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EEE PED 2011 gae 5 - 8 Decebe 2011

A Low-cost Photovoltaic Energy Harvesting Circuit

for Porable Devices

Ian Y.W. Chung and Yung C. Liang

Department of Electrical and Computer Engineering, National University of SingaporeKent Ridge, Singapore 96

AbtactThis pper presens n eien solr energy

hrvesing irui for moile phone ery hrgers whih n

lso e esily dped for oher moile devies. The energy

hrvesing irui is ple of mking Mximum Power Poin

Trking (MPPT) nd lso hs uil-in ery proeion

funion. polyryslline solr pnel whih supplies n verge

of 400mW (under sun solr insolion) of power ws seleedfor use in he proposed irui. MPPT in he irui is hieved y

using he onsn volge rking priniple whih is non

omplex ye highly eien MPPT onrol mehod. I isimplemened wih purely disree nlog omponens wih ulr

low power onsumpion. The proposed design is exremely

omp nd is fesile for ommeril ppliions. The solr

energy hrvesing irui onsumes less hn 330JW of power nd

hieves n overll eieny of pproximely 80-90%.

n nlog Mximum Power Poin TrkingCirui, Moile Phone Chrger, Phoovoli Energy Hrvesing

Cirui, Ulr-low Power MPPT Cirui

I. NTRODUCTION

Today"s consumers expect to enjoy a wide range of services

on their mobile devices, be it a laptop, smart phone or tablet computer. From traditional voice and data services to

faster and more interactive multimedia services, the

unprecedented array of features found on present day mobile

devices has led to an increased reliance on them. This trend

has attracted new efforts to increase and prolong the batter

life of these devices between charges as ultimately, a mobile

device is only as portable as its power source. In most present

applications, the chaging energy is drawn om conventional

AC adapters with power plugs and this limits the Jobilit" of

mobile devices. Consequently, there is an emerging need to

develop autonomous energy sources to supplement these

batteries that have a limited life.The application of photovoltaics (PV) power generation as

mobile phone batter chargers presents a viable solution to the

above problem. Photovoltaic refers to the tecnology used to

achieve the direct conversion of sunlight into electricity via a

solar cell [1]. Currently, photovoltaic power generation is

extensively used to power remote-sensing applications such as

wireless sensor nodes that gather sensor information on

physical or enviromental conditions such as temperature,

sound or vibration and comunicate them back trough a

network to a central location [2].

However, as shown in Figure 1, most existing applications

employ two separate DC/DC converters; the rst converter is

used to implement maximum power point tracking to improve

overall eciency whereas a second converter perfos output

voltage regulation [2]. Additionally, they also require auxiliar

sensors to extract the PV panel"s instantaneous voltage or

current readings om a secondar Pilot PV panel to enable

maximum power point tracking [3]. This paper aims to present

a greatly simplied method that employs only one DC/DC

converter to perfo both MPPT tracking and voltage regulation. Furhermore, no additional sensors are used to

realize the Constant Voltage MPPT Principle. A detailed study

has been conducted to validate the proposed method. A

prototpe of the sola energy harvester circuit has been

developed to facilitate experimental testing of the proposed

design and the results and ndings ae presented in Section III.

D

I:I snfIln Q

"", ·": , ihIi

ru

Figure 1. Conventional two-stage DC/DC converterMPPT circuit [3].

\'

II. ESCRIPTION OF ENERGY HAVESTIG CIRCUIT WITH

PHOTOVOLTAlC CELLS

Descrtion of Ener Harvesting Circuit

The schematic of proposed energy harvesting cirt

shown in gure 2. The system comprises of a solar panel

conected to 4 main modules:

(i) A DC/DC Boost Converter

(ii) A MPPT Controller Circuit

(iii) A Batter Protection Circuit

(iv) A Charge Pump Circuit

978-1-4577-0001-9/11/$2600 ©2011 EEE 334


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Figure 2. Block diagram of the proposed energyharvesting circuit.

The DC/DC boost converter, which is controlled by the

MPPT controller, performs maximum power point tracking as

well as voltage regulation. A boost converter is chosen for this

task as the solar panel maximum power point (MPP) occurs at

approximately 1.65V while the batter charging is enabled at

5V and thus output voltage boosting is required. In addition, a

boost converter is preferred over a buck converter as theformer does not require an isolated gate driver circuit which

will contribute to additional power losses. The Batter

Protection module serves to monitor the mobile phone batter

voltage and disconects the solar panel input om the

charging circuit once the batter is lly charged. A Charge

Pump is also employed in the circuit to provide the circuit

components with the initial startup power supply. It is

subsequently disabled when the DC/DC converter output

voltage rises above the required value.

