Piezoelectric power generation in tires

      The battery range of many electric vehicles is limited, meaning that such methods of transportation can only be used for short trips. However, in extended-range electric vehicles or plug-in hybrids, manufacturers include an internal combustion engine that uses conventional fuels to recharge batteries in motion and, hence, extend their range.1 Although practical, this approach defies the purpose of having an electrical driveline: to reduce environmental impact through the use of renewable energy sources. The need remains for a cleaner power-generation technique that allows maximum range extension with minimal environmental consequences.

      Piezoelectric materials generate electrical energy when subjected to mechanical strain. Power-generation devices based on such materials have surfaced in recent years in the context of vibrational-energy harvesting.2 However, their output has only been sufficient to power sensors and other small, low-energy-consumption gadgets. Benders made of PZT (lead zirconate titanate, the most common piezoelectric ceramic material) attached to a tire3 have also been used but only to supply energy to tire-pressure sensors that operate intermittently. To obtain high-power output from this process, it is imperative to cover as much of a tire's inner surface area as possible with PZT benders. In this way, and because these elements produce power through deformation at the road-tire interface known as contact patch, a reliable and continuous source of energy for the moving vehicle is guaranteed.

      A 4×40 array of low-cost highly bendable PZT elements was bonded to most of the inner surface of a tire using a flexible adhesive (see Figure 1).4 Since the storage capacitor is polarized, the voltage response of these elements, which is an AC (alternating current) waveform, has to be converted into a DC (direct current) signal by a rectifier before it can be cached in the capacitor. Each row of benders, running across the width of the tire, is treated as one generator and is rectified separately with all the PZT lines then connected in parallel. At any given time only two or three rows, depending upon the length of the contact patch, generate power. Since rectifiers provide very high resistance in reverse mode (opposite to the direction of permitted current flow), the remaining PZT benders act like a disconnect in the path of electric current, that is, like an open circuit.

       To test the power output, we designed a dynamometer that permits the tire to rotate at different rates and allows simulation of different vehicle weights using a pneumatic piston. We measured the power at various rotations per minute (rpm) by applying the output voltage across known resistance values. The average voltage changes with both load and rpm.

      We experimentally determined the resonant impedance of the entire harvester module to be 1000Ω and used that value in our measurements. A power of 2.3 watts was produced across this load at 854rpm, roughly equivalent to 100km/h on the road for the tire size we used. Figure 2 shows the power and voltage across this 1000Ω load as a function of rpm. As anticipated, the power output increases with the increasing deformation frequency of piezo benders or, equivalently, with rpm. The well-known approach of stacking various layers of PZT material on top of each other was used to increase the power output. We tried using two layers of PZT benders, which doubled the output to 4.6 watts.

      Power is extracted from the tire-wheel system using a commutator-like assembly, that is, one that continuously maintains the electrical contact between the chassis and the wheel while allowing the latter to spin freely. Such a setup permits the extraction of tire-generated power to run onboard electronics.

      Remon Pop-Iliev is an associate professor and senior chair in innovative design engineering. His research has been funded by the Natural Sciences and Engineering Research Council of Canada and by General Motors of Canada Limited since 2005. His team is actively developing efficient technologies for manufacturing cellular materials and composites, long yet lightweight robotic arms, and innovative sustainable mobility technologies such as hydrogen-powered extended-range plug-in hybrid electric vehicles.