08-03-2011, 02:04 PM
Submitted by:
Aatika Shahwar
Ayushi Agarwal
Kirti Mishra
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POWER DRESSING”FOR SCROLLS-2010
FOREWORD
Energy harvesting technologies that are engineered to miniature sizes, while still increasing the power delivered to wireless electronics, portable devices, stretchable electronics, and implantable biosensors are strongly desired.
Scavenging energy from multiple sources available in our personal and daily environments is
required , not only for powering personal electronics, but also for future implantable sensor-transmitter devices for biomedical and healthcare applications.
Several research groups across the country are working on harnessing energy to power electronic devices from simple acts such as breathing or walking to the refrigerator. These scientists hope that piezoelectric nanowires, nanofibers and crystals—materials that produce an electric potential when bent or stressed—can convert biomechanical processes into electricity and wean people off of using batteries in some daily activities.
The energy-scavenging nanofibers having the literal meaning “power suit” can be used to manufacture clothing and textiles. It will be possible to make clothing that could power different portable devices. The nano-sized generators with piezoelectric properties have the ability to transform energy from mechanical stresses , stretches and twists into power. Thus, if used in clothing, the nanofibers could make use of body movements to power gadgets.
This technology has eventually led to wearable 'smart clothes' that can power hand-held electronics through ordinary body movements.
Although materials vary from lab to lab, researchers aim to sew these piezoelectric fibers into clothes to drive handheld music devices or to install them in the body to power sensors or pacemakers. The latter would eliminate surgeries required every few years to replace dead batteries. Environmental sensors incorporating the fibers could gather energy from their surroundings and wouldn’t need to rely on finicky solar power. Three ways have been listed in which researchers can increase power: They can improve the efficiency of the generator, boost the amount of stress applied to the generator, or increase the size or number of generators.
It works with polyvinylidene fluoride (PVDF), a polymer with high mechanical strength that has been used in fishing lines and insulation on electrical wires. The PVDF nanofibers are roughly 10 times as efficient at harvesting energy as are conventional PVDF thin films.
It has been calculated that his fibers create about 10 mV of electrical potential. With 1 million fibers to convert movements into electrical energy and 1,000 electrodes connected in parallel to capture that energy, he estimates an article of clothing could generate about 50 mW of power. That’s enough to charge and sustain an energy-hungry smartphone.
NANOFIBER GENERATORS
These nano-sized generators have "piezoelectric" properties that allow them to convert into electricity the energy created through mechanical stress, stretches and twists.
Mechanical strain, such as bending a thin piezoelectric wire, changes this electric polarization of the material and causes positive and negative charges to migrate to opposite faces of the material, creating an electric voltage that can be used to do work. The effect can also be reversed.
The advantage of the nanofibers is that they are made using organic polyvinylidene fluoride (PVDF), which allows them to be flexible and rather inexpensive to make.
It is noted that more vigorous movements, such as the kind one would create while dancing the electric power suit, should theoretically generate more power. And because the nanofibers are so small, we could weave them right into clothes with no perceptible change in comfort for the user.
The tiny nanogenerators have diameters as small as 500 nanometers, or about 100 times thinner than a human hair and one-tenth the width of common cloth fibers.
The researchers repeatedly tugged and tweaked the nanofibers, generating electrical outputs ranging from 5 to 30 millivolts and 0.5 to 3 nanoamps, with no noticeable degradation after stretching and releasing the nanofibers for 100 minutes at a frequency of 0.5 hertz (cycles per second).
Researchers have pioneered the near-field electrospinning technique used to create and position the polymeric nanogenerators 50 micrometers apart in a grid pattern.
Surprisingly, the energy efficiency ratings of the nanofibers are much greater than the 0.5 to 4 percent achieved in typical power generators made from experimental piezoelectric PVDF thin films, and the 6.8 percent in nanogenerators made from zinc oxide fine wires.The technology enables greater control of the placement of the nanofibers onto a surface, allowing researchers to properly align the fiber nanogenerators so that positive and negative poles are on opposite ends, similar to the poles on a battery. Without this control, the researchers explained, the negative and positive poles might cancel each other out and reducing energy efficiency
ENERGY HARVESTED FROM THE HUMAN BODY
biomechanical energy represents a feasible source of continuous power for wearable or implantable devices. The human body is a surprisingly rich source of energy—the average adult consumes approximately 2000 kcal per day, equivalent to 100 W. This power is expended during everyday activities, most significantly in fuelling the motions of walking, arm swinging, finger motion, and breathing.
