Using their laboratory prototype, which was developed from off-the-shelf parts, the Penn State researchers can generate 50 milliwatts. Although they haven't tried it, they believe the motion of a runner could be harnessed to generate enough power to run a portable electronic music device. By comparison, simple, un-optimized energy harvesting circuits, for example the type used to power LEDs on "smart" skis, can only generate a few milliwatts.
The researchers say the new circuit offers an environmentally friendly alternative to disposable batteries for wearable electronic devices or for wireless communication systems. In addition, the circuit could be used in sensor and monitoring networks that manage environmental control in office buildings, robot control and guidance systems for automatic manufacturing, warehouse inventory; integrated patient monitoring, diagnostics, drug administration in hospitals, interactive toys, smart home security systems, and interactive museums.
The new circuit is described in a paper, "Adaptive Piezoelectric Energy Harvesting Circuit for Wireless, Remote Power Supply," published in the September issue of the journal, IEEE Transactions on Power Electronics. The authors are Geffrey K. Ottman, former Penn State master's degree student; Dr. Heath Hofmann, assistant professor of electrical engineering; Archin C. Bhatt, former Penn State master's degree student; and Dr. George A. Lesieutre, professor of aerospace engineering and associate director of the Penn State Center for Acoustics and Vibration.
Lesieutre explains that, like other energy harvesting circuits, the new Penn State device depends on the fact that when vibrated so that they bend or flex, piezo-electric materials produce an alternating or AC current and voltage. This electrical power has to be converted to direct current or DC by a rectifier before it can be stored in a battery or used. Hofmann adds that the magnitude of the piezoelectric material's vibration determines the magnitude of the voltage: "Since, in operation, the amount of vibrations can vary widely, some way must also be found to adaptively maximize power flow as well as convert it from AC to DC."
Using an analytical model, the team derived the theoretical optimal power flow from a rectified piezoelectric device and proposed a circuit that could achieve this power flow. The circuit includes an AC-DC rectifier and a switch-mode DC-DC converter to control the energy flow into the battery.
The Penn State researcher notes that using an approach similar to one used to maximize power from solar cells, the team developed a tracking feature that enables the DC-DC converter to continuously implement the optimal power transfer and optimize the power stored by the battery.
The circuit is the first to include an adaptive DC-DC converter and achieves about 80 percent of the theoretical maximum - well above the operating output of simple energy harvesting circuits.
The research was supported by a contract with the Office of Naval Research.