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The energy around us

Features | February 7, 2014 | By:

Researchers are making progress on energy harvesting technologies and are identifying where improvements are needed.

Editor’s Note: I had a phone conversation with prominent energy harvesting researcher Dr. Shashank Priya, Virginia Tech, about the status of kinetic energy harvesting technology. My goal was to get a sense of where the research sits currently, and what we can look forward to in the future. Following that conversation, I put together a few questions. Here are Dr. Priya’s responses:

1. I’m interested in your assessment of the commercial viability of kinetic energy harvesting.

How far away are we from successfully commercializing the technology “somewhere”?

Kinetic energy harvesting prototypes have been demonstrated by several research laboratories and large companies, such as Exelis Inc. There are a variety of platforms where this technology is being targeted, such as trains, aircrafts and buildings for powering the wireless sensor nodes, surveillance components and health monitoring systems. A lot of progress has also been made on water flow energy harvesting systems, and they should find deployment for remote monitoring applications. 

What markets and applications are most likely to be (or will continue to be) early adopters?

Wireless sensor nodes, surveillance components, and health monitoring systems are the prime applications for platforms such as buildings, automobiles, trains, aircraft and ships.

What could happen in the research (what would be “breakthrough”) that could change the picture?

There are two main metrics that need to be improved: power density and bandwidth. In order to do so, more research needs to be conducted on improving the material performance and implementation of these material architectures on the desired substrates. In addition, research is required to further the design of low frequency structures, non-linear concepts and efficient circuits.

2.  You mentioned that the way in which a photovoltaic material is used with a substrate can be quite different. (Obviously, the placement is less of an issue, as long as it can collect light.) You also mentioned that for kinetic energy harvesting, “There are only certain places where you will get stress, which you have to have.” Explain how research is addressing this challenge.

For maximum power transfer, the harvester needs to operate at the resonance. However, as we scale down the size of harvester the resonance frequency continues to increase. Thus, the challenge has been to design structures on the size of a few mm in diameter exhibiting resonance frequency in the range of 10–100 Hz. This is the desired frequency range for most of the kinetic energy harvesters.

The second factor that controls the magnitude of power is volume of the active material. Maximizing the active material volume, while meeting the other structural criterion, such as mechanical strength, resonance frequency and volume of the device, has been another challenge. Depending upon the mechanism, such as piezoelectric, electromagnetic or electrostatic, the operation mode and structure of the harvester is critical in gaining higher efficiencies.

Research has been conducted on all these fronts. Our own results show that arc-shaped beams arranged in a spiral-type configuration are able to lower the resonance frequency dramatically. By designing non-linear structures in both electromagnetics and piezoelectrics, higher bandwidth has been obtained. New processes such as aerojet deposition and 3D printing allow thicker films to be grown on the desired substrates with reduced patterning steps. As this progress continues and university and industry partnerships are built, we will start to see practically relevant solutions.

3. What methods/systems/processes have proven to be the most promising?

It depends upon the mechanism. For example, in electromagnetics, four-bar structures and double-magnet levitation-based harvesters have shown good results. In piezoelectric systems, non-linear cantilever-based structures have provided good bandwidth and power density. Advanced manufacturing techniques are going to be highly relevant in mass manufacturing of these structures, as it requires complex heterogeneous assembly across varying length scales.

4. How are textile substrates (or textiles, generally, because there are conductive fibers that can be woven into e-textiles) figuring into the research? There are textile properties—lightweight, flexible, wearable—that can be engineered using a variety of materials – but maybe I’m missing others that are important?

Kinetic energy harvesting from textiles is a bit more complicated as one has to develop harvesting architectures within the conformal package. Mostly these architectures are off-resonance devices that rely on simple bending or stress pulses. This presents a challenge in designing circuits that can efficiently transfer the generated electric energy into storage components, such as a capacitor or battery. Macrofiber composites and metal–MEMS (micro-electro-mechanical systems) are some approaches that have been experimented with. However, going forward, more research is required on developing nanoscale-based approaches, such as core-shell magnetoelectric fibers, self-poled structures and on developing flexible energy harvesting circuits with high efficiency.

5.  There’s quite a lot of interest in the fashion and design world in kinetic energy harvesting. How might this world and the world of industrial applications intersect? Is there much sharing of research? 

Kinetic energy harvesting refers to the conversion of dynamic stresses into electric signal. Kinetic energy harvesters mostly utilize the same principles on smaller or larger scales. However, most of the large-scale systems are based upon electromagnetics and use either a moving magnet-coil arrangement or an electromagnetic generator. At small scales we have many choices for kinetic energy harvesting, such as piezoelectric, electrostatic, electrets, elastomers and electromagnetics.

This research area is continuously growing and organizations such as the National Science Foundation Industry – University Collaborative Research Center “Center for Energy Harvesting Materials and Systems” (CEHMS) are providing avenues for transition from laboratory development into industry.

Shashank Priya, Ph.D., is professor in the Department of Mechanical Engineering and Turner Fellow in the College of Engineering at Virginia Tech. His research is focused on energy harvesting and bio-inspired materials and devices. He is editor-in-chief of the journal “Energy Harvesting and Systems” and founding chair of the prominent conference in the field, “Energy Harvesting Workshop.”

Active Center-created patents for work related to this article include: Broadband Electromagnetic Energy Harvester [4 bar]; Magnetic Levitation Based Energy Harvester for Low Frequency Vibrations; and Methods for Manufacturing Textured PZT Piezoelectric Ceramics.

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