Here is an article on TechCrunch which does a pretty good job of outlining some of the issues which have to be overcome to get this tech flying routinely:

https://techcrunch.com/2023/03/02/universal-hydrogen-takes-to-the-air-with-the-largest-hydrogen-fuel-cell-ever-to-fly/

Some of the points made are a bit on the daft side in my view, with comments like until both engines are running on hydrogen, it is 'just a show'
Doh! Its an early test flight, with safety paramount.

But substantial points include:

' Part of the problem with today’s fuel cells is that they can be tricky to cool. Jet engines run much hotter, but expel most of that heat through their exhausts. Because fuel cells use an electrochemical reaction rather than simply burning hydrogen, the waste heat has to be removed through a system of heat exchangers and vents.'

It should be noted that Universal's system does not require power boosting with batteries.

And :

' Some experts are skeptical that hydrogen will ever make a meaningful dent in aviation’s emissions. Bernard van Dijk, an aviation scientist at the Hydrogen Science Coalition, appreciates the simplicity of Universal Hydrogen’s modules, but notes that even NASA has trouble controlling hydrogen leaks with its rockets. “You still have to connect the canisters to the aircraft. How is that all going to be safe? Because if it leaks and somebody lights a match, that is a recipe for disaster,” he says. “I think they’re also underestimating the whole certification process for a new hydrogen powertrain.”'

Along with Universal Hydrogen, I don't see the last point about where the green hydrogen is to come from as very substantial.

There are too many options out there.

Of course, the big attraction is that Universal Hydrogen's proposal to retrofit existing planes not only reduces costs but enables speedy GHG reduction.

Reduction of seating through installing hydrogen tanks is at least partly offset by the likely reduction in fuel costs against jet fuel, without counting the benefits of avoiding fried planet.

Safely handling the hydrogen sounds like the biggie to me.

I am looking forward to reading more expert opinion on this, which is almost anyone who isn't me! ;-)

From my link above:

' Today’s 15-minute flight used about 16kg of gaseous hydrogen — half the amount stored in two motorbike-sized tanks within the passenger compartment. Universal Hydrogen plans to convert its test aircraft to run on liquid hydrogen later this year.'

Aircraft are to use liquid, not gasesous hydrogen.
The biggest problems are keeping it cool oboard, not the cost.

I absolutely believe that, in a technological sense, this will work. The bigger question is whether it will be an economical success. What I really doubt is that they will be carrying passengers by the end of 2025. I can not imagine meeting all of the regulations for carrying passengers in next 33 months when they just made a first flight with one engine running on small tanks of compressed hydrogen.

Personally, I believe that future regional aircraft will be battery powered but not by 2025. It will require higher energy density batteries and a redesigned airframe. Lyten has supposedly made lithium sulfur batteries with about 900 Whr/kg and 10 to 20 minute recharge time and has a time-line of 2025 for commercial production. These would work for regional aircraft but probably not before the 2030 time frame. The reason that I believe batteries will be used is because of the economics. Battery electric aircraft are projected to have a seat mile cost of around 1/4 that of existing turboprop aircraft. Maybe hydrogen can come close to meeting the seat mile cost of existing turboprop aircraft but the cost of green hydrogen will have to come down considerably. Even then, I find it hard to believe that hydrogen will ever be lower cost than battery electric. Maybe if we get high temperature fast nuclear reactors and high temperature hydrogen generation. But that would not happen before 2035 at the earliest.

Meanwhile, we will have to see if the battery electric Eviation Alice aircraft is successful. They also have a number of pre-sale orders and have been flying for 5 or 6 months but the plane has only about 9 or 10 passengers and a range with a 30 minute reserve of 250 nm or about 460 km with existing lithium ion batteries.

@sd:

Where the range and payload of battery electric works, that is the way to go, and 'that will do'

Better batteries will of course increase that, when and if they arrive.

Still short of SAF but clearly able to offer a lot more range and payload are fuel cells.

Radically new tech is difficult to predict, so I am still somewhat less than fully confident of the practicality of the specs needed in the fuel cells, although things are looking pretty good, from Universal Hydrogen and Hypoint.

The bit I am way more confident about is the cost reduction curve for somewhat more established technology.

Why I did not fancy and was wary about battery cost projections was that radically different tech was needed , lithium air, sodium sulphur, whatever, and you can't read that from rates of decrease for lithium ion.

I am a lot happier, although not totally confident, now that some cheaper and more energy dense technologies have got going a bit more.

For hydrogen OTOH, without breakthroughs assumed IMO we are already, and have been for some time, at a point where we can make predictions for cost with a high degree of confidence based on cost reduction and increased volume curves.

I would argue that we are pretty much at the same point in the curve as happened for lithium ion batteries some years ago, and we can count on massive increases in volume and substantial falls in cost for many years to come.

That is assuming no breakthroughs, as for instance naturally occuring hydrogen, pyrolysis, or hydrogen from seawater might result in extraordinarily low costs for hydrogen,, as against simply far cheaper than at present in a business as usual scenario.

But then of course I might be wrong! ;-)

Hi sd.

Great insights as usual, and fascinating that you are involved in two builds!

Things on leaded fuel may be moving rather faster in Europe, although for personal aviation the somewhat endearing trait of so many of my American friends of assuming that what goes for the US, goes for the rest of the world is in fact fairly substantially true.

Here is a link to what the powers that be are up to here on it though:

https://www.iaopa.eu/contentServlet/iaopa-europe-enews-july-2022

Getting hydrogen certified is no doubt a fraught process, and I am equally sceptical that the timelines laid out by Universal Hydrogen are practical.

But both they and Zero Avia have serious aerospace backing behind them, who are hardly naive about the process.

The bottom line is, I dunno - and you can quote me on that! ;-)

For very light aircraft of the order of trainers the fuel costs of batteries sound unbeatable, providing you have not got ambitions to go very far....