Of course we don't know yet how mugh the NiFe battery would cost.  I need those document listed on the NiFe page first really to know that.  But I have put a page up titled "comparison of different battery chemistries" on the wiki as I have decided it would be a good idea to look at other chemistries too.  I found only one that looks like a competitor with NiFe for OSE purposes:

ZnBr type of flow battery

 The zinc plates out so litte advantage as a flow battery, still limited by electrode size. Electrolyte the same bothe sides so no contamination issues between the half cells. Various websites says can be made commercially at lower cost than van redox and others.  Bromine is relatively cheap and is extracted commercially from seawater.  Doesn't sound too bad toxicity wise but more research is needed.

From the battery hand book 3rd edition (small quotes allowed under fair use) :

For utility applications battery efficiency is
a primary concern, and percent utilization is about 50 to 70% to maximize efficiency. For
electric-vehicle applications battery size and weight are more important, and the percent
utilization can be as high as 80 to 90%.[of the reactants in the half cell, the electrolyte]

"on total energy requirements of auxiliary systems, although the energy devoted to auxiliaries
is projected to be less than a few percent of the total battery energy.

There is a graph with energy efficiency and it looks like roughly 74% overall  is right for this particular battery, but that can be improved substantially, see below.
Wikipedia says cycle durability a lower than nife, 2000 rather than 2000-3500 but with servicing 3500.  Also it would be trivial to insert/remove the electrodes and electrolyte separator, which should be quite cheap and are presumably the parts that wear out.  The battery could simply have these spare parts included with it, giving it in effect the longer durability figure.

Complexing agents mentioned are just ammonium compounds and should last.
Making the carbon electrodes:
 GEL using"wil-mat" method?  Couldn't find any info on this.
http://www.freepatentsonline.com/6509119.html
http://www.freepatentsonline.com/5626986.html looks good

more patents:
large number of them , just search "zinc bromide battery" "zinc bromine battery" "zinc bromine flow battery" "abst/zinc  abst/bromine  abst/battery"etc. on freepatentsonline.com.
http://www.freepatentsonline.com/3811945.html
issues to be aware of:
http://www.freepatentsonline.com/4343868.html change of ph (1982)
http://www.freepatentsonline.com/4206269.html
http://www.freepatentsonline.com/3811945.html
shape change of zinc layer:
http://www.freepatentsonline.com/3972727.html shape chages of zinc , can also be avoided with deep discharge and refomation of the electrode, could have more than 1 battery and cause one to deep discharge while the other is still charged at a useable level. Could be done with cells in a battery, one cell being given the treatment while the others are in operation.

Doesn't look like the microporous or ion exchange membrane should be too hard to make, many ways to make them, grafted polyethylene maybe? Nafion is IIRC the same stuff as ion exchange beads and should be makeable.  The issue is whether that would last.   

Looks good to me, and reasonably simple, with a better membrane (ionic, probably available off the shelf for prototyping and early production stage) it would be more efficient, could also use separate electrolyte storage tanks for the individual cells, simplifying design too.  Could be easier to make since there is no pasting in or nickel plating, all plastic components in the battery except the electrodes.  (what kind of plastic though?) There are no active materials to convert into fine powders.  We don't need high discharge rates, so maybe we could essentially eliminate the pumps etc. and just make it a passive battery, depend on convection maybe for electrolyte flow as the 15% of energy not converted to electricity would be converted mostly to heat presumably (little gassing, although gassing would cause convection too).   Maybe they could be made of activated carbon or graphite powder rammed into a perforated plastic tube under sufficient force to compress the particles together and get good contact. There might need to be some chemical treatment of the surface to introduce appropriate functional groups as some sort of catalyst apparently.  Zinc sometimes has toxic heavy metal impurities in subsstantial amounts so this needs to be watched but zinc itsself is practically nontoxic.

 Looks like an old technology out of patent http://www.freepatentsonline.com/3811945.html (1974) , materials needed could be partly available from the waste stream due to galvanization
.
Questions to answer:
What material off the shelf would work well for the membrane? Nafion?  Would it last?  What is it's ionic conductivity for the ions of concern here? (Br+ or Br++ I think)

Exactly how should the electrodes be made?

What are the way by which it wears out?  How easy is it to refurbish?

The Efficiency compensated equivalent cost needs to be calculated for various batteries:
Es=Wh that must be delivered/day to socket
Ea= Wh delivered per day by the solar array/watt of array
EFb= Efficiency of battery charge/discharge overall
Cc=cost of collector, in $/watt.
Fsib=fraction of the energy that gets to the socket that had to be stored in the battery


Cb= cost of battery per Wh of storage

ECc=efficiency compensated cost per Wh of battery
 
Cabbi=Total cost added by battery inefficiency


Cabbi=((Es+(((Fsib*Es)/EFb)-Fsib*Es))*Cc/Ea)-Cc*Es/Ea

It's the total energy that needs to be withdrawn from the socket plus the energy wasted by the battery, which is the total energy the array needs to supply, divided by the array cost per unit energy, minus wha the cost woudl be if the battery was 100% efficient.

then
(Cabbi/(Fsib*Es))+Cb=ECc

Cost added per Wh of battery storage needed by the system, plus cost of battery per Wh.

So if
Es=1000Wh
Ea= 5Wh/watt
EFb= 0.65
Cc= $2/watt
Fsib= 0.5


Cb= $0.2


Cabbi=((1000+(((0.5*1000)/0.65)-0.5*1000))*2/5)-2*1000/5

Cabbi=507.69-400 =107.69

(107.69/0.5*1000)+0.2= 0.415

If the nife battery is $0.2 per Wh to start with, the inefficiency of it more than doubles the effective cost of the battery.  Lithium iron phosphate batteries are already available at $0.4 per watt, but only 80-90 percent efficiency they are only just inferior according to wikipedia (might be higher, remember hearing it was, should check battery handbook) search ebay for "thundersky lithium". And they have cycle durability in the same range as nife :http://evolveelectrics.com/Thunder Sky Lithium Batteries.html

But if the cost of the nife batteries was $0.1 per Wh then we'd be ahead of the game even with the low efficiency.  Plus the cost of collectors is going to go down, probably a lot faster than the cost of batteries.  Indeed the cost of batteries may well increase substantially, especially lithium batteries, due to market fluctuations or the cost of materials, especially lithium which is coming into huge demand.  With zinc bromide we might get the best of both worlds, producing an exceptionally cheap system.