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Deep sea compression

A factory for compressing gas at sea
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A factory for gas production at sea could save costs of a compressor and the energy needed for compressing with the following simple apparatus.

An anchored trolley with a circular clothes-line-like rope at sea allows you to lower and then pull back up canisters of gas. The canister is held upright by the rope and by weights, so the compressed gas is trapped on the upper part of the cannister.

During the canister's way up, expanding gas leakage is stopped by a simple pressure valve that closes when the pressure inside is greater than that outside.

To avoid loss of gas desolving into the sea water or to avoid having any sea water in the resulting canister a locking pump could be used, that would compress the air as the canister goes down, and then locks (with a rachet like device) in place once the canister is back on its way up.

Presumably, the cost of pulling the weighted canister with neutral buoyancy down with the rope and then pulling it back up will cost much less than the energy needed for a gas compressor.

pashute, Mar 19 2018

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       Brilliant !
DDRopDeadly, Mar 19 2018
  

       Hang on a moment.   

       The minimum amount of energy you need to put into the system, in order to compress the gas, is the same regardless of how you do it. So, the only question is whether the motor that pulls the canisters down is more or less efficient than the motor that drives a compressor.   

       Plus, compressing a gas will generate heat. In a land- based compressor you could (at least in theory) recover that heat energy to make the system less inefficient. In your deep-sea system, presumably that heat will be lost to the water.   

       Also, [pashute], you need to think about bouyancy. Your canisters will be much less bouyant after the compression, so you will need to put in a lot of work to lift them back to the surface. That's where you are spending the energy budget.   

       Bottom line: this system won't be more efficient than a regular compressor.
MaxwellBuchanan, Mar 19 2018
  

       This sounds suspiciously like an attempt to make an over-Unity machine, which is against the law (Laws of Thermodynamics).   

       If the container at the surface has neutral buoyancy, then at full depth- with the gas compressed- it will have negative buoyancy. You then have to do work to raise it back to the surface against gravity.   

       Add in turbulent drag, and mechanical losses in the transport mechanism, and you'll never recover the energy input to the system from the compressed gas.   

       [suggested-for-deletion] Over-Unity, not a new idea, bad physics, bad harcut, sexist, racist, ageist, bad, naughty and wrong.   

       Oh, and everything [MB] said.
8th of 7, Mar 19 2018
  

       The downwards-travelling canisters won't be neutrally bouyant for their whole journey. They'll act like Cartesian divers and their density will increase as they travel to the bottom.   

       This process won't reverse itself on the way back up, because the ratchet / pressure valve will prevent it. The canisters will remain at their most dense all the way to the surface.   

       So all the floatiness happens on the way down, where you have to work against it, and most of the sinkiness happens on the way back up, where you have to work against it again.   

       I'd hazard a guess that this will be just as much work as running a compressor - even over and above the friction / drag losses you'll encounter moving through seawater.   

       And the constant need to scrape all the limpets off it.   

       Oh, and everything the other two said. Except the bit about the bad haircut.
Wrongfellow, Mar 19 2018
  

       I think the bad haircut bit was probably right.
MaxwellBuchanan, Mar 19 2018
  

       Of course, if anyone were actually dumping lots of dense material at sea, it could be used as a counterweight to drag down the gas bottles, which would have to be sufficiently bouyant after compression to return to the surface. In that case, the gas compression becomes a way to recover the energy otherwise lost by dropping stuff down to the seabed.   

       It might be worth talking to the Mafia, or the National Association of Primary School Teachers. When they send weighted corpses to sleep with the fish, each one could probably carry a gas bottle down with it.
MaxwellBuchanan, Mar 19 2018
  

       Our understanding is that the Mafia don't do that any more.   

       So you'll just have to hope that the primary school teachers can take up the slack.
8th of 7, Mar 19 2018
  

       Instead of cylinders, why not use a continuous hose, and pump the gas down via a series of non-return valves, with it returning to the surface at ten times the pressure? ha (I know why but won't tell, allowing the clever clogs who work for the likes of Exxon to waste trillions of dollars stealing the idea only to find out the hard way that you can't have your gas and heat it)
xenzag, Mar 19 2018
  

       //Our understanding is that the Mafia don't do that any more.//   

       I think that's what they want you to understand.
MaxwellBuchanan, Mar 19 2018
  

       // you can't have your gas and heat it //   

       Cake. It's cake, as in "you can't have your gas and cake it"
8th of 7, Mar 19 2018
  

       Damn it struck me as a great idea .. what about a work around? Move the whole of civilisation to the ocean bed and then set up the business?
DDRopDeadly, Mar 19 2018
  

       I'm interpreting this Idea somewhat differently than other folks here. The sea is not actually needed --only a deep hole is needed (but might be easier to construct if it was in the ocean, a hole lined with steel down to the bottom of the Mariana Trench, for example).   

