WIRELESS POWER TRANSFER (WPT)
by Steve Dent / 05.16.12
“Wireless charging may be all the rage these days, but actually beaming electricity — as sketched above by the man Tesla himself — still has some snags. North Carolina State U researchers have found a way to possibly vanquish the biggest problem: the difficulty of exactly matching resonant frequencies to amplify current. If external factors like temperature change the tuning of a transmitter even slightly then power drops will occur, but circuitry developed by the NC State scientists would allow receivers to detect these changes and automatically re-tune themselves to match. This could make for more potent car and device charging in the future and, if they stretch the distances a bit, maybe we’ll finally get the wire-free utopia Nikola dreamed up 120 years ago.”
“The diagram above shows the basic principles of inductive power transmission.”
Wireless charging technology coming soon to mobile devices, electric cars, medicine
by Rob Matheson / July 10, 2014
“More than a century ago, engineer and inventor Nikola Tesla proposed a global system of wireless transmission of electricity—or wireless power. But one key obstacle to realizing this ambitious vision has always been the inefficiency of transferring power over long distances. Near the end of the last decade, however, a team of MIT researchers led by Professor of Physics Marin Soljacic took definitive steps toward more practical wireless charging. First, in 2007, the team wirelessly lit a 60-watt light bulb from eight feet away using two large copper coils, with similarly tuned resonant frequencies, that transferred energy from one to the other over the magnetic field. Then, in 2010, they shrunk the coils down and significantly increased the efficiency of the system, noting future applications in consumer products.
Now, this “wireless electricity” (or “WiTricity”) technology—licensed through the researchers’ startup, WiTricity Corp.—is coming to mobile devices, electric vehicles, and potentially a host of other applications. The aim is to forge toward a “wire-free world,” says Soljacic. Primarily, this means consumers need not carry wires and power bricks. But it could also lead to benefits such as smaller batteries and less hardware—which would lower costs for manufacturers and consumers. “It’s probably a dream of any professor at MIT to help change the world for a better place,” says Soljacic, a WiTricity co-founder who now serves on its board of directors. “We believe wireless charging has a potential to do that.” He is not alone. Last month, WiTricity signed a licensing agreement with Intel to integrate WiTricity technology into computing devices powered by Intel. Back in December, Toyota licensed WiTricity technology for a future line of electric cars. Several more publicized and unpublicized companies have recently joined in the licensing parade for this technology, including Thoratec for their implantable ventricular assisting devices, and TDK for wireless electric vehicle-charging systems. There’s even talk of a helmet powered wirelessly via backpack, specifically for military applications.
At present, WiTricity technology charges devices at around 6 to 12 inches with roughly 95 percent efficiency—12 watts for mobile devices and up to 6.6 kilowatts for cars. But, with growing research and development, the company is increasing distance, scale, and efficiency. It’s also developed repeaters: passive devices that extend the distance of the power transfer. These can be developed into a wide variety of shapes and can be embedded in a carpet to “hop” the power across a room.
Similar wireless charging technologies have been around for some time. For instance, traditional induction charging, which uses an electromagnetic field to transfer energy between two coils, is used in transformers and wireless toothbrushes. In the past two years, there’s also been an increase in wireless cell phone charging pads based on induction. “These work well, but only over very short distances, so they’re nearly touching,” Soljacic says. “They become dramatically inefficient when the distance increases.” Lasers can also move energy between two points, such as two satellites. But this requires an uninterrupted, continuous path between the transmitter and the receiver, which “is obviously not ideal for consumer products,” Soljacic says.
WiTricity’s system of transmitters and receivers with magnetic coils, on the other hand, “efficiently transfers power over longer distances,” says CEO Alex Gruzen ’84, SM ’86. “It can also charge through materials such as wood or granite, allow freedom to move the devices around, and charge several devices at once.” To make the system more efficient, WiTricity tunes the coils to find a strong electromagnetic highly resonant coupling. This is similar to a tuning fork vibrating when exposed to a sound of the right frequency, or a radio antenna tuning into a single station out of hundreds. The concept took shape in early 2000s, when Soljacic awoke at 3 a.m. to the beeping of his cell phone running out of battery life. Frustrated, and standing half awake, he contemplated ways to harness power from all around to charge the phone. At the time, he was working on various photonics projects—lasers, solar cells, and optical fiber—that all involved a phenomenon called resonant coupling. “The underlying physics could be easily applied to power transfer,” he says.
