A Comparative Survey of Energy Harvesting Techniques

for Wireless Sensor Networks*

Kishor Singh and Sangman Moh†

Dept. of Computer Engineering, Chosun University, Gwangju, South Korea

smmoh@chosun.ac.kr

Abstract. In wireless sensor networks (WSNs), one of major hurdles is the lim-

ited battery power that is unable to meet long-term energy requirement. Energy

harvesting, conversion of ambient energy into electrical energy, has therefore

emerged as an effective alternative to powering WSNs. This paper surveys the

most preferred ambient sources available for energy harvesting and the scav-

enging techniques commonly used in WSNs to deal with energy issues. Fur-

thermore, the energy harvesting techniques are qualitatively compared, and

challenges and open issues are discussed.

Keywords: Wireless sensor network, energy consumption, energy harvesting,

energy conversion, network lifetime.

1 Introduction

A wireless sensor networks (WSN) consists of spatially distributed, inexpensive min-

iaturized devices called sensors which are deployed over a geographical area for the

close monitoring of physical conditions [1]. However, it is not possible for WSNs

with limited power capabilities to afford long-term operation without any external or

supplementary energy source [2]. The lifetime of sensor nodes can also be prolonged

by compromising with the performance parameters [3]. For example, the use of a low-

power processor or low-power transmitter can slow down the depletion of battery

energy but it has to deal with lower processing speed and smaller transmission ranges,

adding up net delay.

The sources of energy in our natural environment exist in many forms, are distrib-

uted widely, and have endless supply which could be exploited indisputably [4]. More

importantly, they are readily available in abundance without requiring any extra effort

to produce them. Energy harvesting from ambient resources stands as reliable and

effective means for empowering the small autonomous sensor nodes. The implemen-

tation of energy harvesting technologies is not simple. A large number of things need

to be taken into account for effective energy conversion and efficient use.

The rest of the paper is organized as follows: The components of an energy har-

vesting system integrated to sensor nodes are overviewed in the following section.

* This research was supported in part by Basic Science Research Program through the National Research

Foundation of Korea (NRF) funded by the Ministry of Education (NRF-2016R1D1A1A09918974). † The corresponding author (smmoh@chosun.ac.kr).

Advanced Science and Technology Letters Vol.142 (GDC 2016), pp.28-33

http://dx.doi.org/10.14257/astl.2016.142.05

ISSN: 2287-1233 ASTL Copyright © 2016 SERSC

The commonly used energy harvesting techniques and the ambient energy sources are

presented in Section 3. In Section 4, the energy harvesting techniques are qualitatively

compared and technically discussed. In Section 5, important challenges and open

issues are summarized. The paper is concluded in Section 6.

2 Energy Harvesting System

Energy harvesting is the technique of converting the unused ambient energy source

into electrical energy. The energy obtained is called harvested energy. The energy

harvesting basically consists of three major components: namely, energy source,

energy harvesting architecture and energy consuming entity [5]. An energy harvesting

system captures and scavenges the ambient energy and then supplies the harvested and

converted electrical energy to sensor nodes. The design of the energy harvesting

system may differ from one application to another depending on the type of load

requirements and the conditions of deployment area [6]. The design and

implementation of an efficient energy harvesting system not only requires proper

selection of devices with appropriate ratings but also the proper integration of those

electronics to the sensor node [7, 8]. Fig.1 shows a usual energy harvesting system

integrated with a sensor node.

Ambient energy

Power

management

unit

Energy storage

Energy

harvesting

device

Memory

Radio

Transceiver

Sensors

Processor

Fig. 1. Sensor node with energy harvesting system

3 Energy Harvesting Techniques

Environmental energy sources have been used to generate electricity for a long time. In

the this section, we discuss the energy harvesting techniques used in WSNs.

Advanced Science and Technology Letters Vol.142 (GDC 2016)

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3.1 Photovoltaic Energy Harvesting

Photovoltaic energy harvesting is the technique of converting solar energy into elec-

trical energy. The significance of the system lies in solar cell, maximum power point

tracker (MPPT), and voltage converter. MPPT makes sure that the maximum power is

extracted from the solar cell by matching the impedance of the solar cell and the volt-

age regulator or converter [9]. The output characteristics of the photovoltaic system

vary nonlinearly with the changing irradiance and time of the day. Under such scenar-

io, MPPT automatically track the value of current and voltage at which maximum

output power is delivered. A combination of a set of ultracapacitors in parallel con-

nection with a series of alkaline batteries is used in [10] as energy storage module.

3.2 Wind Energy Harvesting

Wind is a commonly available ambient energy resource with a tendency of making

WSN completely self-autonomous with sustainable energy. In [11], a wind turbine

generator (WTG) is used as the energy harvesting device, consisting of a wind turbine

coupled to an electrical generator. This combination allows the conversion of kinetic

energy of air flow into mechanical energy by the turbine, followed by conversion of

mechanical energy of turbine into electrical energy with the electrical generator. In

[12], a DC generator has been used, and it simply requires DC-DC converter instead

of the rectifier. The use of MPPT ensures extraction of maximum power from WTG.

A low-power microcontroller (MCU) implements the algorithm for MPPT. For the

purpose of energy storage, battery or supercapacitor or both can be used.

