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Power goes wireless

June 19, 2007
By Frank Hadley, MIT 
and World Science staff

A new sys­tem for trans­mit­ting pow­er could get rid of the tan­gle of ca­bles that keep alive our cell phones, lap­tops and oth­er de­vices, re­search­ers re­port.

Phys­i­cists at the Mas­sa­chu­setts In­sti­tute of Tech­nol­o­gy in Cam­bridge, Mass. found that pow­er could be trans­mit­ted with­out wires us­ing spe­cial “res­o­nant” an­ten­nas. The re­search­ers used the sys­tem to pow­er a 60-watt light bulb more than two me­ters (a­bout two yards) from a wire­less trans­mit­ter at 40 per­cent ef­fi­cien­cy.

Two im­ages of a 60-watt bulb lit from 2 me­ters away by a pow­er-trans­mit­ting coil. Note the ob­s­truc­tion in the low­er im­age. (© Sci­ence)


The find­ings ap­peared in the June 7 on­line is­sue of the re­search jour­nal Sci­ence.

The sto­ry of the group’s re­search be­gan one late night a few years ago, they said. Team lead­er Mar­in Sol­ja­čić (pro­nounced Soul-ya-cheech) of MIT was stand­ing in his pa­jam­as, star­ing at his cell-phone on the kitch­en count­er. 

“It was probably the sixth time that month that I was awak­ened by my cell-phone beep­ing to let me know that I had for­got­ten to charge it. It oc­curred to me that it would be so great if the thing took care of its own charg­ing,” he said.

To make this pos­si­ble, one would need to trans­mit pow­er wire­lessly. Some ways of do­ing this have been known for cen­turies, but prac­ti­cal prob­lems have got­ten in the way. 

One known meth­od uses elec­tro­mag­ne­tic radia­t­ion, like ra­di­o waves. More com­monly used for wire­less trans­mis­sion of in­forma­t­ion, these can al­so trans­mit pow­er. But not very ef­fec­tive­ly. Since radia­t­ion spreads in all di­rec­tions, al­most all the pow­er would end up be­ing wast­ed in­to space. An al­ter­na­tive strat­e­gy is to beam the radia­t­ion spe­cif­ic­ally to­ward the elec­tron­ic de­vice to be charged—but then prob­lems can arise if some oth­er ob­ject gets in the way, or if you move the de­vice.

The MIT con­cept, called “WiTricity” for wire­less elec­tricity, in­volves us­ing so-called cou­pled res­onators. These are ob­jects that, if struck or dis­turbed, tend to nat­u­rally os­cil­late at a def­i­nite rhythm. If two of them tend to have match­ing rhythms, they ac­tu­ally en­hance each oth­ers’ os­cilla­t­ions. 

One ex­am­ple is a child on a swing. If she swings her legs in synch with the nat­u­ral rhythm of the swing it­self, the swing will soon be briskly in mo­tion.

The type of res­o­nance be­hind such a push-pull sys­tem is called me­chan­i­cal, but oth­er types of res­o­nances are pos­si­ble. There are acous­tic res­o­nances, for ex­am­ple. Im­ag­ine a room with 100 iden­ti­cal wine glass­es, each filled with dif­fer­ent amounts of wine. This gives each glass a dif­fer­ent “res­o­nant fre­quen­cy,” or nat­u­ral rhythm of vibra­t­ion. If a sing­er then sings a loud enough note in the room, a glass of the cor­re­spond­ing fre­quen­cy might ac­cu­mu­late enough en­er­gy to ex­plode, while the oth­er glass­es sit un­dis­turbed.

The MIT team fo­cused on yet anoth­er type of res­o­nance, mag­net­ic. 

They set up two cop­per coils, each a self-resonant sys­tem. One coil, at­tached to a pow­er source, is the “send­ing” un­it. In­stead of send­ing out elec­tro­mag­netic waves, it fills its sur­round­ings with an os­cillating mag­net­ic field. This leads to a pow­er ex­change with the oth­er, “re­ceiv­ing” coil. Be­cause the mag­net­ic field, un­like ra­di­o waves, nev­er gets too far from the send­ing un­it, the en­er­gy is­n’t lost in­to space. And ex­tra­ne­ous ob­jects en­ter­ing the field have no im­pact be­cause they nor­mally don’t res­o­nate along with the sys­tem.

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 enough for a lap­top can be trans­ferred over room-sized dis­tances nearly omni-directionally and ef­fi­cient­ly, re­gard­less of what’s be­tween the ob­jects, re­search­ers said.

