The new Hindenburg 747

June 6, 2006 10:20 AM

Could airplanes be powered by fuel cells? Would we need to develop a new type of jet engine or could the same type of turbine, etc be powered by a fuel cell engine.

I don't know much of anything about jet engines, but this seemes like a good place to drastically cut back on fuel costs for the struggling airline industry and perhaps improve safety because airplanes wouldn't need to be flying around with thousands of gallons of explosive jet fuel on board. I suppose they don't have a lot of money for R&D costs and retrofitting lots of airplanes....but that's besides the point of this question.

posted by pithy comment to travel & transportation (27 comments total)

In theory an airplane could be powered by a fuel cell, but I don't see how a jet engine could be.

Fuel cells convert stored hydrogen to electricity. The principle behind a jet engine is that a hot jet of gas is ejected out the rear to create forward thrust. There's no (efficient) way to get a hot jet of gas from electricity.

Now, you could power a propeller with an electric motor that is driven by a fuel cell. The problem at this point is your energy density (not to mention you've got a prop plane, not a jet plane): we don't have very efficient storage technology for hydrogen right now, so that might wind up weighing even more than regular engine fuel or Jet-A kerosene. It's hard to beat petroleum products for energy density, which is why they're so popular.

The more direct process would be to simply burn the damn hydrogen. That would be a jet. But you're back to the energy-density problem.
posted by adamrice at 10:31 AM on June 6, 2006

Once again, fuel cells are exceedingly expensive. The amount of capital investment would be huge on such a propietary technology. Struggling airlines would be the last to benefit from the technology, they make purchases well in advance and operate on thin margins as it is.
posted by geoff. at 10:31 AM on June 6, 2006

Fuel cells convert hydrogen and oxygen into electricity. Plane engines are gas turbines, which work by sucking in air at the front, compressing the air, spraying fuel into the compressed air, and igniting that mixture which then forcefully exits the rear of the engine, providing thrust for forward motion of the aircraft. So, basically, retrofitting is completely out as far as fuel cells are concerned.
posted by pmbuko at 10:33 AM on June 6, 2006

not all plane engines, mind you, just most airliner engines.
posted by pmbuko at 10:34 AM on June 6, 2006

As for the safety factor, why do you think carrying around a bunch of hydrogen would be safer than carrying around a bunch of jet fuel?
posted by Good Brain at 10:44 AM on June 6, 2006

Well, pithy comment is giving mixed signals on safety; his/her title is "The new Hindenberg 747".
posted by evariste at 10:51 AM on June 6, 2006

Fuel cells convert hydrogen and oxygen into electricity. Plane engines are gas turbines, which work by sucking in air at the front, compressing the air, spraying fuel into the compressed air, and igniting that mixture which then forcefully exits the rear of the engine, providing thrust for forward motion of the aircraft. So, basically, retrofitting is completely out as far as fuel cells are concerned.

Modern jet engines are high-bypass turbofans, which means most of the thrust from the jet is used to spin a turbine at the back. That turbine drives a giant fan at the front, which propels the plane. In theory that fan could be spun by electricity.
posted by cillit bang at 10:53 AM on June 6, 2006

It's also worth noting that planes are pretty efficient. The reason airlines spend so much on fuel is because they travel much longer distances than is really practical for people to be doing routinely.
posted by cillit bang at 11:02 AM on June 6, 2006

Weight is a huge issue for aircraft. Turbine engines yield a lot of useable power for their weight. A fuel cell + electric motor would have to be able to offer similar performance/weight.
posted by Good Brain at 11:07 AM on June 6, 2006

During the cold war there were plands and even prototypes of nuclear-electric planes. Of course that's probbaly very differnet in terms of energy density... as well as the potential for turning a Hindenburg into a Hiroshima.
posted by Artw at 11:10 AM on June 6, 2006

I would simply not feel comfortable with 5000 gallons of hydrogen in the wing next to me. Think Hindenberg. One of the major problems with fuel cells is refuelling. How do you safely store, transport and inject enough hydrogen? Aviation fuel (kerosene) has all sorts of additives to make it safer, not burn in puddles, not ignite, etc. There's a hell of a job gearing up airports to use hydrogen to fuel planes instead.
posted by gaby at 11:14 AM on June 6, 2006

If you have enough hydrogen storage to get enough fuel cell output to fly a plane, you'd be far better off just burning the hydrogen directly in the engine -- that way, you don't need to take oxygen tanks as well.

A general rule -- for efficency reasons, never do more energy conversions than you need to. Going Chemical -> Electrical -> Motive is silly when you have a more direct route, namely Chemical -> Motive. There are exceptions (diesel electric leaps to mind) but in general, the case holds.

The big problem, as adamrice notes, is energy density. JET-A fuel is basically kerosene. In terms of volume, you get over 10000Wh per liter. In terms of mass, you get over 13000Wh/kg.

