Transfer efficiency

Dries van Wageningen and Eberhard Waffenschmidt, Philips Research
Figure 2 shows the calculated optimal efficiency of a system according to Figure 1 with an assumed quality factor of 100. Dimensions for these calculations have been scaled to the larger diameter coil 'D', which can be either the transmitter or receiver.
The values are shown as a function of the axial distance of the coils (z/D). The variable is the diameter of the smaller coil D2.
The figure 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, e.g. 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 is competitive with wired solutions under close proximity settings. Wireless proximity power transmission combines comfort and ease of use with today’s requirements for energy saving.