I've been meaning to write a post on flow batteries for some time now and it seems that James Fraser at the Energy Blog has beat me too it.
Flow batteries are a very promising emerging energy storage option that are fully scalable for both power and energy capacity up into the MW and MWh ranges and have load leveling/peak shaving, uninteruptable power supply and power quality management applications and are well suited for coupling with intermittent renewable energy sources.
Rather than be redundant, I'll simply exerpt the beginnings of his post here and refer you to the full post at the Energy Blog for more:
"Flow batteries are emerging energy storage devices that can serve many purposes in energy delivery systems. They can respond within milliseconds and deliver power for hours. They operate much like a conventional battery, storing and releasing energy through a reversible electrochemical reaction with an almost limitless number of charging and discharging cycles. They differ from a conventional battery in two ways 1) the reaction occurs between two electrolytes, rather than between an electrolyte and an electrode and 2) they store the two electrolytes external to the battery and the electrolytes are circulated through the cell stack as required. The great advantage that this system provides is the almost unlimited electrical storage capacity (MWh), the limitation being only the capacity of the electrolyte storage reservoirs."
Head here for the full article.
[Thanks to James Fraser for an excellent post.
Flow battery graphic from www.axeonpower.com/flow.htm]