       Anyway, gravity being what it is, it should be obvious that the air pressure at the bottom of the hole will be rather higher than the air pressure at ordinary sea level. Using a pulley system and a very long cable, we lower an air tank (valve open) to the bottom of the hole. Obviously the air pressure at the bottom of the hole will flow into the tank.   

       Now we remotely close the tank valve. Then we lift that tank out of the hole, while lowering another empty tank (both will have mostly the same mass; one has the extra mass of contained pressurized air, of course).   

       Net effect: filling the tank with compressed air only takes as much energy as lifting the weight of the compressed air in the tank, from the bottom of the hole (with a bit extra to account for friction losses in the pulley system).
Vernon, Mar 19 2018
  

       You don't even need a hole. You can build a tower and have the same Thermodynamics ...   

       <muttering>   

       "Heat engine, Carnot cycle, adiabatic expansion, entropy, General Gas Law, frictional losses, viscous fluid.... "   

       </muttering>
8th of 7, Mar 19 2018
  

       A giant wheel
pocmloc, Mar 19 2018
  

       ... on frictionless axles.   

       In a vacuum...
8th of 7, Mar 19 2018
  

       I detect a certain amount of kneejerk reactions to an assumed number of hammers - none of which have been proven to exist.   

       Fact : a rigid tank which skin is composed of a heavier-than-water substance will sink when it's full of water.   

       Fact : no matter what substance you choose to make the tank out of - steel or Stilton, it can be sized such that it rises when a certain amount of depth-equalized compressed gas flows up into it. (The cheese tank will probably explode after a certain altitude unless the stopcock's open, so don't do that).   

       In both cases the density of the tank (as a whole, including contents) doesn't change. Nor does that of the water.   

       So, in both cases, energy can be taken off during transit.   

       So, energy can be taken off in both directions. Which has little to do with the post, just sayin'.
FlyingToaster, Mar 19 2018
  

       // there's no change in buoyancy at any depth, //   

       Yes, there is, but the effect is very small, since water is not completely incompressible, but only nearly so. Also, the crush depth of the vessel must not be exceeded.   

       // such that it rises when a certain amount of depth-equalized compressed gas flows up into it. //   

       Do you mean a sealed or an open vessel ? We presume the subsequent mention of "stopcock" means an open vessel.
8th of 7, Mar 19 2018
  

       What you appear to be describing, [FT], is a submarine.
MaxwellBuchanan, Mar 19 2018
  

       //the crush depth of the vessel must not be exceeded.// If it's open on the way down there's no problem. There wouldn't be, anyways, but just to be pedantic...   

       //Do you mean a sealed or open vessel// Sealed on the way up of course, the point of the exercise being to fill the container at a high ambient pressure, then keep it that way.   

       //submarine// nope : one of the defining characteristics of such a worthy endeavour is that air pressure remains sea-level'ish, inside.
FlyingToaster, Mar 19 2018
  

       Of course what I've totally failed to notice is that this isn't the electrolysis post... umm... carry on.   

       [mfd - bad science] the energy required to pull the canister up is the same as any harvested from the canister on the way down plus that of the compressed air.   

       Of course, if you happened upon an 82km deep hole in the ground and happened to have 164km of rope handy, you could save yourself the trouble of building a pump for your 250atm scuba tank.
FlyingToaster, Mar 19 2018
  

       [FT], I was thinking of ballast tanks. And so are you now.
MaxwellBuchanan, Mar 19 2018
  

       You would be able to get away with a mere 82 km of rope, shirley ?   

       However ...   

       Your putative tank-filling system requires a length of "rope" capable of supporting its own weight plus the weight* of the air tank.   

       Your homework is to calculate the minimum necessary diameter of carbon nanotube filament line to perform the stated task.   

       *The weight of the air tank will diminish proportionately as it descends, gravity being lower at locations closer to your planet's centre. You may' however, omit this factor from your calculations, and disregard the mass of gas** within the tank.   

       ** which will also change due to pressure fractionation of the constituents.
8th of 7, Mar 19 2018
  

       WIFRT, I was picturing gas which had only just been extracted from sea-bed deposits. In that case, the idea would be to bottle it in situ (a bit like wine - mis en bouteille à la propriété).
pertinax, Mar 19 2018
  

       I concede [Max]'s point. The "bad haircut" charge stands.
Wrongfellow, Mar 19 2018
  
      
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