Auxetic Power Amplification Mechanism for Low Frequency Vibration Energy Harvesting

Lead Research Organisation: University of Exeter
Department Name: Engineering Computer Science and Maths

Abstract

In the last decade, energy harvesting has been an exploding research field in the worldwide by promising of self-powered systems, including wireless sensor networks. The last few years have seen the emergency of several dedicated world class research groups, dedicated international conferences, heavily investments by government research funding bodies, including the UK, and industrial stakeholders. Nevertheless, several critical roadblocks remain that have so far prevented the technology from practical applications. For example, currently, most vibration-based energy harvesters are designed to work at or near resonance frequency. Unfortunately, in the most majority of practical scenarios, ambient vibration is well below the resonant frequency and frequency-varying or totally random over a wide range frequency. The energy harvested reduces significantly outside the resonant frequency. The resolution of these presents scientific challenges, requiring highly innovative solutions.

This EPSRC funded PhD studentship will address this challenge by researching into broadband vibration energy harvesting for low frequencies via a number of innovative approaches, such as frequency-up conversion and novel energy harvester designs. The research outcome of the project will be able for industry to use the developed novel energy harvester for self-powered systems, such as structural health monitoring applications.

Publications

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Studentship Projects

Project Reference Relationship Related To Start End Student Name
EP/N509656/1 01/10/2016 30/09/2021
1783750 Studentship EP/N509656/1 01/10/2016 31/03/2020 William Ferguson
 
Description [short technical version:]
I have identified auxetic(1) metamaterial(2) designs ideal for improving the electrical power output of piezoelectric(3) vibration energy harvesting(4) from small strain(5) oscillations.
[Applications:]
This has great potential to be used to power structural health monitoring(6) devices in infrastructure (buildings' superstucture, bridges, towers/wind turbine supports, etc.), or vehicles (trains, cars, and lorrys particularly; in an aircraft it may be subject to ecessive strain) and vibrating machinery.

[More general definitions:]
(1) Auxetic: A material with a negative Poisson's ratio, i.e. when stretched in one direction it also expands in another; in contrast to most materials which would contract laterally under these conditions.
(2) A Metamaterial is a material who's properties are determined largely by structure rather than by composition. In this case, there are various geometries which can be laser cut from a sheet of normal steel, to give it the auxetic behiviour described in (1). We have used these designs in a substrate under the piezoelectric(3) material, to A) reduce the stiffness of the substrate, which focuses the applied strain(4) into the piezo and B) give an extra lateral strain to the piezo; both of which increase the power output from it (at the cost of making it more stressed, and thus more likely to break; hence our suggested applications revolve around low-strain environments, where vibration energy harvesting(4) has been previously not seen as viable).
(3) Piezoelectricity is a property of certain materials (most crystaline or ceramic) which produce a voltage when strained (and vice versa; produce a reversable deformation when stressed by an electric field). This voltage is casued by an electrically balanced atomic/ionic structure becoming deformed and thus polarised. In materials such as Lead Zirconate Titainate (PZT), this polarisation can be poled in advance, so a deformation in one plane can result in a unified electric field in the perpendicular axis: This is useful for generating an AC electricity directly from vibrations (where the polarisation gets repeatedly flipped by the change in direction of the applied strain).
(4) Vibration Energy Harvesting involves aquiring energy from the ambient vibrations to power some device; typically a sensor with a transceiver and small onboard processor. The amount of energy involved is typically very small, rarely more than a Watt, and more commonly around 100 uW (u=micro=10^-6=1/1,000,000). Vibrations are widely harvested into electrical energy using piezoelectric materials(3) on cantilevers (or being directly strained, as mine; which removes the inertial resonant behaviour of the cantileaver - which works very well at this frequency, but very poorly outside it; hence the need for a broadband harvester where a set frequency cannot be guaranteed in the environment), or with coils and magnets. Many other sources of power for energy harvesting are also being investigated, such as harvesting from thearmal or salinity gradients, micro-solar or -wind power, and human motion.
(5) Strain is a measure of a material's deformation when stretched or compressed. Usually measured as a fraction of the original length. In this case, small strain means less than 300 micro-strain (0.0003 times the sample's Origional Unstrained Length) peak-to-peak (i.e. maximum-to-minimum extension) being applied; this value might be expected in the wing of an areoplane flying through bumby but not especially turbulant conditions.
(6) Structural Health Monitoring is the notion of placing sensors throughout the structure to continuously monitor its condition and structural integrity. There has been a push in recent years to develop self-powered, wireless sensors for this purpose: 'Wireless' allows them to be more easily retrofitted into existing structures (and even newly built ones could benefit from the saved weight (more a concern in aeroplanes) and expensive complexity of fitting all these extra wires). 'Self-powered' means they don't have to be limited by the lifespan of the original battery, and could be fitted in hard to reach places without fear of needing to replace the battery in two years time. The ideal would be to not have to replace the system for ten or more years (by which time many sensors are expected to have failed in any case). A wireless network of these self-powered nodes could watch for changes in the building (or any other structure listed above) and identify a fault before it becomes too serious, allowing for predictive maintenence. The dificulty has been that the availble energy to power such devices has been difficult to harness due to the ambient vibrations typically being of low frequency (<30 Hz) and amplitude (<<300 micro-strain), which is what my design excells at.
Exploitation Route My designs could be applied to many infrastuctural locations without changes being necessary, but using Finite element modeling we have a tool to optimise the design to the specific conditions (particularly the maximum expected strain in that location).
Further research could also be done to use 3D auxetic frameworks around a piezoelectric block to squeeze it in all dirictions when placed under a direct load, and thus increase its power output in a similar way to my 2D auxetic substrates. This aproach would be more applicable to direct loading: such as under /within roads, from various impact sources, shock-absorbers and vibration dampers. Adding a free mass to these (or my current designs) could turn them from direct strain harvesting, to inertial harvesting (whith a resonant frequency and the benefits and issues that brings with it).
Sectors Construction,Digital/Communication/Information Technologies (including Software),Electronics,Energy,Environment,Transport,Other