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The mechanism, which might also be well-suited for powering weather sensors in hostile environments, was created after University of Warwick engineers Sam Tucker Harvey, Dr Igor Khovanov and Dr Petr Denissenko investigated the leaves’ quivering in low wind speed.
The researchers decided to investigate whether the underlying mechanisms could “efficiently and effectively” generate electrical power, finding that it could.
“What's most appealing about this mechanism is that it provides a mechanical means of generating power without the use of bearings, which can cease to work in environments with extreme cold, heat, dust or sand,” said Harvey, the lead author of the research paper.
“While the amount of potential power that could be generated is small, it would be more than enough to power autonomous electrical devices, such as those in wireless sensor networks. These networks could be utilised for applications such as providing automated weather sensing in remote and extreme environments.”
Another application could be a back-up power supply for future Mars landers and rovers, said Denissenko.
“The performance of the Mars rover Opportunity far exceeded its designers' wildest dreams but even its hard-working solar panels were probably eventually overcome by a planetary-scale dust storm. If we could equip future rovers with a back-up mechanical energy harvester based on this technology, it may further the lives of the next generation of Mars rovers and landers.”
The key to aspen leaves' large-amplitude quiver in low wind is not just the shape of the leaf, the researchers said. More importantly, it relates to the effectively flat shape of the stem.
The researchers used mathematical modelling to come up with a mechanical equivalent of the leaf. They then used a low-speed wind tunnel to test a device with a cantilever beam, like the flat stem of the aspen leaf, and a curved blade tip with a circular arc cross-section acting like the main leaf.
The blade was then oriented perpendicular to the flow direction, allowing the harvester to produce self-sustained oscillations at uncharacteristically low wind speeds – just like aspen leaves. The tests showed that the air flow became attached to the rear face of the blade when the blade's velocity became high enough, acting more like an aerofoil rather than the bluff bodies that have typically been studied in the context of wind-energy harvesting.
In nature, the propensity of a leaf to quiver is also enhanced by the thin stem's tendency to twist in the wind in two different directions. However, the researchers' modelling and testing found that they did not need to replicate the additional complexity of a further degree of movement in their mechanical model. Simply replicating the basic properties of the stem and main leaf created enough mechanical movement to harvest power.
Next, the researchers will examine which mechanical movement-based power-generating technologies would be best to exploit the device and how it could be deployed in arrays.
The research was published in Applied Physics Letters.
Content published by Professional Engineering does not necessarily represent the views of the Institution of Mechanical Engineers.