Scientists at Iowa State University created an artificial tree that converts the wind’s kinetic energy into electricity. The group includes associate professor Michael A. McCloskey, associate scientist Curtis L. Mosher, and Professor Eric R. Henderson from the Genetics, Development and Cell Biology Department. Their study, entitled “Wind Energy Conversion by Plant-Inspired Designs,” was published in the Plos One science journal, on January 13, 2017. They used biomimetics, a conceptual approach that involves knowledge transfer from natural sciences to eco-responsible technologies.
Michael McCloskey, associate professor of genetics, development and cell biology, and director of this study, noted that this concept will not replace or optimize wind turbine design because it is supposed to function as an off-grid energy source adaptable to smaller spaces. The device will be able, for example, to supply energy to household appliances. The prototype, developed for the experimental purposes of the project, is an assembly of metal meshes ending with plastic sheets in the form of tree leaves. When the wind blows, the leaves make vertical flapping movements. The sheets contain special plastic strips that release an electric charge when bent by a breeze. According to McCloskey, this arborescent structure can be used to create devices that generate enough electricity to meet the needs of small-scale public spaces, while hiding cellular relay antennas (camouflage artifacts are already being used to this end).
Biomimetic Method: Initial Idea and Development
The researchers took their inspiration from the poplar leaf. Why? According to French legend, poplar leaves stir at the slightest breeze because its wood was used to make the cross of Christ. Since then, the poplar tree always trembles. But in reality, the foliage is highly agitated and rustling because the rounded leaves are connected to the stems by a very long flattened peduncle, which causes regular and intense flapping.
They imitated this structure because the leaves move very energetically at the slightest breeze, which exerts high stress on their surface. This stress is what generates electricity in the strips or, in other words, causes the piezoelectric effect. Piezoelectricity is the property of certain materials that are electrically polarized under a specific force. The piezoelectricity of quartz, discovered in 1880 by the Curie brothers, is still used today in objects such as clocks and automatic lighters. Lighters produce a flame when the force exerted by a push button on the crystal causes an electrical charge, which is instantly released in the form of a spark. There are also piezoelectric polymers that produce electricity when subjected to mechanical stresses.
The scientists chose the poplar leaf also because it oscillates in a regular pattern which optimizes electricity production. According to their calculations, in a 10 mph breeze (16 km/h) a poplar with 500,000 leaves can generate 80 watts per leaf. Obviously, voltage increases with wind speed. The group also conducted comparative tests by stimulating the artificial leaves with nitrogen gas pulses of 34 psi (pounds per square inch). They noticed that the air-generated stress helps regulate electricity production with more precision. The sheets designed by the researchers are laminated polyvinylidene fluoride films (PVDF) and measure 6.15 by 1.22 cm. With this configuration, they react the same way as natural leaves because they perform the same movement, at the same speed, and can thereby produce the calculated voltage.
The study was funded by the National Science Foundation, and Don Halvorsen of Measurement Specialties, Inc. (Hampton, VA).