• 21 Jun 2024 10:07 AM
• 0

Hydrogen Begins to Take Off

By Pat Host
Vertiflite, July/August 2024

Vertical lift companies are breaking new ground in hydrogen propulsion.

Hydrogen is the most abundant element in the universe. It is used as fuel for cars, forklifts and other types of ground vehicles. It also holds promise for ending aviation’s reliance on harmful carbon-based fuels. (Top picture: Alaka’i Technologies’ first prototype Skai flying on hydrogen. Image credit: Alaka’i)

Aviation companies are trying to leverage advancements in ground-based hydrogen fuel cell technology for success in the electric vertical takeoff and landing (eVTOL) marketplace. Hydrogen fuel cells based on a proton exchange membrane (PEM)—also known as a polymer electrolyte membrane—can power vehicles using electricity without the harmful emissions that result from burning fossil fuels.

Hydrogen provides more than three times the specific energy (energy per mass) of kerosene or gasoline (142 MJ/kg vs. 46 MJ/kg) and 30–80 times more energy than today’s batteries. This makes it attractive to aircraft developers looking to provide additional range and flight duration than is currently possible in battery-powered eVTOL aircraft.

Although the energy density (energy per volume) of hydrogen (gaseous and liquid) is slightly better than today’s state-of-the-art batteries, it is less than 25% that of gasoline or kerosene.

Liquid hydrogen has a much higher energy density than compressed hydrogen gas: 71 kg/m³ for liquid vs. 18 kg/m³ at 250 bar and 40 kg/m³ at 700 bar for gaseous, according to a 2022 Department of Energy (DOE) report on liquid hydrogen technologies. This increased density facilitates greater storage capacity within a given volume, allowing for longer ranges and larger payloads, although it requires cryogenic storage and, hence, a potentially greater expense.

Alaka’i Technologies, which is developing its Skai hydrogen-powered hexacopter, didn’t start out trying to be a hydrogen company. Bill Spellane, company COO, told Vertiflite that Alaka’i wanted to provide air-taxi missions ferrying five people in an aircraft with a payload capacity of around 1,000 lb (450 kg) for 400 miles (650 km) without having to develop a tiltrotor due to its very expensive and complicated certification process.

Alaka’i saw two options to meet these requirements: develop an advanced, turbine-powered, rotary-wing aircraft or develop an eVTOL aircraft using hydrogen fuel cells. Alaka’i chose hydrogen.

“The thing to know about Alaka’i and hydrogen in general is that we support long flight times with fast refueling,” Spellane said. “That is the benefit of hydrogen.”

PEM Approaches
John Piasecki, president and CEO of Piasecki Aircraft Corporation (PiAC), told Vertiflite that his company was attracted to hydrogen fuel cells because of the demanding power requirements for vertical flight. After evaluating more than 40 of the most advanced battery chemistries, Piasecki said the vertical flight power cycle of super-high discharge and recharge rates reduced the cycle life of batteries by an order of magnitude. Batteries that would have 4,000 cycles in normal usage, he said, have their lifespans reduced by a factor of 10 when used for vertical takeoff and landing operations. This increased the expected operational cost of battery-powered eVTOL aircraft to where it was only marginally better than current turbine helicopter operating costs for the mission it was pursuing. The company is developing a hydrogen-electric, slowed-rotor compound helicopter, the PA-890, for missions such as emergency medical services (EMS); delivery of high-value, on-demand logistics (ODL); and on-demand mobility (ODM) personnel air transport. The mission that served as the catalyst for the design was for EMS and organ delivery (see “Electric VTOL for Organs on Demand,” Vertiflite, March/April 2019).

“Having been in this business for over 30 years, my experience is that you need at least something like 20–30% of improved capability in order to penetrate a well-established market with a transformational technology,” Piasecki said.

The hydrogen fuel cell technology used by motor vehicle developers, such as Toyota, Hyundai and Honda, is very good, according to Keith Wipke, National Renewable Energy Laboratory (NREL) program manager for fuel cell and hydrogen technologies. Wipke told Vertiflite that modern hydrogen fuel cells are about 60–65% efficient and that automotive developers are on their third or fourth generation vehicles. Additionally, the Million Mile Fuel Cell Truck program, a DOE initiative focusing on long-haul trucks, has an interim goal of 68% peak efficiency by 2030, and an ultimate goal of 72% by 2050 for heavy-duty vehicles, such as tractor trailers.

In a PEM, a catalyst separates hydrogen atoms into protons and electrons, which take different paths to the cathode. The electrons go through an external circuit, creating a flow of electricity. The protons migrate through the electrolyte to the cathode, where they reunite with oxygen and the electrons to produce water and heat.

