|
 In
addition, power and energy are decoupled in certain types of
flow cells. The power (MW) rating being determined
primarily by the amount of cell electrode surface area and
associated power conditioning equipment, and the energy (MWh)
rating being determined by the amount of electrolyte. So,
for certain types of flow batteries, more MWh is a matter of
bigger electrolyte tanks.
General issues with flow batteries include the cost of the
membranes used. Ionic membranes are very expensive on a
square meter basis, and many manufacturers are sacrificing some
efficiency to save a large cost by using separators similar to
those used in other batteries, like lithium ion batteries, that
provide a molecular filter, rather than a true ionic membrane.
Other issues are the fact that whenever you have charged
electrodes in a aqueous solution, there is a potential for the
electrolysis of water into Hydrogen and Oxygen gases. Loss
of Hydrogen gas can also mean a raising of Ph, which may be
detrimental to the battery.
 Other
issues include the quality and purity of metals. For
example, impurities in Zinc may help to cause dendrite
formation, as the dendrites (little spikes that may short
through membranes) may preferentially start on impurities.
Common impurities of Nickel in the Chromium used in Iron
Chromium flow cells may cause issues, and in any flow cell
certain trace species (impurities) may cause side reactions that
reduce efficiency or create hydrogen gas.
One of the most important issues to deal with in a standard flow
battery is shunt or parasitic currents that create self
discharge of the materials in the electrolyte in the cells
themselves. There are generally two main sources of these
losses. The drawing here shows a typical cell stack for a
flow battery. Each cell is very thin, many cells are
stacked together in series to make a typical cell stack.
The cells are joined with bi-polar electrodes, and act much as a
stack of flashlight battery cells in a long flashlight act.
 A
flow cell has a similar voltage to a flashlight battery (1.6
volts DC is fairly typical), so the stacking produces a unit of
high DC voltage, with the current traveling through the stack of
cells. Differences in electrode potential here create
small parasitic currents between a cell and it's neighboring
cell, slowly discharging the cell.
The other and larger current loss occurs because all the anode
sides of the cells of a cell stack are fed with pumped
electrolyte in parallel (as are all the cathode sides of a cell
stack). Now, the electrolyte is conductive, so if two
anode electrodes several cells apart have differing voltages, a
'shunt' current may occur between these two anodes, passing
through the electrolyte fed commonly to both cells.
Many solutions have been offered and used for this parasitic
current issue, including long physical fluid paths to a cell to
reduce the shunt losses, and draining the cell stacks or cell
incoming fluid paths. Various methods of
operation of the battery are also used to mitigate the shunt
current issues, including sporadically pumping new electrolyte into
cells to refresh them during quiescent periods between battery
charging and discharging,
so that the battery may still have 'instantaneous' current
available.
The following are different types of flow batteries, each of
which has their own benefits and issues.
Iron Chromium Flow Battery
First developed by NASA during the Apollo Program, Iron Chromium
(FeCr) has the benefit of using relatively benign chemicals, and
could represent the safest type of flow battery available.
Also, electrolyte chillers are not necessarily required (as in
Zinc Bromine), and the active materials are relatively
inexpensive and widely available.
In a FeCr flow cell, there is a different reactant in each cell
half. The anolyte (electrolyte that flows through the
anode side of the flow cell) contains ionized FeCl3, while the
catholyte contains CrCl2. Issues arise because of the use
of two different reactants, such that transfer of one reactant
(for example Chromium ions) through the membrane can result in a
permanent loss of capacity to the battery. This transfer
might happen due to inefficient or damaged membranes.
Another issue is that the energy density of FeCr is low,
requiring much larger electrolyte tanks than some other flow
battery systems.
Iron Chromium is presently being sold into mostly Telecom
applications by Deeya
Energy, and is also being developed for utility applications
by various startups.
Zinc Bromine Flow Battery
Zinc Bromine is considered a "hybrid flow battery", as during
the charging process, Zinc metal is directly plated on the anode
electrode. This means that the power/energy relationship
in a Zinc Bromine flow battery is more fixed than some other
flow battery systems, as the total energy available in a system
is limited by the available electrode area for plating Zinc.
Zinc Bromine is, along with Vanadium Flow batteries, the most
studied flow battery system for utility applications, and is
actively being manufactured and sold into utility applications
by two companies.
Zinc Bromine does not suffer from the issue of different
reactants, as does Iron Chromium. In a discharged state,
both sides of a Zinc Bromine flow cell contain essentially the
same material, Zinc Bromide (ZnBr2) ionized in solution.
Any issue with crossing of active material species across the
membrane can generally be dealt with by fully discharging the
cell back into it's normal Zinc Bromide solution state.
Zinc Bromine has a relatively high energy density, and uses
relatively inexpensive and widely available reactants.
Bromine gas is a serious health hazard, but in present designs
the elemental Bromine is complexed during the charging process
into a material with a much lower vapor pressure, making it much
less hazardous.
Issues with Zinc Bromine flow batteries include growth of
dendrites on the anode during Zinc plating (tree like spikes of
Zinc metal which can pierce membranes and cause shorting), and
the need for large electrolyte chillers to improve efficiency
and to reduce the potential of Bromine gas production.
Some manufacturers claim unlimited cyclability (30 year life),
while others state that a small degradation of capacity happens
with cycling on the Bromine side electrode.
Zinc Bromine flow batteries are presently being produced in
individual packages as large as 500KW/2.8MWh, and are being
produced by ZBB Energy,
and Premium Power.
Other companies in pre-production are also working on Zinc
Bromine flow systems.
Vanadium Flow Battery
Vanadium flow batteries are another 'true' flow battery,
which has been studied extensively and has been used in a few
utility applications. A Vanadium flow battery uses the
same reactants on both sides of the cell, and so does not suffer
from the ionic transfer issues of Iron Chromium. It also
has potentially the highest efficiency of any flow battery
design, but a relatively low energy density.
The cost of the Vanadium based reactants is higher than that of
FeCr or Zinc Bromine above, and there are questions as to
safety. While the Vanadium flow battery electrolyte itself
is not terribly dangerous, the Vanadium PentaOxide used in one
part of the electrolyte is very poisonous in powder form (should
a spill occur that dries to powder).
Utility testing of Vanadium flow batteries has been done
primarily outside the USA, including a 4MW/6MWh battery used in
output smoothing for a wind farm in Japan. A 500KW/2MWh
battery used for transmission asset deferral in Utah recently went out
of operation. Vanadium flow battery
technology has to be among the most studied of new battery
technologies, as over 50 present US patents and current patent
applications deal with Vanadium flow battery technology.
Utility scale Vanadium flow batteries are presently being
offered by the Chinese company
Prudent Energy, and are
also being developed by several pre-production companies.
Prudent Energy claims unlimited cycles but a 10-15 year life,
limited by degradation of the membranes (pinholes) over time.
Zinc Chloride Flow Battery
In the 1970's and 80's a company called Energy Development
Associates developed a Zinc Chloride flow battery system to the
point of large scale utility tests. Apparently this
company was far ahead of it's time, and could not find a
profitable market at that time. All of the EDA patents
have recently expired, and this technology is being developed or
redeveloped by startups today. Zinc Chloride has the
potential of being somewhat safer than Zinc Bromine, and to have
a somewhat higher energy density. Hydrogen evolution may
be more of a problem, due to the higher voltages at electrodes. |
