layer is an evacuated insulation layer that significantly reduces the heat being transmitted into the capsule. The third layer consists of a thin metal capable of containing the vacuum. One or more capsules are placed in the same lightweight carbon fiber composite frame.

Distribution Logistics

But storage is only part of the solution as there must also be a complementary logistics network. “The infrastructure we currently use to transport fuel took decades to build and was done at a considerable cost,” said Clarke. “Because we don’t have the time—or financing—to start from scratch, our focus is on using existing intermodal freight transport systems.”

For Universal Hydrogen, this means producing hydrogen at an existing hydroelectric dam during the night, when electricity demand is low. This hydrogen is transported via existing intermodal freight transport to the airport, where the modules are loaded onto the aircraft as if they were batteries. Once the modules are spent, they are then taken back to the filling station, refilled, and resent.

“This is a completely closed system that uses existing infrastructure and thus requires very little capital expenditure,” Clarke added.

When it comes to getting hydrogen to vertiports, which may be on top of skyscrapers and thus present unique storage challenges, Universal Hydrogen has developed a hand-loadable, high-pressure storage module that can be easily moved to and from any location.

A Battery-Hydrogen Alternative

Even with storage and transport covered, there’s still the issue of weight. According to tests conducted by Universal Hydrogen comparing hydrogen and battery configurations on eVTOL aircraft capable of flying up to around 100 km (63 miles), a battery-only model carrying four passengers weighs roughly 3,200 kg (7,040 pounds). However, even though hydrogen itself is very light when the batteries are swapped with fuel cells, that same eVTOL configuration weighs significantly more.

“For a 100-km-range aircraft, if you were to use just fuel cells, you’d have to put a lot of weight in just to get to 25 km because the fuel cells are very heavy,” Clarke explained. “However, although batteries are initially lighter, as you increase range, you get very non-linear growth in terms of the weight of the aircraft.”

Based on this analysis, Universal Hydrogen tried a hybrid configuration that uses batteries for take-off and hover and then a hydrogen powertrain for cruise flight. This is a configuration that not only cuts weight in half but increases the vehicle’s overall efficiency.

According to Clarke, this hybrid version gives one the benefit of batteries for high power conditions and of fuel cells for high energy conditions. “We believe that a hybrid battery-hydrogen fuel cell electric powertrain is ideal for urban and regional aerial mobility because high specific power batteries are better for takeoff and hover, while hydrogen fuel cell systems are better for cruise,” he says. “By increasing range, this hybrid configuration could move second-generation eVTOLs from urban air mobility to regional air mobility.”

The Power of Pure Hydrogen

This does not, however, mean that Universal Hydrogen is writing off hydrogen-only eVTOL aircraft. Clarke went on to explain that the company’s core modular distribution technology is very applicable to the pure hydrogen segment.

“Our internal studies show significant weight and volume savings for eVTOL applications that leverage hydrogen as a fuel,” he told FutureFlight. “We’ve had early conversations with several of the emerging leaders in the space and are confident that hydrogen will be a solution for eVTOLs.”

As for weight, HyPoint notes that its air-cooled high-temperature fuel cell system is three times lighter than liquid-cooled low-temperature ones, offering a 61 percent reduction in total weight. “This is important because HyPoint is the only company able to deliver the necessary combination of energy density and specific power that aircraft require,” said Dr. Ivanenko.

HyPoint’s system, which recently cleared significant early testing, is on track for commercial deliveries to customers like ZeroAvia, Piasecki Aircraft, and Urban Aeronautics. ZeroAvia is working on plans to convert 19-seat regional airliners to hydrogen, while Piasecki is working on a new compound helicopter called the PA-860 and Urban Aeronautics is developing its CityHawk eVTOL vehicle.

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