Carbon Nanotubes and another material, combine to form a solar energy capture and storage material that can store heat efficiently for long periods. Compared to lithium ion batteries in energy density:

http://www.sciencedaily.com/releases/2011/07/110713121301.htm

Getting closer to the Holy Grail of batteries.

Or the holy grail of flywheels:

http://www.timesunion.com/business/article/From-a-cornfield-to-clean-energy-s-future-1463583.php

I think this sort of system might be useful for short term fluctuations.

Second hand market for Nissan Leaf batteries:

http://www.dailytech.com/Nissan+Looks+to+Energy+Storage+for+Old+Leaf+Battery+Packs/article22119.htm