While both PiAC and Alaka’i are leveraging hydrogen fuel cell stacks for propulsion, the two companies are utilizing different PEM technologies. Piasecki uses a high-temperature (HTPEM) system, while Alaka’i is utilizing a traditional low temperature (LTPEM). Either fuel cell type can use either gaseous or liquid hydrogen.

Piasecki said the two approaches require different systems to support the reaction of the hydrogen and the oxygen to produce electricity. Piasecki said LTPEMs need a lot of additional equipment for the “balance of plant”—all the supporting components and auxiliary systems other than the fuel cell stack itself—to manage the water and temperature in the reaction chamber, which typically operates at 180°F (82°C). LTPEMs require a liquid-cooling system (including heat exchangers) and more precise quality control of the coolant, as well as the hydrogen and oxygen. This results in additional complexity and weight, which are critical for vertical lift aircraft, Piasecki said.

The reaction chamber in a HTPEM fuel cell, Piasecki said, operates around 300°F (150°C). High-temperature PEMs, he said, have water vapor as a byproduct of the reaction, which is much more manageable than a steady flow of liquid water with an LTPEM. Piasecki said their design also manages the temperature through air cooling, resulting in a simpler and lighter system more suitable for vertical lift applications.

PiAC has partnered with ZeroAvia for its HTPEM. Piasecki said ZeroAvia originally developed LTPEMs for fixed-wing aircraft. But in 2022, ZeroAvia acquired Piasecki-collaborator HyPoint which had been working on its breakthrough HTPEM for several years (see “Future Fuel: HyPoint’s Hydrogen Revolution,” Vertiflite, Sept/Oct 2021). Now, Piasecki and ZeroAvia are working to demonstrate an 80-kW HTPEM on Piasecki’s two-seat Hydrogen coAXial Electric Lift (HAXEL) proof-of-concept demonstrator, by the end of 2024.

Current Testing
Alaka’i Technologies announced its hydrogen eVTOL Skai concept in May 2019, unveiling a “non-flying prototype” and unveiling an ambitious schedule (see “Flying in the Skai with Hydrogen,” Vertiflite, July/Aug 2019). Five years later, reality has recalibrated the company’s outlook to solving the critical technical and business case challenges. The company had been very quiet until showing its impressive progress at the Vertical Flight Society’s Transformative Vertical Flight (TVF) meeting in early February (see “TVF 2024 Reaches New Heights,” Vertiflite, March/April 2024). Alaka’i has been flight testing its two full-scale demonstrators on liquid and gaseous hydrogen fuel cells for the past several years.

Spellane said Alaka’i chose a “known commodity” and high maturity level LTPEM due to its extensive use. Low temperature PEM approaches, he said, also have a high technology readiness level (TRL) and manufacturing readiness level (MRL). Other (non-PEM) fuel cell technologies, such as alkaline have either low TRLs or are too large.

Alaka’i is considering five different vendors for its production hydrogen fuel cell system: Plug Power, Cummins, Hyzon, PowerCell and Intelligent Energy. Spellane said the company is performing testing and development on fuel cells from three different companies.

In addition to flight testing, Alaka’i is performing a variety of testing on its fuel cell system. Spellane said the company is testing how the PEM stack performs when in low power. Alaka’i wants to know this because an aircraft’s pilot essentially pulls no load from the stack when he or she powers on the vehicle. At the other end of the spectrum, Alaka’i is also testing how the stack reacts when pulling a lot of electrical loads—such as in the critical vertical takeoff and landing phases—so the company knows how to avoid any issues. Alaka’i is testing the operating temperature envelopes of its fuel cell systems to evaluate how high and low temperatures can vary without problems and what the temperature gradient can be across the stack.

Reducing the price of liquid hydrogen will be critical to enabling broader use of hydrogen fuel cells in vertical flight aircraft. Danielle McLean, CEO and founder of the hydrogen education non-profit, HYSKY Society, told Vertiflite that operators of airlines and airports will not switch to hydrogen if it is more expensive than jet fuel because they will have to pass price increases on to passengers. Many operators, she said, know hydrogen is better for the environment, but even if certificated hydrogen-powered aircraft were available today, they couldn’t afford to switch at current prices.

