Improving Power Output for Vibration-Based Energy Scavengers

Abstract

P ervasive networks of wireless sensor and communication nodes have the potential to significantly impact society and create large market opportunities. For such networks to achieve their full potential, however, we must develop practical solutions for self-powering these autonomous electronic devices.Fixed-energy alternatives, such as batteries and fuel cells, are impractical for wireless devices with an expected lifetime of more than 10 years because the applications and environments in which these devices are deployed usually preclude changing or re-charging of batteries. There are several power-generating options for scavenging ambient environment energy, including solar energy, thermal gradients, and vibration-based devices. However, it's unlikely that any single solution will satisfy all application spaces, as each method has its own constraints: solar methods require sufficient light energy, thermal gradients need sufficient temperature variation, and vibration-based systems need sufficient vibration sources. Vibration sources are generally more ubiquitous, however, and can be readily found in inaccessible locations such as air ducts and building structures.We've modeled, designed, and built small cantilever-based devices using piezoelectric materials that can scavenge power from low-level ambient vibration sources. Given appropriate power conditioning and capacitive storage, the resulting power source is sufficient to support networks of ultra-low-power, peer-to-peer wireless nodes. These devices have a fixed geometry and-to maximize power output-we've individually designed them to operate as close as possible to the frequency of the driving surface on which they're mounted. Here, we describe these devices and present some new designs that can be tuned to the frequency of the host surface, thereby expanding the method's flexibility. We also discuss piezoelectric designs that use new geometries, some of which are microscale (approximately hundreds of microns). Problem overviewWe first analyze the wireless sensor nodes' power requirements, and then investigate the various sources that can fill those demands. Power demandAssuming an average distance between wireless sensor nodes of approximately 10 meters-which means that the radio transmitter should operate at approximately 0 dBm (decibels above or below 1 milliwatt)-the radio transmitter's peak power consumption will be around 2 to 3 mW, depending on its efficiency. Using ultra-low-power techniques, 1 the receiver should consume less than 1 mW. Including the dissipation of the sensors and Given appropriate power conditioning and capacitive storage, devices made from piezoelectric materials can scavenge power from low-level ambient sources to effectively support networks of ultra-low-power, peerto-peer wireless nodes.

Comparison of energy harvesting systems for wireless sensor networks

“…5 results in a non-uniform distribution of strain in the piezo material while a suitably tapered cantilever formation can ensure uniform strain. This was investigated in an early work by Glynne-Jones et al [55] and subsequent modelling and practical studies by Roundy and co-workers [56,57] showed significantly higher power for a given volume of piezo material using a tapered cantilever. Straight cantilevers, while not giving uniform material strain, are more readily modelled [58] than the tapered form.…”

“…This gives a broadband converter but the size of the device and the complexity of the electronics increases significantly. A more space efficient approach has been adopted by Roundy et al [56,67] in which a single beam has been used but with three masses attached in such a way that the structure is able to vibrate at several frequencies. This gives a broader frequency band but it is not clear how this will affect the peak power output.…”

Bimorph piezoelectric vibration energy harvester with flexible 3D meshed-core structure for low frequency vibration

“…On the other hand, in a bimorph cantilever-type vibration energy harvester [17,18], the elastic layer is sandwiched between two piezoelectric layers. Unlike in the unimorph cantilever, in the bimorph type, the neutral axis is usually at the center of the cantilever regardless of the Young’s modulus and thickness of both piezoelectric and elastic layers.…”

Wireless power transfer system for a human motion energy harvester

“…In addition, a large number of failures in medical devices are caused by the batteries [6,7]. Suitable sources for human energy harvesting are temperature differences [8], photovoltaic systems [9,10] and glucose fuel cells [11]. Body motion is an abundant source of energy that can work for externally worn and implanted devices alike [12,13,14].…”