, Chris Danzl, Mustafa Alnass
Methods of Wireless Energy Transfer
Since the early 1900’s and the invention of electricity, Nikolas Tesla experimented with the idea
of wireless transmitting energy rather than using an electric grid. Stating that an electric grid would have
many losses, he created the Tesla Tower. However,
his methods developed large undesirable electric
fields. Recently there has been an increase in independent electrical devices such as cell phones,
laptops, and PDA’s. These devices all require recharging at some point and thus have created an interest
wireless energy transfer once again. A group from the
Institute of Technology has
created a means for wireless energy called Magnetic Resonance Coupling. They have named this
and have founded Witricity Corp. to add this t
echnique into various products. This
paper will discuss the
methods of wireless energy transfer available now and the theory behind them.
Then will continue by discussing the differences between them.
TRADITIONAL MAGNETIC INDUCTION
A transformer transfer
s electrical energy from one circuit to another through inductively
coupled conductors, the transformers coils.
Faraday’s Law of Induction played a big role in getting
transformers started. The law states that a changing electrical current can produce a ma
gnetic field and
this changing magnetic field within a coil of wire induces a voltage in the secondary coil.
current in the primary windings of the transformer creates a varying magnetic flux in the core of the
transformer. From this a varying ma
gnetic field is produced through the secondary windings. This
magnetic field in the secondary windings induces a varying EMF (electromagnetic force) aka “voltage” in
this secondary winding. This effect is known as mutual inductance.
If a load is attached
to the secondary windings then a current will also be produced. Electrical
energy will then be transferred from the primary circuit to the load, through the transformer. For an
ideal transformer the ratio of voltage between the secondary and primary will e
qual the ratio of turns in
the secondary and primary windings, respectively. Vs/Vp=Ns/Np. From this finding an AC voltage can be
“stepped up” or “stepped down”, depending on Ns and Np.
Most transformers have their windings, usually coils, wound around a f
erromagnetic core. This
core is a piece of magnetic material with high permeability used to confine and guide magnetic fields in
Transformers have multiple ways of energy loss to occur. Some of these include winding
sis, eddy currents, magnetostriction, mechanical losses, and other stray losses. These
losses, however, are usually small. Another disadvantage is the fact that your coils from the primary and
secondary must be very close to one another, otherwise the magn
etic fields dissipate too much. As
distance is increased the loss of energy from one coil to another is also increased. On the other hand,
traditional magnetic induction does not rely on a resonance frequency to transfer energy. Thus, any
frequency can be
used to transfer energy.
Some major applications of transformers come from the ability to transfer electrical energy and
step up or step down currents and voltages. The amount of power transferred with traditional magnetic
induction can be from mW to kW,
and can be transferred thru any medium which a magnetic field can
THEORY OF WITRICITY
Magnetic resonant coupling is similar to traditional magnetic induction
(transformers) but uses the idea of resonance to extend the range of energy tr
resonance can be tuned using capacitors on both the transmitter and receiver.
magnetic induction has a range of only a few mm between antennae and receiver, but with
resonance, the range can be extended to over seven feet and possi
bly further. The basis of
Faraday’s law is still the foundation of this method of energy transfer. But, resonance is a
frequency at which energy can be added to an oscillating system at greatest efficiency. When a
singer shatters a wine glass, it shatters
at resonance. The oscillating wine glass continues to
gather energy at resonance until it shatters.
Resonance occurs within this wireless energy transfer system when both the receiver
and antennae have the same resonant frequency, which can be represented
. Where L1 and C1 are the inductance and capacitance of the receiver and L2 and C2 is the
inductance and capacitance of the antennae. With optimum resonant frequency, an efficiency
of 40 percent was ac
hieved at a distance of 7 fee
t with a transfer of 60 W.
Another important variable to efficient wireless transfer is a strong coupling between
antennae and receiver which is represented by,
. Where k is the coupling coefficient
and Γ is the resonant width due to the objects intrinsic losses. The coupling coefficient, k, can
be estimated by the fractions of magnetic flux between each coil. This energy transfer
application also performs optimal
ly with high values of Q, where
RADIO FREQUENCY IDENTIFICATION TAGS (RFID’s)
There are two types of RFID’s, passive and active. We are less concerned about active tags in
r because they contain a battery
receiver and are less sim
ilar to the Witricity method of
transmitting energy. The passive tags, however, are similar to the Witricity method of transmitting
The basis of Radio Frequency is still similar to transforms in the fact that alternating current
induces magnetic fi
eld which can be transferred back to voltage on a secondary coil or receiver.
