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Running a petrol generator underwater with compressed air tanks

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Andy

Sun Mar 29 2015, 04:09AM

Registered Member #4266 Joined: Fri Dec 16 2011, 03:15AM
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Posts: 874

Dont know weather this should be in the chemistry part, but some numbers on a idea.
NaClO4 makes O2
Diesel = C12H23

168 grams of diesel burnt in a serperate step with oxygen from above make 8.7kg of Sodium chloride
if the specific gravity equals one, one liter at 4kwh for a Lister engine needs 51.8 kg of sodium chloride.

The boat/sub as 0.81m2 area(to water line) so at 3m/sec is 6014Watts, with losses is about double the input fuel and genny.

The thing mentioned might work Bjorn, what would the Lister engine behave like, they normal can take a hit well.

Edit

Dr. Slack

Sun Mar 29 2015, 08:44AM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

nzoomed wrote ...

I dont know if im missing something here, but how did they do it with the early diesel submarines?

big air tanks, big boat, so lots of space for them, short submerged duration, and low submerged speed requiring lower power, some used liquid oxygen to store more oxidiser in a smaller volume. At low depths, exhaust could be vented into the sea directly. At larger depths, or for stealth diesels (something of a contradiction in terms) sea water was brought into the boat through a turbine to reduce the pressure, exhaust CO2 was dissolved into it, then the sea water pumped back out, using the incoming turbine to provide most of the power for the pump.

I think the lesson of the 168g diesel versus 5.4kg sodium chloride could be better expressed as diesel requires two orders of magnitude more mass of oxidiser if the oxidiser is sodium chlorate. It would be interesting to see how much oxygen can be obtained from 10kg of sodium chlorate, or 10kg of diving cylinder charged to 300 bar. With a snorkel, it requires zero times the mass of fuel. That zero multiplier could be very significant.

While in principle there's no reason that an engine should stall even if working into considerable back pressure, in practice, COTS engines are designed to work into one bar back pressure. Working into higher back pressures may well upset gas flows and nice tweaks made tot he engines over years of development, that a COTS engine will probably take a huge hit in efficiency if working into much more than one bar. It should be possible with development to run an engine into higher pressures, but that's not a readily available engine.

If you already have an air input snorkel, then supplying an exhaust output snorkel would allow the engine to run at one bar input and output presures. Just a thought.

hen918

Sun Mar 29 2015, 06:49PM

Registered Member #11591 Joined: Wed Mar 20 2013, 08:20PM
Location: UK
Posts: 556

From what I've read, submarines didn't have air tanks at all, except for the crew's breathing supply. Whilst the sub was surfaced, it used a diesel engine attached to the prop, as well as a generator for charging MASSIVE lead acid batteries, (the sort found in electric fork lifts). The batteries doubled as the ballast for stability and were used for driving the prop via a DC motor whilst the sub was submerged. The engine had snorkels for the exhaust as well as the intake.

GrantX

Mon Mar 30 2015, 05:26AM

Registered Member #4074 Joined: Mon Aug 29 2011, 06:58AM
Location: Australia
Posts: 335

hen918 wrote ...

From what I've read, submarines didn't have air tanks at all, except for the crew's breathing supply. Whilst the sub was surfaced, it used a diesel engine attached to the prop, as well as a generator for charging MASSIVE lead acid batteries, (the sort found in electric fork lifts). The batteries doubled as the ballast for stability and were used for driving the prop via a DC motor whilst the sub was submerged. The engine had snorkels for the exhaust as well as the intake.

Yeah, the Australian Collins-class subs use a 5.4MW DC motor for propulsion, with battery banks and diesel motor/generators.

Andy

Fri Apr 03 2015, 03:05AM

Registered Member #4266 Joined: Fri Dec 16 2011, 03:15AM
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The goal is to trying and go without a snokel, thought about a themite reaction, in little pellets which heat up oil, and themo piles, convert to electricty.
It should have the energy density.
Does anyone have thoughts down that path or another idea, for more freedom from cables.

Sulaiman

Fri Apr 03 2015, 05:24AM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

ultimately you have the same requirement as 'spaceships' ... energy storage density.
I think liquid O2 and H2 are current 'best'.
Fuel cells for electricity/propulsion and fresh water like in space?

how much the crew may enjoy the idea of huge tanks of liquid O2 and H2 I don't know,
some seem comfortable with a nuclear reactor on board.

Patrick

Fri Apr 03 2015, 06:14AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

well i guess a 17,000$, 6 pound fuel cell, at 1kw is out of the question. Though it solves most (or all) of your problems. Sodium Borohydride is becoming available by some of the larger makers.

There are nitro / alcohol / glow engines that could make a generator as -- Im currently trying out. The problem is there easy to overwhelm and just quit. They have such light but fast RPM, those little pistons just stall with load. Nitro adds way more oxidizer once decomposed in the cylinder. however you have big problems in so many directions.

