| 
  • If you are citizen of an European Union member nation, you may not use this service unless you are at least 16 years old.

  • You already know Dokkio is an AI-powered assistant to organize & manage your digital files & messages. Very soon, Dokkio will support Outlook as well as One Drive. Check it out today!

View
 

Electricity Stored

Page history last edited by PBworks 17 years, 2 months ago

 

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

Comments (0)

You don't have permission to comment on this page.