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November 23, 2009

Converting Wasted Kinetic Energy into Electricity

Harvesting energy in the wake of a circular cylinder using piezoelectric materials [Harvesting energy from a flying jet plane Using the new technology automobiles and aircraft could harness currently wasted kinetic energy to generate electricity and power some systems.

About a half-inch by one inch, these piezoelectric devices might be mounted on the roof or tail of a car or on an airplane fuselage where they would vibrate inside a flow, producing an output voltage. Although the power generated would not be enough to replace that supplied by the combustion engines, it could be enough to run some systems, such as batteries that would be used to charge control panels and other small electronic devices such as mobile phones.

The group of researchers from the City College of New York (CCNY) led by Prof Yiannis Andreopoulos, is currently attempting to optimize these devices by modeling the physical forces to which they are subjected in different air flows - on the roof of a car, for instance, or on the back of a truck.

When the device is placed in the wake of a cylinder - such as on the back of a truck - the flow of air will cause the devices to vibrate in resonance, says Andreopoulos. On the roof of a car, they will shake in a much more unsteady flow known as a turbulent boundary layer




The voltage generated by short, flexible piezoelectric cantilever beams placed inside turbulent wakes of circular cylinders at Reynolds numbers of 10,000 is investigated experimentally and computationally. The coherent vortical structures present in this flow generate a periodic forcing on the beam which when tuned to its resonant frequency produces maximum output voltage. There are two mechanisms which contribute to the driving forcing of the beam. The first mechanism is the impingement of induced flow by the passing vortices on one side of the beam and second is the low pressure core region of the vortices which is present at the opposite side of the beam. The sequence of these two mechanisms combined with the resonating conditions of the beam generated maximum energy output which was also found to vary with the location in the wake. The maximum power output was measured at about two diameters downstream of the cylinder. This power drops off the center line of the wake and decays with downstream distance as (x/D)$^{-3/2}$. A three-way coupled interaction simulation that takes into account the aerodynamics, structural vibration and electrical response of the piezoelectric generator has been developed.


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