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.

From the Qi Blog

Beware of fake wireless phone chargers

Wireless chargers are sophisticated, high-tech products – safe, efficient, and easy to use. More than 150 million wireless chargers are used safely every day, and that number is growing fast. The vast majority of these 150 million wireless chargers use our Qi standard. We are proud of Qi’s safety record.
Read more

Dreaming of Power through the Air

Will that far-field dream come true? Or is too much of a nightmare?
Read more

The Importance of Qi-Certification

In early May, the Wireless Power Consortium (WPC) held a meeting for members in addition to a tradeshow and Qi developer forum that were open to the public in Shenzhen for the first time. With more than 400 global experts attending, the event was the perfect opportunity to be in the same room as some of the biggest names in the industry.
Read more