The total available power associated with everyday activities for a 150 lb (68 kg) adult. Because most of this power is lost to wasted heat and vibrations, it may be possible to recover some percentage of this power for use in wearable electronics without significantly increasing the load to human body. For example, the heel strike from walking is a particularly rich source of energy with 67W of power available from a brisk walker. Harvesting even 1–5% of that power would be sufficient to run many body-worn devices. Similarly, lung motion by breathing can generate up to 1 W of power.
GENERAL DETAILS ON PIEZOELECTRICS
Researchers have investigated multiple routes toward harnessing biomechanical motion into electrical power. The first approach utilizes motor-based generators such as spring-loaded
backpacks or knee-mounted gears. Piezoelectrics are a particularly interesting subset of smart materials which become electrically polarized when subject to a mechanical stress, and conversely experience a strain in response to an applied electric field, and in proportion to the strength of the field.
Piezoelectrics thus contain no movable parts or complex assemblies, representing a compact alternative to biomechanical power generation.
From naturally occurring quartz crystals, to flexible piezopolymers, to more exotic and efficient ceramics, the research on piezoelectrics has been extensive. These materials can also be fashioned at a variety of length scales, ranging from largescale bulk materials that can be used to dampen structural vibrations. Piezoelectrics such as lead zirconate titanate (PZT) and the polymer polyvinylidene fluoride (PVDF) have been located inside the soles of shoes for energy harvesting yielding power outputs up to 10 mW from walking. PVDF has also been shown capable of generating 20 mW of power during respiration when implanted inside the rib cage in dogs.
Piezoelectric materials are smart materials which can convert mechanical energy (such as from breathing, or walking) into more useful electrical energy. Yet, traditional piezoelectrics are hard, inflexible crystals which can also be toxic. A new suite of energy harvesting devices are based on novel nanoscale piezoelectrics, allowing for efficient power generation without the limitations of traditional piezos. Specifically, the ability to assemble advanced nanomaterials onto FDA-approved implantable silicones or plastics has led to nanopiezoelectric devices that combine the key qualities of flexibility, energy efficiency, and biocompatibility in a single platform.
Recent experiments involving tapping these chips with a finger or implanting them onto the bodies of rats have shown excellent promise for meaningful energy harvesting capabilities. The excellent performance of the piezo-ribbon nanoassemblies coupled with flexible biocompatible plastics suggests these devices could eventually be incorporated into the soles of shoes to power portable electronics,or even placed on a heart patient’s lungs to recharge a pacemaker battery via breathing
Designing piezoelectric energy harvesters which efficiently convert body motion into power requires fundamental understandings of:
(1) the mechanism of the piezoelectric effect,
(2) the energy conversion efficiency,
(3) the proper selection of suitable materials,
(4) the assembly of these materials into a bio-interfaced component and
(5) the smoothing of the power output .
SPECIFIC PIEZOELECTRICS
Piezoelectric materials generally fall into two classes: piezoelectric polymers and piezoelectric crystals. The most studied piezoelectric polymers are the PVDF family, first discovered in 1969. PVDF is a high-molecular weight polymer with repeat unit [CH2–CF2].
The piezoelectric property of PVDF arises from the strong molecular dipoles within the polymer chain, combined with short- or long-range ordering—the latter obtained by heating the polymer above the glass transition temperature, and then cooling under the presence of an applied electric field. PVDF films are inherently flexible and have been used in energy harvesters. The high electronegativity of fluorine (4.0 for F vs. 2.5 for carbon) gives the carbon–fluorine bond a significant polarity/dipole moment. The electron density is concentrated around the fluorine, leaving the carbon relatively electron poor. This introduces ionic character to the bond through partial charges (Cδ+—Fδ−). The partial charges on the fluorine and carbon are attractive, contributing to the unusual bond strength of the carbon–fluorine bond.
Other pizeo-polymers exist, but are less efficient. Another class of piezoelectric materials are the inorganic piezo-ceramics, commonly perovskite crystals. The most popular of these is lead zirconate titanate, Pb(ZrxTi1_x)O3.
The most important parameter for characterizing the efficiency of piezoelectric materials is the piezoelectric charge constant, d. This value represents the polarization generated per
unit of mechanical stress applied to a piezoelectric material, or, inversely, is the mechanical strain experienced by a piezoelectric material per unit of electric field applied.