What dimensions, duration and mass is your machine expected to come in at? I might be able to make some fiberglass parts. though high pressure containers could be killing devices... the scuba tank might be better. but custom fits to carry them i could do, as well as the body parts. The cylinder with a this 24 ounce FG cloth, would make your body the same diameter as a scuba cylinder, maybe a wire tunnel too.

Once i start my robotics company, ill be doing more than just aerial drone work. Uzzors2k here on the forum, has some umbilical subs on his personal page. Not sure what your planning.

Andy

Fri Apr 03 2015, 07:19AM

Registered Member #4266 Joined: Fri Dec 16 2011, 03:15AM
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Posts: 874

The thing at the moment will be 5 meters by 2 meters by 1.8 meters high, single seater at present, but would like a two seater later.
Duration hopefully over one hour, I havnt run the structal weight or strength at present, assumed a rought esstimate at 4.5 tonnes, but can get it increassed to around 6-7 tonne, with a different door postion.
Will look into Nos and alochol, at present running some numbers on copper sulphate, magnesium and aluminium, with sulfuric acid to start the reaction, and a pallet load into a copper heat sink, submissered in oil as the heat storage.
Pretty much, if I cant get a decent underwater time with a power source it will put a break on the thing.
Dont mind having a $700,1000 refuel if its at one hour, but $70,000 for batterys is out.
Thanks for the offer about parts, but need tests proformed for structual strength, for latter.
Interested in the underwater berings, have you found a way to keep the motors cool without airflow?

Carbon_Rod

Fri Apr 03 2015, 07:20AM

Registered Member #65 Joined: Thu Feb 09 2006, 06:43AM
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Posts: 1155

Diesel electric hybrids have driven massive trains, and some of the earliest submarines.
This method is still the most practical design for economic transportation models.

There have also been mono-propellants around for 60 years, but they are dangerous to handle given the chemical volatility. Every few years one will learn of a forgotten torpedo undergoing catastrophic rupture inside a submarine. IIRC, the last group unlucky enough to have this happen was a Russian crew in the 1990s, and there were no survivors.

@Patrick
Robotics do not sell very well, are highly prized targets for "problem entities", and are often cloned within months. You will start to notice supply shipment boxes sporting yellow-tape from Asia, TSA tape from the USA, and items exempt from duty thoroughly inspected by customs anyway. Even if you are a legitimate tax paying company, the random crushing and theft of parts will continue as part of the business model.
I'd recommend against starting a company like WowWee, as they were a rare success in a hostile market sector.
Watch out for export restrictions, as Elon Musk proved that not filing patents is sometimes more effective at containing trade secrets.

Dr. Slack

Fri Apr 03 2015, 08:57AM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

Andy wrote ...

The goal is to trying and go without a snokel

Sulaiman wrote ...

ultimately you have the same requirement as 'spaceships' ... energy storage density.

That's pretty much the entire argument in those two quotes. You know just as well as the next man that there aren't many options for energy storage. The question is, which system's drawbacks are you comfortable living with. If I am going to put myself in a box, and then cover myself with 20m of water, I'm not sure any of the combustion options would make me feel all that comfortable, especially if to get the oxygen for it required some interesting exothermic chemistry.

But it's not quite energy density that's the issue as for spacecraft, in which lift mass is expensive. In subs, mass carrying ability is cheap, so you need good energy storage per volume, or per dollar.

Do the sums for shaft kWh out per cubic metre of volume, or $1000 expenditure, whichever you feel is the more important. Amongst the options are lead acid, LiPO or NimH driving PM or more advanced motors. Another to consider is a 300 bar scuba cylinder driving an air turbine or reciprocating expansion engine of some type, no combustion involved. That would be very clean and relatively safe. I reckon you could have fun designing and building a water-payload potato-cannon sort of thing to fire water out of the back, using the high pressure compressed air directly. With a multi-barrel design appropriately phased, you could have reasonably steady thrust. I wonder if you could have different sized barrels and operate the set at different pressures, so making best use of the high pressure available directly from the cylinder.

I was astonished when I did the sums for specific energy density for a hammer system for my robot, that rubber beat out air and steel for springs by a long way. Do the sums for 100kg of aeroplane elastic just for the sake of completeness, though I doubt that with your constraints it will figure very highly. My key constraint was very high specific output power with a delivery time of 50ms, which is rather shorter than your (I guess) 5ks.

And then finally do the sums for oxidiser/fuel systems, but do remember to figure in the containers and the safety systems for handling the oxidiser, and the asphyxiant gas the engine produces.

Don't forget to trim your power requirement down by having your top speed target as low as practical, and your energy requirement by similarly limiting your cruise speed. It's worth considering top and cruise speeds seperately, it allows you another degree of freedom, you can fight a strong current for a few minutes, and still cruise out to the fishing grounds and back.

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