Charging a Cell Phone, iPod or Electronic Device by Walking
The Promise of Nanogenerators
Cell phones, iPods and other personal electronic devices play important roles in many people’s lives. These devices require a power supply, which sometimes runs out at very inconvenient times. Researchers at the Georgia Institute of Technology in Atlanta may have the answer to this power dilemma. They’ve created tiny nanogenerators that produce electricity under the influence of human muscle action and can be driven by walking.
The nanogenerators that are being explored create one of two types of electricity - piezoelectricity or triboelectricity. Piezoelectricity is produced by deforming an object. Triboelectricity is produced by rubbing two objects together. Both types of nanogenerator can be driven by other types of mechanical motion in addition to walking. Although the nanogenerators are currently in the experimental stage, researchers predict that they will soon be powerful and convenient enough to charge our personal electronic devices and to do other useful jobs as well.
What are Nanogenerators?
The prefix “nano” means one billionth of the base measurement unit that is being used. For example, a nanometre or nm is a billionth of a metre. In the case of a nanogenerator, nano refers to nanotechnology, which is technology that involves extremely small objects. The definition of what "extremely small" means varies. For some people, it means objects between 1 and 100 nm in diameter. For others, it includes objects the size of atoms and molecules, which are often below 1 nm in diameter.
A nanogenerator is a small but visible device that converts mechanical energy to electricity and contains materials that are active on the nanoscale. The goal is to make nanogenerators as small, lightweight and powerful as possible so that they are both wearable and useful.
Muscle Contraction and Electricity
Electronic equipment is useless when it has no power source. At the moment, obtaining power away from an electrical outlet is often problematic. A replacement battery, a portable charger or a solar-powered charger can be taken on trips. However, these are extra items to carry and may be heavy or awkward to pack. A convenient power source that is always available is muscle energy.
Muscle contraction and relaxation is constantly taking place in our bodies. The heart repeatedly beats and relaxes to pump blood around the body. The respiratory muscles contract and relax to allow the lungs to fill with air and then partially empty. More muscle contraction occurs as we move parts of our body through space. Muscle action is occurring wherever we go and all the time, even in people with mobility problems – it’s a property of human life. Scientists at Georgia Tech have discovered that this muscle action can be used to produce electricity.
What is Piezoelectricity?
Crystals and other materials are made of atoms. Atoms contain smaller particles - protons, which have a positive charge, electrons, which are negative, and neutrons, which have no charge. Under the right conditions, the outermost electrons of some substances move out of their atom and into another one.
Each atom has the same number of protons and electrons and is therefore neutral. When a piezoelectric crystal is pressed and distorted, outer electrons move away from their atoms, leaving these atoms with an unbalanced positive charge. One side of the crystal becomes negative due to the collection of extra electrons and the other side becomes positive due to the loss of electrons. The separation of charges produces a potential difference or voltage.
Voltage can be thought of as a type of force. When charges are separated, electrons "try" to get back to their starting point. The electrons release energy as they do this, which we can use. A flow of electrons (or other charged particles) is known as an electrical current.
Gas burners, stoves and grills use the piezoelectric effect to produce a flame. When an igniter is pressed, a small hammer hits a piezoelectric material, such as quartz. The quartz changes shape and produces an electric spark, which ignites the gas.
Nanogenerators and Piezoelectricity
Professor Zhong Lin Wang and his colleagues at the Georgia Institute of Technology have created a nanogenerator based on piezoelectricity. The nanogenerator contains tiny zinc oxide wires which create electricity when they are bent. Five hundred zinc oxide nanowires placed side by side have the width of one human hair. The nanowires are placed on a flexible polymer film. The polymer layers are then arranged in a sandwich-like structure to create a nanogenerator. The generator creates electricity when a person bends it with his or her fingers.
One zinc oxide wire can create only a very small amount of electricity, but there are millions of the wires in a nanogenerator. In April 2011 the generator had reached a voltage of 3 volts – the same voltage as two AA batteries – and was able to light up the liquid crystal display of a calculator or drive a light-emitting diode, as shown in the video below. In 2013, however, the research team created triboelectric nanogenerators that reached a voltage of 10,000 volts. Piezoelectric nanogenerators may be useful when low voltages are required, but triboelectric generators seem to offer the most potential.
