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Oxy-Fuel Submarine

Run engines underwater and not need batteries
  [vote for,

Internal combustion engines require oxygen to operate, which means that diesel submarines must shut off the engines and run on battery power when they dive. This limits how long they can go without surfacing. Nuclear submarines are expensive to build and maintain, and are only practical for the largest submarine designs.

Rockets operate in outer space by carrying oxygen with their fuel. Also, oxy-fuel torches can burn underwater because they have their oxygen supply independent of the air.

A submarine could carry oxygen tanks for the engine to be used in place of plain air. An engine designed to run on pure oxygen would produce several times the power of one that runs on air. This means that the engine would be much smaller, but would have to be able to withstand the stronger forces. The space occupied by the batteries in a traditional submarine could hold this engine and the oxygen tanks.

Exhaust could be vented to the outside, even underwater, as many surface boats vent their exhaust underwater. The higher pressures at submarine depths may require the exhaust to be pumped out, but it would not need to be stored to vent at the surface.

An engine could not interchange between atmospheric air and bottled oxygen due to the differences in compression and displacement. the submarine would would have to use bottled oxygen on the surface or have a traditional engine for use on the surface in addition to the engine for underwater.

crj900, May 14 2015

This has been thought about before http://en.wikipedia...ependent_propulsion
...And no one is making them anymore. Coincidence? [Custardguts, May 14 2015]

Japanese Li Ion Subs http://archive.defe...itch-Sub-Propulsion
[bs0u0155, May 15 2015]

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       See link. Air Independent Propulsion isn't a new idea.   

       Most or all of the combinations/permutations for using internal combustion engines, and carting oxidiser inside the sub have been tried. Some to moderate success.   

       .... The method you propose isn't, I think, new, or novel, and certainly I would think of it as widely known to exist.
Custardguts, May 14 2015

       Near the end of WWII, the Germans produced several experimental designs by Walter, which used hyydrogen peroxide. They had limited success, mainly because hi-test peroxides in a confined space are a Bad Thing. The British followed up the idea in the 1950's, again with limited success. The idea was obsoleted by the advent of seagoing fission powerplants.   

       It might have some application for subs too small to house a reactor, but as stated it is both Baked and Widely Know To Exist, at least among those aquainted with submarine technology.
8th of 7, May 14 2015

       For the HalfBakery, this Idea should be about using a nuclear reactor to generate electricity to desalt and then electrolyze seawater, to obtain the oxygen that can then be used in a combustion engine to move the submarine.
Vernon, May 14 2015

       ^, but I was thinking fuel cell : on the surface, run the fuel cell on a hydrocarbon and the atmosphere, to electrolyze water into hydrogen and oxygen, then submerge and run off the hydrogen and oxygen. Figuring 65% efficiency of electrolyzation, and 50% efficiency for a fuel cell running on pure h2 and o2, that's not too bad.   

       Or you could run a syngas method to turn a hydrocarbon into hydrogen, and extract (mostly) oxygen from the atmosphere through pressure-swing absorption, which would probably be more efficient.   

       potato, tomato.
FlyingToaster, May 14 2015

       <Air independent propulsion isn't a new idea> Sorry, failed to do adequate research beforehand.
crj900, May 15 2015

       // failed to do adequate research beforehand. //   

       A Mortal Sin, which is rarely, if ever, forgiven, and NEVER unpunished ...
8th of 7, May 15 2015

       hmm, diesel electric submarines are still being built. In fact their low cost relative to nuclear submarines offers advantages. Mainly, that you can go through several generations and incorporate all the latest kit, meaning you have a much more up-to-date sub in most areas, if not propulsion.   

       Anyhow, the traditional diesel-electric submarine is a fairly staid beast. Marine diesels, lead acid batteries, electric motor, done. The energy density of Lead acid is awful, 0.17 MJ/kg or 0.56 MJ/l depending upon which is more important in subs, not sure. Why not go for broke and do some water electrolysis? The energy density of compressed Hydrogen is waaaay better at 142 MJ/Kg. Now you'd have to store the oxygen as well, but it's still way ahead. So marine diesels, electrolysis, compressed gas tanks, fuel cell, electric motor, done.
bs0u0155, May 15 2015

       // compressed Hydrogen //   

       Adiabatic losses ?   

       // electrolysis, compressed gas tanks, fuel cell //   

       Electrolysis, OK. But not outstandingly energy-efficient. Compressed flammable gas on a submarine ? Avoid if possible. Fuel cells ? Not a particularly mature technology.   

       Lead-acid cells may have an awful energy density, but they are very simple and well understood (and reliable) devices. And they pretty much package up the electrolysis/gas production/fuel cell in one unit.   

       Anything you are intending to take with you when you seal yourself in a hermetic chamber and submerge under fifty metres of water needs to be five-nines reliable, simple to repair, and duplicated or preferably triplicated. It's no coincidence that diesel-electric boats still have twin propulsion trains.
8th of 7, May 15 2015

       hmm, H2 at industry standard compression is 4.5 MJ/l, you'll need half the volume again for O2 so, that's 3 MJ/l ish. 60% efficient electrolysis and 70% efficient fuel cell. Ignoring the fuel cell and electrolysis gear, 'cause I don't know how big they are, gives you about 2.6 MJ/l. Which is 15 fold better than batteries. Hell of an advantage.
bs0u0155, May 15 2015

       Aluminum would be a great fuel for a sub of this sort: once the reaction got up to heat it could strip oxygen away from water.
bungston, May 15 2015

       So will carbon, vide the water-gas/produce-gas cycle. Sure you can split water, but you need to desalinate it first.   

       Consider: you are in a hermetically sealed container, outside of which is sea water, available for cooling, so you can reject heat easily. There is a requirement to maintain a habitable environment in a useful proportion of the volume (Oxygen, CO2, CO, no toxic gases, bearable temperatures).   

       Your resources are limited to what's inside the hull when the hatch is closed, plus unlimited free brine.   

       Devise a mechanical, chemical or electrochemical system to provide reliable life-support and propulsion.   

       Answers on the back of an envelope, please. Write in ink, via a pen if at all possible (green crayon is acceptable if your current institution will not permit you access to sharp objects). Show all rough working.   

       Turn over your papers now.
8th of 7, May 15 2015

       No, you'll have to wait until lunchtime now. No snacking between meals.
8th of 7, May 15 2015

       Yeah, not a lot of downsides to lead acid, since you need ballast to make the sub sink anyway. An improvement in energy density in terms of kWh/kg probably isn't important for a sub. An improvement if term of volumetric density (kWh/L) would be worth pursuing.   

       Lithium-Ion provides a huge improvement in kWh/kg, but also provides reasonable improvement in kWh/L, so might be worth considering now that larger batteries are becoming available They still have a significantly higher risk or exothermic self-destruction, especially when damaged.   

       Question: is it true that some submarines use their lead-acid battery banks strategically placed to increase survivability in case of a torpedo strike?
scad mientist, May 15 2015

       //Lithium-Ion provides a huge improvement in kWh/kg, but also provides reasonable improvement in kWh/L, so might be worth considering//   

       The problem with fully baked thinking like that is that other people do it <link>
bs0u0155, May 15 2015


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