Wireless Power Transfer - Online Article

Introduction

It the process that takes place in any system where electrical energy is transmitted from a power source to an electrical load, without interconnecting wires. Wireless transmission is employed in cases where instantaneous or continuous energy transfer is needed, but interconnecting wires are inconvenient, hazardous, or impossible.

Though the physics are related, this is distinct from wireless transmission for the purpose of transferring information (such as radio), where the percentage of the power that is received is only important if it becomes too low to successfully recover the signal. With wireless energy transfer, the efficiency is a more critical parameter and this creates important differences in these technologies.

Unless you are particularly organized and good with tie wrap, you probably have a few dusty power cord tangles around your home. You may have even had to follow one particular cord through the seemingly impossible snarl to the outlet, hoping that the plug you pull will be the right one. This is one of the downfalls of electricity. While it can make people's lives easier, it can add a lot of clutter in the process.

For these reasons, scientists have tried to develop methods of wireless power transmission that could cut the clutter or lead to clean sources of electricity. While the idea may sound futuristic, it isn't particularly new. Nicola Tesla proposed theories of wireless power transmission in the late 1800s and early 1900s. One of his more spectacular displays involved remotely powering lights in the ground at his Colorado Springs experiment station.

Tesla's work was impressive, but it didn't immediately lead to widespread, practical methods for wireless power transmission. Since then, researchers have developed several techniques for moving electricity over long distances without wires. Some exist only as theories or prototypes, but others are already in use. If you have an electric toothbrush, for example, you probably take advantage of one method every day.

Why do we need wireless transmission of power?

A great concern has been voiced in recent years over the extensive use of energy, the limited supply of resources, and the pollution of the environment from the use of present energy conversion systems. Electrical power accounts for much of the energy consumed. Much of this power is wasted during transmission from power plant generators to the consumer. The resistance of the wire used in the electrical grid distribution system causes a loss of 26-30% of the energy generated. This loss implies that our present system of electrical distribution is only 70-74% efficient. A system of power distribution with little or no loss would conserve energy. It would reduce pollution and expenses resulting from the need to generate power to overcome and compensate for losses in the present grid system. The proposed project would demonstrate a method of energy distribution calculated to be 90-94% efficient. An electrical distribution system, based on this method would eliminate the need for an inefficient, costly, and capital intensive grid of cables, towers, and substations. The system would reduce the cost of electrical energy used by the consumer and rid the landscape of wires, cables, and transmission towers.

There are areas of the world where the need for electrical power exists, yet there is no method for delivering power. Africa is in need of power to run pumps to tap into the vast resources of water under the Sahara Desert. Rural areas, such as those in China, require the electrical power necessary to bring them into the 20th century and to equal standing with western nations.

As first proposed by Buckminster Fuller, wireless transmission of power would enable world wide distribution of off peak demand capacity. This concept is based on the fact that some nations, especially the United States, have the capacity to generate much more power than is needed. This situation is accentuated at night. The greatest amount of power used, the peak demand, is during the day. The extra power available during the night could be sold to the side of the planet where it is day time. Considering the huge capacity of power plants in the United States, this system would provide a saleable product which could do much to aid our balance of payments.

The tangle of cables and plugs needed to recharge today's electronic gadgets is very hectic for a home which is not well organized. The offices and factories where space is of big concern we can not allow cables to just hang around all the places so here wireless power transmission can be of big use. All the worries about the chargers to work properly are removed by this technology. Even in near future we are going to see a big leap in this field, for example now a days computers have wireless connectivity, but still we have to carry adapters with it but may be in future we won't require any adapters, we just enter the room n our laptops will start charging on its own after detecting a particular resonant field. All are these some examples which will be benefited by this technology.

How wireless energy could work

The answer is "resonance", a phenomenon that causes an object to vibrate when energy of a certain frequency is applied. "When you have two resonant objects of the same frequency they tend to couple very strongly. Resonance can be seen in musical instruments for example.

"When you play a tune on one, then another instrument with the same acoustic resonance will pick up that tune, it will visibly vibrate," Instead of using acoustic vibrations, the team's system exploits the resonance of electromagnetic waves. Electromagnetic radiation includes radio waves, infrared and X-rays. Typically, systems that use electromagnetic radiation, such as radio antennas, are not suitable for the efficient transfer of energy because they scatter energy in all directions, wasting large amounts of it into free space.

