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Headline: Australian Island Using Flow Batteries To Store Wind Power
King Island is a small island off
the Australian coast, near Tasmania. King Island isn"t connected to the
mainland power grid, and apart from its own small wind farm it relied
for a long time on diesel generators for its electricity. That changed
in 2003 when the local utility company installed a mammoth rechargeable
battery which ensures that as little wind energy as possible goes to
waste. When the wind is strong, the wind farm"s turbines generate more
electricity than the islanders need. The battery is there to soak up the
excess and pump it out again on days when the wind fades and the
turbines" output falls. The battery installation has almost halved the
quantity of fuel burnt by the diesel generators, saving not only money
but also at least 2000 tonnes of carbon dioxide emissions each year.
So what"s new? For years wind
turbines and solar generators have been linked to back-up batteries that
store energy in chemical form. In the lead-acid batteries most commonly
used, the chemicals that store the energy remain inside the battery.
The difference with the installation on King Island is that when wind
power is plentiful the energy-rich chemicals are pumped out of the
battery and into storage tanks, allowing fresh chemicals in to soak up
more charge. To regenerate the electricity the flow is simply reversed.
Flow batteries like this have the
advantage that their storage capacity can be expanded easily and
cheaply by building larger tanks and adding more chemicals. The
technology is already attracting interest from wind farmers, but flow
batteries could also replace all sorts of conventional electricity
storage systems - from the batteries in electric cars to large-scale
hydroelectric pumped storage reservoirs.
Electricity is very different to
commodities like coal or oil that can be stored up in summer ready to
meet peak winter demand. With electricity, generating companies meet
fluctuating demand by adjusting the supply, from day to day and minute
to minute. Typically, they spread the load over large distribution grids
and use a mix of huge, economical, "base-load" power stations
supplemented by smaller, costlier generators that can be switched on and
off at short notice.
Matching supply to demand is
particularly problematical when it comes to renewable energy sources
like the wind and the sun. The wind doesn"t always blow when needed,
which means that electricity companies must keep conventional power
stations standing by so that on calm days, or when electricity demand
leaps, people will still be able to turn on the lights. These power
sources can also be difficult to slot in and out of the generation mix.
An effective way to store electricity on a large scale would give
renewable power sources a welcome boost.
There is no shortage of ways to
do this. Ideas range from storing energy underground using hot rocks or
storing it as electrical charge in "super capacitors" to using off-peak
capacity to pump water into reservoirs where it can drive generator
turbines when demand peaks. Then there are various kinds of batteries.
While each technology has its advantages, flow batteries seem to have
the potential to satisfy the broadest variety of needs - from small
power systems to large-scale grid storage - at a competitive price.
Flow batteries are more complex
than conventional batteries. In a lead-acid battery, the electrical
energy that charges it up is stored as chemical energy inside the
battery. Flow batteries, in contrast, use two electrolyte solutions,
each with a different "redox potential" - a measure of the electrolyte
molecules" affinity for electrons. What"s more, the electrolytes are
stored in tanks outside the battery. When electricity is needed the two
electrolytes are pumped into separate halves of a reaction chamber,
where they are kept apart by a thin membrane. The difference in the
redox potential of the two electrolytes drives electric charges through
the dividing membrane, generating a current that can be collected by
electrodes. The flow of charge tends to even up the redox potentials of
the two electrolytes, so a constant flow of electrolyte is needed to
maintain the current. However, the electrolytes can be recharged. A
current driven by an outside source will reverse the
electrochemical reaction and regenerate the electrolytes, which can be
pumped back into the tanks.
The installation at King Island
has its origins in the 1980s when Maria Skyllas-Kazacos, a young
Australian chemical engineer, started a research programme on flow
batteries at the University of New South Wales in Sydney. This focused
on one of the big weaknesses of these devices. The membranes separating
the two electrolytes allowed molecules of electrolyte to leak across. As
a result, each solution became increasingly contaminated with the
other, reducing the battery"s output.
Skyllas-Kazacos"s solution to
this problem was to use the same chemical element for both electrolytes.
She could still provide the required difference in redox potential by
ensuring that the element was in different "oxidation states" in the two
solutions - in other words its atoms carried different electrical
charges. The element she eventually decided on was the metal vanadium,
which can exist in four different charge states - from V(ii), in which
each vanadium atom has two positive charges, to V(v), with five.
