|Fig. 1: Transferring energy through magnetic field coupling between two coils with identical resonant frequency.|
The wireless transfer of information has brought a lot of convenience to life. Yet people are not going to stop. The dream of a day when electric cables, outlets and adapters are no longer necessary has driven the research of wireless energy transfer.
What kind of energy could be transferred wirelessly? Obviously, energy stored in chemical bonds within molecules like glucose or methane cannot be transferred without the moving of the material itself; Heat can be transferred wirelessly quite easily, but getting your laptop PC to use heat is not that easy. So we will talk about electric energy, which is being used most widely and easily.
Like wireless information transfer, wireless energy transfer is also based on electromagnetic field. The difference is when you're transferring energy, receive-to-transmit ratio rather than signal-to-noise ratio becomes the most important thing. And usually, the former one is much more difficult to achieve than the latter one over long distances. Based on whether radiative electromagnetic field is generated, the methods of wireless transfer are divided into two categories: near-field and far-field.
|Fig. 2: Transferring energy through microwave between two stations.|
Near-field transfer is based on the coupling of two coils within the distance of the coils' dimension. In fact a transformer is transferring energy wirelessly through magnetic field coupling, although it was invented more than 100 years ago. But if you remove the iron core and move the two coils apart, the transfer efficiency drops drastically.  That is why the two coils must be put close enough to each other. This kind of method is already commercialized. For example, most electric toothbrushes today are using wireless chargers, which are much safer than cable chargers in wet environment.
However, if the transmitter and receiver coils have the same resonant frequency, which is determined by the material and shape of the coil, transfer efficiency will decrease much more slowly when they are moved apart. A group from MIT, led by Prof. Marin Soljacic has succeeded in transferring electric energy (60 Watt) between two coils more than two meters apart through non-radiative electromagnetic field, as shown in Fig 1. 
Since near field transfer is usually working at 50 or 60Hz, there is almost no interference with TV, radio or Wi-Fi signals. The major concern is the possible influence on human health. Luckily, almost all materials that form human body are non-magnetic, so they interact very weakly with magnetic field, even to several Tesla like that in a modern MRI machine. [2,3] Thus, such magnetic-field-based transfer is quite safe to people within the transfer range.
To transfer energy wirelessly over long ranges, far-field transfer is used. Far-field transfer is based on electromagnetic wave which is radiative. Different methods use electromagnetic waves within different wave band. In the early times, experiments were carried out with radio and microwaves, around 1GHz.  Electric energy is transferred to a strong beam of radio or microwave by a dish-like antenna, travels through the atmosphere and then received by another antenna which transfers it back to AC electric current, as shown in Fig 2.
|Fig. 3: Diagram of NASA's model plane powered by an infrared ground-based laser beam centered at the photovoltaic cell panel on its ventral.|
Yet according to diffraction, the longer the wavelength is, the larger the antennas must be in order to achieve sufficient directionality. Since speed of light in the air is about 3 x 108 m/s, the corresponding wavelength of radio and microwaves used is about one meter, which requires an antenna with a dimension of several meters to several kilometers. Thus we have to use electromagnetic waves with shorter wavelength if we want to transfer energy to smaller objects. Moreover, since the electromagnetic wave used lies in the waveband of radio, TV, cell phone and Wi-Fi, with a signal intensity several order-of-magnitude larger, you probably won't want it any close to residences or offices - in fact, it was proposed to be used in energy transfer between future solar power satellites and the earth. 
Benefiting from advanced technology in both solid-state lasers and photovoltaic cells (notice that monochromatic photovoltaic cells are more efficient than ordinary solar cells) today, converting energy into laser beams to transfer over long distances is becoming closer to practice. NASA has made a model plane powered by a laser beam focused on a panel of photovoltaic cells on its ventral, see Fig 3. [6,7] Comparing to radio and microwaves, laser has many advantages like short wavelength (shorter than several micrometers), good beam width, perfect directionality and non-interference with radio, TV, cell phone or Wi-Fi signals. But it still has many drawbacks, like relatively lower efficiency during conversion and atmospheric absorption.
© Yue Ma. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
 E. Waffenschmidt et al., "Limitation of Inductive Power Transfer for Consumer Applications," 13th European Conference on Power Electronics (EPE), 8 Sep 09.
 F. Hadley, "MIT Demos Wireless Power Transmission," MIT Tech Talk, 13 Jun 07.
 G. Christoforidis et al., "High Resolution MRI of the Deep Brain Vascular Anatomy at 8 Tesla: Suscpetibility-Based Enhancement of the Venous Structures," Journal of Computer Assisted Tomography, 23, 857 (1999).
 J. Barrett, Electricity at the Columbian Exposition (R. R. Donnelly and Sons, 1894), p. 168.
 G. Landis, "Laser Power Beaming," SPIE Proceedings 2121, 252 (1994).
 M. A. Green et al., "45% Efficient Silicon Photovolatic Cell Under Monochromatic Light," IEEE Electr. Device Lett. 13, No. 6, 314 (1992).
 M. Curry, "Beamed Laser Power for UAVs," NASA Dryden Flight Research Center Fact Sheets.