Energy scavenging and harvesting is becoming a critical element in designing embedded, self-powered and autonomous electronic systems and gadgets in a broad range of commercial, industrial, consumer and healthcare applications. Applications requiring micro-energy sources are especially challenging.
Recently there has been an increase in research related to energy harvesting from non-traditional alternate energy sources such as vibrations. Typically the size of a vibrational energy harvesting device varies from hundreds of microns to several centimeters. There are three primary transduction modes used in vibrational-based energy harvesting devices: 1) Electromagnetic transducers, 2) Electrostatic transducers, and 3) Piezoelectric transducers. This patent disclosure focuses on using MEMs-based piezoelectric cantilever transducers for vibrational energy harvesting.
Piezoelectricity is a property of certain materials, that when subjected to mechanical strain, undergoes an electrical polarization that is proportional to the applied strain. This property can be used to convert mechanical energy to electrical energy. This method is widely used to produce energy, especially for low-power applications. Some of the applications of the piezoelectric effect for energy harvesting are: remote area sensors, structural health monitoring, airplane sensor networks, implanted medical devices, space applications, consumer personal applications such as wearable computers and autonomous tracking devices in industrial and transportation applications.
Materials such as lead zirconate titanate Pb(Zr,Ti)O3 ceramics can undergo a shape change of ˜0.1%, resulting in the generation of a voltage that can be used as an energy source. Vibrations can cause some strain in these materials and, therefore, produce an electric signal. Some of the advantages of using vibrations as an energy source are: the energy source has, in principle, an infinite lifetime; and no physical connections to the energy source are needed (stand alone system). Furthermore, the device can be enclosed and protected from harsh environments. Ambient acoustical energy (either acoustical noise or artificially generated acoustical energy) can be used as an on-demand energy source. Another advantage of a vibrational energy source are that high output voltage can be achieved, and small sizes and relatively simple structures with high efficiency can also be achieved.
Some disadvantages of vibrational energy harvesters include the fact that they are relatively difficult to integrate in a micro-system due to their discrete and large form factors to date, and the energy output is frequency dependent limited by the form factor. Also the availability of vibrational energy can be intermittent.
For energy harvesting from vibration, it is known that the cantilever configuration is the optimal design to maximize mechanical to electrical conversion. Further, the cantilever configuration has been reported in two main variants based on the direction of stress and the electric field—d33 and d31. The d33 cantilever is typically the preferred geometry over the d31 because the d33 piezoelectric constant is approximately two-times (2×) larger than d31, which, when combined with larger spacing between the interdigitated electrode fingers relative to the piezoelectric film thickness, enhances the piezoelectric power generation.
Because the advantages of using vibrational energy outweigh the disadvantages, it would be desirable to develop an energy harvesting system using cantilevers that can be manufactured in an economical way and can be used in various applications.