H2Fly – Breaking the Hydrogen Barrier | Hacker News Reader

H2Fly – Breaking the Hydrogen Barrier

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42 pointsbelter26 comments

“Inherently safer than kerosene and synfuels” - No way. Or at least: I don’t think we truly know yet, until they’ve flown for a few years.

I used to think hydrogen was extremely safe. It’s super light right, so it should quickly vent away if there’s a leak? Then a hydrogen gas station nearby exploded. Didn’t look like it was very airtight to me, but somehow enough gas collected for a big explosion.

It’s extremely hard to prevent hydrogen leaks. The cause of the explosion was a fitting that wasn’t tightened to just perfectly the right torque. Supposedly the procedure to fix this is to have every fitting double checked by a second person. Doesn’t sound inherently safe to me.

There’s no warning if there’s a hydrogen leak. There’s no smell or visible gas.

The right mixture of hydrogen and air will practically self ignite. There was no source of fire at the gas station. The tiniest invisible spark can set it off.

I was also surprised by how loud the explosion was. The explosion is fast. It will rip things apart. On the positive side, it’s over very quickly. You’re not going to have a lingering fire from fuel that has sprayed everywhere like with Kerosene.

I think hydrogen CAN become perfectly safe with the right engineering. I think the solution will be that every single piece of tube or valve that has hydrogen flowing through it needs a double tubing around it with active ventilation, and sensors where it vents out to check that it has the expected air flow and no hydrogen gas is detected.

But inherently safe? No way

> I was also surprised by how loud the explosion was. The explosion is fast.

Here's an example: https://www.youtube.com/watch?v=l9CI6KSV560&t=1625s

One of the things that seems to have been kept very quiet about hydrogen is its ability to self-ignite when leaking from high pressure into air. The jury is out on the exact mechanism but most seem to think it creates a wavefront that has high enough energy content to auto ignite.

This seems to be why automotive hydrogen refueling is relatively slow. At very high pressures there is no reason why refueling should take longer than gasoline refueling.

They made the nozzle so small (less than a millimeter) because if a valve fails open and the hydrogen vents to air, it will limit the length of the invisible flame that probabilistically results to a meter or two.

The temperature limit of the tank is one reason to rate-limit as well, but I don’t think it explains the whole picture.

People would be shocked by how hard it is to ignite Jet A (the fuel that they're seeking to replace). You can take a blowtorch to it and it won't ignite. In many accidents it spills everywhere but it still doesn't ignite. It only becomes seriously flammable when it's atomized.

No way is hydrogen safer! That's just an outright lie.

> There’s no smell or visible gas.

FWIW, this is also true of methane. An odorant is added to the stuff that's put in gas pipelines. Couldn't they add the odorant to H2 as well?

H2 molecules are small and thus prone to leaking through small breaches in the containment. Odorant molecules are much larger and less likely to escape.

At least methane burns with a visible flame in daylight.

IIRC, hydrogen gas will basically detonate at any mixture ratio in oxygen or in air. Quick search says 4% to 75% to burn in air, and 18% to 60% to fully detonate.

By contrast it looks like kerosene will only burn at between 0.7% to 5% in air, so that's a much narrower range that's dangerous. And of course that's just burning, lots of heat and light, but no brisant supersonic shockwave to shred everything around like you get with hydrogen.

Plus apparently if you end up with atomic hydrogen somehow, it will embrittle and structurally ruin a lot of common metals on contact.

It also seems generally just not very practical even ignoring the tendency to leak and explode, needing either cryo or high pressure to be stored at any reasonable density, and even in that case taking up over three times as much space as hydrocarbon fuels by volume, despite also packing three times as much energy by mass. The British designs for Skylon are basically just one massive hydrogen fuel tank with wings and engines.

Hydrogen should be odourised, so lack of smell isn’t an issue https://en.m.wikipedia.org/wiki/Hydrogen_odorant

But yes, inherently safe hydrogen either describes an impressive engineering feat or a bad joke.

A silly curious question.. if contact with air is what we are worried about, could it make a great fuel in space? Or is it already what we use?