“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,” said MIT’s Pe­ter Fish­er.

Al­though the pow­er trans­fer ef­fi­cien­cy re­mains be­low the ide­al, team mem­ber An­dre Kurs said in an e­mail that he’s op­ti­mis­tic it can be im­proved. He al­so ac­knowl­edged that inef­fi­cien­cy raises en­vi­ron­men­tal con­cerns, but ar­gued that the new sys­tem on bal­ance might ac­tu­ally help the en­vi­ronment. That’s be­cause the bat­ter­ies that it would re­place al­so tend to lose ef­fi­cien­cy over time, and con­tain tox­ic chem­i­cals.

Final­ly, he wrote in the e­mail, the small port­a­ble elec­tron­ics mostly af­fect­ed by the re­search ac­count for less than one per­cent of en­er­gy use overall. So pos­si­ble en­er­gy losses from these are “neg­li­gi­ble” com­pared to the gains pos­si­ble from the ef­fi­cien­cy im­prove­ments that ex­perts to­day ad­vo­cate—such as us­ing flu­o­res­cent bulbs, driv­ing fuel-efficient cars, or im­prov­ing build­ing in­sula­t­ion.


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A new system for transmitting power could get rid of the tangle of cables that keep alive our cell phones, laptops and other devices, researchers report. Physicists at the Massachusetts Institute of Technology in Cambridge, Mass. found that power could be transmitted without wires using special “resonant” antennas. The researchers used the system to power a 60-watt light bulb more than two meters (about two yards) from a wireless transmitter at 40 percent efficiency. The findings appeared in the June 7 online issue of the research journal Science. The story of the group’s research began one late night a few years ago, they said. Team leader Marin Soljačić (pronounced Soul-ya-cheech) of MIT was standing in his pajamas, staring at his cell-phone on the kitchen counter. “It was probably the sixth time that month that I was awakened by my cell-phone beeping to let me know that I had forgotten to charge it. It occurred to me that it would be so great if the thing took care of its own charging,” he said. To make this possible, one would need to transmit power wirelessly. Some ways of doing this have been known for centuries, but practical problems have gotten in the way. One known method uses electromagnetic radiation, like radio waves. More commonly used for wireless transmission of information, these can also transmit power. But not very effectively. Since radiation spreads in all directions, almost all the power would end up being wasted into space. An alternative strategy is to beam the radiation specifically toward the electronic device to be charged—but then problems can arise if some other object gets in the way, or if you move the device. The MIT concept, called “WiTricity” for wireless electricity, involves using so-called coupled resonators. These are objects that, if struck or disturbed, tend to naturally oscillate at a definite rhythm. If two of them tend to have matching rhythms, they actually enhance each others’ oscillations. One example is a child on a swing. If she swings her legs in synch with the natural rhythm of the swing itself, the swing will soon be briskly in motion. The type of resonance behind such a push-pull system is called mechanical, but other types of resonances are possible. There are acoustic resonances, for example. Imagine a room with 100 identical wine glasses, each filled with different amounts of wine. This gives each glass a different “resonant frequency,” or natural rhythm of vibration. If a singer then sings a loud enough note in the room, a glass of the corresponding frequency might accumulate enough energy to explode, while the other glasses sit undisturbed. The MIT team focused on yet another type of resonance, magnetic. They set up two copper coils, each a self-resonant system. One coil, attached to a power source, is the “sending” unit. Instead of sending out electromagnetic waves, it fills its surroundings with an oscillating magnetic field. This leads to a power exchange with the other, “receiving” coil. Because the magnetic field, unlike radio waves, never gets too far from the sending unit, the energy isn’t lost into space. And extraneous objects entering the field have no impact because they normally don’t resonate along with the system. 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 enough for a laptop can be transferred over room-sized distances nearly omni-directionally and efficiently, regardless of what’s between the objects, researchers said. “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,” said MIT’s Peter Fisher. Although the power transfer efficiency remains below the ideal, team member Andre Kurs said in an email that he’s optimistic it can be improved. He also acknowledged that inefficiency raises environmental concerns, but argued that the new system on balance might actually help the environment. That’s because the batteries that it would replace also tend to lose efficiency over time, and contain toxic chemicals. Finally, he wrote in the email, the small portable electronics mostly affected by the research account for less than one percent of energy use overall. So possible energy losses from these are “negligible” compared the gains possible from the efficiency improvements that experts today advocate—such as using fluorescent bulbs, driving fuel-efficient cars, or improving building insulation.