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Solar & Alternative Energy

Energy-harvesting power sources for a wide range of applications

Piezoelectric-based sources have the potential to supplement or replace onboard batteries in in many types of devices in which ambient shock, impact, vibration, or oscillatory motions are present.
24 July 2007, SPIE Newsroom. DOI: 10.1117/2.1200707.0810

The choice of batteries for use with munitions, wireless and remote sensors, and other similar devices, raises a number of safety and cost-related issues. Power sources that harvest energy from the environment are often ideal because they are safe, provide long shelf life, and are low-cost compared to many reserve and rechargeable batteries. The patented piezoelectric (PE)-based sources we have developed at Omnitek Partners, LLC1–5 can generate sufficient energy for many applications with low to medium power requirements.

In the past, PE-based devices have generated electrical energy from imparted shock/impact, acoustic noise, and vibration. But quantity has been feeble. By contrast, our power sources can store shock/impact and vibration energy in spring elements, and subsequently harvest it as electricity at a desired rate rate from the vibration of a mass-spring system attached to a PE element.

Figure 1. A 1.0×2.25-inch piezoelectric generator for vibration energy harvesting.

With this approach, during impact or firing of a gun, for example, the spring element of the generator is deformed due to inertial forces acting on the system. Potential energy is thereby stored in the spring elements. The stored potential energy causes the spring-mass element to vibrate at its natural frequency. Piezoelectric elements or generators can subsequently convert the mechanical energy of vibration into electrical energy. The power sources are packaged to withstand high shock and vibration levels, including the firing of large caliber guns.

Figure 2. 0.75×1.75-inch High-G piezoelectric generator.

To date, prototypes of several classes of such PE-based energy-harvesting power sources have been designed, fabricated, and tested. Impact tests, drop tests, and air-gun tests to over 40kGs have been performed to validate performance and survivability. To provide real-time performance data, actual firing tests for power sources integrated into large caliber rounds, with full telemetry instrumentation, are planned for summer 2007. Rounds are to be fired at about 50kGs and are expected to withstand firing accelerations of over 120kGs.

Novel classes of PE-based generators promise ultra-safe harvesting of mechanical energy, from both the firing acceleration and in-flight vibration and other forms of motion, such as spinning.

Current energy-harvesting power sources are suitable for applications that have low to medium power requirements, particularly when safety and long shelf life are critical factors. The quantity of mechanical energy to be stored in the spring can be tuned for a particular application by varying spring design parameters. The resonant frequency of the generator spring may also be tuned to satisfy the rate at which electrical energy is made available.

Energy-harvesting power sources have been designed that can store over 2J of energy obtained from firing shock. Packaged in a 0.75-inch diameter, 2.5-inch long cylinder, units can withstand firing accelerations of more than 100kGs. The efficiency of converting mechanical to electrical energy depends on the generated charge collection and storage electronics, with efficiencies ranging from 30-50%. This energy is usually used to directly drive a load, or it can be stored in a secondary device such as a capacitor or rechargeable battery.

In addition to PE-based energy harvesting power, we are developing other types of impact-based sources to power emergency devices. An innovative two-stage method can harvest energy from low and varying frequency oscillatory or rotary motions. Envisioned applications for these units include power for remote sensors on vehicles or ships. Two-stage rotary harvesters are well-suited to serve as high-efficiency generators for windmills, tidal flow turbines, and other similar turbo-machinery.

This work is supported by the Hybrid Energy Systems Program at The US Army ARDEC at Picatinny Arsenal, New Jersey, with POCs Carlos Pereira, Chris Janow, Maria Allende-Pastrana, Hai-Long Nguyen, Charles McMullan.