The number of ways to harvest energy that would otherwise go unused and wasted is extraordinary. To cite a few of the many examples, there’s the heat given off during almost any physical or electronic process, ambient light which is “just there,” noise, and ever-present vibration. Each of these has different attributes along with pros and cons which are fluid with respect to consistency, reliability, and, of course, useful output power in a given situation.
For example, the harvesting of vibration-sourced energy is attractive (when available) as it is unaffected by weather or terrain conditions. However, most of the many manifestations of such energy are quite small. It requires attention to details and design to extract and squeeze out a useful amount in the energy chain from a raw source to the harvesting transducer.
Most vibrations in daily life are tiny and often not “focused” but spread across a wide area or volume. To overcome this significant issue, numerous conversion devices, typically piezoelectric elements, are often installed in multiple locations that are exposed to relatively large vibrations.
Addressing this issue, a research effort lead by a team at KRISS—the Korea Research Institute of Standards and Science in the Republic of Korea (South Korea) —has developed a metamaterial that traps and amplifies micro-vibrations into small areas. The behavior of the metamaterials enhances and localizes the mechanical-energy density level at a local spot in which a harvester is installed.
The metamaterial has a thin, flat structure roughly the size of an adult’s palm, allowing it to be easily attached to any surface where vibration occurs, Figure 1. The structure can be easily modified to fit the object to which it will be attached. They expect that the increase in the power output will accelerate its commercialization.
Figure 1 The metamaterial developed by the KRISS-led team is flat and easy to position. Source: KRISS
The metamaterial developed by KRISS traps and accumulates micro-vibrations within it and amplifies it. This allows the generation of large-scale electrical power relative to the small number of piezoelectric elements that are used. By applying vibration harvesting with the developed metamaterial, the research team has succeeded in generating more than four times more electricity per unit area than conventional technologies.
Their metasurface structure can be divided into three finite regions, each with a distinct role: metasurface, phase-matching, and attaching regions. Their design used what is called “trapping” physics with carefully designed defects in structure to simultaneously achieve the focusing and accumulation of wave energy.
They validated their metasurface using experiments, with results showing an amplification factor of the input flexural vibration amplitude by a factor of twenty. They achieved this significant amplification largely due to the intrinsic negligible damping characteristic of their metallic structure, Figure 2.
Figure 2 (right) Schematic of the proposed metasurface attachment and (left) a conceptual illustration of the attachment installed on a vibrating rigid structure for flexural wave energy amplification. Source: KRISS
Their phase-gradient metasurfaces (also called metagratings in the acoustic field) feature intrinsic wave-trapping behavior. (Here, the term “metasurfaces” refers to structures that diffract waves, primarily through spatially-varying phase accumulations within the constituent wave channels.)
Constructs, analysis, and modeling are one thing, but a proposal such as theirs requires and is very conducive to validation. Their experimental setup used a vibration shaker and a laser Doppler vibrometer (LDV) sensor to excite and then measure the flexural vibration inside the specimen, Figure 3. For convenience, the specimen was firmly clamped to the shaker instead of being directly attached onto the shaker using a jig and a bolted joint.
Figure 3 (a) Schematic illustration and (b) photographs to demonstrate the experimental setup in order to validate the flexural-vibration amplifying performance of the fabricated metasurface attachment. Using a specially-configured jig and a bolted joint, the metasurface structure is firmly clamped to a vibration shaker. The surface region covering a unit supercell (denoted as M1) and the interfacial line (M2) between the metasurface strips and phase-matching plate are measured using laser Doppler vibrometer equipment. Source: KRISS
The shaker was set to constantly vibrate at frequencies between 3 kHz and 5 kHz at arbitrary weak amplitudes set by a function generator and an RF power amplifier. The phase-matching plate (somewhat analogous to impedance-matching circuit) was another essential component in the structure. It dramatically improved the amplifying performance by assisting coherent phases of scattering wave fields to constantly develop within the metasurface strips in the steady state.
It would be nice to have a summary of before-and-after performance using their design. Unfortunately, their published paper is too much of a good thing: it has a large number of such graphs and tables under different conditions, but no overall summary other than a semi-quantitative image, Figure 4 (top right).
Figure 4 This conceptual illustration graphically demonstrates the nature of the vibration amplification performance of the metamaterial developed by the KRISS-lead team. Source: KRISS
If you want to see more, check out their paper “Finite elastic metasurface attachment for flexural vibration amplification” published in Elsevier’s Mechanical Systems and Signal Processing. But I’ll warn you that at 32 pages, the full paper (main part, appendix, and references) is the longest I have seen by far in an academic journal!
Have you had any personal experience with vibration-based energy harvesting? Was the requisite modeling difficult and valid? Did it meet or exceed your expectations? What sort of real-work problems or issues did you encounter?
Bill Schweber is an EE who has written three textbooks, hundreds of technical articles, opinion columns, and product features.
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