I was excited about the use of hydrogen in aircraft for a while but the more I looked into it the less likely it seems. I think the advantages of liquid fuels are too great and that even if there were no oil production it would still make more sense to synthesize jet fuel or some other liquid fuel for use in aviation.
This has nothing to do with feasibility (the Soviet Union had an airliner that ran on hydrogen, so hydrogen aircraft are absolutely feasible), just with lock-in effects and ease of use.
Hydrogen has a couple of "unique" properties that make it especially disadvantageous.
Namely: It'll diffuse through just about anything on a fairly short time scale if it isn't kept as a liquid at cryogenic temperatures, and it causes almost all metals to become brittle.
I think hydrogen is basically a dead-end as energy storage, except in a few niche applications where it's produced as a byproduct and can be consumed immediately.
This doesn't sound completely ridiculous but is there even any proof of concept for that? The gravitational potential energy doesn't seem all that high - most airplane fuel is burnt maintaining height, not gaining it - to justify the complexity of recovering some small fraction of it. Still, I once thought the same about regenerative braking in electric cars.
> The reaction of hydrogen with OH radicals has a further side-effect of reducing the availability of OH radicals with potential impacts on the build-up of greenhouse gases.
https://assets.publishing.service.gov.uk/government/uploads/...
Both claims are wildly wrong. Neither actually happens except at high temperatures and only with certain metals.
I am not an expert on this, but random internet searches such as [0] indicate that common materials, like high strength steel, are susceptible to both diffusion and embrittlement at room temperature
[0] https://www.energy.gov/sites/prod/files/2014/03/f10/pipeline...
It's hardly a theoretical-only problem, either:
https://en.wikipedia.org/wiki/Eastern_span_replacement_of_th...
That's probably due to them using a vulnerable alloy, and was likely exposed to hydrogen while at a high temperatures. This is nothing like a gas tank filled will hydrogen.
It seems you would try methane before you try hydrogen:
https://energynews.us/2013/08/26/midwest/could-natural-gas-f...
Petroleum-based liquid fuels contain mixed entity hydrocarbons that match a specification; if you have the perfect feedstock and markets for the fractions you don't use this is cheap. Worst-case you have to synthesize them with Fischer-Tropsch chemistry
https://en.wikipedia.org/wiki/Fischer%E2%80%93Tropsch_proces...
which is such a PITA (normally a chem E would be delighted that you can use iron as a catalyst, but it works so poorly that a desperate search of the other 91 elements found just 2 that sorta-kinda work)
The issue there is you are sticking C's onto the end of a chain and you have to deal with a network of reactions that produce products you couldn't care less for, such as petroleum jelly that gums up your catalyst, sticks up your downrisers, etc.
Sustainable motor fuels tend to be single entities such as 1-butanol, dimethyl ether, etc. You might need to blend something in for low temperature starting, but it's possible for a single entity fuel to be synthesized with decent yield.
General av is seen as a backwater that is barely hanging on and couldn't possibly get the lead out. The USAF has done biofuels trials, but commercial av is spooked to try anything that could leave passengers up in the air.
It's important to qualify this: jet fuel contains no lead, and constitutes the vast majority of commercial aviation, and hence of emissions.
Jet fuel itself is kerosene, with some additives that aren't superb in raw form, but nothing so durable in the environment as lead.
A sibling comment does a fine job of explaining why avgas still contains lead, and to be sure, a plan to phase that out except where absolutely necessary would be welcome.
The issue is infrastructure. Modern engines run fine on unleaded fuel, but the little airstrips in the middle of nowhere all have 100LL and/or jet fuel, so that's what pilots use.
I know a few pilots who are prefer to use unleaded in their Rotax engines to prevent spark plug fouling, and they have to jump through some pretty ridiculous hoops to get gas, while their buddies just fill up with whatever is on the field.
Airbus is experimenting with liquid hydrogen: https://www.airbus.com/innovation/zero-emission/hydrogen/zer...
Depending on energy density and power density requirements, I think a combination of batteries and traditional fossil fuels will win in the short term. In the long term, when we have stopped burning fossil fuels for electricity, then we can use electricity to manufacture them. Under this path, hydrogen will have limited importance.
Agreed that doing intrinsic safety for H2 is hell. I did once design a LED sidelight for the relevant class with significant current (redundant electronic energy regulation). So it is possible to do stuff, it just takes more work. The tendency for modern electronics to be very low voltage helps a lot.