To overcome this problem a special class of "non-radiative" objects with so-called "long-lived resonances". When energy is applied to these objects it remains bound to them, rather than escaping to space. "Tails" of energy, which can be many meters long, flicker over the surface.

If you bring another resonant object with the same frequency close enough to these tails then it turns out that the energy can tunnel from one object to another.

Hence, a simple copper antenna designed to have long-lived resonance could transfer energy to a laptop with its own antenna resonating at the same frequency. The computer would be truly wireless. Any energy not diverted into a gadget or appliance is simply reabsorbed. The systems that the team has described would be able to transfer energy over three to five meters. This would work in a room let's say but you could adapt it to work in a factory.

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  1. Power from mains to antenna, which is made of copper
  2. Antenna resonates at a frequency of 6.4MHz, emitting electromagnetic waves
  3. 'Tails' of energy from antenna 'tunnel' up to 5m (16.4ft)
  4. Electricity picked up by laptop's antenna, which must also be resonating at 6.4MHz. Energy used to re-charge device
  5. Energy not transferred to laptop re-absorbed by source antenna. People/other objects not affected as not resonating at 6.4MHz

Example of wireless power transmission

William C. Brown, the leading authority on wireless power transmission technology, has loaned this demonstration unit to the Texas Space Grant Consortium to show how power can be transferred through free space by microwaves. A block diagram of the demonstration components is shown below. The primary components include a microwave source, a transmitting antenna, and a receiving rectenna.

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The microwave source consists of a microwave oven magnetron with electronics to control the output power. The output microwave power ranges from 50 W to 200 W at 2.45 GHz. A coaxial cable connects the output of the microwave source to a coax-to-waveguide adapter. This adapter is connected to a waveguide ferrite circulator which protects the microwave source from reflected power. The circulator is connected to a tuning waveguide section to match the waveguide impedance to the antenna input impedance.

The slotted waveguide antenna consists of 8 waveguide sections with 8 slots on each section. These 64 slots radiate the power uniformly through free space to the rectenna. The slotted waveguide antenna is ideal for power transmission because of its high aperture efficiency (> 95%) and high power handling capability.

A rectifying antenna called a rectenna receives the transmitted power and converts the microwave power to direct current (DC) power. This demonstration rectenna consists of 6 rows of dipoles antennas where 8 dipoles belong to each row. Each row is connected to a rectifying circuit which consists of low pass filters and a rectifier. The rectifier is a GaAs Schottky barrier diode that is impedance matched to the dipoles by a low pass filter. The 6 rectifying diodes are connected to light bulbs for indicating that the power is received. The light bulbs also dissipated the received power. This rectenna has a 25% collection and conversion efficiency, but rectennas have been tested with greater than 90% efficiency at 2.45 GHz.

History

In 1825 William Sturgeon invented the electromagnet, a conducting wire wrapped around an iron core. The principle of electromagnetic induction - that a changing magnetic field can induce an electrical current in an adjacent wire - was discovered by Michael Faraday in 1831. Combining these two discoveries, Nicholas Joseph Callan was the first to demonstrate the transmission and reception of electrical energy without wires. Callan's 1836 induction coil apparatus consisted of two insulated coils - called the primary and secondary windings - both placed around a common iron core. A battery intermittently connected to the primary would 'induce' a voltage in the longer secondary causing a spark to jump across its free terminals.

In an induction coil or electrical transformer, which can have either an iron core or an air core, the transmission of energy takes place by simple electromagnetic coupling through a process known as mutual induction. With this method it is possible to transmit and receive energy over a considerable distance. However, to draw significant power in that way, the two inductors must be placed fairly close together.

If resonant coupling is used, where inductors are tuned to a mutual frequency, significant power may be transmitted over a range of many meters.