The novelty here seems to be that they used liquid hydrogen instead of gaseous hydrogen on a fuel-cell powered, piloted, electric aircraft. This doubled the range that they could achieve on their experimental aircraft to 1500km (~930mi).

Yes, but they only use vague words without any numbers that could support their claims that hydrogen is viable as an aircraft fuel.

They should show the weights, volumes and costs for standard kerosen tanks and their liquid hydrogen tanks with the associated equipment, e.g. pumps, at the same stored energy.

At equal stored energy, the liquid hydrogen tanks must be much bigger and much more expensive, and as soon as they are partially empty they will also be heavier than the partially empty kerosen tanks. The maintenance costs and the risks of failure for the cryogenic equipment will also be much greater.

They do not say any word about any new technique used in their hydrogen fuel cells. All the kinds of hydrogen fuel cells remain too expensive, either due to using too expensive catalysts or because they have parts with a short lifetime, which must be replaced periodically, e.g. fluoropolymer membranes for low-temperature fuel cells or ceramic electrolytes (e.g. of ceria or zirconia) for high-temperature fuel cells.

cryogenic hydrogen was always the centerpiece of hydrogen powered flight.

IIRC gaseous simply didn't have the right energy density to be viable. cryogenic also increased efficiency of jet engines and opened avenues for higher speeds (Tu-244 design, for example, for ~ Ma 3 supersonic transport plane)

"First Piloted Flight of Liquid Hydrogen Powered Electric Aircraft" - https://www.h2fly.de/2023/09/07/h2fly-and-partners-complete-...

Maybe I am naive, but let's say that using liquid hydrogen instead of synthetic kerosene would use 1/3 of the input energy. OK. To me, it seems way more feasible to triple the amount of input energy capacity (build more solar, wind, nuclear, ...whatever), than to completely redesign aviation around a fuel with such unforgiving constraints. What am I missing?

Perhaps that traditional engines shoot greenhouse gas straight into the stratosphere and that’s a bad thing?

I was talking about carbon neutral synthetic kerosene which is a step beyond green hydrogen in the power-to-gas pipeline.

Not sure how I was supposed to get that from your comment or why you think kerosene is a better solution when burning it in jet engines still puts greenhouse gas straight into the atmosphere? I feel like I missed some big piece of your argument here.

Synthetic kerosene was the hint. You can make carbon neutral kerosene in the same way as biofuel, it doesn't add greenhouse gases to the atmosphere since the carbon it contains was captured from the atmosphere. Not a perfect solution as it doesn't solve the impact of contrails and there is the problem of how to generate so much biofuel, but it is a valid way to decarbonize flying.

This study[0] claims that is not viable as it would require 9% of forecast renewable production in 2050. Less net carbon emission can be produced if the electricity is used instead to displace coal plants and the plane run on conventional kerosene. Building lots of nuclear power, which isn't as capacity restricted as photovoltaics, wind, and other forms of power the authors consider renewable in the study.

Hydrogen would still require quite a bit of energy to produce, but if it can be used in fuel cells, it can potentially be more efficient than if burnt in a heat engine. This is a pretty big if for air travel as fast as current jetliners. Fuel cell power per unit weight would need to be drastically increased.

[0]https://www.sciencedirect.com/science/article/pii/S004896972...

Renewables often generate power when it isn't needed. High efficiency reduces costs, but we're already wasting power and it gets progressively worse as the renewable share increases.

Batteries are now improving in efficiency, cost, and availability of materials. This is driven by solar.

You're not missing anything.

What everyone else is missing is that to achieve net zero you don't need to decarbonize everything. The planet absorbs about half of the CO2 that we emit (see the IPCC FAQ 5.1 [1]). If we decarbonize everything but aviation we are well into negative CO2 emissions territory.

[1] https://www.ipcc.ch/report/ar6/wg1/downloads/faqs/IPCC_AR6_W...

I think Hydrogen is a great fuel to replace natural gas for generating electricity. But is it safe enough for commercial airplanes? I don't know, we'll have to see.

Is electric propulsion even viable for passenger jets?

H2. Twin fuselage. Get it?

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