The following information is referenced from the Wireless Power Consortium.

Wireless energy transfer is slightly different from wireless transmission for the purpose of telecommunications (the transferring of information), such as radio, where the signal-to-noise ratio, or the percentage of power received, becomes critical if it is too low to recover the signal successfully. With wireless energy transfer efficiency is the more important parameter.

The most common form of wireless power is carried out using induction, followed by electrodynamic induction. Other technologies for wireless power include those based upon microwaves and lasers.

Non resonant coupled inductors, such as transformers, work on the principle of a primary coil generating a magnetic field and a secondary coil subtending as much as possible of that field so that the power passing though the secondary is as similar as possible to that of the primary. This requirement that the field be covered by the secondary results in very short range and usually requires a magnetic core. Over greater distances the non-resonant induction method is highly inefficient and wastes the vast majority of the energy in resistive losses of the primary coil.

Using resonance can help efficiency dramatically. If resonant coupling is used, each coil is capacitively loaded so as to form a tuned LC circuit. If the primary and secondary coils are resonant at a common frequency, it turns out that significant power may be transmitted between the coils over a range of a few times the coil diameters at reasonable efficiency.

The general principle is that if a given amount of energy is placed into a resonant primary coil, the coil will 'ring', and form an oscillating magnetic field. This will die away at a rate determined by the Q factor, mainly due to resistive and radiative losses. However, provided the secondary coil cuts enough of the field that it absorbs more energy than is lost in each cycle of the primary, then most of the energy can still be transferred.

Because the Q factor can be very high, (experimentally nearly a thousand has been demonstrated with air cored coils) only a small percentage of the field has to be coupled from one coil to the other to achieve high efficiency, even though the field dies quickly with distance from a coil, they can be several diameters apart.

Additionally, because the Q can be so high, even when low power is fed into the transmitter coil, a relatively intense field builds up over multiple cycles, which increases the power that can be received.

Running the secondary at the same resonant frequency as the primary ensures that the secondary has a low impedance at that frequency and that the energy is optimally absorbed.

Unlike the multiple-layer secondary of a non-resonant transformer, our receiving coils are single layer solenoids (to minimize skin effect and give improved q) in series with a suitable frequency tuned capacitor.

induction diagram

The basic principle of an inductively coupled power transfer system is shown in Figure 1. It consists of a transmitter coil L1 and a receiver coil L2. Both coils form a system of magnetically coupled inductors. An alternating current in the transmitter coil generates a magnetic field which induces a voltage in the receiver coil. This voltage can be used to power a mobile device or charge a battery. The efficiency of the power transfer depends on the coupling (k) between the inductors and their quality (Q). The coupling is determined by the distance between the inductors (z) and the relative size (D2 /D). The coupling is further determined by the shape of the coils and the angle between them (not shown).

efficiency diagram

The figure above shows that the efficiency drops dramatically at larger distance (z/D > 1) or at a large size difference of the coil (D2/D < 0.3). A high efficiency (>90%) can be achieved at close distance (z/D < 0.1) and for coils of similar size (D2/D = 0.5..1)

This shows that inductive power transmission over a large distance, i.e. into a space, is very inefficient. Today, we cannot afford to waste energy for general power applications by using such a system. On the other hand, the figure shows that inductive power transmission in the proximity of the devices, i.e. at a surface, can be efficient and competitive to wired solutions. Wireless proximity power transmission combines comfort and ease of use with today's requirements for energy saving.