Even if you're lucky enough to have a renewable energy source that isn't stranded there is still an intermittent power issue. Iowa has it the worst - air conditioning demands in summer time drive the electricity peak in July and August ... when the winds are still.

There are six sensible methods for firming renewable energy sources. These are ...

Pumped hydro storage

This doesn't work in the midwest but it does in some places in the east, like the shores of the Great Lakes. They've got potent wind coming off the lake, 200' to 300' hills not far from the shore, and you can store a great deal of energy by pumping water uphill when the wind blows and then draining the reservoir when the winds are still.

Compressed air storage

If you've got an air tight underground cavern you can fill it up with pressurized air while the wind is blowing and then use it to run a turbine when the winds are still. Iowa has this in some places and we'll be doing an article on one of them shortly (as of 3/11/2008).

Hydrogen production

Hydrogen makes a pretty good storage medium in some situations. When you've got a small scale problem you just hydrolysize water, store the hydrogen in a tank, and use it in a fuel cell or engine when you need it. Hydrogen can also be stored large scale like air is, in a cavern, but these generally have to be solution mined caverns in salt domes. Natural formations are fine for methane or air, but they're generally too porous for the little escape artist hydrogen atoms.

Ammonia production

Ammonia can be produced by either the Haber-Bosch process or solid state ammonia synthesis (SSAS). Hydrogen is 85% efficient and ammonia is only 40% in terms of the input energy being available later, so hydrogen is the first choice, but ammonia makes sense in many cases due to ease of storage, ease of transportation, and having multiple uses.

Flywheels

We see occasionally, but know nothing about (yet) flywheel type storage systems. This will be updated when someone has the time to look into them.

Batteries

We see reference to sodium/sulfur batteries at the small utility scale. This area could use some attention - efficient multimegawatt batteries open interesting doors for smaller municipalities.