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Scientists make electricity
from tap water
By Roger Highfield,
Science Editor
(Filed: 20/10/2003)
Scientists have discovered a new way of generating
electricity using water, the first innovatory method for 200
years.
A team of Canadian researchers has found that an
electrical current can be produced between the ends of a microscopic
channel when a fluid flows through it.
The technique offers a potential source of clean,
non-polluting electric power with a variety of possible uses,
ranging from powering small electronic devices such as calculators
or mobile phones to vast stations that can contribute to the
national grid.
The method, which harnesses the "electrokinetic"
properties of liquids such as ordinary tap water when they are
pumped through microscopic channels, is described today in the
Institute of Physics publication Journal of Micromechanics and
Microengineering.
The research has been led by Prof Daniel Kwok and
Prof Larry Kostiuk at the University of Alberta. They suggest that a
mobile phone could be powered by squirting water at high pressure
through an array of such channels.
Prof Kostiuk said: "This discovery has a huge number
of possible applications. It's possible that it could be a new
alternative energy source to rival wind and solar power, but this
would need huge bodies of water to work on a commercial scale.
"Hydrocarbon fuels are still the best source of
energy but they're fast running out and so new options like this one
could be vital in the future."
The energy source for this device is the work done to
push the liquid through the channel. Although the power generated
from a single channel is tiny, millions of parallel channels can be
used to increase the output.
The channels can be made from any non-conducting
material such as glass, plastics, rocks or ceramics. Standard
commercial filters made of these materials already have the millions
of channels that can be used.
The key to the phenomenon is the way that charges
naturally separate at the interface between the surface of a channel
and a fluid. Scientists believe that this occurs because minute
parts of the solid of one charge (either positive or negative)
dissolve into the water. As a result, the surface becomes
charged.
Opposite-charged ions (charged atoms) in the liquid
are attracted to it, while like-charged ions are repelled, resulting
in a thin liquid layer with a net charge, called the Electric Double
Layer, measuring a few billionths to a few millionths of a metre
across.
The team constructed a channel with a diameter
similar to the distance across the layer and forced liquid through
this channel. When a fluid, such as water which naturally contains
an equal number of oppositely charged ions, is forced down the
channel, a charge separation occurs.
The ions that have a charge opposite to the solid are
preferentially attracted into the channel (remembering that opposite
charges attract each other) and transported to the far end. The ions
of the same charge as the solid are preferentially left behind at
the inlet side of the channel.
Therefore, the liquid at the two ends of the channel
have opposite charges. This produces a voltage difference. If
conducting electrodes are placed at the two ends of the channel and
connected by a wire, then current flows and electricity is
produced.
A typical setup using a ceramic filter and tap water
can produce 10 volts and the current depends on the size of the
filter. Since large water pressures are not needed, natural flows of
water can be harnessed. These could include tidal water flows,
underground aquifers, dammed water, drinking water currently being
filtered by utilities companies to improve its clarity, and rain
falling from roofs. |
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