Comparison of Energy Harvesting Power Management Techniques and Application

Mohd Sofwan Mohd Resali ',Member IEEE, Hanim Salleh 2 , 2Department of Mechanical Engineering

Universiti Tenaga Nasional (UNITEN) Kajang, Malaysia

'Mohdsofwan@gmail.com, 2Hanim@uniten.edu.my

Abstract

There has been a significant increase in the research on energy harvesting device for low power applications in recreant years. This is due to smaller electronics power applications such as wireless and mobile electronics and the demand for better lifespan of batteries. One of the challenges of the harvesting energy from ambient is to convert, transfer and store the usable power effectively. In this context, there is a need to understand and design and efficient energy harvesting power management circuitry. In view of the issues, this paper compares several energy harvesting power management techniques and applications. Based on the comparison, suggestion on the design improvement are also included. This paper proposed improvement on the adaptive circuit as to get better efficiency. This paper will propose by using full bridge AC-DC rectifier to convert AC input voltage to usable DC voltage. In order to reduce power consumption of the circuit and power losses, comparator circuit is implementing as an adaptive approach to the DC-DC step­down converter. Simulation results are presented that output voltage from power management energy harvesting circuit is 3.0V with output power is 30mW. The efficiency reported as 80%. The total power losses are 7.5mW. Lastly this design presents a stand-alone system, single supply voltage and compatibility for micro-scale circuit integration.

1. Introduction

In a few years before, with the increasingly many of electronic company devices start to produce micro-power electronic applications, people start to explore on the energy harvesting technologies as it can promise for the low power source. This technology will become as a big role for the alternative low power technology as it can reduce for the long life operation cycles, reduce cost of maintenance, keep clean of environment and researcher try to prove it can operate reliably as a good system for the environmental and industrial monitoring. This section will discuss about previous work for the power management energy harvesting circuit design. This section will cover on the design technique, key design factor and its application design for as to full fill a task as a good system from the ambient or waste energy sources. This comparison and review paper will be focus on the energy harvesting power management techniques and application for piezoelectric micro generator vibration based.

2. Power Management Energy Harvesting Circuit Concept

Design

In energy harvesting technologies, researcher or engineers start produce or introduce a few approach of the circuit design. The typical circuit consists of voltage rectifier, converter and storage. Below discuss about the components that are will be considered in the power management energy harvesting circuit. The choosing of the electronic device below should also consider the implementation of the low power electronics which is critical to reduce the total power losses.

2.1 AC-DC Rectifier

First of all in piezoelectric energy harvesting research concept, the piezoelectric will work as a harvester to collect a waste energy such as vibration rotational machine and then convert it to electrical energy [1-5]. The extract piezoelectric strain energy will produce an AC voltage as an electrical energy. But most of the low power electronic device nowadays will only use a low DC voltage. Therefore the AC output voltage need to rectify and convert it to the usable DC voltage to supply to the load circuit which is contents with electronic devices.

2.2 Energy Storage

In some electronic applications, the ambient energy or energy harvesting technique, the energy harvested can be considered to replace the usage of the batteries as an energy storage device. In the effort of it, device for the energy storage always promise for the advantages if compare to the batteries. Supercapacitor is one of the devices that can propose to replace of the usage of the batteries. By using supercapacitor it has advantages by more charge and discharge cycles. Other of the advantages are has high power density, high cycle efficiency, longer lifetime and lower toxicity of material used if compare to the usage of the batteries. There a few method of design energy storage circuit after the AC-DC rectifier. One of the method is by directly charge [6, refer IPEC ret] by connecting the supercapacitor parallel across the output AC-DC rectifier.

2.3 Comparator Circuit

In energy harvesting circuit design, the stability and enough targeted power is very important key points need to consider when apply it to the circuit design. The harvester or energy generator for example piezoelectric micro generator always harvest the energy from vibration rotational machine which will vibrating for a long time. The energy that was harvested will always flow to the targeted electronic load. But the challenging is when the input power or energy is lower

than targeted input power it will never to power up the load electronic circuit. That is why many component circuit design for energy harvesting circuit they will use the comparator

circuit. Torah et el [6] reported the comparison by using comparator or it was name as a cold start circuit between by

not using comparator circuit to power the microcontroller and RF sensor node. It was reported that by using the cold start

circuit it can power up the entire circuit and without cold start

circuit it never turn on the entire electronic devices.

