I. INTRODUCTION

Energy Harvesting, as the name implies, is the

process of harvesting energy from different energy

sources. Energy Harvesting may also be known as

Power harvesting or Energy scavenging. The

Different energy sources from which energy can be

derived are Conventional Sources, Renewable

sources such as Solar, Wind, Thermal, Tides etc. or

Manmade Energy sources such as RF Energy.

Deriving Energy from Non-Conventional Power

Sources has numerous challenges and depends upon

the environmental conditions. More over the energy

obtained from such Non Conventional Sources

requires more Space and the results are non

Linear[1].

II. NEED FOR ENERGY HARVESTING

A Survey conducted by WWRF predicted that by

2020, there will be 7 Trillion wireless devices in the

world serving 7 Billion People, which equates to

around 1000 wireless devices per individual. The

energy needed to power these devices will be quite

significant. Even now the energy needed to power

the available wireless devices around the world is

also considered significant. Because, a huge part of

the energy produced in the world, is used to power

these wireless electronic devices.[2] For example,

consider a Mobile communication network. For

seamless communication between mobile devices,

the base stations are to be powered 24X7

throughout the year. Even though the base station is

powered all through the year, the power from the

communication tower is not completely utilized.

Hence most of the energy is converted to futile heat

energy.

III. RADIO FREQUENCY AS A SOURCE OF ENERGY

Radio Frequency Energy(RF Energy) is ubiquitous

and found in large amounts in the environment.

Most of this energy is either absorbed by Living

things or Non Living Things around the world and

finally lost as Heat Energy. This Heat Energy also

contributes a very small extent to the Global

Warming. Hence this ambient RF Energy can be

tapped and utilized to power small wireless devices

and other electronic devices such as home & car

electronic devices, Bio-implanted devices, etc[3].

IV. BASIC RF ENERY HARVESTER

The basic necessary elements required for

harvesting ambient RF Energy are an Antenna, a

Rectifier circuit and a storage device. The

arrangement of an Antenna and a rectifier together

is known as "Rectenna"[4]. The Transmitted RF

Energy from a distant source is received by the

Antenna. The received RF energy is an AC signal

and so this has to be converted to DC Signal for

storage. Hence a rectifier circuit is used to convert

the incident AC Signal to DC Signal. And finally,

the converted DC Signal is stored with the help of a

storage device such as a Battery. The Block

diagram for Ambient RF Energy Harvester is shown

below[5],

Fig : Block Diagram of Basic RF Energy Harvester.

ENERGY HARVESTING USING ANTENNAS

Siva Kumar Subramanian, Student Number: 239077

V. DIFFICULTIES AND POSSIBLE SOLUTIONS IN RF

ENERGY HARVESTING

The power received from the EM waves is very low

and it is difficult to harness such low power energy

using regular diodes. Hence, Schottky diodes are

used in the rectifying circuit to harness such low

power energy. The Schottky diodes used are of

highest speed with lowest voltage drop and low

power loss due to conduction and switching. Hence

it increases the efficiency of the circuit.

The input signal voltage reaching at the rectifier

circuit is less than the threshold voltage of the

diodes used and hence it affects the energy

harvester's efficiency. Hence to overcome this

problem, either the threshold voltage of the diodes

has to be decreased or the input voltage has to be

boosted.

To decrease the threshold voltage of the diodes,

special type of Schottky diodes such as GaAs

Schottky Diodes or specially fabricated discrete Si

Schottky Diodes are used. By using these diodes

low forward threshold voltage of 200mV is

obtained and 20% efficiency is utilized for a input

power of -20dBm. In decreasing the threshold

voltage, voltage biasing of the diodes are to be

taken care off. Voltage Biasing of the Diodes is

necessary for turning on the Diodes and for this

different circuits are designed and used. They are 1.

Distributor Circuits, which has higher charging

current for low input power, 2. Floating gate

devices, 3. Off-chip high impedance resistor

networks.

To Boost the input signal voltage received from the

antenna, different matching networks and various

voltage doubler circuits are used. Few voltage

doubler circuits used to boost the input voltage are

1. Single stage Voltage doubler, 2. Cascaded

Voltage doubler with boosting circuit, 3. Antenna

Coupled Cascaded Voltage doubler with Boosting

Circuit. The fundamental idea behind the voltage

doubler circuit is that by using LC Resonant circuit

large voltages are produced even when the input

voltage is very low. Such a voltage doubler circuit

is used across the MOSFET thereby turning on the

device easily and producing large voltages across it.

Another way of boosting the input signal voltage is

by using high quality capacitor at low input power

level . The Capacitor is placed parallel to MOSFET

and hence at resonance, this capacitor delivers large

voltage and biasing the MOSFET and thereby

increases the harvesting current.

The Matching network helps to reduce transmission

loss and increases the voltage gain by harvesting

maximum transmitted power. Matching Circuit also

avoids reflection of power in the antenna. This

structure of a Energy harvesting device containing

an antenna, matching network, rectifier circuit and a

voltage doubler circuit may be known as Single

Antenna RF Energy harvester. A model Energy

Harvester with voltage doubler circuit is shown

below[6,7].

Fig:Energy Harvester with Voltage Doubler Circuit.

VI. DUAL BAND RF ENERGY HARVESTER

Different models has been experimented to increase

the characteristics which govern the RF Energy

Harvester. Few among them are 1. Dual band RF

Energy Harvester and 2. Multiband Microstrip

Antenna Arrays with Multistage Rectifiers.

