Reply #15: Interesting post on redox flow cells for storing power from wind turbines [View All]

Another battery option is the redox flow cell. This page,


http://www.ceic.unsw.edu.au/centers/vrb/webframe/vanart2a.htm


By the inventor of the vanadium redox battery (VRB), Dr. Maria
Skyllas-Kazacos , University of New South Wales, Sydney AU, gives a good
introduction to this system.


A shorter look can be had at:
http://www.sei.co.jp/sn/97_07.html


The redox system is a unique electrochemical storage system.
one of it's advantages is cost. From the first site:


"Cost estimates by the UNSW group and independent consulting groups (10,
11), place mass production costs at between $100 and $300 per kW for the
stack and $30 to $50 per kWh for the electrolyte."


For a large system,


" typical projected battery costs for 8 or more hours of storage being as
low as US$150 per kWh. "


Of all the options it appears that Redox is the cheapest. This becomes even
more apparent when it is considered that the redox battery has a virtually
unlimited lifespan.

Since all reactions take place in the liquid phase, there are no
irreversable chemical changes. The only part of the system that needs
regular replacing will be the membrane, perhaps every five years. The
battery is not damaged by deep discharge or by rapid charge rates and can be
configured such that off-gassing of H2 does not occur as is the case with
lead acid batteries. The efficiency is remakable, approaching 90%. Total
storage capacity can be sized independent of power by simply increasing or
decreasing the electrolyte volume or by increasing or decreasing the number
of flow cells. Charge and discharge can take place simultaneously and at
different voltages. This in effect makes the system a DC transformer. By
electronically monitoring system voltage and switching cells on and off as
needed, it is possible to track an optimum voltage for PV and wind
generating systems.

There is no energy lost with long term storage since the electrolytes are
stored separately. A feature that holds great promise for electric
transport is the ability to recharge a redox battery by replacing discharged
electrolyte with charged thus recharging an electric vehicle in the same
time it takes to fill a fuel tank on a conventional vehicle. The discharged
electrolyte can be recharged at off peak times from the grid. Instead of
gasoline tankers delivering energy to filling stations we can use the
electric grid.


Even without wind and solar redox makes sense for load leveling, for large
scale consumers of electric power such as heavy manufacturing who can
purchase power at times of low demand. Bulk energy storage will reduce or
end the need for expensive peaking generators by storing excess baseload
capacity in times of low demand. This will also allow more efficient
operation of baseload generators.


For windpower there is another benefit that may not be immediately apparent.
Having a means of storing energy will make it worthwhile to design machines
that can harvest power from higher but less frequent windspeeds. Presently
this would result in short term spikes of power.

The energy of wind increases with the cube of the velocity. There is a lot
of power out there that is presently too erratic to be useful. Having
storage allows this power to be salted away. Also, as wind turbines grow in
size and height they will be capturing energy in larger chunks so to speak,
storage will only increase the efficiency and utility of these machines.
It would appear that the vanadium redox battery is the best choice as a bulk
energy storage option. This combined with wind and solar may be our energy
future.