Wouldn't a strong enough cable be much more expensive and heavy than batteries of the same capacity ? I think I saw that a 4 km anchoring cable runs in the millions of $ and weighs tons. In addition, you lose a significant amount of potential energy from the weight of displaced water. And the power of your system would go down with the weight, litterally, because it wouldn't accelerate past its terminal velocity, which is quite low when underwater.

Gravity allows storing energy at 9.81 Joule per kg per meter of capacity and rates 9.81 Watt per kg for as long as the weight is accelerating, if I don't have my physics all mixed-up. So a 9.81 kW (peak !) gravity battery would require, assuming the weight of the cable is averaged into the ballast's weight over the running length, a 1 ton (net, displaced water discounted) ballast, and would run at that power only a fraction of a second then drop exponentially. It becomes less efficient quickly as soon as yo use it, that's NOT a good thing. In order to have any decent power, you'd need to oversize it grossly. Also, capacity is low: with 2500 m running length, you would store 1000x2500x9.81=24.5 MJ which would give you, with an efficiency of 75% (I'm generously assuming you oversized it by a significant factor and are running it at a fraction of its nominal power rating), an autonomy of 0.75x24500000/9810 = 1873 seconds, which is a bit more than half an hour of power.

• The weight needn't be accelerated.  It just needs to be steadily lowered slowly enough that drag doesn't eat too much of the energy.  The weight could easily be shaped to minimize drag.  So long as it's lowered and raised slowly enough, efficiency should be great.  (Honda's current hybrids convert 97.5% of electrical energy into mechanical energy, and during regenerative braking convert 95.2% of mechanical energy into electrical.  http://corporate.honda.com/press/article.aspx?id=2004091736553 )
• joules = force * distance.
• force = mass * 9.8
• kWh = force/3600/1000
• So a 10 ton weight (after displacement) lowered 1000m would store 27 kWh.  Enough to supply a single family home for a full day.
• 0.952*0.975 = 92.8% initial round-trip efficiency.  Subtract drag losses.  If you're only lowering the weight 1000m in 24 hours, drag losses would be pretty miniscule.  Say 90% total efficiency.
• Anyone actually have a good link for how much would a cable that could support that weight cost?  Have to take into account corrosion, of course.
• I think this is a great idea if you could find a cheap enough cable.
• Whether this is cheaper than the Whole-Ocean Gravity Battery ( http://seasteading.org/interact/forums/engineering/infrastructure/whole-... ) would probably depend on whether pipe or cable are cheaper.  This method is more portable, though, and probably has somewhat better efficiency
•  

 Complex mechanisms are problematic in the ocean environment. A simple solution is to pump air down to an anchored storage bag (resembling a high-altitude balloon). It can be deployed inexpensively with almost no subsea intervention using a suction pile anchor. A 1,000 ft depth yeilds 500 psi air.

“Prosperity is only an instrument to be used, not a deity to be worshiped.”

The mechanism here is not complex, except maybe for the amazing gearbox necessary for connecting a very slowly descending, extremely heavy mass to a 1000s of RPM turning electrical generator efficiently. But then, it does not have to be underwater at all, the cable and weight can be just cast off the board on a pulley.

• Ok this might not be very efficient. The water resistance obviously puts a bit of a damper on things. Also the seastead would bob up when the weight accelerated.
• Anyway let´s assume we have a 100 ton net weight. and assume one tenth of peak power is possible with some degree of efficiency and duration. That would mean 100000kg x 9.81 x 2500m = 2452MJ or 662kWh of stored energy and a continous output of roughly 9,81 / 10 x 100000 = 100 kW.
• Ok I agree batteries are probably way more efficient and practical. A 60Ah car battery contains about 0,7kWh and peaks at around the same figure in kW so you´d need roughly a thousand of them for the capacity and only like 150 for the peak power.

A balloon under water would be subject to a lot of pull from current.. It would probably also be subject to biofouling that could puncture it. It would be a hazard to navigation unless it were pretty deep. The chances of a submarine running into are probably pretty small but not impossible.

This is an interesting concept. I wonder if it can be tied in with some of the external ballast designs we are playing around with. My problem is that ballast is so crucial to the overall balance of the system, that complicating it with moving ballast is kind of scary. Anyhow it is food for thought.

How about pumping seawater to upper part of the spar? All you need is a vater tank, pump/t and electrical motor/power generator.

Possible issues:
a)Efficiency of energy conversion.
b)Stability. Extra weigth on top (tens of cubic meters of water) would elevate the seastead's center of gravity. It should be offset by extra ballast for the bottom.

Why not directly use the seastead itself as the weight for generating current ? On a tall spar design, just let water inside tanks in the lower spar to generate current, "refill" by pumping the water out. You get a comparable height difference, but a MUCH higher mass, without destabilizing the structure.

This is much better in terms of being easier to engineer, install, and maintain, with at least comparable efficiency (probably MUCH higher).

• The best point the original poster had was that unreliable power sources need a cheap energy storage method.
• The elegance of Kiwiserg's idea is that not just the kinetic potential of the water, but the water itself is useful, for flushing greywater systems (perhaps) cleaning decks, fire suppression systems.
• The tank doesn't have to be large enough to affect stability in any meaningful way, a few hundred to a few thousand gallons.
• Having an elevated tank also means smaller, more efficient pumps, but better water pressure. I highly recommend  a similar storage  system for freshwater as well.