Closed Cycle Hydrogen Fuel Cell Question

[davester65]

I seem to remember way back in the spaceship design rules for 2nd edition GURPS Space their was an optional rule whereby you could have a version of the Hydrogen Fuel Cell that cracked its own hydrogen and oxygen from water rather than having to carry liquid hydrogen and oxygen. Does anyone remember the stats for it? I'd like to stat it out for GURPS Spaceships, but my copy of 2nd ed' Space is long gone. In a nutshell it was supposed to provide slightly less power, but be much more efficient in its fuel use than a standard cell. I figured it would be a good choice for fighters and other relatively short endurance spacecraft. Thanks.

[Anaraxes]

My copy (maybe not 2e) mentions options for a fuel processor to crack water, as well as the possibility of extra tankage to store the water produced by the fuel cell, avoiding the need to seek out a source of water for refuelling. The process involves a 10% loss of energy; you need some other power plant to do the cracking.

Which is necessary, of course. A fuel cell that could crack water using the power it got from combining the hydrogen and oxygen, plus some extra power, would be a violation of thermodynamics.

That secondary powerplant could also have a smaller output than the fuel cell, at the cost of running it longer. You could produce 1/10th the power for ten times as long, "charging up" the fuel for the fuel cell which expends it faster. Might be useful for combat and short-endurance ships. But, it seems like they'd leave the cracking to their carrier or ports, and save the expense of carrying the secondary power plant.

[davester65]

I found the rules online that I remembered. What you said was what I remembered.

Originally Posted by Anaraxes

Which is necessary, of course. A fuel cell that could crack water using the power it got from combining the hydrogen and oxygen, plus some extra power, would be a violation of thermodynamics.

By thermodynamics, do you mean the whole perpetual motion thing? The idea I had was that there would be some loss each time the water was reused, so that it would eventually run out, but that it would still use fuel more efficiently. Forgive me if I'm off, my knowledge of science isn't too detail oriented.

Edit. Okay, I reread the rules and it requires more power to produce the fuel than the fuel cell puts out in energy. Like you said. Guess it just took a while to sink in. It was still a nice idea though.

[Ulzgoroth]

Originally Posted by davester65

By thermodynamics, do you mean the whole perpetual motion thing? The idea I had was that there would be some loss each time the water was reused, so that it would eventually run out, but that it would still use fuel more efficiently. Forgive me if I'm off, my knowledge of science isn't too detail oriented.

Related to that, and also to the 'conservation of energy' you may have heard of at some point.

What you're suggesting is flatly impossible. The energy extracted from a fuel cell comes from turning hydrogen and oxygen into water. Turning the water back takes the same amount of energy as making the water released. Except that both steps aren't perfectly efficient, so if you ran it forward and backward that way you'd wind up using up all the energy you got and not having enough to crack all the water to get back to where you started.

[davester65]

Originally Posted by Ulzgoroth

Related to that, and also to the 'conservation of energy' you may have heard of at some point.

What you're suggesting is flatly impossible. The energy extracted from a fuel cell comes from turning hydrogen and oxygen into water. Turning the water back takes the same amount of energy as making the water released. Except that both steps aren't perfectly efficient, so if you ran it forward and backward that way you'd wind up using up all the energy you got and not having enough to crack all the water to get back to where you started.

Yeah, that eventually sunk in. Thanks.

[malloyd]

Originally Posted by davester65

Yeah, that eventually sunk in. Thanks.

Mind you while I think GURPS has always gotten it right, there have been science fiction games that messed it up.

I've seen rules that assume fuel cells run on water for example, and there are plenty of gullible people apparently believe that you can run an engine on water (and more think watering down your gasoline works) in the real world. So I wouldn't be too surprised if you had seen something like that in print somewhere.

[Fred Brackin]

Originally Posted by davester65

The idea I had was that there would be some loss each time the water was reused, so that it would eventually run out, but that it would still use fuel more efficiently. .

Unavoidable losses (or material neverminding the thermodynamics thing) will come after you crack the water and try to hold on to the hydrogen. Hydrogen seeps out of any container you can make in a period of days. Its' molecules are just too small.

They won't just seep pas the gaskets. Under some circumstances they'll seep through the walls. This is one of the reasons the Space Shuttle had an umbilical continually topping off the external tank that was only removed 3 minutes before launch.

The first 230 AD ship supplement I saw had civilian ships powered by a mix of closed fuel cells and solar power. The ships would fly between stars on the fuel cell and then crack all the water produced by the cell when they'd reached a close solar orbit. Then they'd run on the stored H2/O2 on the next FTL run.

It didn't violate thermodynamics but it was operational gibberish. Stored hydrogen is a temporary resource.

There are chemical power plants in Spaceships running on largely unspecified fuel. They could be used by fighters with a 6 to 12 hour endurance on one tank of fuel and might be used for such roles for no other reason that they are cheap.

If you wanted to recycle the spent fuel you'd have to have another fuel tank for the water, another slot for the fuel processor and one more slot for the energy source for the processor. this is nothing like "efficient" but could e required for someone on the ragged edge of minimal spacegoing technology.

[Ulzgoroth]

Of course while we're talking about fuel cells and realism, we should note that fuel cells realistically don't produce enough power for Spaceships power points. Allowing them to is an acknowledged fudge.

[benz72]

Pressurized gas storage of H2 is definitely a problem, but Hydrogen can be stored if it is chemically bonded to something else for the duration and then unbonded and reassembled into H2 for use. Of course extra processing steps consume energy (reducing overall cycle efficiency) and require equipment (weight, expense, maintenance). The appeal of a closed cycle is that one can use external power inputs (solar? RTG?) to regenerate fuel rather than hope one can find it in space.
The question becomes, is the extra headache of recycling worth the effort.

[Fred Brackin]

Originally Posted by benz72

Pressurized gas storage of H2 is definitely a problem, but Hydrogen can be stored if it is chemically bonded to something else for the duration and then unbonded and reassembled into H2 for use.

We went all through this when playtesting Gurps Blue Planet trying to give some credibility to that setting's hydrogen-based vehicles.

Metal hydride totally removes hydrogen's main virtue which is light weight compared to energy content.

The best durable prospect we could come up was a blue sky proposal to store hydrogen in tanks filled with carbon nanotubes and the fellow working n that had a sort of vague hope of hitting a density of 2/3rds cryogenic liquid hydrogen. Nothing's come of his proposal in the dozen years since that I've heard.

Eventually we gave up and the vehicles in G:Blue Planet use cryo H2 with some sort of hope that the technical issues had been overcome. G:Traveller ignores the problem all together.