The ignition zone is larger, risk is higher.
Add in hydrogen's extreme tendency to leak, metal embrittlement, and high pressures and/or low temperatures, and the risks are immense. Particularly at scale, in widespread use, with poor maintenance and inspections.
It’s not strictly better or worse, just different. Diesel for example can be very dangerous in a large open topped container, hydrogen just doesn’t stick around in that environment.
PS: Consider what it would take to make a large hydrogen fuel air bomb that’s as effective as it’s hydrocarbon equivalent.
That and they have very few missions where the increased ISP from LH2 would actually benefit them. The other upside of LH2 and LOX is that you can run it in engines, but you can also run it in fuel cells directly and generate a surprising amount of electricity from a very small package -- the other benefit there is that the Fuel Cells produce pure water as a byproduct.
The Space Shuttle had three fuel cells which provided more than enough power for a full mission. Most missions could be run on two without any compromises. Also, when it was visiting the ISS, it was a convenient means of supplying water to the station.
On non ISS missions, so much water is generated that they have to dump it overboard.
100% this.
IMHO, this is by far the biggest misconception about hydrogen among people.
A LiOn battery is good for many many cycles, and can then be recycled. Hydrogen must be produced and then consumed, and is usually produced from natural gas, itself a greenhouse gas that is on the main a byproduct of the oil industry.
Although, after google search, I see that the Mirai weight is 4075 lb, and the Prius weighs around 3100 lb.
The proof is in the pudding: the curb weight of a Mirai is more than that of the larger and more capable Model 3.
Maybe there's a future world in which we can do it all in orbit -- harvest comets, produce hydrogen in space using solar power or nuclear, and efficiently drop it down to earth in some safe way. That would definitely alter the equation. But by the time we figure that out, it'll probably be too late.
https://about.bnef.com/blog/hydrogen-economy-offers-promisin...
Personally I don't going into competition with laptops, mobile phones and car battery manufacturers as a viable long term strategy for grid scale storage.
Any electric car plugged in is a great buffer for surplus electricity production.
Taking the number above for installed capacity of pumped hydro. You are looking at nearly half a million cars and homes to match the capacity (assuming 50kwh per vehicle), before considering the plugged in factor.
At say a nice round £1000 to upgrade each home you are already looking at a cool half a billion. Already around the cost of the Dinorwig pumped hydro station before you figure out how to compensate electric car owners for the extra wear on their batteries or the fact that when they leave for work their car is flat because it has been powering your neighbors electric showers.
Pretty much all battery research is targeting materials that are common. The DoD made a large bet on Lithium-Sulfer batteries.
Battery recycling can be energy intensive if you go for the full metallurgical extraction. But pretty much all car manufacturing plants want to just 'disassemble' the battery and use other far less energy intensive processes to get battery grade materials out. Also, at least in the west, these factories are generally located in location where there is cheap green energy or at least there is the potential. That is both Tesla/Panasonic and Northvolt (Europe) plan.
With batteries you can reload the battery from your gravitational energy. That is actually quite a significant range extender but it might be a regulatory issue.
But hydrogen requires production, either from fossil fuels, or electrolysis, both can be environmentally taxing. It requires transport. It requires either liquefaction or compression. It require mining to manufacture suitable storage tanks - which will become brittle and have to be replaced pretty often.
> Hydrogen also depletes along the trip unlike battery which will be massive dead weight for planes & ships.
This is indeed a major difference.
Frankly, if you want to burn hydrogen, you're better off synthesizing something like methane. It's a far more effective storage mechanism. After all, if you are willing to spend the massive amount of resources required to do large-scale electrolysis, you might as well go the extra step and setup some sabatier reactors. You'll recapture the CO2 too.
If the former, then what does "H2 can be stored in less volume and less weight" mean? I would assume you would want to compare the mass required to store the same amount of energy, in which case it sounds like H2 takes up more space and weight than Li-ion?
If the latter, then isn't it just a matter of improving our technologies? Or is there some law of nature that makes efficient conversion to electricity impractical?
https://www.sciencedirect.com/topics/engineering/round-trip-...
Roundtrip efficiency is not the reason to use hydrogen for anything.
