This article is a continuation of my series about the innovative applications of Wireless Power Transmission (WPT) technology. In the First Chapter we discussed the history of this technology and some basic principles and uses of inductive coupling. In this post, we will summarize the recent technological advances for another form of WPT: Magnetic Resonant Coupling.

Image courtesy of Pixabay

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Magnetic Resonant Coupling

Wireless Power Transmission is the process of transmitting energy through the use of electromagnetic fields. Energy transfer is achieved when a receiving coil passes through the electromagnetic field produced by a transmitting coil connected to a power source.

Magnetic resonance coupling systems are a form of WPT technology composed of two electromagnetic subsystems with the same natural resonance frequency. The phenomenon of magnetic resonant coupling is inspired by natural processes like the resonant frequencies of musical instruments and the vibrations of ear membranes in animals. The system usually consists of two main parts: the transmitting terminal and the receiving terminal. The transmitting terminal contains the source resonant coil and the transmitting circuit. The receiving terminal is comprised of the load circuit and the resonant receiving coil. A simplified schematic of wireless power transfer is shown in Figure 1.

Figure 1: Schematic of a Magnetic Resonant Coupling WPT system (by the author).

During operation, a high frequency magnetic field is generated by the transmitting circuit and the resonant coil is exposed to this magnetic field. Energy is then transferred to the receiving coil through magnetic coupling. The coils are designed to have same resonant frequency, specific to the magnetic field. Power transfer is accomplished by using electromagnetic couplings rather than radiating electromagnetic waves [1]. This system can be represented by an RLC circuit having Resistor, Inductor and Capacitor connected as shown in Figure 2.

The transmitter coil contains Inductor L₁, Resistor R₁, and Capacitor C₁, while L₂, R₂, and C₂ constitute the receiving coil. R_load is the load resistance of the circuit.

This process is known as “Magnetic Resonant Coupling” because energy exchange occurs when an alternating magnetic field interacts with and resonates the coils. The method is characterized by self-resonant coils in transmitting and receiving circuit. A magnetic resonant coupling system can transfer power over greater distances than the inductive coupling method. In addition, it is a non-radiative method, meaning it can work without ‘line of sight’ functionality and is not harmful to humans [2].

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Applications

Magnetic resonant coupling has a vast number of applications including industrial robotics, electric vehicles, aerospace technology, underwater operations, biomedicine, household appliances, military weapons, and other uses [3].

Long Range

Solar power generating satellites (SPS) were first proposed in 1968. They were designed to be launched into space where solar energy can be collected with an increased efficiency before it is diffused by the Earth's atmosphere. This power is then wirelessly transmitted back to earth as microwave radiation. SPS technology is not yet widely available due to the extreme costs and potentially high risk factors [4].

Short Range

The nearfield implementation of magnetic resonant coupling WPT has become more common in recent years due to improvements in efficiency and intensity. In 2007, a group of MIT researchers succeeded in powering a 60-Watt bulb over a distance greater than 2 meters without any physical connection [5].

Less than a decade later, researchers from Auckland University introduced a wireless charging system for vehicles that can transfer 5 kiloWatts of power over a distance of 20 cm. The efficiency of this wireless charging station approaches 90% [6][7]. Even more recently, a Korean research group at the Korea Advanced Institute of Science and Technology has developed the On-Line Electric Vehicle (OLEV) system capable of transmitting 100 kiloWatts of power wirelessly at 85% efficiency [8].

Remote Sensing

The ability to remotely transmit information and data is one key area where WPT technology is being utilized. In the automotive industry, WPT is used to power up and monitor rotating devices more efficiently. As Magnetic Resonant Coupling does not require direct line-of-sight alignment, this method is used to monitor internal components in various industrial applications.

Wireless Traffic Cones

Wireless power is currently being used to illuminate traffic cones in the Netherlands. Philips Inc. has developed a wire-free charging system for the batteries in traffic cones [9]. The smart sensor-based batteries take in power wirelessly, permitting more flexible use of traffic cones in highway maintenance and construction applications. With no wired connections or batteries to replace, smart traffic cones ensure that construction areas are always well marked, leading to improved safety for the workers.

