A method to boost the efficiency of wireless power transfer over long distances has been suggested by an international research team including scientists from the Moscow Institute of Physics and Technology and ITMO University. This method has been tested with numerical simulations and experiments.
To accomplish this, power was beamed between two antennas, one of which was excited with a back-propagating signal of predetermined amplitude and phase. The research was published in the Physical Review Letters and briefly reported in the American Physical Society journal Physics.
“The notion of a coherent absorber was introduced in a paper published back in 2010. The authors showed that wave interference can be used to control the absorption of light and electromagnetic radiation in general,” recalls MIPT doctoral student Denis Baranov.
“We decided to find out if other processes, such as electromagnetic wave propagation, can be controlled in the same way. We chose to work with an antenna for wireless power transfer because this system would benefit hugely from the technology,” he says. “Well, we were quite surprised to find out that power transfer can indeed be enhanced by transmitting a portion of received power from the charging battery back to the receiving antenna.”
Coils and Transformers
Nikola Tesla was the first to propose wireless power transfer in the late 19th century. He successfully accomplished lighting fluorescent and incandescent lamps from a distance with no wires connecting the lamps to a generator. To achieve this feat, he applied the principle of electromagnetic induction: When an alternating current passes via a coil — that is, a conductor wound in a spiral around a cylindrical core — this produces an alternating magnetic field both outside and inside the coil. Faraday’s law states that if a second coil is positioned in this magnetic field, an electric current is triggered in this other coil, which can then be employed for charging an accumulator or some other use.
It may not be apparent, but wireless power transfer is already extensively used. For instance, unconnected induction coils are at the center of transformers in smartphones, television sets, power lines, energy-saving lamps, etc. By decreasing or increasing the alternating voltage in the electrical grid and separate devices, transformers enable efficient transmission of power and the working of consumer electronics. Moreover, a technology similar to that projected by Tesla has recently been applied in wireless charging pads for electric cars and phones. Inductive charging begins operating the moment a phone or an electric car supporting the technology comes within range.
Starting today, however, “in range” means right on top of the charger, and that is one of the key failings of the presently available technology. The issue is that the strength of the magnetic field produced by the coil in the charger is inversely proportional to the distance from it — that is, the field fades rapidly with distance. Thus the second coil, which is fabricated into the device, has to be quite close for a perceptible current to be induced. That is the reason why magnetic cores are used to restrain and guide magnetic fields in transformers. Moreover, that is also why wireless chargers work over distances less than 3-5 cm. That range could, obviously, be improved by increasing the size of one of the coils or the current in it, but that would mean robust magnetic fields potentially detrimental to humans around the devices. In a majority of countries, there is a legal limit on radiation power. For instance, in Russia, the density of radiation around cell towers cannot go beyond 10 microwatts per square centimeter.
Transmitting Power Over the Air
There are other means of transferring power without wires that function over longer distances. These methods, referred to as far-field energy transfer, or power beaming, employ two antennas, one of which transmits energy in the form of electromagnetic waves to the other, which then changes radiation into electric currents. The transmitting antenna cannot be significantly improved, as it fundamentally just produces waves. In contrast, the receiving antenna has a lot more room for improvement.
Significantly, the receiving antenna does not absorb the incident radiation totally but reradiates a portion of it back. In general, antenna response is established by two main parameters: the decay times τF and τw into free space radiation and into the electrical circuit, respectively. These decay times specify the amount of time it takes for the amplitude of a wave to be reduced by a specific factor — typically the e number is used. The ratio between these two values establishes how much of the energy carried by an incident wave is “extracted” by the receiving antenna.
A maximum amount of energy is extracted when the two decay times are equal. If τF is smaller than τw, reradiation starts too early. On the other hand, if τF is greater than τw, the antenna is very slow to absorb the incident radiation. When the two times are equal, engineers say that the conjugate matching condition has been realized. In other words, the antenna is modified. Although antennas are manufactured keeping that condition in mind, attaining 100% precision is quite tough. Also, even a faultless antenna can easily be detuned because of an alteration in signal reflections from the terrain, temperature, and other external factors. Lastly, the amount of absorbed energy also relies on radiation frequency and is maximized for waves whose frequencies equal the resonant frequency of the antenna.
Notably, the above is only accurate for a passive antenna. If, however, the receiver conveys an auxiliary signal back to the antenna and the signal’s amplitude and phase match those of the incident wave, the two will impede, potentially changing the proportion of extracted energy. This configuration is mentioned in the paper authored by a team of researchers featuring MIPT’s Denis Baranov and led by Andrea Alù.
Exploiting Interference to Amplify Waves
Before executing their projected power transmission configuration in an experiment, the physicists theoretically estimated what enhancement on a standard passive antenna it could offer. It turned out that if the conjugate matching condition is matched initially, there is no enhancement whatsoever: The antenna is impeccably tuned to start with. However, for a detuned antenna whose decay times vary greatly significantly — that is, when τF is several times larger than τw, or the other way round — the auxiliary signal has an obvious effect. Based on its phase and amplitude, the proportion of absorbed energy can be several times greater compared with the same detuned antenna in the passive mode. Actually, the amount of absorbed energy can go as high as that of a tuned antenna.
To verify their theoretical calculations, the team numerically modeled a 5-cm-long dipole antenna linked to a power source and irradiated it with 1.36-gigahertz waves. For this arrangement, the dependence of energy balance on signal phase and amplitude commonly coincided with the theoretical predictions. Remarkably, the balance was maximized for a zero phase shift between the incident wave and the signal. The explanation given by the researchers is this: In the presence of the auxiliary signal, the effective aperture of the antenna is improved, so it absorbs more propagating energy into the cable. This increase in aperture is obvious from the Poynting vector around the antenna, which specifies the direction of electromagnetic radiation energy transfer.
Besides numerical simulations, the researchers did an experiment using two coaxial adapters, which acted as microwave antennas and were placed 10 cm apart. One of the adapters radiated waves with powers about 1 milliwatt, and the other tried to pick them up and convey the energy into a circuit through a coaxial cable. When the frequency was fixed to 8 gigahertz, the adapters worked as tuned antennas, conveying power with virtually no losses. At lower frequencies, however, the amplitude of reflected radiation grew sharply, and the adaptors worked a lot like detuned antennas. In the latter case, the researchers managed to increase the amount of conveyed energy nearly tenfold with the help of auxiliary signals.
In November 2017, researchers including Denis Baranov theoretically showed that a transparent material can be made to absorb most incident light, if the incoming pulse of light has the correct parameters (particularly, the amplitude has to increase exponentially). Back in 2016, physicists from MIPT, ITMO University, and the University of Texas at Austin built nanoantennas that scatter light in various directions based on its intensity. These may be employed to develop ultrafast data transmission and processing channels.