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I propose building a submersible geothermal powerplant and sinking it on an existing underwater hot spring (ie geothermal vent). To my (and Google's) knowledge, using undersea geothermal vents for power is a new idea.
To withstand the high external pressure from the water, the powerplant/submersible
will be remotely-controlled from the surface with the same controls used for deep-sea scientific probes. The liquid and gas inside the generator turbine would be at pressure close to the surrounding pressure at the seafloor. The turbine would use a working fluid that boils at seafloor pressure and about 400 C. Thus, all systems will either be very strong or will have internal pressure close to external pressure.
Efficiency calculation: Water emerging from hydrothermal vents on the ocean floor is often 400 deg C. The surrounding water is about 1 deg C. A heat engine (ie steam turbine) can be used to generate electricity from this temperature difference. The maximum theoretical thermal efficiency given the source temperatures is 59% (n=1-Th/Tc). Assuming 90% turbine efficiency and 50% transmission losses in the seafloor power lines, the system can use 26% of the heat energy in the hot water from the undersea vent.
Electricity would be carried back to shore along an underwater high-voltage DC cable. The power cable would be over 200mi long, depending on the location of the vent. Undersea DC power cables are currently used to deliver electricity to islands and offshore oil rigs.
Regions with electricity shortages and offshore hydrothermal vents include Japan and the west coast of the United States.
To increase power output, blasting or drilling might be used to increase the water flow from the hydrothermal vent.
If the vent ever stops producing power, the entire submersible plant can be motored / towed to a new undersea geothermal vent.
World Map of Hydrothermal Vents
http://departments....thermal-vents-c.jpg
I became curious about [sninctown]'s proposed 200 mile long DC transmission cable and went looking for a map to show the location of known hydrothermal vents and their proximity to population centers where generated electricity might be useful. This is the best map I found in a cursory search, but it is really inadequate to judge the workability of [sninctown]'s idea. I'll remove this link if a better map is added by another reader. [jurist, Mar 05 2006]
How Steam Turbines Work
http://library.thin...mpfturbine.php3?v=2
[sninctown, Jun 22 2006]
Nexans Canada Inc.
http://www.nexansenergy.com/
Company that makes and lays undersea power cable [sninctown, Jul 20 2006]
[link]
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Not sure you'd want to drill on the ridge
itself since you'd be drilling directly into
the mantle - that'd be a sure fire way to
melt all your equipment. Better then to
drill a few kms away from the ridge. The
rocks are still thin and warm there. |
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Your Stirling engine would need to be
very big to exploit the difference in
temperature which ultimately makes
this idea a no-goer. Sadly, Stirling
engines are not scaleable to a size
where they could generate useful
energy - they lose efficiency. For that
reason, it's a fish. |
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Where's that Google Ocean we asked for? |
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[note: I updated this idea and deleted some annos. Apologies to Texticle and others for not noticing my unclear explanation. I have also made changes suggested by some annos, including amending my spelling and not using Stirling engines.] |
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[jonthegeologist]: If the midocean ridge is already hot enough to melt equipment, then there's no need to drill. |
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I vote no just because you deleted annotations. I hate people who do that, it ruins the whole experience. Oh and what the others said(the ones that are still here and the ones you deleted) |
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DC power transmission is very inefficent. I would think loss over the required distances would cancel any benefit of having gone there in the first place(which there is little reason to do, as the same features can be found just by drilling from the surface or around volcanos) |
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You also have an issue with your generator fluid, The increased presure at that depth will very likly make it very hard to boil any liquid, you will need to hold the system at a subambient pressure. |
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Hey, I poured my heart and soul into that anno. You bastard. |
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Nah, just kidding. You're all right. |
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The question I posed originally is still relevant though - why go to the trouble of tackling all those deep-ocean design problems when you could simply utilise geothermal energy on land? You don't see a lot of industrial installations two miles below, and I can't help but think there's a good reason for that. |
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[jhomrighaus], I think that HVDC power transmission is cost-effective in certain circumstances. Something to do with cable and insulation sizing, I think. Many seabed cables are HVDC - 100% of the ones I'm aware of, anyway. |
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The longest undersea HVDC cable I've read about is 24mi; most geothermal sites would need more, increasing inefficiency. Wikipedia says that AC is worse since seawater+insulation gives the wire significant capacitance. Well, since this is hb, one option is to use superconducting HVDC cable. |
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I am fairly sure a suitable generator fluid could be found, since I suspect that helium is still a gas at oceanfloor pressures and 1degC. I shall check... |
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[Texticle] A submersible geothermal plant is less vulnerable to sudden increases in volcanic activity than a land-based one is. If volcanic activity at the site suddenly increases, the underwater plant can just float up off of the bottom a bit, avoiding damage while still catching the rising hot water. Also, the deep oceans are largely unregulated, unclaimed and significantly unexplored. |
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I think large submarine/tankers could fill up on the hot water. They could then travel to a shore based station and off-load the water into a huge tank where it would be easier to handle. |
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Maybe this could be used for OTEC too. To avoid the pumping losses in the long pipes. |
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At the risk of being pertinent, a question.
What's the volume of water coming out of
these vents? |
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I dun a lot of thinks on dis one myself. |
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One feature of these vents is that there are often many in a small area - perhaps 20 in an area the size of a football park. |
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The water is very hot - even supercritical - and if I got my sums right one smoker could easily produce a GW of power, let alone 20 of them. It's also loaded with metal salts, so I think there's some chance that a finely woven glass fibre net tube close to the smoker outlet will become saturated with minerals and, so long as buoyancy's kept neutral, will be able to insulate and bring the water up near the surface. |
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However, I'm not sure this is required. A toroid, with the hot water going up the inside and cold on the outside, would give a large heat difference from one side to the other. It's also a perfect shape to take the pressure. |
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Inside, a bit like the scroll ball on a Mac Mighty mouse, four sealed cylinders lie around the toroid. Inside each cylinder a horizontally mounted revolving door with eg leather flaps making the air seals. The instant air expansion on the inside of the toroid should spin this bigtime. |
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There would be enough room left in the toroids to house the generators & rectification gear, and also a virtually perfect heatsink is available if required. These generating toroids would be simple, neutrally bouyant, hopefully no-maintenance, and stackable. |
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Output would diminish nearer the surface, but wiring in series might produce HVDC without any extra effort. My understanding is that HVDC is pretty efficient, and subsea it's brilliant - cooling is not an issue. |
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It's doable, but the capital requirement is large. The mineral salts would also reduce efficiency very quickly, and would have to be removed and/or mined. There would be no need to get near the sea surface though, which avoids a great deal of hassle. |
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Whoops - should have added that the inside of these toroids would be at normal air pressure. |
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pps Might just screw up the Gulf Stream / North Atlantic Drift too? |
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Damn. The coordinates of the sunken treasure was encoded in, and only in, that now deleted anno. |
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