For now, the most mature technology for storing energy appears to be electrochemical batteries. While they are expensive, the alternatives (flywheels, ultracapacitors, redox batteries, creating hydrogen to power fuel cells) are generally still experimental. However, redox batteries are rapidly approaching usefulness.
With miles of ocean below it is possible to store significant potential energy with a weight on a rope. Lowering the weight generates energy and you put energy in by lifting it up. The motor/generator from a hybrid car could be used.
Compressed air energy storage
Normally CAES uses expensive tanks to store the compressed air. One can instead use the high pressure of the deep ocean to let you store compressed air in a cheap bag. Here the main cost are the hose or pipe to take the compressed air down to the bag and a method to hold the bag down.
If large steel pipe is used for the structure of a seastead, it might also be used to store compressed air without adding to the cost.
If the air socket used to make the platform float is of variable volume it is possible to pump in extra air when there is a energy surplus, we could use that air power an turbine for when there is a under meassure in power.
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