Hydrogen is much less dense. You get 2600Wh/l, which means you need about 4 times as much space to store the same energy as JET-A does. In terms of mass, though, you get almost 40,000Wh/kg -- over three times as much energy. Looks impressive.

But that density kills you -- yes, you get three times the energy, but it takes four times the space. That means you need vastly larger tanks, tanks have mass. You also need insulation, since LH2 is a cryogen. That's more mass. Mass in tanks and insulation is mass that isn't in passengers and cargo.

This is the reason that many space boosters use RP-1 -- a very clean version of kerosene -- for lower stages. LH2 is much more powerful per kilogram, but the tanks end up being so large that the rocket doesn't fly well. The boosters that are all LH2 tend to use solid boosters to help keep the size down. The Saturn V used RP-1/LOX in the first stage for this very reason.

Gaseous H2 is even worse -- at 150 bar, which is a scary pressure to work at, you get 405Wh/l. You get more energy per volume from burning wood than H2 at 150 bar.
posted by eriko at 11:18 AM on June 6, 2006 [1 favorite]

Thanks for the insightful discussion. I guess I had heard/imagined that hydrogen could be put into some kind of block from that is safer. And I probably assumed that you could use less hydrogen than jet fuel. As I said, I don't know much about this. Maybe to further the direction of my question, what will planes use for fuel assuming oil runs out? Biodiesel jetfuel?
posted by pithy comment at 11:27 AM on June 6, 2006

Won't/cant work with current technology: See the energy density charts - the entire contents of the Hindenburg's gas bags (assuming 100% purity - Something that NEVER happened) has about the same energy output as 1,563 litres of gasoline - or less than 1%(.008522) the tank capacity of a 747.(183,380 litres)

[geek] RE/OT: Hindenburg(LZ-129) safety - 35 dead out of 97 passengers is probably better than most jet crashes - especially since it was designed for helium.[/geek]
posted by Orb2069 at 11:42 AM on June 6, 2006

Brilliant comment eriko.
posted by WinnipegDragon at 12:18 PM on June 6, 2006

Note that there have been serious plans for fuel-cell-based UAVs. Charge on solar during the day, run the fuel cell at night. However, last I heard, the project had stalled out.
posted by Ptrin at 12:44 PM on June 6, 2006

Eriko didn't touch on the biggest problem with hydrogen as a fuel: where does it come from?

Hydrogen should be thought of as being like electricity: a way to move energy from one place to another. It isn't an energy source because there's no significant naturally occurring source where we can get it.

Note: "significant" means on the order of 1% of American energy consumption. Currently the US uses power at an average rate of about 3.5 terawatts, so a "significant" energy source needs to be at least 30 gigawatts, i.e. about 9.5*10¹⁷ joules per year.

There is no natural source of hydrogen that large -- or even a thousandth that large. Most hydrogen now is made by conversion of natural gas, in a process which wastes a lot of the energy present in the methane. (It's also possible to make it through electrolytic breakdown of water if you have spare electricity, but that's hard to scale up and it's too expensive.)

...hydrogen could be put into some kind of block form that is safer... You're referring to metal hydrides. Nickel can be used to do that, but all metal hydrides are extremely heavy relative to the amount of energy you get from the hydrogen. If you managed to get one hydrogen atom stored per nickel atom (and I think that's way beyond what's possible) then less than 2% of the total weight would be hydrogen, with the remaining 98+% being nickel.

There's no way you could afford to shovel a hundred tons of nickel into a jet for every 2 tons of hydrogen fuel storage capacity.

Hydrogen as a fuel is one of those things that looks really, really nifty -- as long as you don't know anything about the practical aspects of trying to implement it large scale.
posted by Steven C. Den Beste at 1:52 PM on June 6, 2006

There are electrically powered planes today - mostly UAVs, but some some very large gossamer high-altitude contraptions as well. All have been powered by batteries to date. There are quite a few fuel cell systems that outperform batteries, even when taking into account the weight of fuel tanks and auxiliaries (pumps & so forth).

Lockheed very nearly got the go-ahead to convert and L-1011 to all hydrogen fuel, but the project was terminated due to cost factors. Hydrogen fuel for aircraft, no matter what engine it powers, appears to be impracticable but by no means infeasible, as some commentors have suggested.

But the limiting factor for aircraft power systems that has not been addressed here so far is power output per unit volume. All fuel cell systems that I'm aware of produce a fairly low amount of power per unit volume - so you'd need really big fuel cells to power a plane, even if they weighed a lot less than batteries or engines.

Which is why all those electric UAV's a powered by batteries, not fuel cells.
posted by Jos Bleau at 2:34 PM on June 6, 2006

Oh, and by the way - there are direct carbon conversion fuel cells - no hydrogen involved. They are more energy efficient than hydrogen fuel cells, have a better power-to-volume ration than hydrogen fuel cells, and are cheaper to build and operate. The fuel consist of turbostratic (yes, spelled correctly) carbon particles, which have MUCH better power to volume ratios than hydrogen, and are easier and safer to store. But still, the figures of merit are not enough to make aircraft propulsion viable.

