Ferrofluids used to generate power

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ebeuerle
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Ferrofluids used to generate power

Post by ebeuerle » Thu Mar 23, 2006 4:01 pm

From NewScientist magazine:

Everlasting power in the offing

"IT LOOKED like a slug moving along the lab bench," says Jeffrey Cheung, a materials scientist at Rockwell Scientific in Los Angeles. "My first reaction was - oh my goodness someone forgot to turn off the sprinkler outside, and this thing has crawled into the lab. The strange thing was, when I moved to the right or the left, it always followed my movements." Then he leaned over to take a closer look. To his surprise, the slug shot off the workbench and rocketed straight at his midriff.

That day, Cheung had been doing some experiments using a commercial ferrofluid. As fate would have it, he made two crucial errors. First he lost a bar magnet, which he had borrowed from a colleague for the experiment. Then he spilt a beakerful of the fluid over his lab bench, leaving it covered with a thick layer of reddish-brown goo.

What happened next led not only to the acrobatic slug, but the beginning of an intriguing new technology which could soon be used for anything from constructing executive toys to large-scale electricity generation.

"I was a mess," says Cheung, recalling the accident. "My lab coat looked like exhibit A from a crime scene." But already the possibilities opened up by the gloop-covered magnet were racing through his head. "Instead of going to wash my face I grabbed a piece of paper and a pencil and started to jot down a lot of ideas - I think I wrote about two pages."

The slug, it turned out, was the missing magnet with globs of ferrofluid tightly bound to each end. Ferrofluids are simply a suspension of magnetic nanoparticles in an inert liquid of some kind. Pour a little fluid around a magnet, and it quickly migrates to the magnet's poles and stays there, in the same way as iron filings cling to the ends of a bar magnet. It was the coating of liquid on each end that dramatically reduced the friction between the magnet and the bench, so when Cheung's metal belt buckle came into range, the attractive force faced little resistance and the magnet launched itself straight at it.

Cheung's eureka moment came with the realisation that the ferrofluid can act as a super-efficient lubricant, and that there are a wealth of ways to exploit it. One of his favourites is to use a ferrofluid-covered magnet as the heart of an electricity generator that needs no input except gentle motion.

The idea relies on simple high-school physics: move a magnet close to a copper coil and the changing magnetic field experienced by the coil will induce an electric current to flow through it. Cheung placed a magnet in a tube filled with ferrofluid, wrapped a coil around the tube, and stuck a magnet at each end to keep the magnet inside moving. The result is a system that turns random motion into electricity, with almost no loss of energy to friction. The key is the exceptional slipperiness of the ferrofluid coating - around 40 times as slippery as ice.

Now Cheung needed to test out his device in a place that would provide free random motion, and that is not difficult to find. "The ocean waves are always changing," he says. So, armed with a grant from the Pentagon's Defence Advanced Research Projects Agency, Cheung and his team set about designing a system to provide power for the buoys used for oceanographic monitoring.

Existing buoys use battery packs or solar panels to power their monitoring and communications equipment. Clean solar panels work well when the sun is high in the sky, but have to be backed up by batteries for the rest of the day and all night. After a while, the spattering of guano from perching sea birds will cut down their efficiency even in bright sunlight. Batteries are bulky, need reliable waterproofing and eventually have to be changed - not an easy option for a buoy in the middle of the ocean. Cheung's generator, by contrast, will run day and night on the smallest waves in the glummest of weather, and since it is hermetically sealed corrosion should not be a problem.

Oceanographers at the Scripps Institution of Oceanography in La Jolla, California, tested the design in the summer of 2004. "Just a few watts of power is all that's required to run most marine instruments and to transmit their data to a satellite," says Robert Pinkel, head of the buoy development team at Scripps. The team designed a float that amplifies its movement to deliver maximum acceleration to Cheung's device, and electronics to store the electricity it generates in a super-capacitor. The generator proved itself even in calm conditions: in a gentle sea with waves of around 60 centimetres it generated 0.3 watts. With further work to optimise the transfer of wave power to the generator, the team hopes it will be able to deliver on average 1 watt of electricity.

Cheung's latest design uses coils mounted at right angles to the direction of the magnet (see below). The challenge now is to increase the power output by tweaking the design of both the buoy and the generator. Cheung is planning to work with Malcolm Spaulding and Stephan Grilli from the University of Rhode Island in Kingston to model the most efficient designs and test them in a wave tank.

The aim is to generate not merely watts, but megawatts. "Our goal is to build an energy farm on the ocean," Cheung says. Stephen Salter, who heads the wave power group at the University of Edinburgh, UK, thinks that may be a step too far. Though the iron particles in the ferrofluid maximise the current induced in the coil - by increasing the flux density around it - Salter is sceptical about the technology's viability for large-scale generation. "I think this is one technology that's going to run out of puff as you scale it up." Nevertheless, Cheung says he could have a prototype of a wave power system ready within three years.
“Our goal is to build an energy farm on the ocean”

Even if Cheung doesn't manage to scale it up efficiently, he has dreamed up plenty of other applications for his invention. So many, in fact, that he has set up a company to commercialise them. They include self-powered tyre pressure monitors, computer mice and TV remote controls.

He has an idea for an executive toy, too. Put an even number of magnets coated in ferrofluid into a doughnut-shaped tubular ring - with their north and south poles facing each other - and they will shuttle around chaotically as they repel one-another. The result is somewhere between a stress ball and a pocket-sized lava lamp.

Cheung's first commercial product is going to be much more useful. In the coming months his company will launch a holster-style mobile phone charger. "The first goal is to have enough power to keep the phone on standby forever," he says. All you'll have to do is shake it.
From issue 2544 of New Scientist magazine, 25 March 2006, page 46

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