In the following sections the circuit design, component

descriptions and principle of operation of each module

mentioned will be discussed in detail.B. Characteristics of a Solar Panel

R

Figure 3. Solar panel equivalent circuit [4].

A solar panel represents the most ndamental powerconversion unit of a photo voltaic power generation system [4].

It is a device that directly converts the energy of sunlight into

electricit by the photovoltaic effect. As solar panel

characteristics are important considerations in the design and

development of the energy harvesting circuit, they will be

briey discussed in this section.

A solar panel is a non-linear device and can be modeled as a

current source as shown in Figure 3. Preliminar experiments

carried out to nd the I-V characteristics of a tpical

polycrstalline solar panel yielded the following results shown

in Figure 4 below. It is observed om the results that the

output of the solar panel is non-linear and is greatly inuenced

by the solar insolation level; the MPP of the solar panel varies

as ambient conditions change [5].

r rrssa

0

�

a2

5

a2

======

=

:�

� -

+

W

l

-----------

.

._

a0

�

ao

a a 1- 1 5 25o

t e

{

Figure 4. I-V characteristics of a typical solar panel underdifferent solar insolation levels.

Selection of Solar PanelIn this section, the performance of comonly available solar

panels are compared and evaluated for their suitabilit for

implementation.

TABLE IOARISON OF DIFFERENT TYES OF SOLAR ANELS AT

Type of Solar Amorphous Polycrystalline

Panel

Panel Size53.0m x 76.0m x

44.7m 45.0m

Maximum Power139 mW 403.2 mW

OutputMaximum

Power/Effective 58.6 W/m 117.9 W/m Area

Noting the inuence of solar insolation levels on solar panel

performance, the maximum output power of an amorphous and

a polycrstalline solar panel are measured under the same solar

insolation level. As seen om Table 1, for a sola panel of

approximately similar size, the polycrstalline solar panel

produces more than 2 times the power produced by the

amorphous solar panel and has a maximum power per effective

area that is 100% more than the amorphous variant. A similar

conclusion on the polycrstalline solar panel"s superior performance was also drawn in an extensive study carried out

in [6]. The results reected are largely expected as amorphous

solar panels in production today have an eciency of

approximately 5-7% while poly crstalline solar panels enjoy

efciencies ranging om 13-15%. Another advantage of

polycrstalline solar panels is their longer lifetimes as

compared to the aohous varieties.

Therefore the polycrstalline solar panel is favored in this

experiment as a result of its signicantly higher maximum

power per effective area and longer lifetime.

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D. Discussion and selection ofMP PT methodPerformance of any PV based system is largely dependent

on the system"s abilit to identi and operate at the solar

panel"s MPP. As such, many MPPT techniques have been

developed and implemented over the years. They range in

terms of complexit, cost, hardware and effectiveness [7].

However these various MPPT methods all seek to achieve a

comon objective, which is to ascertain the solar panel"s V

MPPor MP and to maintain the circuit operation at that given MPP under varing ambient conditions. This section will aim to

briey discuss 3 popular MPPT methods and describe the

MPPT method selection process for this paper

Hill Climbing/Perturbation and Observation P&OThese 2 methods are essentially the same ndamental

method but are implemented differently. The Hill Climbing

MPPT methods as seen in literatures [8-11] involve a

perturbation in the dut ratio of the power converter while the

P&O method requires a perturbation in the operating voltage

of the photovoltaic cell.

The Hill Climbing and P&O MPPT methods are mostsuitable for implementation in applications where perfoance

and reliabilit are key considerations as optimization is only

achieved when implemented with a Digital Signal Processor

(DSP) or microcomputer controller which increases the cost of

the system.

I ncremental ConductanceThe Incremental Conductance method is based on the

variation in the slope of a solar cell"s power curve at different

points [ 12- 14]. It rther relies on the fact that the slope of the

solar cell power curve is zero at the maximum power point,

positive on the le of the MPP and negative on the right of the MPP. The MPP can then be tracked by comparing the

instantaneous conductance (I) to the incremental

conductance (�I!�V).

A major disadvantage of this method is the high cost

involved due to the need for microcomputers or DSPs and the

need for two sensor probes to constantly measure the

instantaneous voltage and current of the solar panel.

Fractional The Fractional Voc method is a result of the near linear

relationship between VMPP and Voc under varing solarinsolation and temperature levels [15-17]. The relationship can

be represented by the following equation:

(1)

The constant of proportionalit k is dependent on the solar panel being used and has to be determined empirically. With

k known, MPP can be computed using equation (1). If thesolar panel is used with an application that is exposed to

varing ambient conditions and precise tracking of the MPP is

desired, Voc must be measured periodically.