In 1864 James Clerk Maxwell mathematically modeled the behavior of electromagnetic radiation. Some early work in the area of wireless transmission via radio waves was done in 1888 by Heinrich Hertz who performed experiments that validated Maxwell's mathematical model. Hertz's apparatus for generating electromagnetic waves is generally acknowledged as the first radio transmitter. A few years later Guglielmo Marconi worked with a modified form of the Hertz-wave transmitter, the main improvement being the addition of an elevated conductor and a ground connection. Both of these elements can be traced back to the 1749 work of Benjamin Franklin and that of Mahlon Loomas in 1864.

Nikola Tesla also investigated radio transmission and reception but unlike Marconi, Tesla designed his own transmitter - one with power-processing capability some five orders-of-magnitude greater than those of its predecessors. He would use this same coupled-tuned-circuit oscillator to implement his conduction-based wireless energy transmission method as well. Both of these wireless methods employ a minimum of four tuned circuits, two at the transmitter and two at the receiver.

As wireless technologies were being developed during the early 1900s, researchers further investigated these different wireless transmission methods. The goal was simply to generate an effect locally and detect it at a distance. Around the same time, efforts began to power more significant loads than the high-resistance sensitive devices that were being used to simply detect the received energy. At the St. Louis World's Fair (1904), a prize was offered for a successful attempt to drive a 0.1 horsepower (75 W) air-ship motor by energy transmitted through space at a distance of least 100 feet (30 m).

"...the air will serve as a conductor for the current produced, and the latter will be transmitted through the air with, it may be, even less resistance than through an ordinary copper wire." This was given by Tesla.

Modern day usage

Wireless power transmission over room-sized or community-sized distances has not been widely implemented. Rightly or not, it has been assumed by some that any system for broadcasting energy to power electrical devices will have negative health implications. With focused beams of microwave radiation there are definite health and safety risks. Considering the hazards associated with powerful radiation, the physical alignment and targeting of devices to receive the energy beam is of particular concern. However with the use of resonant coupling, wavelengths produced are longer, making it no more dangerous than being exposed to radio waves.

Ways of transmitting power wirelessly

Inductive coupling -

Inductive coupling uses magnetic fields that are a natural part of current's movement through wire. Any time electrical current moves through a wire, it creates a circular magnetic field around the wire. Bending the wire into a coil amplifies the magnetic field. The more loops the coil makes, the bigger the field will be.

A toothbrush's daily exposure to water makes a traditional plug-in charger potentially dangerous. Ordinary electrical connections could also allow water to seep into the toothbrush, damaging its components. Because of this, most toothbrushes recharge through inductive coupling.

If you place a second coil of wire in the magnetic field you've created, the field can induce a current in the wire. This is essentially how a transformer works, and its how an electric toothbrush recharges. It takes three basic steps:

  1. Current from the wall outlet flows through a coil inside the charger, creating a magnetic field. In a transformer, this coil is called the primary winding.
  2. When you place your toothbrush in the charger, the magnetic field induces a current in another coil, or secondary winding, which connects to the battery.
  3. This current recharges the battery.

You can use the same principle to recharge several devices at once. For example, the Splash power recharging mat and Edison Electric's Power desk both use coils to create a magnetic field. Electronic devices use corresponding built-in or plug-in receivers to recharge while resting on the mat. These receivers contain compatible coils and the circuitry necessary to deliver electricity to devices' batteries.

An electric toothbrush's base and handle contain coils that allow the battery to recharge as shown in fig.

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  1. Resonance and Wireless Power -

 

Household devices produce relatively small magnetic fields. For this reason, chargers hold devices at the distance necessary to induce a current, which can only happen if the coils are close together. A larger, stronger field could induce current from farther away, but the process would be extremely inefficient. Since a magnetic field spreads in all directions, making a larger one would waste a lot of energy.

In November 2006, however, researchers at MIT reported that they had discovered an efficient way to transfer power between coils separated by a few meters. The team, led by Marin Soljacic, theorized that they could extend the distance between the coils by adding resonance to the equation. Research at MIT indicates that induction can take place a little differently if the electromagnetic fields around the coils resonate at the same frequency. The theory uses a curved coil of wire as an inductor. A capacitance plate, which can hold a charge, attaches to each end of the coil. As electricity travels through this coil, the coil begins to resonate. Its resonant frequency is a product of the inductance of the coil and the capacitance of the plates. As with an electric toothbrush, this system relies on two coils. Electricity, traveling along an electromagnetic wave, can tunnel from one coil to the other as long as they both have the same resonant frequency. The effect is similar to the way one vibrating trumpet can cause another to vibrate.

