1. BOWLING GREEN STATE UNIVERSITY
ECET 4450
PROJECT REPORT
WIRELESS POWER TRANSFER
By: Yogitha R Malkireddy
2nd
December, 2014.

2. ABSTRACT
Wireless power transmission is a technique used to transmit power from one place to another
without using wires. WPT is generally opted in order to overcome the losses during
transmission. The percentage of loss of power during transmission is 26%. Main reason for
this loss is the resistance of the wires used in grid and disturbances during distribution. To
overcome these, electricity is transmitted wireless using electromagnetic induction. This
phenomenon is known as “Witricity”. The different techniques involved in wireless power
transmission are Inductive coupling, Resonant Inductive coupling and Air Ionization. These
three comes under near field technologies. The far field technologies involve Microwave
Power Transmission and LASER Power Transmission.
INTRODUCTION
The idea of wireless power transfer was first proposed by Nikolas Tesla in 1897 and is
considered as the pioneer of Induction techniques. Tesla was successful in transferring
energy from one coil to another and lighting up 200 lamps within a distance of 40Km. This
phenomenon was taken into consideration by University of MIT and given the name as
“Witricity”. This technology lead to the development of a way to charge cell phones, laptops,
and many other electronic devices wirelessly. Solar Power satellite system is also being
discovered from the ideas of wireless power transmission. SPS is a technology in which solar
energy from space is collected and utilised on earth. WPT technology is in high demand
because of its convenience to consumer and industrial marketplaces.
Why WPT?
It is reliable, fast, efficient, Low maintenance cost, and can be used for short and long ranges.

3. Energy coupling methods:
The transfer of energy can be done with
1. Magnetic Coupling
2. Inductive Coupling
Magnetic Coupling:
In general Magnetic coupling is implemented for transferring torque. But in this case for
electrical engineering two conductors are considered to be mutual inductively coupled or
magnetically coupled when change in current flow through one wire induces a voltage across
the ends of other wire through electromagnetic induction.
Inductive coupling: In inductive coupling generally the primary and secondary coils are not
in contact with each other. The transfer of energy takes place due to mutual induction. In
Inductive coupling technique, the higher the frequency at which device is transmitting the
smaller the transmitting and receiving coils must be. In this method, it has two coils which
are configured to have same resonant frequency as oscillator, which sends a sinusoidal signal
transmitting the power at resonant frequency.

4. The picture is an example of Transformer coupling which is inductively coupled.
Technologies involved in Wireless Power Transmission:
1. Near field technologies
a. Inductive Coupling
b. Resonant Inductive Coupling and
c. Air Ionization
2. Far Field technologies
a. Microwave Power Transmission(MPT)
b. LASER Power Transmission
1. Inductive Coupling:
In inductive coupling as mentioned above an alternating current in the transmitter coil
generates a magnetic field which induces a voltage in the receiver coil. This voltage can
be used to power a mobile device or charge a battery. The efficiency of the power
transfer depends on the coupling (k) between the inductors and their quality (Q).

5. The coupling is determined by the distance between the inductors (z) and the ratio of D2
/D. The coupling is further determined by the shape of the coils and the angle between
them.
2. Resonant Inductive coupling:
In this method both resonant and inductance of the coils interact with each other.
Resonance is used to improve the interaction between both the coils whereas inductance
induces the current.
Resonant Inductive Coupling vs Inductive coupling:
1. RIC is highly efficient, greater range than inductive coupling.
2. Directional when compared to inductive coupling,
3. RIC can be one-to-many whereas inductive is one-to-one coupling.
4. Devices using RIC are highly portable.
3. Air Ionization:

6. As a known fact ionized gas is a conductor, and the conductivity of a gas is the measurement
of ionization. This is the toughest technique, because Air ionizes only when there is high
field. (Required field is 2.1MV/m). Practically this cannot be implemented. An example with
high voltages is: Lightning
The following picture shows ionization between two wires due to high field.
Advantages and disadvantages of near field technologies:
Advantages Disadvantages
No wires Distance constraint
No e-waste Field strength might be harmful at times
Need for battery is eliminated Intial cost is high
Efficient energy transfer High frequency signals are necessary
Maintenance cost is less Air ionization technique is hard to implement
Far Field Technologies:

7. Far field technologies basically involve radiative transfers. The waves are either transmitted
in the form of magnetic or light. The following two techniques are implemented in far field:
1. Microwave power transmission
2. LASER power transmission
Microwave Power Transmission:
This technique involves the conversion of energy into microwave and then transferred using
rectenna ( Rectifier and antenna) from transmitter side and converted into useable electrical
form on the receiver side.
This is a cyclic process which involves:
 Conversion of electrical to microwave energy
 Receiving microwave using rectenna
 Transmitting the microwave energy
 Conversion of microwave to electrical energy

8. The power source generates the microwave power and it is controlled by the electronic
control circuits. The wave guide circulator protects the microwave source from
reflections which is connected to it using a Coax-wave guide adaptor. Tuner is used to
match the impedances between the source and transmitting antenna. After the attenuation
signals will be seperated using directional coupler according to direction of propogation.
The transmitting antenna radiates the power into free space to the receiving antenna.
On the receiver side, the received microwave power is converted to DC. The output
impedance of the source and rectifying circuit is matched with the filter and matching
circuit. The rectifier circuit is built using the schottky barrier diodes which converts
microwave to dc.
Fig: Microwave Power Transmission
Due to attenuation on the surface of the earth there is a limited availability of
transmission frequencies. Therefore frequencies above 6GHz are not efficient and
frequencies below 2GHz requires very large apertures for receiving and transmission.
Because microwave power transmission works on the principle of line of sight.
LASER power transmission:

9. Laser power transmission technique uses directional property. The emitted energy is of
high frequency and coherent. The greater advantage of this power transmission technique
is the aperture collection effeciency which makes a small antenna requirement.
In this transmission the energy doesn’t get dispersed but can get attenuated when
passed through atmosphere. The receiver can be built as simple as photovoltaic cell and
this makes it cost efficient than micorwave. LASER technology is applied for solar
power satellite stationed in the geostationery orbit containing lots of photovoltaic cells.
Solar energy trapped by this can be transmitted to earth rectenna as a light beam.
Comparing LASER and MPT:
Size of antenna is reduced in LASER MPT has interference and safety problems.
High Attenuation, interference and
Diffraction by atmospheric particles.
First Bird effect is a big concern.
Advantages and Disadvantages of Far field technologies:
Efficient Need line of sight
Easy Initial cost is very high
Need for grids, substations are eliminated Radiative, and absorption loss is high
Low maintenance cost Interference problem with microwave transmission.
Effective when Line of sight is perfect MPT cannot convert ac directly to microwave.
First it converts to dc then using magnetron.
DC is converted to Microwave.
Can reach places which are remote

10. Works Cited:
Benson. “Wireless Transmission now possible.” March 1920.
Biswa. “Feasibility of wireless power transmission” May 2012.
Bomber. “Wireless Power Transmission: An Obscure History, Possibly a
Bright Future.” March, 2006.
Princeton Blog “Solar Power Satellite.” Web.
Deller et al. “Wireless power transfer.” Dec, 2008.