Chemical Engineering December 2011 - 22

ZBB Energy
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plates where reduction-oxidation reactions
occur. Several flow-battery electrochemistries
are under investigation
in an effort to capitalize on their
advantages - including the ability to
store electricity in a modular design,
fast response rates and the ability to
vary durations of discharge.
An example in this area is ZBB
FIGURE 2. Flow
battery chemistries,
like the one shown
here from ZBB Energy,
use pumps to
circulate electrolyte
solutions within
cell stacks
ChemEngg-1111-half-ol.pdf 1 11/17/2011 8:40:28 AM
Energy, which has developed a zincbromine
flow battery that is currently
in commercial production. The electrochemical
cell consists of a zinc anode
and a bromine cathode, separated by a
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Circle 4 on p. 62 or go to adlinks.che.com/35072-04
22 CHEMICAL ENGINEERING WWW.CHE.COM DECEMBER 2011
microporous membrane. Aqueous electrolyte
solutions of zinc and bromide
ions are circulated through the cell
stacks. The battery design employs a
replaceable cell stack of high-density
polypropylene spacers, and 60 bipolar
electrodes. As the battery charges, zinc
metal is plated onto the anode in the
cell stacks, and bromide is converted
to bromine, which is stored in a phaseseparated
manner.
ZBB's technology targets applications
that require power discharge in
the 2-8 h range. " The ZBB flow battery
is ideal for time-shifting renewable
energy, " says ZBB's Chris Kuhl.
" It is also useful as a ramp-control device
to stabilize power demand peaks, "
he says. Among the advantages of the
ZBB flow battery is its ability to operate
at full output over a wide temperature
range (-30 to 50°C). The
electrolytes never need replacing, and
although the high-surface-area anode
will wear out, it can be changed out
with the stack after five years.
In a DOE-funded project, flow batteries
using a different chemistry are
under development by United Technologies
Corp. (UTC; East Hartford,
Conn.; www.utc.com), along with partners
including the University of Texas
(Austin; www.utexas.edu). The team of
scientists has designed a unique flowbattery
cell that offers power densities
an order of magnitude higher than
those published so far for flow batteries.
" The electrochemical cell stacks
dominate the cost of batteries, " says
UTC engineer Craig Walker, so if we
can increase power output, the batteries
can produce the same power at
smaller cell sizes, which lowers costs.
The flow battery, which is targeted
at improving power quality from
wind farms and time-shifting offpeak
power, has been demonstrated
at small scale, and could be described
as a hybrid between a fuel cell and a
conventional battery. Liquid reactants
flow over electrodes separated by an
ion-exchange membrane. Leveraging
UTC's expertise in fuel cells, the cell
stack was designed for higher performance
and optimized efficiency. The
http://www.utc.com http://www.utexas.edu http://adlinks.che.com/35072-04 http://WWW.CHE.COM
Chemical Engineering December 2011

Table of Contents for the Digital Edition of Chemical Engineering December 2011
