Energy Storage

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.

The book covers these technologies extensively.

Potential energy

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.