3.3 Piezoelectric Energy Harvesting

Piezoelectric devices convert mechanical force, stress or vibration into electrical en-

ergy. The low-power vibrations are amplified to enhance output power. In fact, a

higher output power is possible only when the system is in resonance with the exter-

nal exciting vibration [13]. The piezoelectric energy harvesting system consists of a

piezoelectric material bonded to a cantilever beam [14]. When the beam gets excited

by some ambient vibration, a significant strain develops in the piezoelectric patch

which is transformed into electrical energy. In [15], a trapezoidal piezoelectric har-

vester has been used for harnessing electrical energy from ambient vibrations. The

device has an excitation frequency of 100 Hz and the resonance is achieved with a

software operated trapezoidal cantilever.

3.4 RF Energy Harvesting

A radio frequency (RF) energy harvesting system allows the energy-constrained

WSNs to harvest energy from the radio signals for performing their operations. It is a

combination of impedance matching, rectifier, and DC-DC converter, and it is devised

to collect radio signals and convert them to corresponding electrical equivalent with

maximum efficiency. The use of Schottky diode in the rectifier can improve the effi-

ciency of power conversion significantly [16]. In [17], far-field RF energy harvesting

has been implemented to energize a low-power and low-duty-cycle WSN with radio

Advanced Science and Technology Letters Vol.142 (GDC 2016)

30 Copyright © 2016 SERSC

signals in gigahertz band. Data transmission, which is the most energy consuming

operation, is not performed continuously. Hence, harnessed energy is stored during

the periods of no data transmission.

3.5 Thermal Energy Harvesting

The inevitable component of thermal energy harvesting is a thermoelectric generator

(TEG). TEG basically consists of a pair of p-type and n-type semiconductor connect-

ed in series electrically but in parallel thermally between two ceramic layers [18]. A

thermal powered WSN is presented in [19], with a simplified power manager (PM).

The PM balances the performance of the system by changing the duty cycle of the

node such that the energy is in proportion the harvested energy. Supercapacitor is

used as a main storage element, the voltage level of which determines the active or

sleep period of the sensor node.

4 Comparison of Energy Harvesting Techniques

The most common form of energy harvesting techniques have been reviewed and

compared qualitatively and presented in Table.1.

The major advantage with solar energy lies in the fact that it is green energy with

limitless availability. However, the point to be noted that solar based system are func-

tional only when get exposure to the sunlight. Wind being uncontrollable and unpre-

dictable needs special caring and techniques to get the best out of it. Usually, wind

based energy scavenging system consist of mechanical parts which may produce un-

necessary noise. Piezoelectric energy harvesting is the most appropriate form of ener-

gy harnessing mechanism for fabricated environment. It is more or less dependent on

the environment or apparatus set for excitation or generating vibration. The develop-

ment and implementation of RF based system needs to consider the characteristics of

propagation channel, distance from the RF transmitter, design of the rectenna, antenna

size and transmit power. Since the thermal energy systems are more prone to charge,

leakage care must be taken to prevent the energy loss.

Table 1. Comparison of different energy harvesting techniques for WSNs.

Harvesting

Technique

Energy

Source Energy type

Harvesting

device Merit Demerit

Solar energy

harvesting [9] Sun

Radiant

energy Solar cell

Limitless avail-

ability

Needs exposure

to light

Poor indoor

efficiency

Wind energy

harvesting [11] Wind

Kinetic

energy

Wind

turbine High power

density

Noisy

Unpredictable

Piezoelectric

energy harvest-

ing [13]

Stress or

vibration

Mechanical

energy

Piezoelec-

tric film or

ceramic

Equally efficient

indoor and out-

door

Piezoelectric

devices are brittle

Advanced Science and Technology Letters Vol.142 (GDC 2016)

Copyright © 2016 SERSC 31

RF energy

harvesting [16]

Radio

signals

Electromag-

netic energy Rectenna

No mechanical

parts

Fading with

distance

Thermal ener-

gy harvesting

[18]

Heat Heat energy

Thermoe-

lectric

generator Easy and simple Current leakage

5 Challenges and Open Issues

Integrating energy harvesting systems to WSNs requires a lot of processes that in-

volves capturing, storing, conditioning, regulating, and managing. The miniaturization

of the energy harvesting system to fit the requirements of WSNs is always a challeng-

ing job. Followed are the challenges and open issues in this regard. Followed are the

challenges and open issues in this regard.

The power conversion efficiency of an energy harvester and the overall system is

always an issue of interest. For most of the harvesting techniques, the overall

conversion efficiency turns out to be low.

In addition to harvesting power from the ambient sources, it is also important that

the harnessed power is efficiently stabilized, stored and delivered to the targeted

destinations.

The excess energy obtained from the harvesting system need to be stored efficient-

ly. Because the energy supplied by harvesters is discontinuous, the task of generat-

ing a continuous and constant supply goes to energy storing devices.

The control over the transmit power and regulation of convenient wake-up and

sleep schedules for sensor nodes can help in efficient conservation of hard-earned

energy.

Energy harvesting in itself is a great effort to address the energy constraint in

WSNs. Implementing the harvesting technique is just not enough to reap all the

benefits of energy-harvested WSNs.

6 Conclusion

In this paper, the most widely used and efficient techniques for energy harvesting in

WSNs have been extensively surveyed and qualitatively compared. Undoubtedly, the

recent advancement of the energy harvesting systems is offering a huge supplement to

the conventional battery-dependent systems. The challenges and important issues on

the way to more efficient harvesting have been discussed as well. The need for smart

grid technologies to deal with variability would be an effective solution for extracting

energy in a more efficient way. To sum up, energy harvesting with the ambient energy

resources is reliable, safe and clean, and it has the potential to power WSNs endlessly.

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