Might change they way your land-power gets made, but yes, the cheapest source of fuel would be fossil, so you don't get the big greenhouse savings like you'd like.
posted by Jos Bleau at 2:49 PM on June 6, 2006

None of the current hydrogen fuel cell technology is efficient for powering a car, let alone a jet engine. I really think unless there's some great advance in biofuels, we'll be riding around in dirigibles and nuclear-powered ships. And there'll be a lot less traveling.
posted by lunalaguna at 3:09 PM on June 6, 2006

Take a (spruce) gander at the Aeroscraft, being prototyped by Igor Pasternak in California. Partially powered by fuel cells, the next-gen bleppelin is designed to be stronger, safer, faster, and more capacious than your Großvati's airship: capable of "whisking" 250 passengers across North America in 18 hours. (Company info page)
posted by rob511 at 6:25 PM on June 6, 2006

Let's just cut out the (fuel cell) middleman here, shall we?

Behold~

The Nuclear Powered Zeppelin, circa 1959!!!!

Now everybody's happy, right?
posted by Jos Bleau at 6:47 PM on June 6, 2006

Maybe to further the direction of my question, what will planes use for fuel assuming oil runs out? Biodiesel jetfuel?

My department is currently testing what we think may be the world's first jet turbine to run on biodiesel (It would be very useful to me to see references if anyone knows different). There are two problems with the use of biodiesel. The first is the technical difficulty, biodiesel solidifies at relatively high temperatures, notably well above the -50C that a commercial jet plane experiences. Since solidification of fuel on a jet flying at 30,000 feet would be somewhat unhealthy this will have to be addressed before biodiesel is a realistic alternative. It's possible some form of additive would allow it to become useful but my understanding is that this is not currently being researched.

The second problem is that of producing the required volume of fuel for an aircraft fleet from a finite amount of land that is already under pressure to produce food and for other uses. Additionally, if biodiesel becomes economic then there will be other fuel users queuing up to take it as well as the airlines, so further pressure on the resource.
posted by biffa at 3:55 AM on June 7, 2006

The problem with "alternate energy sources" is scaling. It's not that they cannot be made to work, it's that they cannot be made to work big enough to actually make a difference.

Biodiesel is an example of that. It's a great idea as a way of getting rid of things like fryer grease from restaurants, since otherwise the only way to get rid of the stuff is to dump it in the sewer. But we're never going to produce enough biodiesel to actually substantially offset our usage of petroleum.

what will planes use for fuel assuming oil runs out? Biodiesel jetfuel? First, "oil runs out" isn't going to be a stairstep event. It'll happen gradually, "peak oil" aficionados notwithstanding.

But what we'll use instead? Hard to say for sure, but the most likely thing is a fuel created from coal through some sort of gasification process.
posted by Steven C. Den Beste at 11:40 AM on June 7, 2006

The problem with "alternate energy sources" is scaling. It's not that they cannot be made to work, it's that they cannot be made to work big enough to actually make a difference.

That might apply to biofuels but in general its rubbish, there is plenty of opportunity for large-scale application of other renewable energy sources.
posted by biffa at 1:22 AM on June 8, 2006

That might apply to biofuels but in general its rubbish, there is plenty of opportunity for large-scale application of other renewable energy sources.

Name one renewable energy source that you think it's feasible to scale up to our current energy demands.
posted by evariste at 1:47 AM on June 8, 2006

Well leaving aside your call to name one source (as the solution obviously requires a basket of technologies) and to do so relevant to all current energy demands rather than 'make a difference' which is the comment I replied to then its pretty obvious that various technologies have considerable potentials.

Wind power can make a significant contribution to electrical production globally and already does so at a national level. It already accounts for over 20% of generation in Denmark, 13% in Germany and 12% in Spain, it is likely to become a considerable contributor in a number of other EU countries in the next 5-15 years, and there is little reason that it could not make an equal contribution in the US and elsewhere. Up to 20% of electricity from wind is economically and technically feasible in Europe.(And that study has probably been superseded by higher estimates since its publication). The majority of wind energy generation currently in operation is onshore and large potential remains to be exploited offshore.

Wave energy and tidal stream technologies have massive potential should sufficient funding for R&D and deployment be attached to their development.

14% of Swedish energy demand comes from biomass, and many countries have decent potentials for use of biomass.

Solar thermal in Greece produced an estimated 1TWh of heat energy in 2001 and has continued to grow since. There is considerable potential for the technology across Southern Europe and many parts of the US, displacing fossil fuels.

Personally I'm not a big fan of PV but a lot of estimates for the future suggest it may be a large scale energy contributor and production & installation is starting to hit high levels in Germany (500Mw p.a.) so its worth mentioning.

There are a bunch of other technologies that have contributions to make.
posted by biffa at 5:46 AM on June 8, 2006