Your fuel of choice also must be transmitted from the point of generation to the point of consumption. For electricity, this is easy, you can run it down a wire. For gasoline, you need a tank that can hold liquid. For hydrogen, you need to cool it to −253°C and compress it in a cryogenic tank.
Just a side note, you could ship it in hydride tanks.
Again, it's kind of stuck in the middle: energy density by volume sucks compared to hydrocarbons, and if it isn't cryogenic, it really sucks, and cryo is expensive. Transporting fuel by truck is volume limited, not weight limited.
If we're comparing to batteries, then... ok, but you kinda only have to move the batteries once, and then you can either recharge on site with solar or connect to grid. With hydrogen, you have to keep bringing it, and you may as well have hydrocarbons at that point.
The lack of carbon emissions on-site is what makes it tempting, but it's just easier to work around this problem in various ways than it is to pay the hydrogen penalty on an ongoing basis. To a first approximation, all commercial hydrogen is cracked off of methane anyway, and electrolysis is a pretty inefficient way to use renewable electricity, so even that just displaces emissions to fossil fuel plants unless none of those exist anymore.
Usage wise: Batteries have lower gravimetric density than h2 tanks. Batteries have about the same gravimetric density than h2.
Economic wise: Batteries have cobalt which is more scarce than oil/nat. gas from which h2 is made.
Unlike flowing water which may turn a generator, hydrogen isn't a continuing source of generating power.
I can't believe that we went to the moon in the 1960's and now we cannot get support 60 years later for trying to economically exploit space.
Once in orbit, it takes very little energy to go to an asteroid and to launch that asteroid to a parking orbit compared to dredging mountains of soil and rock on Earth. The yields would be extremely high; there's more gold and platinum in some Inner Belt asteroids than we've ever mined on Earth so far, and it's just floating there for you to grab and bite chunks out of.
This is not true at all. This is laughably absurd. Getting from Earth's surface to LEO takes about 9.8km/s of delta-v but then getting from LEO to another destination requires more delta-v. To get to the Moon requires another 6km/s of delta-v on top of the 9.8km/s you needed to get from the ground into orbit. There's a reason the Saturn V was a giant rocket with a teeny tiny capsule mounted on top. Getting places in space requires a lot of propellant.
Here's a nice list of NEAs and the delta-v required to reach them (one way) from LEO[0]. The list is very helpfully sorted by delta-v requirements. You'll notice that the easiest NEA (2018 AV2) requires an additional 3.758km/s of delta-v. The easiest to reach NEA on the least requires more than half the delta-v that it would take to land on the Moon. Once you rendezvous with an asteroid you need to come home which requires yet more delta-v (thus more propellant). Moving an asteroid to orbit Earth is not in any way as simple as you might think from reading science fiction.
So there's nothing "very little" about the energy required to land on an asteroid let alone land on one, mine anything useful, and then return it to Earth. There's also the complexity of the vehicle needed to do the mining. Platinum (pure platinum mind you) goes for up to $37k per kg.
Even if we pretended that all of the technology existed and was donated and we just paid for a Falcon Heavy launch (about $150m to get about 17t of payload to an asteroid) we would need to ship back 4t of pure platinum to Earth just to break even. Even if you assumed that in the best case platinum was a thousand times more abundant in an asteroid than Earth's crust[1] that's still only 5ppm! You'd need your asteroid miner to process about eight million tons of ore (if my math is right) to get that 4t.
If our asteroid miner is 17t or so how long do you think it will take to process that much ore? The loan for the Falcon Heavy would have defaulted long before it would be even close to finished. There's just nothing asteroids have that ends up being worth the cost to mine them instead of mining them on Earth. Don't forget besides raw ore Earth also has lots of landfills literally filled with stuff than can be recycled and valuable materials extracted.
Materials in space are only really useful to an entirely space-based industry and the only way to bootstrap a space-based industry is to launch it from Earth which is basically just lighting money on fire.
[0] https://echo.jpl.nasa.gov/~lance/delta_v/delta_v.rendezvous.... [1] https://en.wikipedia.org/wiki/Abundance_of_elements_in_Earth...
Therefore the relevant comparison is between H2 fuel cells and Li ion batteries. Batteries have much higher full cycle efficiency (energy input -> storage -> energy output), but they are large and heavy. H2 can be stored in less volume and less weight, but it is less efficient. In my opinion, batteries will be a technically and economicly superior solution for all uses other than where weight is extremely critical such as in aircraft.