Micro waterflows make power

By Kimberly Patch, Technology Research News

Energy systems are all about converting relatively abundant forms of stored energy to forms of energy that produce useful work.

Energy can be stored mechanically, in the pressure of water behind a dam, chemically, in the bonds that link atoms to form molecules, and atomically, in the forces that hold together the particles that make up atoms.

The trick to tapping into the stores is coaxing energy into a form -- like a flow of electrons -- that can be easily directed to perform work, and to do so efficiently and without creating toxic waste.

Researchers from the University of Alberta in Canada have made a device that taps the same principles of charge separation used by fuel cells in order to directly convert water pressure to electricity.

The device contains millions of microscopic channels, and gleans a small amount of electricity from the interaction between flowing water and the surface of each channel. The researchers' prototype tapped the water pressure generated by a hand-operated syringe to power a light-emitting diode.

In principle, the method could be used to convert any type of water pressure, including river flows and tides, to electricity.

For several years now, microfluidics researchers developing devices like biochips have used electricity to move tiny amounts of fluids. As with many things in nature, the opposite is also true. Water pressure can produce a voltage.

Microfluidics researchers have tended to concentrate on manipulating fluids rather than generating power, however. "I have been working on electrokinetic phenomena for years, but not power generation," said Daniel Kwok, a professor of mechanical engineering at the University of Alberta.

The idea of using microfluidics to generate power came as a surprise when talking to a colleague about separating charges by pushing water into extremely small channels, said Kwok. The two realized that if they were able to separate charges they should be able to make an energy conversion device, said Kwok.

The conversion process depends on the way ions naturally congregate at the boundary between water and the glass channel. The chemical reaction at the boundary between water and most solids leaves the solid surface negatively charged, which in turn attracts positive hydrogen ions contained in the water.

Ions form when atoms gain or lose an electron and so have more or fewer negatively-charged electrons hovering over the atomic nucleus than positively-charged protons contained in the nucleus.

This charged liquid-solid interface, known as the electrical double layer, is a focus of research aimed at using electricity to manipulate liquids. Applying a voltage to a water-filled microscopic glass tube causes the positively charged ions to flow, and friction drags the rest of the water along.

Conversely, when water is forced through a microchannel it produces a streaming electric current by pushing the positively-charged ions in the direction of the water current.

The researchers realized that if they put many parallel microchannels together, creating a large surface-area-to-volume ratio, the streaming current could add up to a potentially useful force. The larger the ratio, the more movable ions are available, and the larger the streaming current.

The researchers tested the idea using a glass filter that contained millions of microscopic channels. They produced electrical currents of 1 to 2 millionths of an amp by flowing water through the filter from a 30-centimeter hydrostatic pressure drop. The researchers were able to increase the electrical flow by increasing the salt concentration in the flowing solution.

The device produced enough electricity to power a light-emitting diode, according to Kwok. "So far, by a simple push on a syringe using my hand... we can light up LEDs," he said. The hand pressure on the syringe was about three pounds per square inch, he said.

It should be possible to scale the method up to produce larger amounts of electricity, said Kwok. "In principle, one can scale up the channel to do [more] work -- that should not be a problem." The method should also work with naturally porous materials, he said.

Research into better understanding interactions at small scales is needed before practical devices can be made, said Kwok.

Using microchannels to create streaming current drag and harnessing energy from it is an interesting idea, said Bor Yann Liaw, an associate specialist at the Hawaii Natural Energy Institute. "I think the idea will work in principle; the question is how to make it work efficiently," said Liaw.

It will take at least 8 to 10 years for electrokinetic microchannel power generators to become practical, according to Kwok.

Kwok's research colleagues were Jun Yang, Fuzhi Lu and Larry W. Kostiuk. The work appeared in the November, 2003 issue of the Journal of Micromechanics and Microengineering. The research was funded by the Alberta Ingenuity Establishment Fund, the Canada Research Chair Program, the Canada Foundation for Innovation, and the Natural Sciences and Engineering Research Council of Canada.

Timeline:   8-10 years
Funding:   Government
TRN Categories:  Energy; Microfluidics and BioMEMS
Story Type:   News
Related Elements:  Technical paper, "Electrokinetic Microchannel Battery by Means of Electrokinetic Microfluidic Phenomena," Journal of Micromechanics and Microengineering, November, 2003


November 5/12, 2003

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