Researchers charge 40 smartphones wirelessly from 5m

22 Apr 2014


Engineers from the Korean Advanced Institute of Science and Technology (KAIST) have developed a system that can power devices up to five metres away.

The Dipole Coil Resonant System (DCRS) extends the range of inductive power transfer to a distance of five metres between the transmitter and receiver coils. It was demonstrated to charge 40 smartphones simultaneously, even when the power source was metres away.

The system improves upon MIT's Coupled Magnetic Resonance System (CMRS), which used a magnetic field to transfer energy over 2.1 metres using magnetically coupled resonators -- large coils. The team demonstrated the technology back in 2007 and was able to light a 60W light bulb from a power feet seven feet away. They have called the concept "WiTricity" (wireless electricity).

These sorts of systems work by coupling two objects that resonate at the same frequency in order to exchange energy efficiently while weakly interacting with off-resonant objects.

This resonant coupling takes place in the acoustic arena when an opera singer sings a note sufficiently loudly amongst glasses of different frequencies: only the glass with the same frequency will explode, while others will remain unaffected. By coupling two electromagnetic resonators -- copper coils -- with their magnetic fields, researchers can identify a "strongly coupled" regime of operation, which allows for very efficient energy transfer.

The first copper coil is attached to the power source and is the sending unit. It fills the space around it with a magnetic field oscillating at a specific frequency. The non-radiative field mediates the power exchange with the other coil, which acts as the receiving unit.

This approach to power transfer is not that dissimilar from that used in power transformers, although this tends to take place over very short distances. An electric current running in the sending coil induces another in the receiving coil, but they are very close together. It is much harder to transfer energy when the space between the coils is increased. That's why the resonant coupling is important -- non-resonant magnetic induction would be much less efficient.

On 16 April of this year, Professor Chun Rim and team showcased their DCRS system, which aimed to address some of the limitations of previous versions of the technology: the system's bulkiness and low transfer efficiency. The coil structure has been optimised to feature two magnetic dipole coils -- one primary one to induce a magnetic field and a secondary one to receive power. They swapped the large loop-shaped air coils used in the CMRS with compact ferrite core rods with winding coils at the centre. A high frequency AC current in the primary winding generates the magnetic field which induces voltage at the secondary winding.

The DCRS system is three metres in length, 10cm in width and 20cm in height, making it quite a bit smaller than CMRS. It works better at lower frequencies and is more resistant to environmental changes, such as temperature. When the sytem was operated at 20 kHz, the maximum output power was 1,403 watts at a three-metre distance, 471 watts over four metres and 209 watts over five metres. For 100 watts of electric power transfer efficiency was 36.9 percent at three metres, 18.7 percent at four metres and 9.2 percent over five metres. "A large LED TV as well as three 40 watt fans can be powered from a five-metre distance," said Rim in a press release. "Just like we see Wi-Fi zones everywhere today, we will eventually have many Wi-Power zones at such places as restaurants and streets that provide electric power wirelessly to electronic devices. We will use all the devices anywhere without tangled wires attached and anytime without worrying about charging their batteries."

You can read about the research in IEEE Transactions on Power Electronics