Gregory Polek has spent his entire career in aviation journalism with AIN, starting as a proofreader and assistant to then-managing editor Mary Mahoney in 1995 after serving an internship with&nbsp;New Jersey Monthly&nbsp;magazine and completing his B.A. degree in English/Writing at New Jersey’s William Paterson College. By 1997 Polek accepted a position as an associate editor, covering the regional airline beat for&nbsp;Aviation International News&nbsp;in place of retiring industry veteran Don Anderson. The assignment took Polek across North America and Europe to profile regional airlines varying in size and mission from the likes of floatplane operators Kenmore Air and Chalk’s Ocean Airways to regional jet operators such as SkyWest and American Eagle. Today, in his dual role as Air Transport Editor and International Airshow Editor, Polek writes, edits, and manages AIN’s commercial aviation content while overseeing each of the company’s daily international air show publications in Paris, Farnborough, Singapore, and Dubai. Most recently Polek has assumed oversight of daily coverage of the Helicopter Association International’s annual Heli-Expo convention.

Senior Editor

Of all the various renewable energy sources under development, hydrogen represents the so-called “holy grail” in the effort to eliminate carbon emissions from future aircraft, say many scientists. But while the technology exists today to build a hydrogen-powered aircraft, prompting several startup enterprises to develop fuel-cell-based vehicles for the burgeoning advanced air mobility (AAM) sector, for example, scaling the production and distribution of hydrogen for the wider air transport market presents its own set of challenges. 
With several companies working to advance fuel cell technology using hydrogen gas for generally small aircraft, the application of liquid hydrogen, or LH2, appears to carry the most promise for larger airliners due to its superior energy density and versatility. The use of liquid hydrogen stored in a cryogenic tank onboard to power turbine engines presents the most feasible means of introducing a passenger-carrying aircraft to the market, according to many experts in the field.
NASA Cryogenics Test Laboratory principal investigator Adam Swanger explained to FutureFlight&nbsp;that although researchers continue to study pure hydrogen fuel cell-electric architectures, the use of onboard liquid hydrogen to power a combustion engine remains a comparatively more straightforward and economically practical solution for large commercial aircraft. “These engines [represent] billions and billions of dollars, and decades of investment,” he said. “And they’re super-optimized for their application. They’re fantastic…some of the most amazing technological achievements ever, but you can’t just swap over to another fuel that easily. On the other hand, fuel cells are great because there are very few moving parts and they have high efficiencies. It’s not that pure fuel cell electric can’t be done, and it might be in the future, but for large aircraft that would most likely require a full, ground-up design. So, if you’re trying to get hydrogen into your aircraft as soon as possible, that’s going to be a hard sell I think.”
A recently published report by the UK’s Aerospace Technology Institute (ATI) named “green” liquid hydrogen as “the most viable zero-carbon emission fuel.” However, it added that generating, transporting, and storing the vast amounts of hydrogen needed for future use will require “unprecedented” renewable energy capacity. Delivering hydrogen to airports will present another challenge, whether through gaseous pipelines or liquid hydrogen tanker deliveries, while the refueling and servicing of hydrogen-powered aircraft will have to take place safely and efficiently alongside conventional aircraft.
Swanger explained that for hydrogen to remain in a liquid state, at a normal boiling point of -253 degrees Celsius, requires cryogenic storage in a vessel designed to manage large heat loads and to vent boil-off gas in a safe manner. But by tackling the thermal management challenges, one benefits from the fact that liquid hydrogen carries around 800 times the density of gaseous hydrogen, making it far more volumetrically efficient for powering passenger-carrying airplanes.
“If you leave liquid hydrogen in a container, any container—it doesn't matter how well insulated it is—it will eventually all boil away,” said Swanger. “So you have to manage that. Conversely, using ambient temperature, high-pressure gas you don’t have to deal with the heat management as much, but the vessels [that contain the gas] are generally heavy and you can’t get as much density out of them.”
Swanger added that two of the biggest technical challenges presented by liquid hydrogen center on minimizing boil-off losses and safely managing boil-off gas.&nbsp; The former typically requires a double-walled vessel, or a tank within a tank, with a high-performance vacuum insulation system between the two. “High-performance tanks help minimize boil-off, but they can’t completely eliminate it without active cooling,” he noted.&nbsp; “So, you’re left to deal with safely managing a flammable gas flow, and you can’t just let it go wherever it wants,” explained Swanger. “And then, during loading and unloading, you're taking equipment that's sitting outside warm and you have to chill it down to liquid hydrogen temperature. So you have to flow liquid through it, which boils really quickly, and it chills the hardware down until you can finally start to build up quality liquid flow through the system. You have to deal with all that boil-off gas. So boil-off gas management's probably the most difficult technical aspect.”