The actional Voc method is most suited for applications which require a simple and cost effective MPPT method as

this method does not require the use of DSP or microcomputer

control.

Based on the review of the various MPPT methods above,

the Hill Climbing/P&O method and the Incremental

Conductance method are deemed to be unsuitable due to their

complexit, high cost and high power consumption that are a

result of the need for a micro-controller interface. The MPPT method selected for implementation in this paper is thus a

variation of the Fractional Voc method - the Constant VoltageTracking Principle.

la ll haacttc45

f ------- i f

r

I -

3�

f

�

02

6V age (

�W/

W/lWj

Figure 5. Solar panel output power curves under differentsolar intensity levels.

Under constant solar intensit, the output voltage of the

solar panel can be varied by changing the load resistance

terminated at the solar panel. The output power of the solar

panel under any solar intensit is thus a nction of the solar panel output voltage as shown in Figure 5. From the gure, it

can also be observed that the MPP increases with increasing

solar intensit; this trend is highlighted by the black diagonal

line. Therefore, it is reasonable to conclude that a

microcontroller would be required to accurately track the MPP

during circuit operation of the solar power mobile phone

charger. However, the ndings above also highlight that the

variation of the MPP under different insolation levels is

minimal and any power loss due to MPP mismatch is small.

Consider the following:

If constant voltage tracking IS implemented with the MPPvoltage set at 1.65V;

• At 1000 W/m - Power loss of less than 3.8mW(or loss of 1.0% of maximum power)

• At 800 W/m - Maximum power is obtained• At 500 W/m - Power loss of less than 2. 1mW

(or loss of 1.2% of maximum power)

As it is not ecient and cost effective to employ a digitalcontroller to constantly track the insolation level and adjust the

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MPP voltage accordingly (which the Fractional V method

requires), the Constant Voltage Tracking Principle which is

considered to be one of the simplest control methods, is asuitable alteative as it assumes that any variations in the

insolation levels are insignicant to the maximum power

drawn om the solar panel. Furthermore, as precision is notcritical in the operation of the mobile phone solar energy

harvester, the constant reference voltage is an adequate

approximation of the solar panel"s true MPP [18].E. MPPT Controller DesignThe MPPT controller is designed to implement the constant

voltage MPPT method discussed above. Under varying solar

insolation levels, the MPPT controller circuit will ensure that

the solar panel operates close to 1.65V so that the efciency of

the charging circuit can be maximized. The controller circuit is

implemented together with a DC/DC Boost Converter that

basically performs as an input voltage regulator to constantly

maintain the solar panel"s operating voltage at 1.65V.

�_M2 C2

+O=BATT

Figure DC/DC Boost Converter as an Input RegulatorThe solar panel"s operating voltage or V is regulated by

controlling the dut cycle (D) of the boost converter. From

Faraday"s Law, for a DCDC boost converter:

I-D

r( D) = (2)

The nction (D) is different for different tpes of DC/DCconverter but as seen above, for a boost converter, it is given

by _. Eqn. (2) can then be re-written as:

(3)

where v _, and the dut cycle D is calculated usingC j{

Eqn. (4) as shown below:

D

r

v

�

J

(4)

Therefore a non-linear feedback linearization block

'(�) is included in the control loop so that the resultingcontrol loop becomes linear and a conventional Proportional

Integral (PI) controller can be used to get the desired performance as shown in Fig 7.

,----------------

-

OAm 1

Figure Implementation Block Diagram

The above control system can be divided into tree sections

and is realized by tree different Operational Ampliers (OpAmps). The PI controller is implemented by the Op-Amp 1;

the proportional gain K is determined by the ratio of andp while the integral gain K; is given by the capacitor C• The

reference signal VR is provided by the built-in voltage reference in the MAX 921 which serves primarily toimplement the circuit"s Batter Protection nction. A variable

resistor, ' is conected between the solar panel and the inputof Op-Amp 1, it allows for the precise setting of the desired

solar panel operating voltage or VMP The operation performed by Op-Amp 1 is expressed in Eqn. (5).

(5)

(6)

Subsequently, Op-Amp 2 in the form of a differential

amplier serves as the control loop"s feedback linearization

block. And its operation is represented by Eqn. (6) shownabove. The task of Pulse Width Modulation (PWM)

generation is le to Op-Amp 3. The dut cycle of the PWM is

proportional to the output of Op-Amp 2, V, and the equencyis controlled by and C•

Figure 8. MPPT Control Circuit Schematic Diagram

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The timing diagram of the PM generation is shown inFigure 9.

