Power from the rain

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20 February 2008 News

Information from Frost & Sullivan Technical Insights

As wireless devices and electronic sensors become ever smaller and more distributed, developers are increasingly searching for ways to harvest energy from the environment to reduce the need for power connections or batteries.

In recent months, several groups have explored using micro-electromechanical systems (MEMS), piezoelectric systems to harvest energy from acoustic vibrations.

Other researchers are looking into thermoelectrics or miniaturised photovoltaics. Frost & Sullivan has however taken particular notice of a team of researchers in France that proposed a new source of environmental energy - falling raindrops - in the journal 'Smart Materials and Structures'. "Our system scavenges the vibration energy from a piezoelectric flexible structure impacted by a water drop," explained Thomas Jager, one of the authors of the report.

In the device, strips of a piezoelectric ribbon are suspended across a gap. When a drop of water splashes onto the band of material, it vibrates, creating a small amount of electrical energy in the piezoelectric material. "It appears that to effectively recover energy from raindrops using piezoelectricity, the material used must be very thin, not pre-stressed, and with a width slightly smaller than the maximum diameter of the impacting drop," Jager said. The strips should also be completely covered with conducting electrodes to yield the maximum effect.

The researchers found that about 1 nJ of electrical energy and 1 μW of instantaneous power could be recovered using raindrops. "This is the worst-case scenario, as simulations show that it would be possible to recover 25 microjoules and 12 milliwatts from a downpour drop," Jager explained.

Raindrops in a downpour are larger in size and fall more rapidly, delivering greater kinetic energy to the piezoelectric band. The researchers are now working to determine the effect that different kinds of dropping action have on the power generated, as they believe that 'splashing' could reduce the power available to the system. They are also exploring ways to conveniently store the electrical power produced by such a system to allow a device to make use of energy from multiple raindrops.

For more information contact Patrick Cairns, Frost & Sullivan, +27 (0)21 680 3274, [email protected]

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