2.4 DC-DC Step Up or Down Regulator

Most of the electronic device for low power application

needs for the low input voltage. The ranges of the low input

voltage usually range 3V until 5V. DC-DC step up or step down regulator are functioning first to supply a very stable supply voltage to the electronic load circuit which can

eliminate the voltage ripple from piezoelectric micro­

generator. The second function is it also can step down the

high input voltage to the targeted low voltage as an example from 9V to 3V supply voltage and vice versa.

2.5 Load or Application Circuit

The last parts of the energy harvesting circuit that needs to consider is the application or load circuit. Basically researcher

or engineers will defmed first what is the targeted energy that

will be supplied to the application circuit and prepared the electronic device which can be utilize the energy that will be

supplied. Torah et al. [6] present also the power or budget energy consumption for the every single electronic application

device and by accumulates the energy consumption. By that it

can help engineers or researcher to provide the energy storage

that can store enough energy as requested.

3. Factors Affecting the Performance of the Power

Management Energy Harvesting Circuit

3.1 Circuit Efficiency

Many times of the research paper will report for the

optimum power energy extracted from a harvester such as from piezoelectric micro-power generator but many times one not reported for the circuit ineffectiveness. When comparing

techniques of power management energy harvesting circuit

solutions for a given application, the most important thing is to focus on the effectiveness of the circuit. The efficiency will

report on the actual energy effectively delivered to the application or also as a report of the energy harvested lost before delivered the energy to the targeted application. The

effectiveness of the circuit is calculated and measured by its efficiency converting power between input power (Pin) which

is the power generated from the micro-power generator and the output power (POIlI) which is the output power from storage in the power management circuit which will deliver the power

to the targeted application. It is also important to know what

the budget's power of the targeted application is and comparing with the actual power since the actual power will

be different from the optimwn power produce from the

harvester.

3.2 System Operations Independent

One of the applications by using energy harvesting circuit is the wireless sensor node (WSN) monitoring system. This

system is functioning to monitor as an example the vibration

rate for cooling fan's motor in the power plant by transmit vibration data to the control room. As to full fill this task

sometimes power management circuit requires for the external

sensor to trigger the energy harvesting circuit before can start to operate. This scenario shows this power management circuit

is not stand-alone. Stand-alone here means that this power

management energy harvesting circuit can operate independent of the other components of the electronic load.

One of the advantages by using the stand-alone operation

energy harvesting circuit is to save more energy or power.

3.3 Simple Management Circuit

Monitoring system by using energy harvesting circuit

always need for mUltiple electronic loads such as accelerometer sensor, microprocessor, RF module, switching

circuit and other more. Some electronic loads need for a more than one supply voltage for sensing current before can start operate. It makes the power management energy harvesting

circuit become more complex and need for a high power

application and reduces the efficiency due to non-scalable. A

good energy harvesting circuit which apply for a low power application is by applying only a single supply voltage and at

the same time makes the circuit more simple and increase the efficiency.

3.4 Small Size Circuit

These characteristics requests for the final integration of the power management energy harvesting circuit which will

improves the efficiency, energy density and reduce the cost. When applying adaptive circuit to the energy harvesting circuit avoid using the external sensor for sensing current. By

that this circuit can reduce for the circuit complexity, multi

supply voltage, and more simplicity and at the same time can

reduce for the cost.

3.5 Circuit Adatively

Adaptive approach is needed as to achieving the maximum

power flow from the rectified voltage. Since the amplitude of

the ambient energy such as the beam of vibration is not fixed, the power flow from the micro-power generator to the storage device is not optimum and is inefficient. By using an adaptive

circuit approach will maximize the power transferred from

vibrating piezoelectric transducer to the storage device.

4. Previous Energy Harvesting Circuit Design

In this section it will discussed on the previous works by previous researcher on the energy harvesting circuit design.

Some consideration that was takes by previous researcher or

engineers are depending by the specification that was earlier

set-up by them. This previous works are being review as to get look in depth understanding from previous works concept design and adapt to the new design.

The first findings research on the piezoelectric movement was reported by Kymissis et al. [7]. It proposed piezoelectric

generator the direct forced in a running shoe. Using direct discharge method by connecting capacitor to the AC-DC

rectifier is cannot extract maximum power. The output voltage

was regulated by a step-down switch mode power supply as to

get the stable DC output voltage and supply it to the targeted

load electronic circuit.

