redox batteries

[bob golding]

hi all,

i have been trying to get some idea of the relative long term costs of vanadium redox cells compared with lead acid. it seems they they rate redox cells in KW rather than amp hours. each cell is 1.6 volts so if i wanted a 24 volt battery i would need 15 cells. not sure how this relates to lead acid which would need 12 cells. is there any  meaningfully relationship between the 2 systems or do you just calculate your power requirements over time and use that to work out the size system needed.? from what i have read the system costs seem to be related to the time from fully charged to discharged with no power input as in range of an electric vehicle say. i can see how this would work in this instance, but not for our normal use where the input and discharge are constantly changing. am i trying to compare chalk and cheese here? it does seem vanadium redox systems are staring to become available at last. there is a contract to supply 5 kw systems to mobile phone relays in Africa. that sounds almost viable for small RE use if it is cheap enough. waiting for a price for a 5 kilowatt system but not sure i know what i am talking about when  say 5 kilowatt!!

cheers

bob golding

[Ungrounded Lightning Rod]

In lead acid cells the energy is stored in the plates of the cells.  So the cells are rated primarily by the amount of energy they hold, not just by how fast they can absorb or release the energy.  Also, that energy absorption/release rate is proportional to the storage capacity for any given type of construction.

In vanadium redox cells the energy is stored in the oxidation state of the vanadium solutions, which are stored in tanks and pumped into and out of the cells.  So the amount of energy the system can hold depends solely on the size of the tanks.  The only rating that is significant for the cells (and pumping system) themselves is how fast they can absorb or release the energy.

Vanadium redox cells are essentially a type of fuel cell.  The cell is the engine and the storage is in the "fuel tank(s)".

[bob golding]

thanks ulr,

that was what i thought. i know the voltage but not the current density of each cell. more reading required. i understand about the tank size and pumping speed. i have a figure for the watts per kg of electrolyte so i just need to find the current per cell to get a comparison. i have looked at some figures but am always very suspicious of marketing dept excel graphs. so when they say a 5KW system do they mean the total energy contained in the fully charged electrolyte in litres? i know "fully charged" is not the right terminology but you know what i mean.

cheers

bob golding