Transfer efficiency

Dries van Wageningen and Eberhard Waffenschmidt, Philips Research

Figure 2 shows the calculated optimal achievable efficiency of a system according to Figure 1 with the assumption of a quality factor of 100. All dimensions are scaled to the diameter of the larger coil D, which ever it is (transmitter or receiver coil).
The values are shown as a function of the axial distance of the coils (z/D). The parameter 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 in the proximity of the devices, e.g. at a surface, can be really efficient and competitive to wired solutions. Wireless proximity power transmission combines comfort and ease of use with today’s requirements for energy saving.