Dallago et al. [8] present for the AC-DC voltage doubler

retire to rectify the input power from micro-generator. It

implements exclusively a fraction of the harvested energy to

supply itself. The comparator of power conditioning circuit is

replaced with operational amplifier as to sense the voltage

across the switching MOSFET and drive their gate terminals.

The function of the OP-AMP was design so that they are able

to work with the minimum possible value of the supply

voltage. It was found that by implement independent supply

bias circuitry in order to make the current consumption of the

driving circuitry independent of the supply voltage the total

current consumption is 500nA. Meanwhile the efficiency is 90% in a wide range of load value. This design approach is

standalone system and compatible for a small size circuit.

Lesieutre et at. [9] proposed for the two mode of energy harvesting circuit. The first mode for low excitations, the

rectifiers charges the battery directly. The second mode, for

higher excitations the battery runs the DC-DC converter. At

the higher level of excitations the DC-DC converter delivers

more than four times the power to storage than direct charging from the rectifier, this ratio increase with excitations. The

hardware experimental results on a base on piezoelectric

cantilever had shown that having 26% coupling coefficient

and yielded an effective loss factor for the fundamental 2.2%

of vibration mode. This design approach presented the

standalone system but it has a big size of complete circuit since apply for the two mode of energy harvesting circuit.

Le et at. [10] proposed for the conversion circuit with

synchronous rectifier for voltage doubler, full bridge and

passive full bridge rectifier which are connected to the

piezoelectric micro power generator. Synchronous circuit here

means that the circuit is the digital control circuit in which the parts are synchronized by a clock sognal. It was reported that

for synchronous voltage doubler rectifier efficiency was

measured as 88% efficiency with output power exceeds

2.5f.lW at 100KQ, 100nF load. Meanwhile the synchronous

full bridge rectifier produced the efficiency 86% with peak power 22f.lW at 68KQ. For the passive full bridge rectifier it

was reported that the efficiency is achieve only 66% at 220KQ

and the peak output power was achieve 16f.lW at 68KQ load.

This paper reported that the best performance achieve at the

power conversion circuit based on the full wave synchronous

rectification architecture.

Yi et al. [11] proposed energy adaptive maximum power

point tracking (EA-MPPT) as to allow power management

circuit unit to activate at different operation modes according

to the available power level. AC-DC voltage doubler rectifier

circuit was used as a conversion circuit and then followed by

on chips charge pumps circuit with variable up or down conversion ratios for higher efficiency. It was reported that the AC-DC voltage double produce the open circuit voltage from

1.4 V to 5V and to make the efficiency comparable to a

switching converter the charge pump has to be highly

reconfigurable according to the input voltage. This deign method by using charge pump as power management circuit at

last not mention for the efficiency achievement but this is an

adaptive circuit implementation and standalone system with

small size circuit.

Hehn et al. [12] proposed for the two modes energy

harvesting circuit. The first mode at low excitations, the rectifiers charge the battery as a storage device directly. For

the second mode, at the higher excitations the battery will runs

the DC-DC converter. At the higher levels of excitations the

DC-DC converter delivers more than four times the power to

storage than direct charging from the rectifier, this ratio

increase with excitations. For this design method it use the full bridge AC-DC rectifier circuit with filter capacitor, a

switching DC-DC converter and battery as a storage device.

This paper presented operates as a standalone system which Observe that by fix duty cycle provides near-optimum

performance when persistent excitations exceed a certain

levels. The circuit design is not compatible for the small size

circuit.

Ottman et al. [I3*7nm] proposed the adaptive control

technique for the DC-DC converter. The objective of the design method to develop an approach that maximizes the

power transferred from vibrating piezoelectric transducer to the electrochemical battery. This paper initially presents the

simple model of a piezoelectric and added AC-DC rectifier

and then used to determine the point of optimal power flow for

the piezoelectric element. At lasts this paper present adaptive

approach to achieving the optimal power flow through the use

of a switch mode DC-DC converter. Experiment result show adaptive DC-DC converter increase power transfer by over

400% as compared to when the DC-DC converter is not used.

