The Power of Induction

Cutting the last cord could resonate with our increasingly gadget-dependent lives

By Davide Castelvecchi
From Science News, July 21, 2007

Marin Soljacic was understandably nervous. The young physicist was about to give his first public presentation of an idea that sounded almost too good to be true. There was no telling how his audience, at a Berkeley, Calif., symposium, would receive his daring proposal. Design two antennas to be as inefficient as possible at transmitting radio waves, Soljacic began.

Separate the antennas by a few meters and, with some fine-tuning, you can safely and efficiently transfer electricity from one to the other—without wires. Put this system inside your home, and you would have a wireless network for electrical power. You could recharge your laptop or turn on a light without plugging anything in.

The crucial bit would be the fine-tuning: The two antennas would have to be tweaked so that one would create a pulsating magnetic field with a specific frequency and geometry, which the other would then transform into an electric current.

When Soljacic first presented the principle, it was unproved. All he could show were his calculations. “I expected that some people would think I was a crackpot,” says Soljacic, a physicist at the Massachusetts Institute of Technology (MIT). “This was pretty far out.”

Perhaps it also didn’t help that the participants at the symposium—a celebration of the 90th birthday of Charles Townes, who pioneered the laser in the 1950s—included 18 Nobel prize winners and dozens of other luminaries. Much to Soljacic’s relief, he sold the scientists on his presentation.

A year and a half later, a bulb lit up in an MIT lab—unplugged. Soljacic and his collaborators had demonstrated a new way of coaxing magnetic fields into transferring power over a distance of several meters without dispersing as electromagnetic waves. The demonstration ushered in a technology that might eventually become as pervasive as the gadgets it could power. Laptops, cell phones, iPods, and digital cameras might someday recharge without power cords. With the proliferation of wireless electronics, perhaps it was just a matter of time before power transmission would go wireless, too.

The device that Soljacic and his collaborators put together had a disarming simplicity. On one side of the room, hanging from the ceiling, was a ring-shaped electrical circuit, about half a meter across, plugged into the wall. Hanging adjacent to the circuit, but with no physical connection to it, was a slightly larger copper coil looking like an oversize mattress spring. A few meters away hung a similar system with an ordinary lightbulb attached to the circuit. When the physicists sent power through the first circuit, the bulb lit up.

As expected, some energy was lost on its way to the lightbulb. However, a surprising amount reached its destination, the team reports in the July 6 Science. “The efficiency was 40 percent at the biggest distance we probed [more than 2 meters],” Soljacic says. At shorter distances, the efficiency was much higher.

The coils of this demonstration device would be too big to fit inside a laptop, let alone a cell phone. But this was only the first and simplest of several prototypes that the physicists have in mind. More tests are to come. The MIT team and other physicists say that in principle they see no obstacle to making such devices more compact and more efficient.

Making no waves

The idea of transmitting energy wirelessly isn’t new. For almost two centuries, scientists have known that rapidly changing magnetic fields, such as those produced by an alternating current flowing through a wire, can induce an electric current in another wire. That’s how the coils inside power transformers transmit energy from one coil to another without touching. But this form of induction usually works efficiently only when the two coils are very close to each other.

In the early 1900s, long before the power grid made electricity widely available, electricity pioneer Nikola Tesla devised a grand scheme to transfer large amounts of power over long distances from a tower 20 stories tall, to be built on Long Island in New York. To this day, historians puzzle over how Tesla’s system was supposed to work, or whether it could have worked at all, says Bernard Carlson, a historian of science at the University of Virginia in Charlottesville who is writing a biography of the great engineer. “We can’t even begin to understand what he was doing with this power stuff,” Carlson says.

The project died when Tesla’s financial backers pulled the plug, possibly because Tesla seemed unclear as to how to bill customers receiving wireless power. Ironically, Tesla also invented the alternating current (AC) system of power production, transmission, and distribution that would become the standard for the modern grid.

But electromagnetic radiation can indeed carry energy through air or empty space and over large distances. One familiar example is the energy we receive from the sun, mostly as visible light. Another is radio waves, first detected by Heinrich Hertz in 1888. An electromagnetic wave is a synchronized dance of an electric field and a magnetic field. Because an oscillating magnetic field generates an oscillating electric field, and vice versa, the two fields sustain each other as the wave propagates.

Radio waves and light waves, however, tend to shoot out in all directions. This makes for very inefficient power transmission, because the farther the waves travel, the larger the volume of space throughout which their energy spreads. Technologies such as lasers and parabolic antennas can confine the energy of electromagnetic waves in tight beams, that can transfer power. But beams have disadvantages. One problem is that anything that happens to cross a beam’s path may get fried.

Soljacic’s wireless power system harnesses oscillating electric and magnetic fields in a novel way. Although it doesn’t radiate energy as a radio antenna does, it transmits power across greater distances than a conventional transformer can.

