Engineering an Aircraft Hydrogen Powertrain
ZeroAvia’s journey into the world of the hydrogen powertrain began with a mission to decarbonize one of the most challenging sectors in existence due to the complexities in recreating the synthesis of chain reactions that aircraft require to fly securely with hydrogen fueling. Currently, the balance of engine-triggered events that power, heat, pressurize, and so forth is not necessarily the result of the high-efficiency performance of fossil fuels. Yet, aircraft use engine inefficiencies to fill other needs such as thermal management. One of the most significant challenges in designing an effective hydrogen powertrain system is keeping the aircraft capabilities intact by using efficient and non-efficient fossil-fuel turbine engine performance metrics as a benchmarking tool for peak hydrogen performance.
As a physicist and pilot (both fixed-wing and helicopter), I started ZeroAvia because I wanted to work with engineers to move the needle towards zero-carbon aviation. To do this, we needed to target a significant existing segment. That means relatively larger aircraft (1020 seats) for a relatively long distance (500 miles). The most practical way to do this anytime soon is via hydrogen fuel cells, which are currently about four-times more energy-dense than the best available batteries, even with compressed gas hydrogen storage. Liquid hydrogen brings about a two to three times increase in energy density, which is close to the density of jet fuel. In five years, ZeroAvia expects liquid hydrogen storage to become safety-qualified in aircraft, allowing achievement of 1,000+ mile ranges in even larger aircraft.
Multiple engineering disciplines are needed to create both the physical hydrogen powertrain and the avionics suite that powers the engine. This requires full awareness of the system's physical principles throughout the team, right down to every technician. ZeroAvia uses a rapid prototyping technique that has proven to be highly effective in meeting targeted deliverables. We’ve achieved flight milestones and become the only hydrogen company with working hardware. The engineering teams focus on tangible demonstrations and push to create physical components, which helps the team learn through action. The engineering teams use 3D printing for custom parts and balance between precise modeling (as often as possible), empirical characterization, and iteration.
Communication is one of the critical building blocks of rapid engineering. The team commonly does three engineering standups a week with an even distribution across mechanical components, hardware, and software. Engineering teams are purposely kept small to maintain agility, and each group is loaded with cross-functional representation that includes mechanical, software, and hardware engineers along with dedicated technicians. Teams lean on a waterfall methodology at the highest level of the project and then use a hybrid agile method when tackling internal group design and development tasks. This hybrid development approach supports the rapid prototyping protocol that requires a massive amount of elasticity when it comes to completing the necessary project tasks needed to drive the testing phase.
Challenges in Decarbonizing the Aviation industry
Besides ZeroAvia’s efforts, there are currently many valiant efforts underway worldwide to create sustainable solutions for the aviation industry. Some of the most significant obstacles include meeting the intense fuel demand that the commercial aviation space requires and determining an approach to generate enough power that does not disrupt air vehicles’ intricate weight sensitivity parameters.
Biofuel and battery-powered aircraft have significant roadblocks that impede their progress in becoming a complete fossil-fuel replacement for the aviation industry. The amount of biofuel needed to keep the entire aviation industry running would dramatically increase nitrogen oxide (NOx) emissions and compete with the scarce resources needed for farming; meanwhile, batteries (including lithium-ion) have 40-times lower energy density in comparison to jet fuel.
Even if you do decide to go the hydrogen route, you must still deal with the heaviness of compressed hydrogen tanks. The best way to do this is to leverage a hydrogen-electric powertrain's higher efficiency, develop lightweight composite gas tanks, and create evaporatively cooled liquid hydrogen tanks with 30%+ mass fractions to exceed jet fuel energy density per kilogram of the entire fuel system. Through this example, you can begin to see how hydrogen-electric powertrains have a clear advantage over all other alternative propulsion types because there are no blockers, and secondary issues can be leveraged similarly to what is already taking place with jet fuel-powered turbines.
A longtime bottleneck in progressing sustainability within aviation has been attempting to create entirely new aircraft equipped for hydrogen-powered flight. Although this will need to happen for long-haul widebody aircraft at some point in the future, the powertrain technology that ZeroAvia has engineered will be a drop-in replacement for existing turbine engines in smaller airplanes up to 20 seats. The powertrain matches mounting points and supports pre-existing weight and balance configurations.
ZeroAvia’s powertrain technology utilizes surfaces of the aircraft for thermal management through actions such as coupling the cooling channels to wing surfaces. This can be done because of the significant surface area planes have compared to ground vehicles, higher airspeed, and lower ambient temperature. Bleed air from the fuel cell compressor can be used for de-icing, similar to today's turbine engines, and bleed air valves have the capacity to catch compressed air and mix it with cool air for cabin temperature control.
This is a critical component missing from battery-electric systems because batteries must generate more power dedicated to performing separate thermal functions.
Safety Protocols and Redundancy
Far lower probability of failure and less severe consequences of engine failure are among the many perks that come with flying a hydrogen-powered aircraft for the following reasons:
Electric propulsion is inherently more reliable and has a small number of moving parts.
Hydrogen tank integrity is superior to any liquid fuel tanks in use today. It's crash-resistant and has been tested by firing high-caliber guns at the tanks.
Hydrogen is an ultra-light molecule, which dissipates very quickly if leaked, unlike liquid fuels.
Hydrogen is harder to ignite than most fuels and has a 500-degree celsius auto-ignition temperature, compared to just 210 degrees celsius for jet fuel. Even if it ignites, hydrogen flames emit much less radiative heat in comparison to jet fuel. This results in a decreased probability of a secondary fire.
ZeroAvia’s powertrain has full redundancy built into all major components, such as stack, balance of plant, motors, and inverters, and is dual redundant per prop, and engine unit. Failure of any one component results in only a 25% loss of power. Refueling is also simplified, resulting in minimal operator schedule disruptions.
ZeroAvia’s Hydrogen Airport Refueling Ecosystem (HARE unit) is a mobile hydrogen refueling solution containing a re-deployable modular electrolyzer, compressor, and a dispensing truck. Transfer pumps move hydrogen from the temporary storage section to a 500 bar storage container onboard the fueling truck. ZeroAvia's HARE refueling truck stores 60kg of hydrogen at 500 bar and refuels planes at a 350 bar target vehicle pressure in under ten minutes.
Milestones Prove Why Hydrogen Always Wins
This past September, ZeroAvia completed the U.K.’s first-ever commercial-scale battery-electric flight in June 2020 and the world’s first hydrogen fuel cell commercial flight in September 2020. ZeroAvia is aiming for commercialization as early as 2024 for flights up to a 500-mile range in aircraft containing up to 20 seats. By 2026, they intend to run flights over a 500-mile range in aircraft with up to 80 seats, and by 2030 over 1,000-mile range flights in aircraft with over 100 seats. The company currently has approximately 15 letters of intent from mainly regional airline companies.
The impact of the existential crisis that the coronavirus pandemic unearthed and the changing of the guard in the U.S. government has recently propelled the commercial aviation industry to take critical steps towards zero-carbon aviation. California has even added aviation to the qualifying list for low-carbon tax credits; meanwhile, the United Kingdom’s green recovery is well underway, and the aviation sector has vowed to play a large role in this movement. Hydrogen will continually have the advantage over battery-electric systems, turbine electric, biofuel, synthetic fuel, and hydrogen-turbine propulsion, and mainstream momentum is moving towards this truth.
This article was written by Valery Miftakhov, Founder & CEO, ZeroAvia (Hollister, CA). For more information, visit here .