Although used in industries such as steel manufacturing and pharmaceuticals as a large source of high purity hydrogen gas, until now LH2 has propelled only rockets in an end-use application, and the largest liquid storage tanks in the world reside at NASA’s primary launch site at Kennedy Space Center (KSC).&nbsp;The product comes in tanker trucks to KSC from liquid hydrogen plants located roughly 700 miles away on the Gulf Coast. Safety regulations do not allow the trucks to vent hydrogen in transit, so the LH2 absorbs heat and pressure builds in the tank during transport.&nbsp; After crews transfer the liquid to the massive holding tanks at the launch pads, the heat dissipates naturally through boiling, but still constitutes a loss of product due to transport. 
“Of course, airport logistics present wholly different challenges, and debate continues on the best method for fueling airplanes,” said Swanger. Ostensibly, tankering liquid fuel on trucks to the airport and fueling the airplanes through basically the same method used now for jet fuel would present the least complicated scenario, but maintaining the cold temperatures to reduce chill-down losses and venting gases would lend more complexity, he added.
Today, Latham, New York-based Plug Power ranks as the biggest purchaser of liquid hydrogen in the world. Like NASA, it stores liquid hydrogen in tanks at its facilities, but it then re-gasifies it for use in fuel-cell-powered forklifts at large warehouses for customers such as Amazon and Walmart. 
Plug Power’s model of shipping LH2 to its own storage facilities could, in fact, serve as a model for airports, regardless of whether the liquid gets shipped via tankers, sent via a pipeline, and liquified at the airport, or both produced and liquified on site. According to the ATI report, most airports would initially prefer tanker deliveries due to their lower capital costs. However, when the frequency of tanker deliveries increases to a level that causes congestion on local roads or the off-load point, the two other options could prove the preferred solution. 
The choice between the second scenario, in which gaseous hydrogen gets sent via a pipeline for liquification at the point of use, and the third scenario, in which the fuel gets produced and liquified on-site, will depend largely on the size of the airport, said the report. For large airports, the energy requirement for producing and liquifying enough hydrogen will eventually prove economically unattractive, leaving the pipeline option as the most likely scenario for hubs, it added. A pipeline could also supply gaseous hydrogen for other airport uses such as heating and ground support equipment.&nbsp; 
Swanger explained that although hydrogen is the only fuel option that industry can produce and consume without causing greenhouse emissions, doing those things in a way that makes sense economically and at the needed scale presents another obstacle to overcome.&nbsp; The method to produce hydrogen today predominantly involves a process called steam methane reformation (SMR), which uses high-temperature, high-pressure steam to crack methane and extract hydrogen, thereby releasing carbon. “Really, most of the hydrogen, regardless of whether it gets liquified, comes from SMR,” said Swanger. “It's a well-established, large-scale industrial process. Methane is carbon and hydrogen, so you're splitting off and capturing the hydrogen and you're left with CO2 [carbon dioxide] as a byproduct.”
Meanwhile, the process requires a lot of electricity, which, although getting cleaner with more renewable energy sources, still creates a large amount of carbon itself. “If you can have all of your electricity coming from renewables, you clean up that part of it, and then you can use that electricity to power a green SMR process to create hydrogen, which people are working on,” explained Swanger. “So you’d basically have SMR with carbon capture, and you don’t emit any CO2.”
Another option involves using electricity to run a device called an electrolyzer, which extracts hydrogen gas from water through electrolysis and releases the left-over oxygen into the atmosphere, or captures it for some additional use. Swanger called the process completely green when paired with renewable electricity, but not yet widely used on an industrial scale to make hydrogen. “There seems to be a lot [of study] going on in that space,” he noted. “The technology is very well understood. So right now it seems to be mostly about improving efficiency and scale-up.”
But while Swanger said he doesn’t see any technical problems he’d describe as “show stoppers,” liquid hydrogen’s challenge lies with the sheer number of hurdles to overcome related to sourcing and logistics in a relatively short period of time. 
“The new hydrogen “wave” is primarily driven by global climate change initiatives, so there’s a very big sense of urgency across the board, which is a good thing, but it places an additional burden on a problem with an already huge technical scope,” he explained.
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Gregory Polek

An 850,000-gallon liquid hydrogen storage sphere used to provide LH2 for the Apollo missions resides at NASA's Kennedy Space Center. (Photo: NASA)

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