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Comment by janef0421 | Hacker News Reader

I don't think any of these green fuels are viable, because the thermodynamics don't make sense. Kerosene makes aviation viable because it has a high energy return on investment and is readily available because it is produced by fractional distillation that would occur regardless. Conversely, hydrogen and e-fuels involve substantial energy losses even in ideal circumstances, and require dedicated supply chains that would not otherwise exist, which adds additional energetic costs. In a world where available energy is going to become scarcer, I don't think using these fuels is going to make sense.

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Hydrogen has been used to get rockets to orbit. The thermodynamics make lots of sense. It's the pressures and temperatures at which you need to store the hydrogen that make it hard. Synthetic methane might be a much more practical fuel. That too is used for rockets and storing it is much easier. The reason it is a preferred fuel for lots of new rocket companies is that it is easy to handle and still has excellent energy density. Which is important if you are trying to get a few tonnes of useful payload to orbit.

The reason the aviation industry is dragging its feet with hydrogen is that they are currently making money with selling kerosene burning jets. The transition away from that is going to be very disruptive for them as they'd be killing the thing that makes them money now. So, like some ICE car manufacturers, they study hydrogen endlessly without ever really committing to doing anything with it. They are not in a hurry. Methane, which I mentioned before, works fine in jets. There have been flying prototypes using that as a fuel. It might be much more practical than hydrogen even. And it can be synthesized just like hydrogen. The latter is often actually synthesized from methane. That's currently the cheapest way to produce it.

Energy is not going to be scarce but abundant and dirt cheap long term. You are confusing the short term shortage of fossil fuels with the long term growth of renewables. We're not running out of wind and solar basically. And we are long past the point where exploiting either economically is a problem, even when you consider necessary storage mechanisms. The main thing we are short of here is production capacity, which is growing very rapidly.

Ammonia is another option. That's a bit easier to make than methane, because there is a lot more nitrogen in the atmosphere than carbon.

Something working for rockets doesn't mean it will work for planes. The physics are very different, and rockets can use inefficient and scarce fuels because there are relatively few rocket launches and they carry high-value payloads. Also, the idea that renewables will bring energy abundance is based on a shallow economic analysis that ignores material factors.

I'm not so sure. For very long haul flights, I see hydrogen making sense.

There are 2 things going for liquid hydrogen: the square-cube law and the rocket equation.

The square-cube law: the mass of the hydrogen in the tank goes up with the cube of the size, while the heat loss rate by the square of the size. Which means for bigger tanks, the refrigeration overhead goes down.

The rocket equation: the actual equation doesn't matter, but the idea is that you need fuel to carry the plane, then you need fuel to carry the fuel, than fuel to carry that one, etc. It all converges, but the lowest the density energy of the fuel, the more fuel you need.

Hydrogen has an energy density 3 times as high as kerosene, by mass. By volume, it's about 8 times lower. But mass matters more than volume.

A long range airplane, like the Dreamliner, starts with a lot of fuel at takeoff, about 45% of its total mass. If you replace that fuel with a fuel that's 3 times as energy dense, then you don't need only 3 times less fuel, but about 3.7 times less fuel. Which means the takeoff weight gets reduced by about a third. Which means you can reduce the structural weight of the airplane by a lot. Maybe not a third, but by maybe 20%.

The compounding benefits of having a fuel that's 3 times more energy dense are considerable. Will they be enough to overcome the need for cryogenic tanks and much larger volume of fuel? I don't know, I think it's a close call. But it doesn't sound absolutely crazy to me.

The issue is that you're going from a fuel that produces net energy to a fuel that loses net energy. No amount of efficiency gains can overcome that.

what do you mean by "loses net energy"?

Presumably that it costs more energy to produce hydrogen than you get back when you use it as a fuel, which is strictly true for all fuels; however, with fossil fuels, the energy cost of production has mostly been "paid" already.

But it's still not true as stated.

Well to Wheel analysis reliably shows that hydrogen FCEV are more energy efficient than ICE vehicles.

(Battery EV are better again, because Hydrogen productiom does use some energy).

So efficiency improvements make FCEVs more energy efficient overall, despite the Hydrogen production step replacing fossil fuels.

I don't think a "well-to-wheel" analysis is relevant here, for two reasons. First, because it involves electrical energy as an intermediate product, which could be converted to kinetic energy in ways that are orders of magnitude more efficient. Second, because I'm analysing this based on the amount of energy that is initially invested, not the total quantity of energy that exists in nature.

EDITED because I said converted to electrical energy when I meant kinetic energy.

I'm actually on a project looking at this and it's not so rosy. My area is trying to predict maintenance costs for the hydrogen PEM fuel cells. The cells get contaminated pretty fast.

Regarding hydrogen suitability while it has excellent gravimetric efficiency it has terrible volumetric efficiency. You either need huge tanks and high pressure or cryogenic tanks to store it in its liquid phase.

Like anything else its doable we just need to accept higher airliner prices and slower speeds/shorter ranges.

There are iirc substances like graphite(?) sponge that will take in more H2 at a given pressure. It's still high, but it's better. The tanks can be made long and thin and stacked. Is the extra tank material too much in either cost or mass?

