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Cutting the last cord

Wireless power transfer (WPT) allows the transfer of energy from a transmitter to a receiver across an air gap, without any electrical connections. We briefly address some advantages and drawbacks of WPT.

The advantages of wireless power transfer

WPT has several advantages compared to wired charging:

  • The most important advantage, certainly for portable electronics, is the improved convenience and user experience. Billions of people worldwide use smartphones, laptops, tablets and smart watches. These devices have their mobility in common, due to an internal power source. But paradoxically, they need to be connected periodically to a power chord to recharge and ensure their operation, making them non-mobile for a certain amount of time. With WPT, the user no longer has to hassle with connecting cables to charge the device. For example, a smartphone can be charged automatically and wirelessly by simply putting it each night on the bedside table, which is equipped with a wireless charger. In the future, an abundance of wireless charging points (at the office, in restaurants, at home…) may make these devices perpetually mobile.
  • Wireless charged devices can achieve a higher durability and robustness since no open connections are necessary. The device can be sealed entirely, making it water and dust proof.
  • Wired charging can generate sparks when (dis)connecting the charging cable to the device. WPT can increase safety in hazardous industrial environments where flammable or combustible atmospheres are present, for example at chemical plants.
  • WPT can facilitate the miniaturization of devices due to the omission of a large charger connector or a reduction in battery size.
  • For certain applications, it can be costly, hazardous or infeasible to replace the batteries of the device or to connect charging cables (e.g., the charging of medical implants or Internet of Things sensor networks).
  • Smart devices are able to detect their low battery supply and can automatically report to a charging station to reload (e.g., automated guided vehicles, robots, drones,…). WPT allows the charging of these smart devices without any human involvement, leading to more flexible, reliable and energy-efficient operations.

The disadvantages of wireless power transfer

However, it is not all good news. WPT also has some drawbacks:

  • Charging points for wired power transfer (electrical outlets) are abundant and standardized within each country. Wireless charging points are more scarce.
  • Even though high efficiencies (>90%) are realized with WPT, the efficiency is lower than wired charging.
  • Since extra transmitters and receivers are necessary, the implementation cost of WPT is generally higher than wired charging.
  • In general, wireless power transfer will produce more heat, which can lead to higher requirements for the material selection and/or a faster device degradation.

Applications

Technically, any device that needs power can become an application for WPT. The current list of applications where WPT is applied, is therefore very diverse. We limit ourselves to some examples.

Most known WPT applications are portable electronics and household devices (e.g., electric toothbrushes, smartphones, smart watches, wearables). Cutting the last cord is especially important for biomedical implants such as neurostimulators, pacemakers, and spinal cord stimulators.

The above examples are located in the low to midrange power levels, from a few watt to 100 W. However, WPT also allows for high power transfer, well above kilowatt level (at short distance) and is therefore used by industrial automation (e.g., automated guided vehicles, robots, and automated underwater vehicles).

An important issue hindering the breakthrough of electric vehicles is the limited action radius and the hassle to frequently connect the power cable. Wireless charging could improve the adaptation to electric vehicles significantly by providing wireless charging points at parking lots, in the garage, or at bus stops for electric public transport. Even wireless charging while driving is being pursued by installing transmitters in motorway lanes.

Other promising markets for WPT include the roll-out of Internet of Things (IoT) sensor networks, machine-to-machine communication systems and powering unmanned aerial vehicles.

References

Ben Minnaert, Franco Mastri, Mauro Mongiardo, Alessandra Costanzo and Nobby Stevens. Constant capacitive wireless power transfer at variable coupling. MTT-S International Wireless Symposium (IWS), 2018 IEEE. Chengdu, China, 6-10 May 2018, pp. 1-4. Paper: [pdf]

X. Lu, P. Wang, D. Niyato, D. I. Kim, and Z. Han, “Wireless charging technologies: Fundamentals, standards, and network applications,” IEEE Communications Surveys & Tutorials, vol. 18, no. 2, pp. 1413–1452, 2016.

A. M. Jawad, R. Nordin, S. K. Gharghan, H. M. Jawad, and M. Ismail, “Opportunities and challenges for near-field wireless power transfer: A review,” Energies, vol. 10, no. 7, p. 1022, 2017.

S. D. Barman, A. W. Reza, N. Kumar, M. E. Karim, and A. B. Munir, “Wireless powering by magnetic resonant coupling: Recent trends in wireless power transfer system and its applications,” Renew. Sust. Energ. Rev., vol. 51, pp. 1525–1552, 2015.