New category of magnetic resonance
Seeing use for consumer devices, Soljacic and a team of five MIT researchers—including physics professors Peter Fisher and John Joannopoulos—published a proof-of-concept experiment in Science in 2007, and founded WiTricity that same year. In the experiment, the researchers used two copper coils, about two feet across, each a self-resonant system. One transmitting coil was connected to an AC power supply, while another connected to a 60-watt light bulb. The transmitter emanated a magnetic field, oscillating at megahertz frequencies, which the receiver matched, ensuring a strong coupling between the units and weak interaction with the rest of the environment, including nonmetallic materials—and humans. In fact, they demonstrated that they could light the bulb, at roughly 45 percent efficiency, with all six researchers standing in between the two coils.
Gruzen uses the following analogy: A room is packed with 100 wine glasses, each filled with a different level of wine to ensure a different resonant frequency. “If an opera singer belts out a note inside that room, the glass with the corresponding frequency accumulates enough energy to shatter, but none of the other glasses will resonate enough to break,” he says. A 2010 paper published in Applied Physics Letters by Soljacic and colleagues made another breakthrough: They found that when adding more receiver coils, power transfer efficiency climbs by more than 10 percent. In that experiment, they used larger transmitting coils, but receiving coils that were only a foot across, resulting in a power output of 50 watts from several feet away. “This enabled the development of a whole new category of magnetic resonance,” Gruzen says. From there, the company focused on finding the optimum design of the coils and electrical control systems for commercial applications.
Wireless charging: An expectation
These days, Gruzen sees wireless charging as analogous to the evolution of a similar technology—WiFi—that he witnessed in the early 2000s as senior vice president of global notebook business at Hewlett Packard. At the time, WiFi capabilities were rarely implemented into laptops; this didn’t change until companies began bringing wireless Internet access into hotel lobbies, libraries, airports, and other public places. Now, having established a standard for wireless charging of consumer devices with the A4WP (Alliance for Wireless Power) known as Rezence, WiTricity aims to be the driving force behind wireless charging. Soon, Gruzen says, it will be an expectation—much like WiFi.
“You can have a charging surface wherever you go—from a kitchen counter to your workplace to airport lounge and hotel lobbies,” he says. “In this future, you’re not worried about carrying cords. Casual access to topping off power in your devices just becomes an expected thing. This is where we’re going.” With an expected rise of wireless charging, one promising future application Soljacic sees is in medical devices—especially implanted ventricular assist devices (or “heart pumps”) that support blood flow. Currently, a patient who has experienced a heart attack or weakening of the heart has wires running from the implant to a charger—which means risk for infection. “In our case, a patient could lie on the bed and, while he or she is sleeping, our technology could charge the device from a distance,” Soljacic says. “We expect to have much more of these embedded electronic devices in people over the next decade or so.”
“When a highly resonant transmitting copper coil, connected to an AC power source (top, left), is tuned to the same frequency as a highly resonant receiving copper coil (bottom, left), the two coils exchange energy efficiently over distances via the magnetic field (right).”
