Wireless Charging: Turning an Age-Old Idea into Reality
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Figure 2: Illustration of magnetic induction between Rx and Tx coils in wireless charging. Devices utilizing magnetic induction are commercially available in common household items such as electric toothbrushes and more recently, in the smartphone and automobile industries [5]. As shown in Figure 3, some of the newer smartphone models have wireless charging capability that consumers can use if they buy a compatible charging pad. In Genoa, Italy, magnetic induction is used in a larger scale to charge electric buses “by parking over flat charging coils built into the road surface” [1]. 
http://www.samsung.com/uk/consumer/mobile-devices/smartphones/smartphone-accessories/EP-WI950EWEGWW Figure 3: Charging pad for a Samsung smartphone, one of many out in the market.
Although this technology is simple to execute, the device must be in direct contact with or within proximity of the charging pad [6]. This limits the orientation that the device must take on—flat against the pad—to charge its battery effectively; it also restricts consumers’ mobility when using their device while it charges. Compared to wall charging, magnetic induction coupling charges devices slower and generates more heat, which negatively affects its efficiency [7]. Adding Resonance to the Mix In 2007, a team from the Massachusetts Institute of Technology (MIT) discovered a new wireless charging technology that uses magnetic resonant coupling. In their experiment, they used two resonant objects to light a 60-Watt light bulb seven feet from a power source without physical contact [8]. As a result of this breakthrough study, WiTricity Corp.—the brainchild of the MIT team—was created to realize the technology’s market potential [1]. Energy transfer in resonant coupling is similar to inductive coupling except that it also utilizes resonance to increase efficiency. The resonant frequency is the natural frequency that an oscillating system tends towards [3]. A simple example of resonant frequency is a tuning fork. Suppose two tuning forks with the same natural frequency are placed on a table spaced apart as shown in Figure 4 [9]. When the first tuning fork is struck, it vibrates at its resonant frequency; the vibrations from the first tuning fork then cause the second tuning fork to vibrate at the same frequency. The energy transfer happens quickly and easily because the tuning forks are identical [9]. In other words, objects that have the same resonance transfer energy more efficiently than objects that do not have the same resonance [3]. Resonant charging works in a similar fashion to the tuning fork example; the transmitter and receiver coils are tuned to the same frequency so that electrical power can be exchanged between the coils with minimal loss to the environment [10]. 
http://www.cliffsnotes.com/assets/10126.jpg Figure 4: Two tuning forks vibrating at the same resonant frequency. Pros and Cons of Magnetic Resonance Besides increased efficiency, energy transfer in resonant coupling can occur at greater distances. In the MIT study, the distance at which energy can be exchanged between two resonant objects is “several times larger than the sizes of the resonant objects” [8]. Thus with magnetic resonance, devices will no longer have to be in contact with or in near proximity to a charging source. Furthermore as shown in Figure 5, multiple electronic devices can be simultaneously charged as opposed to one [8]. To get a better understanding of this technology, imagine that you walked into a room that supported wireless charging. Once you are within range, all your devices—your phone, your laptop, your tablet, etc.—will begin to charge automatically; of course, this is contingent on whether the resonant frequencies are the same. 
http://electronics.howstuffworks.com/everyday-tech/wireless-power2.htm Figure 5: Multiple electronic devices can be charged from a single source as long as they all have the same resonant frequency and are within range. Despite the benefits, there are issues that need to be resolved before resonant coupling can be incorporated into our daily lives. According to the MIT study, the magnetic field measured midway between the transmitter and receiver coils was about 14 times the maximum value established by the International Commission on Non-Ionizing Radiation Protection (ICNIRP) [1]. The electric field was about 7.5 times over the limit. To make matters worse, the magnetic and electric fields closer to the coils had even higher readings [1]. Currently, magnetic resonance may pose a radiation risk for humans, which in return, may limit the power and range of a product utilizing this technology. However, engineers are working towards making the technology non-radiative and safe for public use [11]. A Wireless Future Despite the current limitations of wireless charging, the future implications are huge. Companies like Starbucks, McDonald’s, and Delta Sky Club have recently installed wireless charging docks in select locations for customers. If results are promising, the public will see more of these charging stations nationwide [12]. Not only can wireless charging change the way consumers interact with their electronic devices, it can also herald a new age in transportation, particularly for electric vehicles. According to a market report, wireless charging will help increase sales of electric vehicles from “120,000 in 2012 to more than 280,000 by the end of the decade” because it makes charging vehicles more convenient [13]. Furthermore, engineers in Berlin are developing an electric streetcar that is wirelessly powered from roadways embedded with coils, eliminating the hassle of overhead cables [1]. This development may lead to powering all electric vehicles while driving on the road and make the world greener [1]. As engineers continue to research and improve the technology, charging devices on the go will no longer be a distant dream. References [1] D. Schneider. (2010, Apr. 30). Wireless power at a distance is still far away. IEEE Spectrum [Online]. Available: http://spectrum.ieee.org/green-tech/mass-transit/a-critical-look-at- wireless-power [2] R. Bhutan and S. Spare. Wireless Energy Transfer using Magnetic Resonance. presented at 2009 2nd Int. Conf. Computer and Electrical Engineering [Online]. Available: http://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=5380190 [3] WiTricity. WiTricity Technology: The Basics [Online]. Available: http://www.witricity.com/ pages/technology.html [4] BBC. Direct current and alternating current [Online]. Available: http://www.bbc.co.uk/ schools/gcsebitesize/science/add_aqa_pre_2011/electricity/mainselectrev5.shtml [5] Wireless Power Consortium. An introduction to wireless charging: changing the way we think about power [Online]. Available: http://www.wirelesspowerconsortium.com/what-we- do/how-it-works/ [6] J. Zhao, G. Xu, C. Zhang, W. Zhang, X. Li. A Contrastive Studies between Magnetic Coupling Resonance and Electromagnetic Induction in Wireless Energy Transmission. presented at 2012 6th Int. Conf. Electromagnetic Field Problems and Applications [Online]. Available: http://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=6310324 [7] S. L. Ho, J. Wang, W. N. Fu, and M. Sun. (2011 May). A Comparative Study Between Novel Witricity and Traditional Inductive Magnetic Coupling in Wireless Charging. IEEE Trans. Magnetics [Online]. 47(5), pp. 1522-1525. Available: http://ieeexplore.ieee.org/stamp/ stamp.jsp?tp=&arnumber=5754800 [8] F. Hadley. (2007, June 7). Goodbye wires. MIT news [Online]. Available: http://web.mit.edu/ newsoffice/2007/wireless-0607.html [9] Resonance and Standing Waves. Sound Waves and Music-Lesson 4 [Online]. Available: http://www.physicsclassroom.com/class/sound/u11l4b.cfm [10] A. Kurs, A. Karalis, R. Moffatt, J.D. Joannopoulos, P. Fisher, and M. Soljačić, “Wireless Power Transfer via Strongly Coupled Magnetic Resonances,” Science, vol. 317, no. 5834, pp. 83-86, Jul., 2007. [11] WiTricity. WiTricity is More Than… [Online]. Available: http://www.witricity.com/ pages/more.html [12] J. Friedman (2013, Aug. 2). Companies, public connect with wireless smartphone chargers. USA TODAY [Online]. Available: http://www.usatoday.com/story/tech/2013/08/02/ starbucks-mcdonalds-wireless-charging/2600699/ [13] W. Ferguson (2012, Dec. 28). Wireless Power May Cut the Cord for Plug-In Devices, Including Cars. National Geographic Daily News [Online]. 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