Julia Chen
Wireless Charging: Turning an Age-Old Idea into Reality
Abstract
Wireless charging, or charging devices without plugging them to the wall socket, is a fairly old
concept that never gained ground until recently. Using magnetic induction and magnetic
resonant coupling, engineers have created a way to make wireless charging a reality. While in
its nascent stage, wireless charging has several problems that need to be solved and
improvements to be made. Nonetheless, with products already in the market, the wireless
charging movement is moving towards the next generation of electronic products, changing the
way people interact with their devices.
Introduction
Have you ever run into a problem with having too many devices and too few outlets to plug them
into? As trivial as it seems, this is a growing frustration for consumers in the 21st century.
Although a power strip helps alleviate that issue, it also creates another problem: a nest of
tangled cords. However, wireless charging—powering devices without cables—can solve all
these problems associated with conventional electronics.
As advanced as wireless charging sounds, the concept is not new. This idea was first conceived
by Nikola Tesla, who was best known for his work in alternating current, the type of electricity
that runs through our power sockets [1]. In 1891, Tesla developed a resonant transformer known
as the Tesla coil, which could transmit electrical power through air [2]. During one of his
experiments in 1899, he sent “100 million volts of electric power wirelessly over a distance of 26
miles to light up a bank of 200 light bulbs and run one electric motor.” Although the numbers
were impressive, the Tesla coils were not safe to use commercially because the resulting high
voltages damaged other electrical devices [2]. Now more than a hundred years later, wireless
power development, specifically in magnetic induction and magnetic resonant coupling, is
gaining momentum and holds a promising future.
Basic Physics Terms Defined
In order to understand the physics behind wireless charging, it is important to first define certain
keywords in electricity and magnetism. Electricity is the flow of electrical charge and has two
types: direct current (DC) and alternating current (AC) [3]. As shown in Figure 1, DC—obtained
from batteries and cells—flows in one direction, while AC—obtained from wall sockets—flows
in both directions; AC plays an integral part in wireless charging [4].
http://www.allaboutcircuits.com/vol_2/chpt_1/1.html
Figure 1: Illustrates DC (left) and AC (right).
Magnetism is a force by which certain materials attract or repel one another; an example of this
is a simple refrigerator magnet. Electricity and magnetism are interdependent; a magnetic field
produces an electrical field, and vice versa, in a process known as induction [3]. Another
important term to define is energy or power coupling. Coupling refers to the exchange of energy
between two objects; in the case of magnetic coupling, direct contact is not necessary to
successfully transmit energy [3]. This is another key component of wireless charging.
How Magnetic Induction Coupling Works
Most wireless charging technologies use magnetic induction coupling. Magnetic induction is a
type of energy transfer that involves two coils of conductive material [3]. Conductive material
like copper carries electrical current. As shown in Figure 2, the transmitter (Tx) coil is embedded
in the charging pad, and the receiver (Rx) coil is embedded in the mobile device [3]. An AC
source is first connected to the Tx coil, generating an alternating magnetic field around it. The
Rx coil picks up some of the magnetic field created by the Tx coil, which induces a current in the
Rx coil [3]. In return, the induced current charges the mobile device.
http://www.epcos.com/web/generator/Web/Sections/Components/Page,locale=en,r=263282,a=23
86330,principals=none_21none_21_20_21_20_21_20,print=1.html
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/EPWI950EWEGWW
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
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[Online]. Available: http://spectrum.ieee.org/green-tech/mass-transit/a-critical-look-atwireless-power
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