Biomedical Applications

WPT research has made progress recently in the field of biomedicine. One example is in high powered wireless systems that are now implemented in CT scanners. The conventional powered slip rings have been replaced by a WPT system, enabling efficient power transfer to the high-speed rotational system of the CT scanner. This wireless system poses up to 98% coupler efficiency and 90% end-to-end efficiency [9].

Another common application of WPT is the wireless charging of biomedical implants inside the human body. Developing non-radiative and harmless wireless conduction is the main obstacle preventing the widespread adoption of wirelessly charging body implants. Researchers expect a biologically safe conduction system to be developed for use with body implants within a few years.

Wireless Power Plugs

Magnetic resonance coupling is also used in wireless power plugs. These plugs are connected to the main electrical line without the use of any risky bare pins. The plugs can be used in a variety of home devices ranging from appliances to cellphone chargers. Philips Inc. has developed a universal wireless plug that is capable of delivering 50 Watts of power continuously with an efficiency of up to 90% [9].

Conclusions

Magnetic resonant coupling has the potential to overcome some of the major the drawbacks of wired electrical transmission including flexibility issues, supply line exposure, and fire hazards. However, there are some remaining hurdles that are preventing the widespread implementation of WPT. Developing coils capable of transmitting power over long distances and ensuring safety by using non-radiative systems are two critical challenges that future researchers must overcome.

References

_{[1] O. Jonah and S. Georgakopoulos, “Wireless power transfer in concrete via strongly coupled magnetic resonance,” IEEE Transactions on Antennas and Propagation, vol. 61, no. 3, pp. 1378–1384, iEEE Xplore, Accessed on 5th April, 2018.
[2] L. Xie, Y. Shi, Y. Hou, and A. Lou, “Wireless power transfer and applications to sensor networks,” IEEE Wireless Communications, vol. 20, no. 4, pp. 140–145, iEEE Xplore, Accessed on 5th April, 2018.
[3] B. Zhu, J. Li, W. Hu, X. Gao, “Review of Magnetic Coupling Resonance Wireless Energy Transmission”, International Journal of u- and e- Service, Science and Technology, Vol.8, No.3 (2015), pp.257-272. Accessed on 5th April, 2018.
[4] T. Hatsuda, K. Ueno, M. Inoue, “Solar power satellite interference assessment”, IEEE Microwave Magazine, vol. 3 no. 4, iEEE Xplore, Accessed on 5th April, 2018.
[5] F. Hadley, “Goodbye wires!”, MIT News, Accessed on 5th April, 2018.
[6] S. Li, C. Mi, “Wireless Power Transfer for Electric Vehicle Applications,” IEEE Journal of Emerging and Selected Topics in Power Electronics, vol. 3, no. 1, pp. 4-17, iEEE Xplore, Accessed on 5th April, 2018.
[7] Y. Geng, H. Sun, Z. Yang, B. Li, F. Lin, “A High Efficiency Charging Strategy for a Supercapacitor Using a Wireless Power Transfer System Based on Inductor/Capacitor/Capacitor (LCC) Compensation Topology,” Energies, vol. 10, no. 135, pp. 1-17, energies, Accessed on 5th April, 2018.
[8] H. Hao, J. Boys G. Covic, “An Approximate Dynamic Model of LCL-T Based Inductive Power Transfer Power Supplies,” IEEE Transactions on Power Electronics, vol. 29, no. 10, pp. 5554-5567, iEEE Xplore, Accessed on 5th April, 2018.
[9] Dr. ir. F. Sahin and Dr. ir. A. Mehendale, “Innovations in wireless power transfer: the future is now!”, Philips Innovation Services, Accessed on 5th April, 2018.}

Thanks to @gra for the huge support in mentoring this article.

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