WiTricity Studies and Experiments
- The 1st Experiments of Witricity from Massachusetts Institute [View Experiment]
- MIT Witricity Not So Original After All [View Experiment]
- Wireless energy transfer [View Experiment]
- WiTricity Technology Basics [View Experiment]
- WiTricity: non-radiative wireless power transfer [View Experiment]
- Experiments on the Tesla Wireless Transmission Method for Submarine Communication [View Experiment]
- Wireless Power Transmission Using Magnetic Resonance [View Experiment]
- A Low-Cost Wireless Power Transmission Experiment [View Experiment]
- World System of Wireless Transmission Of Energy – Nikola Tesla [View Experiment]
- Concepts for wireless energy transmission via laser [View Experiment]
WiTricity, stands for wireless electricity, is a term coined initially by Dave Gerding in 2005 and used by a MIT research team led by prof. Marin Soljačić in 2007, to describe the ability to provide electricity to remote objects without wires (wireless power transfer). This could be useful to power consumer and industrial electronics like cell phones, laptops, etc. Scientists and engineers have known for nearly two centuries that transferring electric power does not require wires to be in physical contact. Electric motors and power transformers contain coils that transmit energy to each other by the phenomenon of electromagnetic induction. A current running in an emitting coil induces another current in a receiving coil; the two coils are in close proximity, but they do not touch. Later, scientists discovered electromagnetic radiation in the form of radio waves, but transferring energy from one point to another through ordinary electromagnetic radiation is typically very inefficient: The waves tend to spread in all directions, so most of the energy is lost to the environment. Serious interest and effort was devoted in this direction, most notably by Nikola Tesla in the late 19th century, but with little success.
Every technician working at a radio transmission plant knows that holding a fluorescent lamp a few meters from the antenna pole will light it. But of course this method is not efficient and safe since high radiation energy is involved. In any case, don’t try this experiment by any means since it could be very dangerous. One can also 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.
Soljačić realized that the close-range induction taking place inside a transformer – or something similar to it – could potentially transfer energy over longer distances, say, from one end of a room to the other. Instead of irradiating the environment with electromagnetic waves, a power transmitter would fill the space around it with a “non-radiative” electromagnetic field. Energy would only be picked up by gadgets specially designed to “resonate” with the field. Most of the energy not picked up by a receiver would be reabsorbed by the emitter.
Non-radiative wireless power is based on evanescent waves that are found in the nearfield region within one-third wavelength (a few meters in our case) of any radio antenna. During normal operation, an antenna emits electromagnetic fields into the surrounding nearfield region, then a portion of the field energy (evanescent waves) decays since it is re-absorbed by the antenna, while the remainder is radiated into the environment as EM waves.
The novelty of WiTricity is that Soljacic and his MIT team built energy emitters (transmitters) whose electromagnetic evanescent waves radiated greater distances without significant decay. In addition, the emitter and receiver resonate with each other (resonant evanescent coupling (REC)) and the energy transfer rate is high. And no energy is transferred to other objects with different resonances, even if those objects directly block the line-of-sight between emitter and receiver. Non-radiative wireless power would have limited range, and the range would be shorter for smaller-size receivers. But the team calculates that an object the size of a laptop could be recharged within a few meters of the power source. Placing one source in each room could provide coverage throughout your home.
The MIT researchers successfully demonstrated the ability to power a 60 watt light bulb from a power source that was 2 meters (7 ft) away at roughly 40% efficiency. They used two capacitively loaded copper coils, 60 centimeters (24 in) in diameter, oriented along the same axis, The coils were designed to resonate together at 10 MHz (relatively safe for living tissue). One was connected inductively to a power source, the other to a bulb. The setup powered the bulb on, even when the direct line of sight was blocked using a wooden panel. As for now (July 2007), the researchers plan to miniaturize the setup enough for commercial use in three to five years and suggest that the radiated power densities can be brought below the threshold for FCC safety regulations.”
Getting in Tune: Researchers Solve Tuning Problem For Wireless Power Transfer Systems
by Matt Shipman / 05.15.2012
“Researchers from North Carolina State University have developed a new way to fine-tune wireless power transfer (WPT) receivers, making the systems more efficient and functional. WPT systems hold promise for charging electric vehicles, electronic devices and other technologies. Researchers have shown that it is possible to transmit power wirelessly by using magnetic resonance. Even minor changes in how the transmitter or receiver is tuned, however, can result in faulty power transmission. A new prototype developed at NC State addresses the problem by automatically – and precisely – re-tuning the receivers in WPT systems. The researchers focused on receivers because methods already exist that allow researchers to use electronics to precisely tune the transmitters. “We’re optimistic that this technology moves us one step closer to realizing functional WPT systems that can be used in real-world circumstances,” says Dr. Srdjan Lukic, an assistant professor of electrical and computer engineering at NC State and co-author of a paper on the research.
