Electrical energy storage

Overview
Electrical energy storage options include batteries of course, but ultracapacitors, pumped water storage and flywheel should be investigated at least with some basic calculations too.

Details
Carbon gels suitable for use in ultracapacitors may be possible to make but it is not trivial. The gel is a network of extremely fine fibers, these need to be coated with an insulating material then surrounded buy a conductive fluid (can be an organic fluid or water with solutes), the aerogel forms one electrode and the electrolyte the other, similar to an aluminum or tantalum electrolytic capacitor but with even higher surface area.

http://en.wikipedia.org/wiki/Electric_double-layer_capacitor says a basic one can be made with activated carbon somehow, activated carbon is not only cheap on the order of a couple dollars a kg, but it can be made from biomass or a wide range of sorts, nut shells, peach pits, maybe even grass. They also last forever.

Pumped water storage more dependent on the topography, if it is right then the water tower is easy, if it is not then it could be substantially expensive.

Pumped water efficiency would be 65% or so at most (electric motors are 70% tops) unless connected directly to a shaft e.g. shaft of steam engine in which case might be 90%.

Supercapacitors, can be combined with batteries to d short term load leveling, reduces the charge/discharge cycling on the battery and increases energy efficiency (since they are quite efficient).

Compressed energy storage, thermodynamic reversibility might be a problem. Can the heat of compression not be stored in a regenerator?