B1'-

.

V

B-yz

ie

G2

ie

Figure PWM Generation by Op-Amp 2 & 3

The power consumption of the proposed Constant Voltage

MPPT control method is minimized by the use of ultra-low

power Op-Amps, MAX 4289 and NCS 2220A. This ensures that majorit of the energy harvested by the PV panels are

chaneled to achieve the primar task of the device which is tocharge the mobile phone batter. Although the Constant

Voltage tracking method does not allow for precise tracking of

the MPP, the accuracy achieved by the control circuit issufcient to successlly accomplish the controller"s intended

objectives.

Battery Protection Controller DesignThe Batter Protection controller circuit is realized using

the ultra-low power compaator MAX 921. This module

continuously senses the conected batter voltage and

activates MOSFET M when the batter has been llycharged. This disconects the solar panel om the charging

circuit and protects the rechargeable batter om the adverse

effects of overcharging. These effects include depleted batter

capacit and excessively high temperatures which may in t

lead to damages to the batter or electronics in the charging

circuit. While the solar panel is disconected om the

charging circuit, the 921 draws its power om theconected rechargeable batter to enable it to keep MOSFET

M in the ON state.

Charge Pump CircuitA S882Z ultra-low voltage operation charge pump is utilized

to provide the initial power requirements to the energy harvesting circuit"s components. This enables the circuit to

be used for charging of devices where power canot bedrawn om the batteries. The charge pump achieves this by

storing stepped up electric power drawn om the solar panelinto a startup capacitor before discharging it as startup power

to the DC/DC converter when the discharge start voltage

level is reached. Furhermore, a built-in shutdown nctioncan be activated once the voltage of the conected DC/DC

converter rises above the required value, thereby allowing

for signicant power savings.

III. XPERIMENTA ESULTS FIIGSAn experimental prototpe was developed to assess the

performance of the solar energy harvesting circuit under

various conditions. The results of the tests are presented in the

following section.

Solar Simulator testing of Ener Harvesting Circuitand MP PT circuit

The experimental waveforms obtained are shown in Figures10 and 11. The waveforms in both gures are of the PV panel

operating voltage (pv PV panel current (fpv EnergyHarvestng Circuit output voltage ( and output current (f.

TABLE IIFFICIENCY OF NERGY HARESTING DEICE UNDER

ARIOUS SOLAR INTENSITIESInput Output Eciency

Solar Power Power

n=l JIntensity n = p xI p P =V x11.65V x 3.818V x1000W/m 24m= 85m= 82.0%396mW 324.7mW1.65V x 3.78V x

500W/m 48m= 58.1m= 89.9%244.2mW 219.6mW

The experimental results in Figure 10 are measured under a

solar intensit of 1000W/m• It must be noted om the gure

that the solar panel"s constant voltage maximum power point

of 1.65V is successlly tracked by the prototpe. The energy

harvestng device operates close to an eciency of

approximately 82.0% at this insolation level as highlighted by

calculations in Table II.

( /dv)0

l �

'

,

o( /dv _ t . • I :. t + 1 I


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operating voltage maintains at the desired voltage level of

.65V and as shown by calculations in Table II, the solar panel

operates at an eciency of almost 89.9%. At lower levels of

solar insolation, the corresponding batter charging cuent is

also reduced and this results in a lower forward voltage drop

across the Schottk diode (D) in the DCDC boost converter

and hence lower power losses. This phenomenon accounts for

higher efciencies achieved at lower solar insolation levels.

Z V/di) Vp -

(100 mA/d)

f ' .�q r?. v!i) ,Vo - t • . I

I I

- 5S1mA(1 mAi)

Figure 11. Waveforms of PV panel voltage p & currentIp, Output voltage o and current () under a solar

intensity of 500 W/m (at 20s/div)

IV. ONCLSION

In this paper, a low-cost yet highly ecient analog MPPT

converter circuit was implemented to realize the Constant

Voltage MPPT Principle. The developed prototpe has

demonstrated its abilit to constantly operate at the solar

panel"s MPP to allow maximum power to be harvested

regardless of solar insolation levels. By limiting component

selection to only analog components with ultra-low power

consumption, the proposed energy harvester is capable of

operating at eciencies between 80-90% and draws less than

330 W of power. An additional Batter Protection feature has

also been incoorated into the energy harvesting circuit to

enable it to directly charge comercial rechargeable batteries

which do not have built-in protection features.

COWLEDGMENT

The authors would like to thank Ko Ko Win for his technical

assistance and discussion in this project.

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