A Piezoelectric Generator in Action
The Triboelectric Effect
The Georgia Tech team is currently creating nanogenerators that produce electricity based on the triboelectric effect. In this effect, two surfaces are rubbed together and then separated, which produces electric charges. The charge is created when electrons move from one of the surfaces to the other. The surface that receives the electrons becomes negative, while the surface that loses the electrons becomes positive.
The triboelectric effects results in static electricity, or electricity that doesn't travel through a circuit. The popular act of rubbing a balloon against someone's hair and then finding that the balloon sticks to a wall is an example of the triboelectric effect.
In static electricity, the charge eventually dissipates by flowing to a nearby area or by a visible electrical discharge and its energy is wasted. In the case of triboelectric generators, however, the charge (in the form of electrons) is captured and transported through a circuit. The electrons have energy and can do work.
The researchers decided to create triboelectric generators when they noticed that a piezoelectric generator was producing an unexpectedly high power output. They discovered that the generator had been assembled incorrectly and that two surfaces were rubbing together, generating the additional power.
A Description of the Triboelectric Effect
A Triboelectric Generator
The current triboelectric generator is worn as a backpack. It contains two pairs of plastic cards. One card in each pair is coated with a material that has the ability to donate electrons while the other is covered with a material that accepts electrons. In addition, one card in a pair contains tiny, nano-sized pores while the other is covered with tiny nanowires. These irregularities in the card surface increase the friction when the cards contact each other.
The cards are arranged in a rhombus shape, as shown below. The rhombus is placed in a box containing weighted springs, which becomes a backpack. When the body movement of walking causes the weights to move and the box to collapse, the surfaces of the cards are brought together. The nanowires on one card are pushed into the holes on the opposite card, creating electric charges as the surfaces rub together. When the springs cause the box to return to its original size, the rhombus expands and the charges are separated, creating a voltage.
An electrode is connected to one of the plastic cards in a pair. Another electrode is connected to the other card. The electrodes are connected to each other via an electrical circuit outside the cards. An electrical load, such as a personal electronic device, is part of this circuit.
When the cards are separated after being rubbed together, a small current of electrons flows through the circuit from one card in a pair to the other in order to equalize the charge on the cards. The electrons pass through the electrical load as they travel and give up some of their energy to the load. The process of charge creation, charge separation and electron flow through the circuit occurs repeatedly as the person walks.
The National Science Foundation article in the "Further Reading" section below includes an illustration of the backpack tribogenerator. By the time the generator is ready to sell to the public it will probably look very different than it does today, however.
Potential Uses of Nanogenerators
Nanogenerators may be used for more than simply charging personal electronic devices. In the future, the generators may be placed inside the body where the heartbeat, the activity of the breathing muscles or even the flow of blood could trigger electricity production. The electricity could then be used to drive medical instruments. For example, the muscle movement of the heartbeat might be used to stimulate nanogenerators that power an insulin pump for diabetics. In addition, pacemakers might be charged by nanogenerators.
Nanogenerators could also be used as environment sensors. They may detect movement due to water leaks, vibrations and explosions. They may also be used to provide power for other environmental sensors.
Harvesting the World's Mechanical Energy
Nanogenerators in the Near Future
In the near future, piezoelectric or triboelectric nanogenerators may be placed in the soles of shoes so that a person’s footsteps will compress the substance and generate electricity. Our future clothing may contain nanogenerators that produce electricity as the clothing moves on our bodies.
Any object that moves could be used to produce electricity. For example, nanogenerators may be placed in car tires or in flags that blow in the wind. The energy of ocean waves could also be used to compress crystals, generating electricity.
Nanogenerator research is progressing rapidly and the devices are becoming much more powerful - especially the triboelectric versions. Professor Wang predicts that they will be ready for commercial use in about three years. Nanogenerators for powering or charging our mobile phones, media players and other personal electronic devices will be very useful. Nanogenerators for medical devices and environmental sensors could be very important.
© 2011 Linda Crampton
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