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science:energy: water
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# oceanic MHD

DC electricity from ocean currents
 (+1, -1) [vote for, against]

a moving conductor in a magnetic field will produce a voltage V= ul X B so an equatorial current moving east-west across the earth's magnetic field would produce a top to bottom voltage difference in the ocean. The idea is to float a net a few fathoms down, sink another to the bottom, and harvest the energy of the moving water.

u is whatever the current speed is - single digit MPH. L is the depth of the ocean, or more correctly the thickness of the current - say about 10,000 ft. B is variable but varies from .3 to .6 gauss. so we could use .45 as an average. That gives a voltage of about .306V The current is dependent on the area, but you could cover square miles, (or not).

OH yeah, MHD stands for MagnetoHydroDynamics, and is a known way to produce DC electricity.

 — maniacal_engineer, Jul 07 2005

[daseva, Jul 11 2005]

So what are the values for ul and B? And what current can be drawn? (ie, what is the available *power* here?)
 — Basepair, Jul 07 2005

We have merely begun to suck energy from the waves - I think that a lot of potential before we start worrying about deep ocean currents.
 — DrCurry, Jul 07 2005

True, but if you could get raw electricity without all those dumb bobbing things...
 — Basepair, Jul 07 2005

With a voltage as low as that you'd lose it all in the conductors.
 — elhigh, Jul 08 2005

Nearer the poles, the angle of the flux is nearer vertical, so the angle of the cable would be nearer horizontal - and could be longer.
 — Ling, Jul 08 2005

\\u is whatever the current speed is - single digit MPH. L is the depth of the ocean, or more correctly the thickness of the current - say about 10,000 ft. B is variable but varies from .3 to .6 gauss\\
Err, OK, but mph x feet x gauss is wrong. You have left out some constant of proportionality or are using the wrong units, methinks.
 — Basepair, Jul 08 2005

 I'm lost. How is electricity generated? Is the net conductive? Does the net have conductive ends or something? Where's this freaking conductor you speak of? What's the net for? what kind of net is it? what's it made out of? You speak little of properties.

I think your screwing up current with current. Possibly from drinking too much currant, but who knows.
 — daseva, Jul 08 2005

 [Man] - noble first effort. Help me understand. When you say current up there, since you use mph I guess you mean physical movement of the water.

 Either link me up some MHD reading, describe how MHD works, or (best) describe a conventional example which uses this property so I can appreciate the analogy.

 I understand that if I pull a bar along through a magnetic field, I will induce an electrical current in the bar. Here I would suppose the bar is the water and the magnetic field that of the earth. I am not 100% sure about the nets but Isuppose they would be like alligator clips on the bar.

If all that is right, then I would say you should position the nets in the Bay of Fundy or somewhere similar where tidal forces will increase your flow rate.
 — bungston, Jul 08 2005

I once saw a demo of a simple electric motor - an annular trough containing brine (a conductor), in a magnetic field. An electrical current passed though the brine caused a circulation of the brine. Now, reverse the experiment, cause a circulation of brine in a magnetic field, and generate current.
 — AbsintheWithoutLeave, Jul 08 2005

 [AWL] - how did they pass the current through the brine?

It occurs to me that another place this could be used in in the Panama Canal. The good thing about the canal is one could install fixed magnets at each lock, and so not rely on the puny magnetic field of the earth.
 — bungston, Jul 08 2005

Oh damn, I forgot about the salt. But still, what are the nets for? How will they harness the current?
 — daseva, Jul 08 2005

Yeah, I don't want this to fall of the recent list before someone explains whether the nets could work.
 — bungston, Jul 10 2005

The voltage is already being generated, right now, but I think it must contribute to circulating current. This is because different tidal currents must generate different voltages; maybe in some places there is no voltage (and maybe even opposite voltage).
To make use of the voltage, one needs to get the electrical current to flow along a parallel path of much lower resistance than the seawater return path. A large net would have lower resistance compared to a cable end, and would help achieve the result.
 — Ling, Jul 10 2005

Is [Maniacal Engineer] still in the house? Or was this a hit-and-run posting. I'm intrigued, but this is still a pointless discussion without the numbers (see earlier anno).
It could be that a metre-square net will yield 3 amps at 0.3V (about 1 Watt) which is interesting and perhaps useful.
It could also be that a metre-square net will yield 3mA at 0.3V (1mW), which is interesting but not useful.
Can we please, Maniacal, have the correct equation for voltage and for current, in the right units and with the relevant constants, so we can start?
 — Basepair, Jul 10 2005

 Good plan. You don't need to use a net, surely? You just need any conductor with large surface area to collect that amount of current - remember that to get any useful amount of power at 0.3V you will need MASSIVE amounts of current ([Base] - by "useful" I'm thinking power stations). Something like a cubic version of a car radiator.

