# Wireless Power Efficiency

## Introduction

 Fig. 1: Simplified block diagram of an inductive wireless power system. (Source: Wikimedia Commons)

In the 1890s, Nikola Tesla first demonstrated power transmission by radio waves. This marked the beginning of Wireless Power Transfer (WPT) technology. Nonetheless, not until a few decades ago where WPT technology has advanced enough to start attracting public interest. Even though WPT can be used in wide range of situation where wired connection is simply not suited, to the public, WPT is equivalent to convenience of never having to plug smartphones, laptop, and electric cars to charge.

While it is true that the term "Wireless Power Transfer" actually describes a collection of different technologies for transmitting power via electromagnetic fields, for most cases when WPT is mentioned, it almost always means inductive or inductive resonant coupling, such as that shown in Fig. 1. Inductive (and inductive resonant) coupling WPT utilizes alternating current (AC) to energize a transmitter coil, which generates a magnetic field. This field in turn is coupled into any nearby receiving coils, forming transformers and thus creating AC currents in receiving coils. Typically, this AC is then passed through a rectifying circuit to convert it back to direct current (DC). One good example of everyday used device that utilizes WPT is electric toothbrush chargers.

## Losses in WPT

The convenience of not having the wires, however, does come at a price. While it is true that conduction loss in conventional copper wired connection is not negligible, losses in WPT is much worse. In order to generate strong enough magnetic field to couple two separated coils, a large alternating current is needed. As we know, circulating current in conducting material (other than superconductor) generates conduction loss. Therefore, the further the separation distance between the two coils in WPT is, the larger magnitude of current is needed to generate enough magnetic field, and the lossier the whole system becomes. Not only that, for real world application, the transmitting coil not only sends energy to the other coil, but also to any other metallic object that happens to be nearby.

The current state of the art for high power system with relatively short separation such as for vehicle charging has the efficiency of 97%. [1] On the other hand, a system for high separation, for example - 5 times the radius of the coil, such as demonstrated by a group at MIT in 2007 only has the efficiency of 53%. [2] And that is already at the limit of what efficiency we can physically get from metal conductor at room temperature.

## Vision for the Future of Wireless Power Transfer

Companies such as WiTricity are marketing the dream of the world where power cords are obsolete. People simply drop their phones into cup holders that has built in wireless charger to charge their phone. People simply drive their cars into their garages with wireless charging mats installed on the garage floors. No need to plug anything in to charge. No need for any special procedures. [3]

## Extra Losses

This future; however, will come at a price of extra and unnecessary losses generated every time someone charges their cars. Passenger or personal mobility-related energy consumption are projected to be at 60 quadrillion Btu by 2040. [4] Even with the state of the art WPT technology, when taken into account the auxiliary circuits needed to convert DC to AC and back to DC again, we are looking efficiency of about 90%. [3] If WPT vehicle charging becomes the new standard and half of the vehicles on the roads in 2040 adopted this technology, we will be looking at 3 quadrillion Btu of extra losses generated. This is roughly the same as burning away 410 million barrels of crude oil simply for the convenience. While it makes sense that everyone loves convenience of not having to walk over the plug a cord into their car, there actually are other solutions to do just that without generate such losses. Companies such as Tesla have developed a robotic "snake" to automatically search for the charging outlet in the vehicle and plug in the charging cord for you. [5] Though funny looking and not as attractive to the public, this is a much better solution from energy standpoint.

## Conclusion

While there are many areas, such as in medical implants, where contactless power transfer is a necessity, electric vehicle charging is not one of them. Wireless power transfer is flashy and attracts attention of public, but it does generate unnecessary losses. The question to ask is, is the convenience and garishness of WPT worth the extra energy cost, or can we compromised to another solution that might not be as flashy, but will offer the same convenience without the added energy cost.

© Kawin Surakitbovorn. 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.

## References

[1] X. Yu et al., "Wireless Energy Transfer with the Presence of Metallic Planes," Appl. Phys. Lett. 99, 214102 (2011).

[2] A. Karalis, J. D. Joannopoulos, and M. Soljacic, "Efficient Wireless Non-radiative Mid-range Energy Transfer," Ann. Phys. 232, 34 (2008).

[3] M. Kesler, "Highly Resonant Wireless Power Transfer: Safe, Efficient, and over Distance," WiTricity Corporation, 2013.

[4] "International Energy Outlook 2016," U.S. Energy Information Administration, DOE/EIA-0484(2016), May 2016.

[5] "Tesla's Creepy Robot Snake may be the Future of Charging Electric Cars." The Telegraph, 7 Aug 15.