As long as both coils are out of range of one another, nothing will happen, since the fields around the coils aren't strong enough to affect much around them. Similarly, if the two coils resonate at different frequencies, nothing will happen. But if two resonating coils with the same frequency get within a few meters of each other, streams of energy move from the transmitting coil to the receiving coil. According to the theory, one coil can even send electricity to several receiving coils, as long as they all resonate at the same frequency. The researchers have named this non-radiative energy transfer since it involves stationary fields around the coils rather than fields that spread in all directions.

Figure below explains the inductor coil.

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  1. Long-distance Wireless Power -

 

Whether or not it incorporates resonance, induction generally sends power over relatively short distances. But some plans for wireless power involve moving electricity over a span of miles. A few proposals even involve sending power to the Earth from space. In the 1980s, Canada's Communications Research Centre created a small airplane that could run off power beamed from the Earth. The unmanned plane, called the Stationary High Altitude Relay Platform (SHARP), was designed as a communications relay. Rather flying from point to point, the SHARP could fly in circles two kilometers in diameter at an altitude of about 13 miles (21 kilometers). Most importantly, the aircraft could fly for months at a time. The secret to the SHARP's long flight time was a large, ground-based microwave transmitter. The SHARP's circular flight path kept it in range of this transmitter. A large, disc-shaped rectifying antenna, or rectenna, just behind the plane's wings changed the microwave energy from the transmitter into direct-current (DC) electricity. Because of the microwaves' interaction with the rectenna, the SHARP had a constant power supply as long as it was in range of a functioning microwave array. Rectifying antennae are central to many wireless power transmission theories. They are usually made an array of dipole antennae, which have positive and negative poles. These antennae connect to semiconductor diodes. Here's what happens:

  1. Microwaves, which are part of the electromagnetic spectrum, reach the dipole antennae.
  2. The antennae collect the microwave energy and transmit it to the diodes.
  3. The diodes act like switches that are open or closed as well as turnstiles that let electrons flow in only one direction. They direct the electrons to the rectenna's circuitry.
  4. The circuitry routes the electrons to the parts and systems that need them.

Other, longer-range power transmission ideas also rely on rectennae. David Criswell of the University of Houston has proposed the use of microwaves to transmit electricity to Earth from solar power stations on the moon. Tens of thousands of receivers on Earth would capture this energy, and rectennae would convert it to electricity.

Power transmission via radio waves can be made more directional, allowing longer distance power beaming, with shorter wavelengths of electromagnetic radiation, typically in the microwave range. A rectenna may be used to convert the microwave energy back into electricity. Rectenna conversion efficiencies exceeding 95% have been realized. Power beaming using microwaves has been proposed for the transmission of energy from orbiting solar power satellites to Earth and the beaming of power to spacecraft leaving orbit has been considered.

Power beaming by microwaves has the difficulty that for most space applications the required aperture sizes are very large. For example, the 1978 NASA Study of solar power satellites required a 1-km diameter transmitting antenna, and a 10 km diameter receiving rectenna, for a microwave beam at 2.45 GHz. These sizes can be somewhat decreased by using shorter wavelengths, although short wavelengths may have difficulties with atmospheric absorption and beam blockage by rain or water droplets.

4. Light -

With a laser beam centered on its panel of photovoltaic cells, a lightweight model plane makes the first flight of an aircraft powered by a laser beam inside a building at NASA Marshall Space Flight Center.

In the case of light, power can be transmitted by converting electricity into a laser beam that is then fired at a solar cell receiver. This is generally known as "power beaming". Its drawbacks are:

  1. Conversion to light, such as with a laser, is moderately inefficient (although quantum cascade lasers improve this)
  2. Conversion back into electricity is moderately inefficient, with photovoltaic cells achieving 40%-50% efficiency. (Note that conversion efficiency is rather higher with monochromatic light than with insulation of solar panels).
  3. Atmospheric absorption causes losses.
  4. As with microwave beaming, this method requires a direct line of sight with the target.

NASA has demonstrated flight of a lightweight model plane powered by a laser beam.

