# Regenerating Fuel Cell

I'm working on a powered exoskeleton design for my universe and I had an idea about how to power it.

It works in this cycle:

1. Water enters an electrolyzer powered by a massive betavoltaic battery and is split into hydrogen and oxygen.
2. The hydrogen and oxygen are run through the fuel cell to create power, releasing water to be put back into the electrolyzer.

So as long as the betavoltaic battery makes up for the energy loss it could create a regenerative cycle that could power the exoskeleton for many decades or at least until the fuel cell wears out or the betavoltaic battery runs out.

Would this be feasible?

• Why?! Each transformation is lossy. If you need electricity, why won't you just use electricity generated by betavoltaic battery? Why are you adding additional steps, each creating its own losses to the system? I assume you wanted to solve some problem with it, but without knowing what problem would that be assessing feasibility is hardly possible. Mar 4, 2019 at 11:33
• Jules Verne, Five Weeks in a Balloon, 1863. The balloon was powered by a battery electrolizing water followed by burning the hydrognen; it was good enough for a young adult novel published in 1863. Mar 4, 2019 at 11:55
• @AlexP Well that works because a hot-air balloon is based on the concept of, you know, hot-air, which is less dense and a buoyant force exceeding it's weight. And flying like a hot-air balloon but in a exo-skeleton suit diametrically opposes classy and fun. Mar 4, 2019 at 12:55
• @VaradMahashabde: It sort of works, except that the process throws away two thirds of the energy of the battery, instead of using it directly to produce heat... Just like the question throws away two thirds of the energy of the battery instead of using it to power the exoskeleton. Mar 4, 2019 at 13:24
• @AlexP Using hydrogen directly, and heating it (and throwing away oxygen), you get a clever (and very flammable) variant of the Rozière. Assuming a magical future battery, of course.
– Eth
Mar 4, 2019 at 15:38

## 4 Answers

Yes and no. As pointed above is not the most efficient method but on the other hand...

Betavoltaic batteries are not very efficient in terms of output, so probably you cannot power the android on them alone. Massive, after all, would probably mean "too massive to load into an exoskeleton", as there is a good reason betavoltaic haven't seen much use beside buildings (space probes don't use betavoltaic, is more like a peltier effect on a very radioactive source; you don't want this on your mech). In this case, the hydrogen cell would be like an accumulator that drives extra energy into the exoskeleton (and that could be refueled externally before use), the betavoltaic acting like an auxiliary power source that while it's not in use can regenerate small amounts of fuel, not enough to make a difference in action but let's say you find yourself stranded from the group, enough to ensure that you can go back to the base if you let it work overnight.

For any of this to be useful, it would make more sense if your exo has access to plenty of water (let's say a very humid environment) so the system can get water from ambient via condensation, instead of a closed system. A purely closed system would probably have less problem using batteries or supercondensers, unless the water or the hidrogen is used for something else (exos are equipped with hydrogen torches, or they have rockets, the water is used in life support systems, etc).

So, without having much information about the context where they will be used I will say that (sorry for yodaspeak) feasible... it is, sense does not make much.

If you provide better context I may be able to expand the answer with better information.

• This is the answer. The battery cannot supply the energy density needed for suit operation and so must accumulate hydrogen, which can provide the needed density. Mar 4, 2019 at 20:20

Hydrogen fuel-cell cars, an alternative to both fossil-fueled and electric vehicles, are use the hydrogen gas to power an internal combustion engine. A great advantage it has over electric cars is that there no need for big, bulky, expensive batteries that can be easily damaged, or high-torque electric motors which are also costly, instead carrying hydrogen in a pressurized tank. This also means static hydrogen plants, which are guaranteed to be more efficient by virtue of static infrastructure, can be used to fuel HFCs just like petrol plants do currently.

Clearly, carrrying the water AND the battery to make the hydrogen is pure wastage. And there are many statements for the Second Law of Thermodynamics, one of which is no process fully converts input energy to output energy, with no heat waste.

Weeeeeeelllll, this doesn't mean the exoskeleton should be scraped. After all, this is a story, so you may as well use the hydrogen for fireblasts or something. could also make the person OP underwater.

Would this be feasible?

No, it makes little sense.

For starters, if you have an energy source, any intermediate step you add between the source and the user simply lower the net efficiency.

To use some number, you have two steps: betavoltaic to hydrolysis and then combustion cell to user. Imagine each of the step has a 90% yield. The net yield will be $$0.9 \cdot 0.9 = 0.81 = 81$$%. And, before you object to it, no process can have a 100% yield.

It's like paying good money to buy apples from the supermarket when you are growing those very apples in your own orchard.

Your battery produces electricity and your user consumes electricity, so just lay a couple of wires between them.

• I am simply using the battery as an auxilary source to fuel the electrolysis of the water to use the hydrogen and oxygen in a fuel cell to get power, and then using the water to be re-electrolyzed to continue the cycle. The betavoltaic battery isn't doing anything other than recovering the losses within the cycle so the fuel cell can create more power. Mar 4, 2019 at 20:10
• @Efialtes The trick is there is losses from powering the suit too, and the battery has to be supplying more than that, so why not just use the batteries directly.
– John
Mar 4, 2019 at 20:18

Redox Flow Battery

Do you know about flow batteries? Right now, one that could power your exoskeleton might need to be towed behind you in a caboose, but it is the kind of tec. I think you are considering, and they are improving with research.

They recharge infinitely.

• hmmm.... i will look it up, however when working on worldbuilding please try to add more info and sources with your answers. I fell into that trap (and still do) many times and got called out by mods to no end. Mar 4, 2019 at 21:49
• Well, I did some googling and it seems redox flow batteries are great since they are very hardy and are less prone to degradation than other batteries, but they currently have a much lower specific energy (energy per unit of mass) than lead-acid batteries. And they are reversible, the thing is they behave more like conventional rechargeable batteries and don't help much when it comes to creating a regenerative fuel cycle. I may be wrong because of course my sources are wikipedia, thats just my 2 cents. Mar 4, 2019 at 22:02