I haven't looked at the various sponge or chemical binding concepts personally. To be used in an aircraft to be delivered say in 10 years time they would already need to have been approved by EASA or FAA or have a clear and straightforward approval pathway. Otherwise they represent a large schedule risk and new airliner types cost several billion to launch so they are pretty risk averse.

That's not to say those concepts won't work. Regarding long thin tanks wings don't actually have that much suitable internal volume for that. Lots of ribs, stringers, etc. That's why most hydrogen aircraft concepts use under wing pods or fuselage tanks to store the hydrogen even though it reduces safety margin. Civil aircraft regulators are allergic to storing fuel in the fuselage near the passengers for good reason.

The containers for the hydrogen can be made to vent safely for a lot of impacts. Fiberglass, carbon fiber, or other composites.

> You either need huge tanks and high pressure or cryogenic tanks

According to Airbus all 3 demonstrator planes will have liquid hydrogen [1].

As for the fuel cells, they will basically not be used.

The engines will be regular engines that burn hydrogen instead of jet fuel. Fuel cells will only be used for the electricity needs of the airplane, but not for moving the airplane through the air. They are probably going to use less than 1% of the liquid hydrogen.

[1] https://www.airbus.com/en/innovation/zero-emission/hydrogen/...

That gets to the point I was trying to make. Aviation initially became popular because it had advantage in cost, speed, and range. If climate change adaptation means aviation loses all or most of those advantages, then it would be logical to transition to other forms of transport, like rail, which have other advantages, such as capacity and comfort.

Why is available energy becoming scarcer? Renewable energy is not expensive.

> Kerosene makes aviation viable because it has a high energy return on investment and is readily available because it is produced by fractional distillation that would occur regardless.

Kerosene (and liquid petroleum-derived fuel generally) works well as an aviation fuel because it is relatively stable and energy-dense in both volumetric and mass terms. That it often has a high EROI means it can be dollar-cheap as well, which doesn't hurt, but it's still used even when the EROI is terrible (eg. military aviation - delivering fuel to a war zone costs a lot of energy) because the other advantages are still overwhelming relative to the energy cost.

> Hydrogen and e-fuels involve substantial energy losses even in ideal circumstances

Absolutely, though this is true of every industrial activity.

> and require dedicated supply chains that would not otherwise exist, which adds additional energetic costs.

While this is certainly true for hydrogen, e-fuels (which I take to mean synthetic electrochemically-produced hydrocarbon fuels, though perhaps you mean something different) can be produced and distributed using the much of the same infrastructure that exists today - for example, if you're making Sabatier methane, there's no reason you need to build new pipelines and tanks for it, though I'll grant that you do need new production facilities.

Of course, the rub is that the energy to drive it all has to come from non-carbon-producing (in operation) sources - solar, wind, hydro, or nuclear - and there's no denying that building out that infrastructure is going to be a massive project.

> In a world where available energy is going to become scarcer, I don't think using these fuels is going to make sense.

Available fossil energy is indeed going to decrease, but what makes you think that's true for renewables too?

As I was writing this comment, I took a small detour to do a bit of rough figuring here:

- Assume $77/MWh (Q1 2021) for utility-scale solar+storage [1]

- Assume 75 cents/L (today) for wholesale gasoline [2]

- Assume 105 L of gasoline contains 1 MWh (roughly) [3]

Then: $77 of solar energy can, if 100% efficiency were possible, produce 105 L of gasoline having a wholesale value of $78.75 at today's prices.

Obviously, 100% energy efficiency (or anything close to it) is impossible. But if solar energy instead cost $30/MWh, it suggests that an industrial process with an overall efficiency of 38% in energy terms (i.e. 2.625 MWh is needed to make 1 MWh of fuel) would be able to produce gasoline that matches the current market wholesale price.

Unfortunately I wasn't able to find anything quickly that describes industrial process efficiency in quantitative terms, so I don't know if 38% overall efficiency is reasonable to expect; personally, I think that probably would be considered high efficiency, so solar would need to get even cheaper (which it will) and/or gasoline will need to get more expensive (which it will) to improve the economics.

[1] https://www.nrel.gov/docs/fy22osti/81325.pdf

[2] https://markets.businessinsider.com/commodities/rbob-gasolin...; price quoted in gallons and converted

[3] https://hextobinary.com/unit/energy/from/gasoline/to/megawat...

EDITED because I can never remember to double-space when I want newlines

I think energy is going to be scarce based on analysis of the material circumstances, such as land and resource availability and the involved thermodynamics. Price is irrelevant, because price is a social phenomenon, not a material one.

While land and resource availability are definitely reasonable concerns in any discussion about, say, shifting humanity's primary energy source to direct solar (lots of panels means lots of factories to build them and lots of land to put them on) as opposed to indirect solar (oil and coal) I'm struggling to understand why you think "thermodynamics" means that usable energy will become scarcer on a timeline that is meaningful to human civilization. Happy to listen if you care to explain, but no worries if not.

Land is plentiful and solar panels are (by weight) almost exclusively made from sand (followed by aluminium). What problem to do you anticipate?

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