WPT systems work by transmitting magnetic waves on a specific frequency from a transmitter to a receiver. These magnetic waves interact with a coil in the receiver to induce an electric current. If the coil is tuned so that its resonant frequency matches the frequency of the magnetic waves, the current it produces is amplified. However, if the receiver and the transmitter are out of tune, the system becomes inefficient and doesn’t transfer a significant amount of power. The receiver coil still picks up a trace amount of current, but it is not amplified. This is a problem because many factors can affect the tuning of a receiver or transmitter, such as temperature or proximity to other magnetic objects. In other words, a hot summer day could wreak havoc on the tuning of a receiver.
A small prototype charger from North Carolina State University is shown that can transmit power wirelessly from a stationary source to a mobile receiver. The goal is to create highway “stations” that can recharge electric vehicles wirelessly as the vehicles drive past.
Lukic and NC State Ph.D. student Zeljko Pantic developed an electronic prototype that incorporates additional circuitry into the receiver that does two things: it injects small amounts of reactive power into the receiver coil as needed to maintain its original resonant frequency; and, if the transmitter’s tuning changes, the prototype can read the trace amount of current being transmitted and adjust the receiver’s tuning accordingly. “Because we are using electronics to inject reactive power into the receiver coil, we can be extremely precise when tuning the receiver,” Lukic says. “This degree of fine-tuning maximizes the efficiency of the WPT system. “The next step is to try incorporating this work into technology that can be used to wirelessly charge electric vehicles.”
Lukic and Pantic, who co-authored a paper on the research, developed a prototype that incorporates additional circuitry into the receiver that does two things: it injects small amounts of reactive power into the receiver coil to maintain its original resonant frequency; and, if the transmitter’s tuning changes, the prototype can read the trace amount of current being transmitted and adjust the receiver’s tuning accordingly.
Framework and Topology for Active Tuning of Parallel Compensated Receivers in Power Transfer Systems
Authors: Zeljko Pantic, Srdjan Lukic, North Carolina State University
Published online, IEEE Transactions on Power Electronics
Abstract: “Wireless power transfer (WPT) based on magnetic coupling is becoming widely accepted as a means of transferring power over small to medium distances. An unresolved issue is the source and receiver resonance matching in multi-receiver systems where the source operating frequency adjustment is not possible. This paper presents a framework to analyze the effect of parallel-compensated receiver detuning on the power transfer in WPT systems. Building on this analytical study, we present a new receiver design for WPT systems. The proposed design combines a parallel compensated resonant tank with a tri-state boost converter. By synchronizing the tri-state boost switching period with the half-period of the resonant tank voltage, we position the inherently discontinuous current pulse drawn by the tri-state boost to control both active and reactive power flow from the resonant circuit to the tri-state boost. Controllable reactive current can be used effectively to emulate appropriate inductance or capacitance to tune the resonant tank and achieve optimal power transfer.”
“Rohm’s Wireless Power Transmission system also allows you to charge devices by placing them anywhere on the pad, with one prototype outputting up to 100 watts.”
WIRELESS POWER STANDARD
Soon, Power Will Be Delivered to Your Device by Air
by Christopher Mims / 10/5/2015
“In 1902 workers completed a mysterious tower, 187 feet high and shaped like a giant mushroom, on which rested the hopes of one of the 20th century’s most prolific geniuses. Facing the beach in the hamlet of Shoreham, N.Y., on Long Island, the Wardenclyffe Tower was, according to its inventor, Nikola Tesla, the key that could unlock an age of wonders. As Mr. Tesla later wrote, the tower’s ability to transmit information to the far side of the Earth would someday allow the creation of “an inexpensive instrument, not bigger than a watch, [which] will enable its bearer to hear anywhere, on sea or land, music or song however distant.” Sometime in 2016, Tesla’s other prediction—that it isn’t only possible, but commercially viable, to transmit power as well as information through the air, without wires—is expected to come true. What is coming are hermetically sealed smartphones and other gadgets that charge without ever plugging into a wall. And soon after there will be sensors, cameras and controllers that can be stuck to any surface, indoors or out, without the need to consider how to connect them to power. Wireless power will be, in other words, not just a convenience, but a fundamental enabler of whole new platforms. The players in this field are myriad, but their technology can be boiled down to four basic types.