However, Spellane said that in an operational scenario, Alaka’i could enter into a forward contract with a liquid hydrogen supplier and pay roughly $8/kg. This would make Alaka’i profitable, he said, and cheaper to operate than a battery-powered vertical flight aircraft or a fossil fuel-powered helicopter. In contrast, Alaka’i is paying $32/kg for liquid hydrogen on the spot market. Spellane said there are three major suppliers of liquid hydrogen: Air Liquide, Linde and United Hydrogen Group, a subsidiary of Plug Power. He declined to say where Alaka’i is buying its liquid hydrogen.

Spellane’s $8/kg long-term rate is also with no changes in government subsidies or technology. The DOE’s Hydrogen Shot seeks to reduce the cost of producing clean hydrogen by 80% to $1/kg by 2031. Spellane said this would result in a market cost of around $3/kg.

Other Interest
The Pentagon is also interested in potentially leveraging hydrogen fuel cell aircraft for military missions. Matthew Clouse, spokesman for the US Air Force’s (USAF) venture unit, AFWERX, said that the service awarded contracts to PiAC and Joby “for full-scale prototype and testing of novel hydrogen powerplants for airborne applications.”

The two companies aim to bring zero-emissions aviation to the military, along with other benefits, such as a quiet noise profile and the cost savings that come with operating and maintaining an aircraft fleet not dependent on traditional fossil fuels. AFWERX awarded PiAC a $37M Strategic Funding Increase (STRATFI) contract on Sept. 12 that included work on both its hydrogen fuel cell technology and its ARES cargo uncrewed aircraft system (see “Piasecki Aircraft: Carrying on the Spirit of Innovation,” Vertiflite, May/June 2021).

Joby and the Air Force declined to provide specifics, but Clouse noted that Joby’s AFWERX contract was worth as much as $131M, including delivery of up to nine aircraft to the government. “These contract values represent the total amount released, which includes more than just the testing of novel hydrogen powerplants for airborne applications,” AFWERX said, “and looks forward to the results of these efforts.” Joby Aviation purchased leading hydrogen aviation developer H2FLY, based in Stuttgart, Germany, which has flown extensively with both gaseous and liquid hydrogen fuel cell systems (see “Joby Delivers,” Vertiflite, Nov/Dec 2023).

Piasecki said under its AFWERX funding, his company will perform iterative development and testing of a single power module fuel cell stack. Piasecki will also perform ground testing of these fuel cell stacks on its HAXEL helicopter, before performing a flight demonstration on its own funding. Piasecki will also develop two 330-kW turbo air-cooled power units (TAPUs), which he called sub-elements of the overall system. The company will first perform ground testing of one TAPU before building the second unit and integrating both into the full 660-kW test rig.

At the Vertical Flight Society’s 80th Annual Forum in May (see “Forum 80: Ideas Earn Their Way,” pg. 26), Piasecki’s Program Manager for Hydrogen Propulsion Technology, John Scott, explained that the full-scale, 660-kW “iron bird” test rig will be sized for the PA-890 to run it through full testing cycles. The testing, planned for 2026, will mature the HTPEM technology at full scale and validate performance, safety and reliability, as well as inform the development of Federal Aviation Administration (FAA) certification requirements and burn down technical and schedule risk for PA-890 development.

Mikaël Cardinal, Vice President of Unither Bioelectronics, announced that the company — a subsidiary of public benefit corporation United Therapeutics (see “Martine Rothblatt: Serial Entrepreneur,” Vertiflite, July/Aug 2022) — had begun ground-testing a Robinson R44 converted to run on liquid hydrogen fuel cells. At press time, flight testing was expected to begin in the coming weeks, but the company declined to provide more specifics.

H2 for the Future
Not all eVTOL developers are excited about hydrogen fuel cell’s potential in aircraft. Rani Plaut, CEO and co-founder of AIR EV of Israel, told Vertiflite that while hydrogen is an excellent source of energy, battery-electric and hybrid-electric aircraft developers should showcase their approaches with proven technologies. This would also reassure the public, insurers and regulators, which Plaut said are more comfortable with a lower rate of technology innovation. AIR EV is developing its 550-lb (250-kg) payload capacity, multirotor, battery-powered AIR ONE.

Plaut said while hydrogen fuel cell technology is not as futuristic as it was a few years ago, it is still not as mature as battery-electric aircraft. Plaut also said while battery-powered eVTOL aircraft provide inferior range to those powered by hydrogen fuel cells, a considerable number of customers for cargo and private use will utilize battery-powered aircraft.

About the Author
Pat Host is an experienced Washington media relations and news professional. He’s previously covered aviation technologies for Janes, Defense Daily, Rotor & Wing and other news sites, and his work has appeared in publications such as FlightGlobal, Vertical and Aerospace America. He can be reached at patrick.host@gmail.com.