A typical antenna—the simplest type being essentially a rod—has a size comparable to the wavelength of the radiation it emits. The electric and magnetic fields it creates are in phase. They rise and fall in sync with each other, a property that’s crucial to the self-sustaining feedback that allows a wave to propagate.

The circuit in Soljacic’s device carries an alternating current with a frequency of about 10 megahertz (MHz). It generates a magnetic field that induces a current in the adjacent coil, which then amplifies the magnetic field.

Electromagnetic waves of 10 MHz have a wavelength of about 30 m. Because the coils are much smaller than that, they don’t generate conventional waves, explains Aristeidis Karalis, an MIT graduate student who helped with Soljacic’s theoretical model and computer simulations. Instead, “the electric field is at its maximum when the magnetic field is zero, and vice versa,” which is the opposite of being in phase, Karalis says. This arrangement means that the fields’ energy stays mostly in the vicinity of the coil, and only a small percentage of the total power disperses as waves.

The MIT team introduced two additional ingredients into its design, the first to make it safe and the second to make it efficient.

For safety, they took the advice of John Pendry, an Imperial College London physicist who visited the MIT lab in 2005. Pendry recommended designing the system to minimize exposure to electric fields, since rapidly changing electric fields can heat up the surroundings, including any people close by. “With the electric field you’d get hot, like in a microwave oven,” Pendry says, whereas the body “hardly responds to magnetic fields.”

In the team’s designs, the magnetic fields change slowly enough to not create strong electric fields. The magnetic fields themselves are comparable in strength to Earth’s magnetism, Karalis says, and only one-thousandth as strong as the field inside a magnetic resonance (MRI) machine. On the other hand, both MRIs and Earth have constant, not rapidly oscillating, fields. But the MIT scientists say that their fields stay within safety guidelines issued by the Institute of Electrical and Electronic Engineers.

Resonating power

The second ingredient is Soljacic’s use of resonance—the innovation that makes efficient energy transfer possible. Just as guitar strings and wine glasses vibrate at specific frequencies, electric circuits have their own natural AC oscillation modes. The diameter of the MIT coils and the spacing between their turns are suitably adjusted, so the coils act as electrical circuits with a natural AC frequency of 10 MHz, putting them in sync with the magnetic oscillations and with each other. One coil can then transfer energy to the other by the same principle that enables a violin played at just the right pitch to break a wine glass.

When Pendry revisited the MIT lab this March, he got a firsthand view of the bulb lighting up. “What they’ve done is take some very basic physics concepts [and] brought these ingredients together. It’s the synthesis which is the novel thing,” says Pendry.

Shanhui Fan, a physicist at Stanford University, says that the use of magnetic resonance as a means of transferring energy is a completely new concept, and “very clever.” Although it’s a simple principle, nobody seems to have thought of it before, he says. “Many great things look simple from hindsight.”

Soljacic and his colleagues have applied for two patents, and they have branded their idea with the name WiTricity to suggest an electrical-power version of Wi-Fi wireless-Internet technology.

But if the physics is simple, why didn’t anyone think of it sooner? Soljacic suggests that before the spread of cell phones and laptops, there was little need for a wirefree power source. In fact, Soljacic admits that what got him thinking hard about wireless power was the frustration of being awakened at night by a beeping cell phone that needed to be recharged.

In a smaller way, wireless power has already crept into our lives and our wallets. The access cards of many office buildings and public-transportation systems now carry embedded radio frequency identification (RFID) tags. RFID tags have no batteries. They are semiconductor chips that draw a tiny amount of energy—typically microwatts—from radio waves generated by the device that reads them, and in response beam back an identification code.

In a similar vein, Powercast, a start-up company in Ligonier, Pa., recently began marketing a new kind of chip that can harvest several milliwatts from radio waves. The company’s chips have a patented design that converts up to 70 percent of the radiofrequency energy picked up by a small antenna into direct current (DC) power, says Powercast’s Keith Kressin.

Powercast’s small, dedicated radio sources can be hidden in fixtures such as desk lamps. One chip can provide enough power to keep a cell phone charged while it sits in standby mode a few inches from the emitter, Kressin says. Eventually, the technology could be used in environmental sensors and in medical implants.

In comparison, the MIT team’s system could potentially furnish a room with hundreds of watts of wireless power, which could drive a wide range of devices. The system’s ultimate limitation derives from the physics of the magnetic fields. A few meters from the source, the fields’ strength quickly drops. “Eventually, you have to face the fact that the fields decay very fast,” Soljacic says.

Efficiency is limited primarily by the power dissipated as heat in the copper coils. The physicists plan to experiment with different materials and designs to reduce electrical resistance.

If Soljacic’s “far-out” idea bears fruit and engineers manage to squeeze WiTricity into electronics products, then in a few years homes, workplaces, and coffee shops could be pulsating with magnetic energy, greatly reducing the tangles of cords that clutter floors and eliminating the need to plug gadgets in. A simple, relatively low-tech idea could make everyone’s life a little more hasslefree.

As Pendry puts it, “The power cord is the last cord that needs to be cut. Everything else has been severed.”

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