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<H1 class=inline>A bank for wind power</H1></DIV>
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<LI>12 January 2007 </LI>
<LI>Exclusive from New Scientist Print Edition. <A
href="http://environment.newscientist.com/subscribe.ns?promcode=nsenvarttop">Subscribe</A>
and get 4 free issues </LI>
<LI><B>Tim Thwaites</B> </LI></UL></DIV>
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<DIV class=straptext style="WIDTH: 120px">Power to the people</DIV></DIV>
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<DIV class=straptext style="WIDTH: 120px">Catching the wind</DIV></DIV>
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<P>Sitting at the western end of Bass Strait between the Australian mainland and
Tasmania, King Island might not seem like a beacon to the future. Yet inside a
large metal shed close to the island's west coast is an electricity storage
system that promises to transform the role of wind energy.</P>
<P>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.</P>
<P>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.</P>
<P>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.</P>
<P>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.</P>
<P>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.</P>
<P>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.</P>
<P>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.</P>
<H5>No more leaks</H5>
<P>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.</P>
<P>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.</P>
<P>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 <FIGREF
refid="mg25861401.jpg">(see Graphic)</FIGREF>. 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.</P>
<P>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.</P>
<P>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.</P>
<P>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.</P>
<P>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.</P>
<P>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.</P>
<P>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.</P>
<P>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.</P>
<P>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.</P>
<P>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.</P>
<P>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.</P>
<P>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.</P>
<DIV class=authoraff></DIV>
<DIV class=straptext>From issue 2586 of New Scientist magazine, 12 January 2007,
page 39-41</DIV>
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