A Redox flow battery is when the positive and negative electrical charges are both held in a liquid electrolyte in two separated tanks. In charging and discharging, the electrolytes are flowed by pumps past a membrane that allows the charge to be drawn off.
As main storage, or a "back-up" Uninterrupted Power supply (UPS) it can be recharged by any normal supply (grid, wind, solar, Diesel, etc)
A short-range prototype golf cart is recharged by draining the spent fluid, and re-filling with charged fluid.
http://www.vrb.unsw.edu.au/
Australian University VRB overview
http://en.wikipedia.org/wiki/Vanadium_redox
Wikipedia VRB info
There are several very large VRB power back-up systems for large wind-farms around the world. Some of the early research was done by NASA and the Australian University of New South Wales.
"VRB Power Systems" (Vancouver, Canada) saw potential in VRB technology, and found a cheaper way to make the membrane and increased the energy-density capability of the electrolyte, so a storage system could be much smaller and more affordable.
They recently sold 80 of their $5,000 telecom UPS units that are the size of a closet and are rated as 5 KW. (5 KWH?)
In theory the electrolyte never "wears out", and once a system is installed, the storage capacity can be increased by simply adding more tanks and electrolyte.
They are capable of large current discharges and they respond rapidly to variable loads.
Russia, China, and the US all have significant Vanadium mining capacity, so it is not especially rare or strategic, and it is a common steel alloy component.
http://www.madreline.com/colorado_plateau.php
VRB electrolyte is relatively benign, environmentally safe, and available. If any future problems occur, there are other chemistries that can be swapped in.
http://www.rsc.org/publishing/journals/CC/article.asp?doi=b701629h
Other redox flow battery chemistries being investigated
Most of this was paraphrased from the Oct 2008 issue of "Discover" magazine.
Any corrections or additional useful info appreciated...
