The New Scientist
January 13, 2007
http://environment.newscientist.com/article/mg19325861.400
A bank for wind power
By Tim Thwaites
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.
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.
No more leaks
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.
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.
--
Cold Mountain, Cold Rivers
Working at the Crossroads of Environmental and Human Rights since 1990
PO Box 7941
Missoula Montana 59807
(406)728-0867
posted to ClimateConcern
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