5. Electrical conduction -

Electrical energy can also be transmitted by means of electrical currents made to flow through naturally existing conductors, specifically the earth, lakes and oceans, and through the atmosphere - a natural medium that can be made conducting if the breakdown voltage is exceeded and the gas becomes ionized. For example, when a high voltage is applied across a neon tube the gas becomes ionized and a current passes between the two internal electrodes. In a practical wireless energy transmission system using this principle, a high-power ultraviolet beam might be used to form a vertical ionized channel in the air directly above the transmitter-receiver stations. The same concept is used in virtual lightning rods, the electro laser electroshock weapon and has been proposed for disabling vehicles.

WiTricity (wireless electricity) - The new advancement

We moved one step closer to a world where plugging in your laptop, cell phone, iPod, and even a lamp for power is no longer required. Researchers at Massachusetts Institute of Technology have developed wireless electricity called "WiTricity" that can transmit electricity wirelessly.

In a proof of concept demonstration of the technology MIT researchers were able to light a 60W light bulb from a power source seven feet away with no physical connection between the source and the lamp.

In this image the coil on the left is transmitting electricity to the coil on the right powering a 60W light bulb. Members of the team that performed the experiment are obstructing the direct line of sight between the coils. However, the technology up until now has been hamstrung by issues such as line of site requirements between source and destination. As MIT puts it: "One can envision using directed electromagnetic radiation, such as lasers, but this is not very practical and can even be dangerous." MIT has broken the line-of-site requirements. Researchers explain: "WiTricity is based on using coupled resonant objects. Two resonant objects of the same resonant frequency tend to exchange energy efficiently, while interacting weakly with extraneous off-resonant objects."

Various methods of transmitting power wirelessly have been known for centuries. Perhaps the best known example is electromagnetic radiation, such as radio waves. While such radiation is excellent for wireless transmission of information, it is not feasible to use it for power transmission. Since radiation spreads in all directions, a vast majority of power would end up being wasted into free space.

One can envision using directed electromagnetic radiation, such as lasers, but this is not very practical and can even be dangerous. It requires an uninterrupted line of sight between the source and the device, as well as a sophisticated tracking mechanism when the device is mobile. In contrast, WiTricity is based on using coupled resonant objects. Two resonant objects of the same resonant frequency tend to exchange energy efficiently, while interacting weakly with extraneous off-resonant objects. A child on a swing is a good example of this. A swing is a type of mechanical resonance, so only when the child pumps her legs at the natural frequency of the swing is she able to impart substantial energy.

Another example involves acoustic resonances: Imagine a room with 100 identical wine glasses, each filled with wine up to a different level, so they all have different resonant frequencies. If an opera singer sings a sufficiently loud single note inside the room, a glass of the corresponding frequency might accumulate sufficient energy to even explode, while not influencing the other glasses. In any system of coupled resonators there often exists a so-called "strongly coupled" regime of operation. If one ensures to operate in that regime in a given system, the energy transfer can be very efficient.

While these considerations are universal, applying to all kinds of resonances (e.g., acoustic, mechanical, electromagnetic, etc.), the MIT team focused on one particular type: magnetically coupled resonators. The team explored a system of two electromagnetic resonators coupled mostly through their magnetic fields; they were able to identify the strongly coupled regime in this system, even when the distance between them was several times larger than the sizes of the resonant objects. This way, efficient power transfer was enabled.

Magnetic coupling is particularly suitable for everyday applications because most common materials interact only very weakly with magnetic fields, so interactions with extraneous environmental objects are suppressed even further. "The fact that magnetic fields interact so weakly with biological organisms is also important for safety considerations," Kurs, a graduate student in physics, points out.