In Dual Band RF Energy Harvester, the energy

from two different bands are harvested by using two

different impedances simultaneously. The Block

Diagram for the Dual band RF Energy Harvester is

shown below[7],

Fig : Block Diagram of Dual Band Energy

Harvester

VII. MULTIPLE ANTENNA MULTI BAND RF ENERGY

HARVESTER

In Multiband Microstrip Antenna Array with

Multistage Rectifiers, Multiple Antennas along with

Multiple Rectifiers are coupled together to harvest

maximum energy from the environment. Each

antenna has high gain at a particular band. Hence

the antenna collects high RF energy at a particular

band. Also each Rectifier has a different impedance

matching for each antenna, thereby providing good

matching and converts maximum power from the

received power. The rectifier is also chosen to have

less thermal and electrical loss. This method has

major advantage over multiband antenna, as

multiband antenna scavenges power from multiple

bands at a time. Hence designing a matching

network is very difficult and so loss of power is

maximum. Also designing a multiband antenna with

high gain for several bands is difficult.

The difficulties in Multiple antenna structure is that

choice of antenna with maximum gain for each

narrow band is tiresome. Block Diagram of

Multiband Microstrip Antenna Arrays with

Multistage Rectifiers is shown in figure[8]

Fig: Multiband Microstrip Antenna Arrays with

Multistage Rectifiers

VIII. CHOICE OF ANTENNA WITH THE HELP OF GSO

To choose an antenna with maximum gain in a

multistage antenna energy harvester, Genetic

Swarm Optimization(GSO) algorithm may be used.

GSO algorithm combines the primitive algorithms

like Genetic algorithm(GA) and Particle Swarm

Optimization(PSO). Genetic Algorithm uses fitness

function to derive the suitable individual, whereas

the PSO uses position of each individual to obtain

the suitable individual. Now the GSO, divides the

individuals chosen for testing into two parts and

iterates each part with GA and PSO. Then GSO

combines the result and reiterates the process and

finally obtains the suitable individual. In this case,

the suitable individual will be the suitable antenna

for the particular band. The GSO provides the

Fitness Function(FF) and this FF is decoded to

geometrical dimensions to determine the design of

the antenna. The GSO can also be used to analyze

the S Parameters of wireless systems like S11 and

S21 parameter along with its feasibility. The S11 is

decreased to reduce the return loss and S21 is

increased to maximize the coupling performance.

This GSO is also used in optimization of antennas,

wireless systems, complex electromagnetic

problems etc. GSO reduces the computational effort

and number of iterations required to find the

suitable individual.[9] The flow chart of GSO is

shown below,

Fig : Flow Chart of GSO Algorithm.

IX. CONCLUSION

Even though field of RF energy harvesting using

antennas had begun in early 1950's, it still has major

challenges and is still under research to harness

maximum power from the available ambient

Electromagnetic wave energy which includes RF

Energy. New fabrication techniques and

development of nano technology opens this field to

new dimension. New devices such as Nanoscopic

Rectifying Antenna(Nantenna) are developed and

are used to harvest RF energy by converting Light

signals. With more advancement in the scaling

structure in the future and more availability of

ambient sources, new applications and techniques

will be developed[10]. It is quite evident that

Energy Harvesting using Antennas will play a

major part in the energy industry.

REFERENCES

[1] Energy Harvesting

http://en.wikipedia.org/wiki/Energy_harvesting

[2] Angeliki Alexiou, WWRF - Research views on

IMT Technology Evolution

http://www.itu.int/dms_pub/itu-

r/oth/0A/06/R0A060000500005PDFE.pdf

[3] K Pirapaharan1, WLADA Gunathillake, GI

Lokunarangoda, MV Nissansani, HC Palihena, PRP

Hoole, CV Aravind and SRH. Hoole, " Design of a

Battery-less Micro-scale RF Energy Harvester for

Medical Devices"

"http://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumbe

r=06498086

[4] Rectenna, http://en.wikipedia.org/wiki/Rectenna

[5] D. Bouchouicha, M. Latrach, F. Dupont, L. Ventura, "An

experimental Evaluation of Surrounding RF Energy

Harvesting Devices"

http://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber

=05616298

[6] W.M.D.R.Gunathilaka, G.G.C.M.Gunasekara,

H.G.C.P.Dinesh, K.M.M.W.N.B.Narampanawe

,"Ambient Radio Frequency Energy Harvesting

"http://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumbe

r=06304789

[7] Bo Li, Xia Shao, Negin Shahshahan, Neil Goldsman,

Thomas Salter and George M Metze “An Antenna Co-

Design Dual Band RF Energy

Harvester,”http://ieeexplore.ieee.org/stamp/stamp.jsp?tp=

&arnumber=06525423

[8] J. Michael Barcak and Hakan P. Partal1 "Efficient RF

Energy Harvesting by using Multiband Microstrip

Antenna Arrays with Multistage Rectifiers,

http://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber

=06404327

[9] M. Roscia, D. Zaninelli, G. C. Lazaroiu, and H.

Shadmehr, "Using Design Optimization in Radio

Frequency Energy Harvesting Systems"

http://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber

=06586939

[10] Nantenna, http://en.wikipedia.org/wiki/Nantenna