To be useful as even a tiny power station it would need to put out about 1MW. Delivered from under the sea that would need cables that could carry 3.3MA.
 — wagster, Jul 10 2005

 I can confirm the voltage is correct (I converted to metric and used 1.78m/s for 'conductor speed'(tidal current), 3000m length, 0.5 Gauss (10,000 Gauss = 1 tesla or webers per square metre). Just multiply them all together to get electromotive force. I found 0.267V.

 The electrical current is much more difficult. I think, if we can get low enough resistance, it will increase until the opposite electromagnetic field created by the electrical current causes a limit. That is, the magnetic field by the electrical current in the wire acts to cancel out the Earths magnetic field in the area of the tidal current. I think that current will be quite large (but I don't know how to calculate it).

 Practically the limit will probably lie in the following: F=B x I x L. If we were to assume a low power output of just 100kW, the current in the cable would be 374,530 Amps, and the force on the cable would be 56,180N (or in American money: 1,212,476 lbs-force).

 The copper cable would be 150,000mm^2, at least, depending on how low the resistance needs to be, and would weigh approx 4,000t. Copper costs about \$3,000/t, so the cable would cost \$12m. Annual income would be (at 10c/kWh (!)) \$87,600, so a minimum payback period of 136 years.....

When can we start?
 — Ling, Jul 11 2005

 / The voltage is already being generated, right now / This got me thinking about fish. Underwater, In open ocean, it is hard to perceive if there is a water current - there is not a visible point of reference which is not being swept along with the current. However, the earth's magnetic field is a stable point of reference. I wonder if the speed and direction of water movement in a stable magnetic field could be determined by the strength of the electrical field generated? Sounds like a college physics problem. I worked really hard for that C in Intro to Physics.

 I suspect that the direction of the generated electrical current would be the same regardless of the direction of the water current - the electrical current generated produces a magnetic field to oppose the earth's magnetic field.

If this principle works, it could be used by open ocean sensors to detect oceanic current.
 — bungston, Jul 11 2005

 //However, the earth's magnetic field is a stable point of reference// Unfortunately it is not. A magnetic field can have no velocity (i.e. it does not follow the rotation of the earth, and cannot be said to be stationary, either).

So why does the idea of the cable work? Because of the relative speeds of the water and the cable.
Tricky stuff indeed. That's why I call it electrickery.
 — Ling, Jul 12 2005

Hah, all the power that is generated would need to be used for cathodic protection...
 — Ling, Jul 12 2005

Ling - ah! Many thanks for the numbers. I like numbers :-)

Bungston - I like the idea of using MHD-generated voltages as sensors (either by fish or otherwise) hugely, and much more than the power-generating idea.
 — Basepair, Jul 12 2005

 sorry for the delay in posting - no hit and run intent, but just had a busy work day ( or days - I am confused) The units work, as someone else shows, but I was too lazy to show all the conversions. The "nets" of which I wrote were envisioned as being metallic and anchored a few fathoms down to avoid problems with shipping. Lings analysis of the size of cable required definitely earns this idea a fishbone, at least until someone comes up with 4deg C superconductivity.

 zannen nagara

 bungston - your surmise is correct.

MHD has been used experimentally with high temp coal combustion. It has also been used for marine "jet engines" by the japanese. It was the basis for the caterpillar drive in "hunt for red october" except that what they didn't say was that the necessary magnetic equipment would drive a MADMAN.... wel mad.
 — maniacal_engineer, Jul 13 2005

Would an MHD turbine be more efficient than a bladed electromagnetic one?
 — omegatron, Oct 26 2006

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