Most transducers designed to harvest vibrational energy throw away a lot of the energy they harvest. That's because these devices typically send their output to voltage rectification circuitry and regulator ICs that themselves draw appreciable power.
But that's not a problem with a novel harvesting mechanism from researchers at New York's Stony Brook University. The device is designed to sit on railroad tracks and harvest the energy from track deflections as trains roll by. To do so, researchers devised a mechanical mechanism that includes two roller clutches and a flywheel which turn a conventional generator.
The apparatus thus makes use not only of the downward deflection of the track, but also of the upward motion as the track springs back after the train wheel rolls away from the point of contact.
The harvester is the brain child of Professor Lei Zuo and graduate students Gopinath Reddy Penamalli, Teng Lin and John Wang from Stony Brook's Dept. of Mechanical Engineering. They say their device can harness 200 W of electric energy from train-induced track deflections (typically on the order of a half inch, more for poorly maintained tracks). In a prototype they've devised, they've been able to measure a mechanical efficiency of between 55 and 72%. They also say they've licensed the technology to a company called Electric Truck/Harvest NRG for commercialization.
The transmission mechanism includes three shafts with three spur gears, a pair of rack and pinions, and two roller clutches. The two racks move together up and down. The roller clutches transmit motion only in one direction. When the two racks move downward together, they drive both pinions to rotate in opposite directions, but only one gear is engaged to drive the output shaft. When the two racks move upward together, the racks drive both pinions to rotate in the opposite direction but only the other roller clutch is engaged to drive the output shaft. So regardless of whether the racks move up or down, the output shaft always rotates in one direction.
Moreover, if the output shaft and generator spins faster than a preset speed, both roller clutches will disengage. This lets the researchers incorporate a flywheel to make full use of the erratic pulse-like vibrations that characterize train tracks. At high vibration velocity some kinetic energy will be stored in the fly wheel; when the vibration velocity is low or zero, the kinetic energy stored in the flywheel can drive the electric generator.
Researchers say their design is actually a mechanical motion rectifier‖, where the roller clutch is analogous to a diode in a voltage rectifier. It rectifies the irregular reciprocating vibration the way a voltage rectifier regulates an ac voltage.
Researchers say their design permits use of a generator that is smaller than the usual case for energy harvesters. In typical harvester applications, the electric generator must be selected to handle peak power which is usually 10 times or more higher than that generated by the average vibration, and most of the time the generator works in a speed or load far from its most efficient operational condition. As a result, the efficiency is super low. Use of a flywheel gets around this problem because the electrical generator will rotate in one direction with only small fluctuations. Therefore, the generator will work mostly in a more efficient speed range.
This feature is especially valuable for track vibration harvesting because the train-induced deflection is in the form of short-time large-peak pulses. The traditional harvester designs are extremely inefficient.
Researcher Penamalli wrote his master's thesis on the train track harvesting problem and goes into some detail on the harvester operation: http://dspace.sunyconnect.suny.edu/bitstream/handle/1951/56088/Penamalli_grad.sunysb_0771M_10694.pdf?sequence=1
Stony Brook Dept. of Mechanical Engineering: http://me.eng.sunysb.edu/