Figure 1. Acceleration input vs. the time to charge to 5V in a 200 µF capacitor.

Recent advances in low-power electronics have generated tremendous interest and provided new opportunities for the use of piezoelectric materials to power electronic devices without the need for batteries.^1,2,3,4 Ceramic fiber spinning technology has been refined to produce inexpensive piezoelectric fiber composites, which have generated sufficient power to run wireless sensor networks and electronic devices from waste mechanical energy.

Piezoelectric ceramics generate electricity when exposed to a mechanical stress, and, conversely, they change shape when an electric field is applied. Piezoelectric fibers take full advantage of this phenomenon, and piezoelectric fiber composites are being produced for their maximum energy-harvesting efficiency. These ceramic fibers are flexible and, in composite form, very robust. Typical successes include powering wireless sensors from low-level environmental vibrations as the source of power, and recharging or eliminating batteries in remote locations. Figure 1 shows the energy generation capability of these devices under vibration. At the low acceleration level of 0.1 g, it takes just 5 minutes to charge sufficient energy to power a wireless sensor and transmitter. At about 1 g, the charging time reduces to under 1 minute.

This green technology produces useful amounts of electricity from environmentally available sources of mechanical energy and eliminates the need for expensive wiring or the installation of heavy, expensive, difficult-to-replace and polluting batteries. In addition, ceramic fiber-powered products* are priced competitively with rechargeable batteries.

*Harvestor^TM, developed and produced by Advanced Cerametrics, Inc.

Energy Harvesting

Energy harvesting (EH), sometimes referred to as energy scavenging, has gained tremendous attention as a means to lessen or eliminate the need for battery power. Novel EH approaches are available to generate power by utilizing energy from human and environmental sources.⁵ Given battery limitations, wider adoption of EH is coming but requires a clever combination of design skills. A multi-discipline approach is required that leverages knowledge in several fields of engineering, including electrical, mechanical, material and process.

The concept of EH is not new. Consider hand-cranked radios, flashlights and watches powered by shaking, as well as windmill farms, solar energy and even sailboats. What is new is the application of EH for ultra-low-power embedded electronics that require no moving parts. The convergence of high-charge piezoelectric ceramics and ceramic fiber composite process technology has enabled the application of self-powered systems for an array of electronics. Added to the obvious advantages associated with utilizing free mechanical energy to power many types of systems, EH eliminates concerns regarding battery replacement and disposal.

Converging Technologies

The science of piezoelectric devices is fairly well understood in the engineering world, but their application remains a nascent field rich with possibilities. The emergence of piezoelectric ceramic fiber “super transducers” that offer significantly increased deliverable power, combined with electronic components that measure performance in nano-watts, is opening a range of new products and services.

The need for constant and/or long-term power for numerous electronic systems and devices is fueling extensive research, development and growth. Piezoelectric ceramic fibers, given their unique properties of flexibility, light weight and higher output per pound of material, offer the greatest potential for enabling the wide-scale deployment of self-powered piezoelectric ceramic systems.

Conventional piezoelectric ceramic materials are rigid, heavy and produced in block form. A new low-cost technology, termed the viscous suspension spinning process (VSSP), can produce fibers that range in diameter from 10 to 1300 microns.^6,7,8,9 When formed into user-defined (shaped) composites, the ceramic fibers possess all the desirable properties of ceramics (electrical, thermal, chemical) but eliminate the detrimental characteristics (brittleness, weight).

The VSSP generates fibers with 20-30% more sensitivity (higher output voltage) than traditional bulk ceramics due to their small cross-sectional area. Mechanical-to-electrical transduction efficiency can reach 70%, compared to the 10-15% common with solar energy harvesting. In addition, vibrations can be harvested 24 hours per day, almost anywhere.

Piezo Power Generation

Figure 2. Output voltage signal of PFCB when 4 N force was applied (a), and peak voltage generation of a 1-3 fiber composite for an igniter (b).

Piezoelectric fiber composites (PFCs) open the door for an array of energy-harvesting applications.¹⁰ The fiber can recover (harvest) waste energy from mechanical forces such as motion, vibration, compression and/or tension. With simple, low-cost analog circuits, the piezo power can be converted, stored and regulated as an independent power source or a direct replacement for batteries. A typical PFC can generate voltages in the range of 80 Vp-p from vibration, while a typical PFCB (bimorph) can generate voltages in the range of 500 Vp-p, with some forms reaching outputs of 4000 Vp-p. The converted energy can be stored in a capacitor to power electronic devices.

Figure 2a shows the frequency dependance of such power. The full width at half maximum (FEHM) is fairly wide (10 Hz), and the frequency can be tuned as desired. Further, the FWHM increases significantly as the frequency increases.  Piezoelectric multilayer composites (PMCs), however, are designed to withstand large impact forces and accelerations up to a few thousand g’s. This type of transducer is a good sensor that can produce large voltages, even with the moderate force of 1 N impacting the transducer (see Figure 2b).

Figure 3. Real-time road experiment results storing a threshold 1.44 mJ of energy for a toll transponder device using the piezoelectric fiber transducers in a Dodge Truck (a) and a Honda Civic (b).