The investigated design consists of two copper coils, each a self-resonant system. One of the coils, attached to the power source, is the sending unit. Instead of irradiating the environment with electromagnetic waves, it fills the space around it with a non-radiative magnetic field oscillating at MHz frequencies. The non-radiative field mediates the power exchange with the other coil (the receiving unit), which is specially designed to resonate with the field. The resonant nature of the process ensures the strong interaction between the sending unit and the receiving unit, while the interaction with the rest of the environment is weak. The crucial advantage of using the non-radiative field lies in the fact that most of the power not picked up by the receiving coil remains bound to the vicinity of the sending unit, instead of being radiated into the environment and lost." With such a design, power transfer has a limited range, and the range would be shorter for smaller-size receivers. Still, for laptop-sized coils, power levels more than sufficient to run a laptop can be transferred over room-sized distances nearly omni-directionally and efficiently, irrespective of the geometry of the surrounding space, even when environmental objects completely obstruct the line-of-sight between the two coils. Fisher points out: "As long as the laptop is in a room equipped with a source of such wireless power, it would charge automatically, without having to be plugged in. In fact, it would not even need a battery to operate inside of such a room." In the long run, this could reduce our society's dependence on batteries, which are currently heavy and expensive.

WiTricity is rooted in such well-known laws of physics that it makes one wonder why no one thought of it before. "In the past, there was no great demand for such a system, so people did not have a strong motivation to look into it," points out Joannopoulos, adding, "Over the past several years, portable electronic devices, such as laptops, cell phones, iPods and even household robots have become widespread, all of which require batteries that need to be recharged often."

Powercast Systems

Shearer came in. A former physicist based in Pittsburgh, he and his team spent four years poring over wireless electricity research in a lab hidden behind his family's coffee house. He figured much of the energy bouncing off walls could be captured. All you had to do was build a receiver that could act like a radio tuned to many frequencies at once.

This system turns radio waves into D.C voltage.

The transmitter plugs into the wall socket and broadcasts safe, low power radio waves.

The radio waves change their frequency as they bounce off objects and walls.

Tiny receivers in your devices "hear" frequencies around the original one capturing up to 70 % of radio signal's energy. That energy is converted into DC electricity.

Its tiny but hyper efficient receiving circuits can adjust to variations in load and field strength while maintaining a constant DC voltage. Thanks to the fact that it transmits only safe low wattages

Broadcasting power through the air isn't a new idea. Researchers have experimented with capturing the radiation in radio frequency at high power but had difficulty capturing it at consumer-friendly low power. "You'd have energy bouncing off the walls and arriving in a wide range of voltages," says Zoya Popovic, an electrical engineering professor at the University of Colorado who works on wireless electricity projects for the U.S. military.

Powercast says it has signed nondisclosure agreements to develop products with more than 100 companies, including major manufacturers of cell phones, MP3 players, automotive parts, temperature sensors, hearing aids, and medical implants.

The last of those alone could be a multibillion-dollar market: Pacemakers, defibrillators, and the like require surgery to replace dead batteries. But with a built-in Powercast receiver, those batteries could last a lifetime.

Could Powercast's technology also work for larger devices? Perhaps, but not quite yet. Laptop computers, for example, use more than 10 times the wattage of Powercast transmissions. Industry trends are on Shearer's side: Thanks to less energy-hungry LCD screens and processors, PC power consumption is slowly diminishing. Within five years, laptops will be down to single-digit wattage--making his revenue potential even more electrifying.

The Wireless Transmission of Electrical Energy Using Schumann Resonance

It has been proven that electrical energy can be propagated around the world between the surface of the Earth and the ionosphere at extreme low frequencies in what is known as the Schumann Cavity. The Schumann cavity surrounds the Earth between ground level and extends upward to a maximum 80 kilometers. Experiments to date have shown that electromagnetic waves of extreme low frequencies in the range of 8 Hz, the fundamental Schumann Resonance frequency, propagate with little attenuation around the planet within the Schumann Cavity.

Knowing that a resonant cavity can be excited and that power can be delivered to that cavity similar to the methods used in microwave ovens for home use, it should be possible to resonate and deliver power via the Schumann Cavity to any point on Earth. This will result in practical wireless transmission of electrical power. The successful resonating of the Schumann Cavity and wireless transmission of power on a small scale resulting in proof of principle will require a second phase of engineering, the design of receiving stations. On completion of the second phase, the third and fourth phases of the project involving further tests and improvements and a large scale demonstration project will be pursued to prove commercial feasibility. Total cost from proof of principle to commercial prototype is expected to total $3 million.

