H₂ Mobility, Transportation and Infrastructure

J. SCOTT, Piasecki Aircraft Corp., Essington, Pennsylvania (U.S.)

Watch the television in the U.S. today and it will be mere minutes before you see a commercial for an electric or hybrid vehicle. Even young children have begun to understand the difference between a gas engine and a hybrid, even if they cannot explain how the powertrain actually works. The point is this: the world is rapidly changing, and electric propulsion—in whatever form you choose—is here.  

There are, however, options of how to generate, manage and store that electricity for the “transport layer.” In this context, the transport layer includes planes, trains, ships, recreational vehicles and automobiles. These transportation methods move goods and people and must carry their source(s) of energy (fuel) with them. The challenge is how to create that fuel, store it for the haul and efficiently convert it to useful energy when needed—all while on the move. Since the advent of the first internal combustion powered automobile in 1886, the solution has been tried, tested and true using fossil fuels.

The hydrogen (H₂) factor. Fossil fuels have helped propel the journey into the modern world, but there is an element with far more power in its punch: H₂, which has about three times the energy by weight and when converted to electricity using a fuel cell, the only byproduct is water vapor. An all-electric transport layer will demand highly efficient methods of transporting and converting electricity into motion.  

Today, batteries and generators are the primary electrical sources, but H₂ fuel cells will emerge as the dominant source sooner than you think. H₂ fuel cells are simple, reliable and efficient. Fuel cells using oxygen (O₂) and H₂ have been around since 1932 when first invented by Sir Francis Thomas Bacon. They gained some momentum during the mid-1960s when the U.S. National Aeronautics and Space Administration (NASA) used them for the space program. They built some intermittent momentum during the 1970s gas embargo and from the 1990s forward with the Zero Emissions Vehicle (ZEV) act.  

What is the catalyst driving this renewed interest in H₂ at a time when fossil fuels are readily abundant and low-cost? H₂ conversion is 15% more efficient than an internal combustion engine, far cleaner to create (via electrolysis or reformation) than fossil fuels, has zero emissions and six times more energy density than lithium-ion batteries, making it an ideal energy source for aviation and the transport layer.  

Two dominant types of H₂ fuel cells are viable for the transportation industry: low-temperature and high-temperature proton exchange membrane (LT-PEM and HT-PEM). The PEM is an ultra-thin, rubber-like material that facilitates the chemical conversion of O₂ and H₂ into water while releasing an electron. LT-PEMs have been around for many years and have been put into production systems for automobiles and recently small unmanned aerial vehicles (UAVs). 

These low-temperature membranes require high-purity H₂ gas, a humidification system for the PEM, and operate at temperatures below 80°C (176°F), which requires liquid cooling to maintain adequate thermal performance. Conversely, high-temperature membranes require operating at temperatures above 140°C (284°F) to maintain their chemical reaction, which allows them to utilize a lighter-weight air cooling system. Additionally, HT-PEM fuel cells require no humidification and can consume cheaper, less pure H₂ gas. A comparison of LT-PEM and HT-PEM systems is shown in FIG. 1. 

Ideal for aviation. Both are viable systems, but the significant advantages of HT-PEMs—such as lower system weight, fewer moving parts and lower parasitic power consumption—will solidify their position as the only choice for aviation. Another often overlooked advantage is how fuel cells are built and packaged. They are assembled in power stacks that can be daisy-chained serially to produce the exact power and voltage needed. Users are no longer bound by power plants that come in pre-set ratings (e.g., 150 hp, 250 hp). Users can select the stack size and then procure the number of stacks needed to provide the exact power required. This offers huge flexibility for new aircraft design, improves redundancy, reduces the burden of carrying unneeded power and weight, and provides “snap-in, snap-out” upgrades and replacements. An example of HT-PEM fuel stacks packaged into Piasecki’s future PA-890 Pathfinder III is shown in FIG. 2.

Direct operating cost (DOC) in aviation consists primarily of fuel cost, parts reliability and replacement costs, and overhaul maintenance cost. Fuel cells have significantly fewer moving parts, resulting in fewer opportunities for part failure as well as fewer maintenance hours, leading to significantly reduced DOC (roughly 50% cost reduction).  

In addition, less weight burns less fuel and provides more payload for operations, which translates to increased revenue. Both HT-PEMs and LT-PEMs use gaseous H₂ delivered to their stacks, but storage tank technological advancements into liquid hydrogen (LH₂) tanks and systems will benefit both system architectures. Tank technology and LH₂ have the potential to double the range of H₂ fuel cell-based flight.  

Ultimately, HT-PEM technology will provide the power density to exceed 4.5 kW/kg at the stack level, which is the starting point at which the fuel cell begins to overtake the turbine engine. The author’s company and its partner ZeroAvia are in the midst of executing a U.S. Air Force contract to generate > 750 kW of power in an aviation grade Iron Bird. This system will come online in mid-2026 and demonstrate the HT-PEM capability to meet the demanding requirements of electric vertical take-off and landing (eVTOL) applications.

Takeaways. H₂ is the key to clean, abundant and efficient flights with zero emissions. Using high-temperature fuel cells takes advantage of efficient power generation, providing eight times the energy of batteries by weight and reducing DOC by as much as 50%. Clean electrical power generation on par with turbines is here now, and the commercial expansion into related new fuel cell technology is increasing exponentially. Aviation will continue to lead the way due its unique position of using hub-based operations and its economic need for lower lifecycle DOC and stringent zero-emissions standards. H₂T

About the author

JOHN SCOTT has spent 32 yr in the aerospace industry, including 24 yr at Boeing where he championed model-based definition (MBD) as the cornerstone of digital design and build. Scott is a plank member of Boeing Phantom Works, and continued to lead as Chief Engineer and Program Manager of multiple development and production programs in fixed wing, rotorcraft, space and intelligence, surveillance and reconnaissance (ISR). He also served as the engineering lead for digital factory implementation across F18, CH-47, V-22 and AH-64 programs.    

Scott works as the Program Manager for Piasecki’s future H₂ fuel cell programs, including the PA-890 eVTOL, HAXEL Prototype, AFWERX Hydrogen Propulsion System, and the U.S. Department of Energy’s aviation-focused initiatives.