Using a constant vibration source with the frequency matched to the resonance of a standard PFCB at 30 Hz, piezo-fibers have the proven ability to produce enough energy with maximum efficiency to run many electronics continuously. Energy levels sufficient to power wireless systems for sensing, monitoring and control of security equipment, appliances, medical devices, buildings and other infrastructure elements are in development or have been commercialized.

Power output is scalable by combining two or more PFC elements in series or parallel, depending on the application. The composite fibers can be molded into unlimited user-defined shapes and are both flexible and motion-sensitive. The fibers are typically placed where there is a rich source of mechanical movement or waste energy. Figure 3 shows the results of real-time road tests to power a toll transponder device using the piezoelectric fiber transducer. The charging time varied depending on the vibration level transferred from the road, but in all cases provided suitable power within the required time.

Wireless Sensor Networks

Figure 4. Bridge with sensors deployed through a self-power system of piezoelectric generators.

Sensors that measure everything from process temperatures to system pressures and machine vibrations have been historically expensive to deploy in manufacturing, infrastructure and industrial environments. The sensors require expensive wiring or batteries and are expensive to service as well. With the emergence of the new ZigBee standard, based on IEEE 802.15.4 (and other low-power RF protocols), the availability of large, low-cost, low-power wireless sensor networks (WSNs) that are self-managed is becoming a reality.

Sensors, signal conditioners, controllers and RF transceivers continue to become smaller, lower power and more highly integrated. The combination of wireless networking, intelligent sensors and distributed computing has created a new paradigm for monitoring the health of machines, buildings and environments.

A low-cost, renewable energy source is critical to ubiquitous deployment of WSNs. New piezoelectric ceramic fiber-based energy harvesters will, in some cases, obviate the need for batteries in WSNs (see Figure 4). In other cases, the harvesting technology can be used to recharge batteries to enhance service life.

The power comes from the vibration of the system being monitored. Piezo fiber-based products require no maintenance, significantly reduce life cycle costs, improve the overall quality of industrial and machine control, and enhance security systems by responding to pressures as small as sound pressure.

Figure 5. Example of PZT fiber acting as an energy harvester to convert waste mechanical energy into a self-sustaining power source for a ZigBee wireless sensor node.

Figure 5 shows an example of the PZT fiber acting as an energy harvester to convert waste mechanical energy into a self-sustaining power source for a ZigBee wireless sensor node. The piezoelectric fiber captures the energy generated by the structure’s vibration, compression or flexure. The resulting energy (after conversion to current) is used to charge a storage circuit that then provides the necessary power level for the sensor node electronics.

In this example, energy is harvested by the vibration of PZT fiber composites. The energy is converted and stored in a low-leakage charge circuit until a certain threshold voltage is reached. Once the threshold is reached, the regulated power is allowed to flow for a sufficient period to power the micro-controller and the RF data transmission.

Additional Applications

PFCs can convert mechanical energy directly into light energy with no intervening electronics. By harvesting energy from ambient vibrations, PFCs can provide electroluminescent, LED or fluorescent lighting on vehicles, bridge decks, digital signage, buoys and other low-power lighting loads using waste energy. Some of the frequencies of electroluminescent lights can be readily seen through smoke or fog and could serve as signaling devices that have no electromagnetic signature.

PFCs also offer solutions for vibration damping and structural morphing. To enable self-adjusting systems, a smart structure containing PFCs can sense a change in motion. The motion produces an electrical signal that can be sent to a control processor that measures the magnitude of the change in motion and returns an amplified signal that either stiffens or relaxes the active fiber actuators/sensors. Just a 2 g PFC can create a 20 lb blocking force.

For self-powered RFID tagging systems, incident evidence can be collected in a flash memory that is powered by the incident and then evaluated later, so that penetration, damage or other activities can be sensed and recorded without the need for external power sources. In addition, one version of the energy harvester can create a shoe-powered GPS/homing beacon. Identical devices can be easily installed on containers, inventory or other places where RFID is being considered for active tracking. Similar designs are being assembled for Homeland Security border control using tree or grass motion as the source of power for long-scale wireless sensor networks.

Unmanned aerial vehicles (UAVs) are finding wide use in many surveillance systems, and power sources for both guidance and telemetry have been a nagging problem. The batteries currently in use are heavy and fail frequently. One PFCB design uses vibrations from the engine and airframe to power these electronic systems. The system can either help keep the batteries charged longer or eliminate them altogether by using the present waste energy without debiting any ongoing operations.

You Have the Power

This unique power source for low-power electronics uses environmental energy and flexible piezoelectric ceramic fiber composites to provide broad synergy with many applications. Wireless sensor networks, embedded systems, active structural control and other mechanical and electronic functions are being powered by this technology and its products from ambient sources of mechanical energy, such as human movement, wind or water, vehicle passage, or unexpected occurrences. Thousands of potential applications for self-powered systems could take advantage of flexible piezoelectric fiber ceramic composites.

For more information, contact Advanced Cerametrics, Inc., 1700 Fostoria Ave., Findlay, OH 45840; (800) 261-1208; fax (609) 397-2708; e-mail advcer@aol.com; or visit www.advancedcerametrics.com.