There are those power mats you may have seen at Starbucks, an older technology that hasn’t been widely adopted. The second, pioneered by the 8-year-old company WiTricity, is slated to show up in Intel-chip-powered laptops sometime in 2016. It uses “magnetic resonance” to efficiently transmit power, over distances ranging from centimeters to a meter. “It’s really charging that Intel notebook as if you’d plugged it in,” says WiTricity Chief Executive Alex Gruzen. But it is the third and fourth kinds of wireless power that are the most intriguing, because they involve beaming power over significant distances. One, which the startups Energous and Ossia are racing to commercialize, involves transmitting power more or less as Mr. Tesla envisioned—through radio waves. And the last, pioneered by uBeam, involves transmitting power through sound waves.
The challenge with these approaches isn’t technology, but physics. Radio waves, after all, are in the same range as the waves generated by a microwave oven. There’s only so much energy you can beam through the air without cooking whatever gets in the way. Energous CEO Michael Leabman claims his two-year-old company has this problem licked, and not because of breakthroughs in focusing radio waves. The key, experts say, is that mobile devices use less power than ever. “It’s really the chip makers who deserve most of the credit for this stuff,” says Gregory Durgin, a Georgia Institute of Technology professor who is an expert on wireless transmission of power. He says the claims that Energous is making for its technology are in line with his own experience.
A typical smartphone might be able to charge quickly from a wall outlet putting out 5 watts, but if you can—as Energous claims—beam up to 2 watts of power over a distance of 10 feet, to a small radio antenna embedded in that phone, you can “trickle charge” it in a matter of hours. If you think about how much time we typically spend in our offices and homes, this is a perfectly reasonable way to almost guarantee that we’ll never have a dead phone again, especially if our devices start charging automatically the moment we walk in the door. Energous has shown off a workable demo, and Mr. Leabman says the company’s technology will be a mass-market product by the end of 2016 or early 2017.
Just as exciting is the potential of wireless energy to solve the problem that has always plagued the Internet of things—or the idea that we will cover our entire world in sensors and tiny motors that control devices, leading to “smart” everything. The hitch is, how to power all those little chips and their electronics, some of which may be as small and thin as a stick-on price tag. Energous already has a patent on the idea of putting a power transmitter into the base of a light bulb, allowing its technology to cover an entire room, and putting out enough power that a device 15 feet away could absorb one watt.
“Wireless power could enable a whole new class of devices,” says Mr. Durgin. Those devices will include sensors on all the mechanics of a home, business or factory; detectors for heat, light and motion; and cameras and controls that we can move and upgrade at our convenience, without ever having to touch the building’s wiring. These controls will include “peel and stick” light switches and thermostats, which are already a common senior design project among Prof. Durgin’s students. Meredith Perry, CEO of uBeam, says that her company’s technology will be able to beam more power, via ultrasound waves, over greater distances than what is claimed by companies like Energous, and that uBeam will unveil a working prototype by the end of 2016. Both of these technologies face major issues. In the case of uBeam, experts I spoke with were skeptical that, based on the physics involved, the company can deliver on its promises.
Moreover, energy transmitted via radio waves represents a major pollution of the bands of unregulated spectrum that already are crowded with everything from microwave ovens to Wi-Fi routers. That could limit the places wireless power can be used. So it isn’t likely that the three-prong outlet will be obsolete anytime soon, but it is likely that in the near future ambient power could be commonplace. It would be the ultimate vindication of Mr. Tesla, a hundred years after his tower project shut down for lack of funds.”
MAXWELL’s REAL EQUATIONS