The wireless power transmission sheet

A team of Japanese researchers has created a novel wireless power-transmission device that is thin, flat, and flexible. Based on a sheet of plastic, the device can be put on desks, floors, walls, and almost any other location, delivering power to electronics placed on or near it without the use of cables or connectors.

The electrical components are deposited onto the plastic via state-of-the-art inkjet printing technology using "electronic ink." The finished product is about one millimeter thick and 21 centimeters square, although power sheets large enough to cover entire walls or floors could potentially be created.

The sheet can deliver up to about 40 watts; enough to power light bulbs and small electronics (cell phones, clocks, etc.) equipped to accept wireless power. The sheet has an impressive 81 percent efficiency, meaning 81 percent of the emitted power is received by devices. The sheet is a significant step forward for the field of electronics. Corresponding researcher Takao Someya, a scientist at the University of Tokyo, told PhysOrg.com, "Our power-transmission sheet addresses two of the issues facing the electronics field: creating ecologically friendly power systems and developing power-transmission technologies that further the imminent trend of 'ambient electronics' - electronic networks, such as sensors, built into our homes and offices to increase our day-to-day security and convenience."

The sheet is an example of "organic electronics," a fast-growing field in which circuits are based on conducting plastics rather than conventional silicon. Organic electronics have several advantages, including being cheaper to manufacture, more environmentally safe to produce, physically light, and, as in this case, are often thin and bendable. However, many organic electronic devices can only be integrated into low-power applications because organic transistors have high electrical resistances and cannot handle large amounts of electricity. By combining organic transistors with a technology traditionally used to fabricate silicon circuits, Someya and his colleagues have boosted the amount of power their organic power-transmission sheet can handle. The finished product consists of several layers. These include a layer printed with an array of thin, flat copper coils, which sense the position of nearby electronic devices, and a layer of sender coils that deliver the wireless power. This process occurs via electromagnetic induction, a physics phenomenon in which a magnetic field can induce a current in a nearby conductor. Here, a voltage applied across the sender coils produces a magnetic field, which induces current flow in nearby devices that need power, as long as those devices are equipped with receiver coils.

A large-area power transmission sheet by using printing technologies

The position of electronic objects on this sheet can be contactlessly sensed by electromagnetic coupling using an organic transistor active matrix. Power is selectively fed to the objects by an electromagnetic field using a plastic MEMS-switching matrix.

Ubiquitous electronics or ambient intelligence is attracting attention because of its potential to open up a new class of applications. This paper reports the first implementation of a large-area wireless power transmission system (Fig. 1) using organic transistors (1-9) and MEMS switches. The system realizes a low-cost sheet-type wireless power source of more than several watts. This is the first step toward building infrastructure for ubiquitous electronics where multiple electronic objects are scattered over desks, floors, walls, and ceilings and need to be powered. These objects may be mobile or located in the dark and therefore solar cells cannot be used to power them. On the other hand, the periodic replacement of the primary batteries could be tedious since there may be too many objects. The proposed wireless power transmission sheet may directly drive electronic objects and/or charge a rechargeable battery in the objects without a connector, thereby providing an easy-to-use and reliable power source. The wireless power transmission sheet has been manufactured on a plastic film by using printing technologies. The effective power transmission area is 21 × 21 cm2. The sheet contains a two-dimensional array of 8 × 8 cells comprising position-sensing and power transmission units. The position of electronic objects on the sheet can be contactlessly sensed by electromagnetic coupling using an organic transistor active matrix. Then, power is selectively fed to the objects by an electromagnetic field using a two-dimensional array of copper coils that are driven by a printed plastic MEMS-switching matrix. Due to selective power transmission, we achieved a coupling efficiency of power transmission of 62.3%, and a power of 29.3 W was wirelessly received. The thickness and weight of the entire sheet are 1 mm and 50 g, respectively.

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From Author

This report deals with the advancement in technology related to the wireless transmission of electric signals or power. It the process that takes place in any system where electrical energy is transmitted from a power source to an electrical load, without interconnecting wires. Wireless transmission is employed in cases where instantaneous or continuous energy transfer is needed, but interconnecting wires are inconvenient, hazardous, or impossible.

This technology has showed the various advancements in the fields of science which have been provided in t his report. Various references are given for any further requirement of the reader.

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