Nowadays the demands for the energy harvesting

application especially for the industrial wireless sensor system

monitoring network start to increase. Many company starts to

introduce their products and try to become a pioneer company who start explore this technology. Advance Linear Device

Inc. (ALD) produces the VEH300, VEH360 series power

management circuit that was ready to use. The portable power

management has a small size WxLxH (l4mm, 50.8mm, and

17.8mm) which the input generator is form piezoelectric.

Meanwhile Perpetuum Ltd produces the product PMG 17

which the generator is from electromagnetic. Their product is

including with the complete RF wireless sensor node and

commercialized with complete monitoring system especially

to the vibration machinery.

5. Design Proposal

0: 0: 0

U'!! � u �D DU- o: Enables D 'f- t!: 0 Uu

"'UJ ::;; D 0: 0 u

Figure I. Proposed power management energy harvesting circuit

Figure 1 show the proposed power management energy

harvesting circuit vibration based for wireless sensor nodes. The power management circuit apply for the full bridge AC­

DC rectifier circuit as to convert from AC voltage to the DC

usable voltage. The supercapacitor then connected to the

rectifier as to store the energy flow from piezoelectric

transducer. The comparator is used as and adaptive circuit to

the DC-DC step-down converter. The output from the DC-DC

step-down converter then connected to the electronic load

circuit. This proposed power management energy harvesting

circuit was simulated and will be reported on the simulation results.

Energy Harvesting and Storage

Figure 2. Proposed block diagram for the energy harvesting circuit wireless sensor node

Microcontroller

RF Module

Figure 2 shows the block diagram for the propose design

energy harvesting circuit wireless sensor node. These propose

design is the early stage as to complete the proposed power

management energy harvesting in the figure 1. The

implementation of the microcontroller and RF module as the

load to the power management energy harvesting circuit vibration based were tested with hardware experimental test.

The result and discussion will be explained in the next chapter

of result and discussion.

6. Result and Discussion

Figure 3 show the simulation result for the propose power

management energy harvesting circuit in the figure 1.

12

10 -:::-- 8 III tI.O 6 �

- � �V(regulator)

... '0 4 :::-

2

0

,

,1 "'---

�V(Co rnparator)

_ V(Capac i tor)

o 27 54 81 108135

Time ( s )

Figure 3. Simulation results for the proposed power management energy harvesting circuit

Simulation results are presented that output voltage from

power management energy harvesting circuit is 3.0V with

output power is 30mW. The efficiency reported as 80%. The

total power losses are 7.SmW.

Figure 4. Piezoelectric generator to harvest energy from vibration.

Figure 5. Hardware experimental setup.

Figure 6. Storage capacitor charge and discharge.

Charge and Discharge

16

14

12

10 �

"0 8 >

__ Volt, 6

4 ........ Transmit

0

0 4 8 10 1 2

Seconds

Figure 7. Signal waveform of the system.

Figure 4 and S shows the hardware setup for the propose of block diagram energy harvesting circuit as mentioned in the

figure 2. Piezoelectric cantiliver was choose as the harvester which connected to the vibration shaker. The open circuit voltage was measure to the piezoelectric and the maximum voltage was measured is 16V AC.

Figure 6 and 7 shows the wavefonn for the experimental

test for the figure S. The supercapacitor will start charge until

maximum DC voltage supply at 14.SV DC and discharge until

SV DC voltage. When storage capacitor reached 9.SV threshold voltage the LED was blinking ON and OFF. The

cycle of ON and OFF is depending to the how long the storage

discharge and charge again. The power input after rectify the

AC voltage is 9mW. Mean while the power output that was measured is around 7mW.

The power losses before the energy flow reached the targeted electronic devices are 2mW. It means that the

efficiency of the system is 77.7%. The authors believe that the power losses can be decrease by using more low power

consumption for the DC-DC step-down converter. The

challenging for the author is also when it will start use the actual RF module load, which is almost require for the stable power supply.

The full bridge rectifier use MIC W06M device with the 47�F storage capacitor. The PIC 16F877A microcontroller are programmed to blink the light emitting diode (LED) once the

microcontroller are turn on. The LED function as an indicator

to demonstrate an autonomous blinking LED to represent

wireless condition monitoring system powered by energy harvested vibrational energy.

Acknoedgement

The authors would like to acknowledge the funding from TNB

(grant project number RJO 10090427) for the project an

energy harvesting power management module for wireless sensor network.

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