Energy harvesting textiles are not new, but often the yarns developed in a lab environment, whether they have piezo-, tribo- or thermo-electric properties, are susceptible to short-circuiting when in presence of high humidity. That is simply because they are designed with both electrodes exposed to their environment, notes Dr Anja Lund, lead author of a paper titled “Energy harvesting textiles for a rainy day: woven piezoelectrics based on melt-spun PVDF microfibres with a conducting core” published in npj Flexible Electronics.
In her paper, the researcher focused on the use of Poly(vinylidene fluoride) (PVDF) known for its high piezoelectric coefficient when the polymer contains a high ratio of the β cristalline phase (among other phases). In order create worthy PVDF fibres containing the β phase while encasing a conductive element, the researchers first melt-spun a bi-component fibre, comprising a mix of carbon black in polyethylene at its conductive core and the PVDF as its sheath. The co-extrusion carried out on a multichannel spinneret producing a yarn of 24 bi-component filaments with an average diameter of 60µm for a core 24µm in diameter. Next they used a cold-drawing stage to strain the fibre and convert the non-polar α-crystals in the PVDF to the polar β phase which exhibits a higher electromechanical coupling for energy harvesting. The researchers also took care to anneal the bi-component fibres (at the melting temperature of polyethylene but well below that of PVDF) so as to ensure that the inner carbon black/polyethylene core would retain its electrical conductivity even after fibre drawing. The core conductivity was measured at 0.2 S/cm.
Adding an outer electrode to characterize the yarn’s piezoelectric properties, the researchers demonstrated that the yarns could function as sensors for heartbeat and respiration or to monitor motions, generating a piezoelectric voltage amplitude of 1V at less than 0.5% strain.
To produce a robust two-electrode textile for energy harvesting, Lund and her colleagues used an industrial-type band weaving machine, feeding it with 60 piezoyarns in parallel as the warp and inserting conducting yarns as the weft. While the piezoyarns fully embed and protect their conductive core, the conducting yarns of the weft forms the outer
electrode of the fabric. This means that no short-circuiting occurs even when holding the strap by hand, and the textile retains its piezoelectric properties even when submerged in tap water. Better still, adding a conducting liquid to the surface of the textile substantially increases the contact area between the piezoyarn (consisting of 24 fibres) and the conducting yarns, leading to a higher capacitance. In such conditions, the researchers calculated that the textile could output in excess of 7µW.
Interfaced to an energy harvesting circuit (consisting of a rectifier and an energy storing capacitor of 22 µF), the 2.5cmx20cmm woven bands were later used as a strap for a laptop case, replacing a 20 cm length of its shoulder strap. For a modified a commercial laptop preloaded with books, the paper reports that stair-walking with the bag over the shoulder was enough for a wet piezoelectric strap to produce a peak output of up to 8V.
“After an initial charging period of approximately 1min we were able to produce an average output voltage of 1V, corresponding to an average output power PEHC = 1µW for as long as walking continued” the authors wrote.
Interestingly, carrying the bag by hand during a brisk walk produced the same peak voltage at a steadier pace, increasing the energy produced and providing a continuous power of 4 µW after an initial charging time less than 15 seconds. Connected to a 1.6V LED, the strap made the LED blink continuously with each step (after the initial charging time).
Other observations were that the twill fabric construction resulted in the highest generated voltage, independent of the choice of conducting yarn for the outer electrode.
The authors anticipate that if the generated power scales linearly with area, then fabricating an entire piezo-strap for the load carrying part of a laptop case would yield about 0.1mW of harvested power, enough to power several RFID tags or wireless sensor nodes. Through further optimization of their piezoelectric yarn, by increasing the core’s conductivity with carbon nanotubes or graphene, the researchers hope to reach a 100-fold increase in generated power to 10mW. A combined approach would be to decrease the resistance of the outer electrode, either by increasing the yarn conductivity or simply by adding more yarn.
The researchers envisage that large area woven piezoelectric fabrics could be used in hammocks, shoe soles and in the upholstery of passenger vehicles, but also as a filler in structural composites, to produce relevant levels of electrical power. Research collaborators and co-authors from Swerea IVF have secured a patent on spinning the piezoyarns.
Chalmers University of Technology – www.chalmers.se
Swerea IVF – www.swerea.se