The Seasteading Institute

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 0.75×24500000/9810 = 1873 seconds, which is a bit more than half an hour of power.

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.”

• 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.

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.

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.

• Current- There may or may not be current on the bottom. Either way a storage bag that holds tons (lots of tons) of buoyancy would not be a problem. It would just sway a bit. The bottom is a very calm place relative to the air-sea interface. If you want to reduce stresses, put it on the bottom.
• Biofouling- yes it would be. Everthing you put in the ocean is subject to biofouling. This would be less of a problem than one might think. As the current causes sway and the bag is filled and emptied, most encrustation would drop off the slippery composite surfaces. Also replacement is cheap.
• Pretty deep- Correct, they should be tethered a short distance above the bottom to maximize capacity and minimize motion.
• A submarine? Running into the lift bag?
• “It is a mistake to think you can solve any major problems just with potatoes”. -Douglas Adams

Me driving a submarine. . .

It probably is more efficient in a theoretical sense that considers only energy efficiency. But the amount of ballast change available within the stability limits of the platform is very limited. You wouldn’t get much capacity unless you had a very, very large platform. You’d probably have more with a tension leg platform than a semi-submersible.