A RFID works as follows. The “reader” or the transmitter sends power to the receiver or “tag”
and then the tag sends data and information back to the reader.
The power sent can
be related to this
The power received at the tag can be related
RFID’s can operate at many different frequencies based on resonance of both the
can send mW
amount of power and the design of the tag relies heavily on the distance
between tag and reader.
The RFID works in the near field which is dominated by a magnetic field from
the reader. Thi
s field can be a distance from 2
3 feet with high fr
equency signals (13 MHz)
A range of
Figure to the left shows
current through a circuit with
a capacitance of 100 Pico
Farads, an inductance of 66
micro Henry’s and a
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25 feet can be achieved with UHF signals.
This range is determined by this equation,
Efficiency of the RFID method depends on the mutual inductance of the antennae and receive
as with Witricity method
. This can be related by,
The efficiency also depends on the
alignment of the receiver coil in the magnetic field
. Misalignment can cause a loss in
RFID are used to transmit data from the tag back
to the reader such as a bar code would do.
They are beneficial when the tag is within th
e range of the reader or about 1 meter
A benefit of RFID’s
is they don’t have to be in the direct line of sight with the reader. RFID’s can also be used when there
different mediums which exist between reader and tag
and allow magnetic fields to pass through.
The set back is the range of the RFID’s and the amount of power transferred.
Infrared Method (IR’s)
It is known that light can be converted to electricity by
using a PV converter. The PV converter
can then power and also recharge a batter. One constraint to infrared radiation or laser to transmit
energy is the sender and readers have to be in direct line of sight. However, the distance between the
two can be v
ery large as long as the receiver or PV converter is in contact with light
Charge times and efficiencies depend on the amount of current being absorbed by the PV
converter. But, for a photo diode with a surface area of 2.1 cm
, emitting an infrared ligh
wavelength of 810 nm with a power density of 22 mW/cm, a regular pacemaker can run for 24 hours
with a charge time of 17 minutes. (2) During the charging time, the temperature of the skin near the
irradiation was found to rise only 1.4 degrees C.
is method of recharging
has been found to be around
the order of 20 % efficient depending on battery voltage and skin depth. (3)
The amount of current supplied by the PV converter is represented by
the normal amount of current supplied to the battery by the PV converter at normalized
light power density
is the amount of light impinging on the PV converter, and
is the time
ration that the converter is illuminated.
A unique feature of this method of recharging
is the possibility of sunlight replacing a
infrared laser. In one study, an illumination time of 49 hours per week and a fiber opti
c diameter of
900 microns can increase the life of a battery by 5%. This compared to an increase of 145% with only 21
hours per week with a near
infrared laser. Consequently, sunlight and ambient illumination can
supplement the use of a laser, but cannot
be relied on as the sole illumination exposure. (1)
Compare and contrast section
Although both RFID’s and the
method use a resonance frequency, the amount of
power sent from the
method is much greater than that of an RFID.
The main dif
between these two means of wireless energy transfer is in the word resonance. Resonance is the natural
frequency of an object where energy can most efficiently be added to the system.
This resonance allows
more energy to be transferred.
One way to think of resonance in a mechanical sense is with an opera singer and a wine glass.
When the singer hits the natural frequency of the wine glass, or resonance, the vibrating glass will
vibrate more and more until it shatters. The magnitude of the
vibrations in the glass is the energy being
added to the system. In electrical sense the imaginary parts to an RLC circuit offset each other during
resonance and thus a larger current can be added to the system with the same applied voltage.
l Magnetic Induction works in the near field, usually only mm between antennae and
receiver. Witricity, however, works in the mid
range which has been found to work at a distance of over
and can send similar amounts of power (1
e range of traditional magnetic
induction is used for wireless energy transfer without a core, the efficiency decreases much greater than
that of Witricity.
With IR’s mW of power can be send just as with RFID’s but the range can be extended as long as
e PV converter is in contact with light. IR’s however do need to be in direct line of sight of the power
source/laser where as with Witricity, misalignment is less of an issue.
Type or Wireless energy transfer
Amount of Power Delivered
Both Near And