Dissolving vanadium pentoxide in dilute sulphuric acid creates a
sulphate solution containing almost equal numbers of V(iii) and V(iv)
ions.
When Skyllas-Kazacos added the
solution to the two chambers of her flow battery and connected an
outside power supply to the electrodes, she found that the vanadium at
the positive electrode changed into the V(v) form while at the negative
electrode it all converted to the V(ii) form. With the external battery
disconnected, electrons flowed spontaneously from the V(ii) ions to the
V(v) ions and the flow battery generated a current (see Graphic). Best
of all, it didn"t matter too much if a few vanadium ions on one side of
the membrane leaked across to the other: this slightly discharged the
battery, but after a recharge the electrolyte on each side was as good
as new.
After more than a decade of
development, Skyllas-Kazacos"s technology was licensed to a
Melbourne-based company called Pinnacle VRB, which installed the
vanadium flow battery on King Island. With 70,000 litres of vanadium
sulphate solution stored in large metal tanks, the battery can deliver
400 kilowatts for 2 hours at a stretch. It has increased the average
proportion of wind-derived electricity in the island"s grid from about
12 per cent to more than 40 per cent.
It hasn"t all been plain sailing,
though. For example, engineers have had to solve a perennial problem
with flow batteries - how to prevent leaks that allow energy to
literally dribble away - as well as working out how to construct
long-lasting membranes.
With the installation at King
Island up and running, it shows the advantages of vanadium flow
batteries over conventional electricity storage. Their working lifetime
is limited only by that of the membrane and other hardware, and is
expected to be several times the two to three-year lifespan of a
lead-acid battery. Like lead-acid batteries, they deliver up to 80 per
cent of the electricity used to charge them, but they also maintain this
efficiency for years.
One of the key advantages of flow
batteries is their scalability. To increase peak power output you add
more battery cells, but the amount of energy they will store - and
therefore the time they will operate on a full charge - can be expanded
almost indefinitely by building bigger tanks and filling them with
chemicals. The result is that the batteries can be used in a wide range
of roles, from 1-kilowatt-hour units (like a large automotive battery,
say), to power-station scales of hundreds of megawatt-hours.
Small vanadium flow batteries are
already operating in Japan, where they are used for applications such
as back-up power at industrial plants. In the US, a 2-megawatt-hour
battery installed in Castle Valley in south-east Utah has allowed the
local power company PacifiCorp to meet increasing peak power demands
without needing to increase the capacity of the ageing 300-kilometre
distribution line that feeds the area.
The vanadium-based technology
developed at the University of New South Wales is now being put to use
by VRB Power Systems, based in Vancouver, Canada. Last year the company
signed a $6.3 million contract to construct a 12-megawatt-hour vanadium
battery at the Sorne Hill wind farm in Donegal, Ireland. The idea is to
offer a guaranteed supply of wind-generated electricity, and improve the
economics of the wind farm by selling stored electricity to the grid at
peak times when prices are highest.
The company has commissioned a
new production line with the capacity to turn out 2500 5-kilowatt
batteries each year. The first dozen of these new batteries are
currently under evaluation by customers including the National Research
Council Canada and one of North America"s biggest cellphone companies.
This is an important stage of
development. At present, as with any new technology lacking economies of
scale, flow battery systems are more expensive than competing products,
but that could change once the new production line is running.
Basic research is continuing too.
Vanadium sulphate solutions cannot be made very concentrated so the
energy stored in a given volume of vanadium flow batteries is about half
that of lead-acid batteries. This rules them out for applications where
compactness and low weight are at premium - electric cars being a prime
example. So Skyllas-Kazacos and her team want to replace vanadium
sulphate with vanadium bromide, which is more than twice as soluble. She
expects that research to be completed by 2008.
VRB Power Systems has already
tested its units in electric golf carts. Just as with existing electric
vehicles, a car equipped with a flow battery could be charged by
plugging it into an electric socket. Enticingly, though, flow batteries
might one day allow drivers to refill the tank with energised
electrolyte. The spent solution can be recycled.
“Drivers could refill the tank
with energised vanadium”Whether or not we will one day top up our cars
with vanadium, King Island has proved that flow batteries already have a
practical role to play, keeping wind-generated electricity humming
through the wires even when the breeze drops. You might not even notice
it"s there - but that"s probably the biggest compliment you could pay
it.
Via: New
Scientist
