Measurement 45 (2012) 1499–1509

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Measurement

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Design and implementation of a low-cost multi-channel temperaturemeasurement system for photovoltaic modules

Rustu Eke ⇑, A. Sertap Kavasoglu, Nese KavasogluMugla University, Clean Energy Research & Development Centre, 48120 Kotekli, Mugla, TurkeyMugla University, Faculty of Sciences, Department of Physics, Photovoltaic Material and Device Laboratory, 48120 Kotekli, Mugla, Turkey

a r t i c l e i n f o

Article history:Received 3 October 2011Received in revised form 7 February 2012Accepted 25 February 2012Available online 8 March 2012

Keywords:NTC12-bit A/DTemperature measurementsSolar cell temperaturePC-controlled switchSensor amplifier

0263-2241/$ - see front matter � 2012 Elsevier Ltdhttp://dx.doi.org/10.1016/j.measurement.2012.02.02

⇑ Corresponding author.E-mail address: erustu@mu.edu.tr (R. Eke).

a b s t r a c t

An efficient and low-cost temperature logging system with a 16-channel input was devel-oped for measurements of photovoltaic module temperature. This paper reports the prin-ciple of operation, design aspects, as well as the experimentation and performance of thesimultaneous temperature measurement of 16 solar cells/modules. The system consistsof a 16 channel multiplexer, a 12 bit A/D, a differential amplifier and NTC temperature sen-sors. The temperature range of the sensor is from �20 �C to 120 �C. The simplistic designrequires no large internal memory to store data but incorporates a high degree of sensitiv-ity and dynamic range (according to climate condition), thus the cost of the design remainslow and makes it suitable for field applications. The system was successfully tested for theoperating temperature of a 40-cell mono crystalline Si photovoltaic module under realisticoutdoor conditions during a summer and a winter day. The temperature Instrumentationdeveloped for avoidance of special interface card use enabled the successful collection ofdata from long distances with negligible level of noise.

� 2012 Elsevier Ltd. All rights reserved.

1. Introduction

Controlling and monitoring of accurate and reliablemeasurement of temperature is particularly essential invarious fields, including the environmental, industrial,food, agricultural, clinical and biotechnology sectors. Inaddition research labs, clean rooms and nuclear reactorsare all environments that are highly affected by tempera-ture levels and require constant monitoring. It is crucialto understand the role of sensors in a circuit and what er-ror they may introduce into the measurement. We will,therefore, point out the advantage and disadvantage ofthe two most common industrial temperature sensors:thermocouples and NTC’s. The sensor choice may play alarge role in how cost effective the system becomes. Everytemperature measurement application has different per-formance requirements and it is important to know exactlyhow your resolution is being affected by noise. The issue of

. All rights reserved.9

the noise and the presence of measurement errors are notinvestigated in details for this work. However the issue ofthe error analysis has been discussed in some publicationand the theories behind these analyses are well explainedin literature [1–7].

Thermocouples are the most widely used type of tem-perature sensor in industry. Extremely rugged, they canbe used from sub-zero temperatures to temperatures over4000 �F. A thermocouple takes advantage of the voltage in-duced between two different metals as they are heated. Atechnology known as cold junction compensation (CJC) isused to remove unwanted junction voltage. CJC essentiallyuses a direct-reading temperature sensor to measure thecold-junction temperature, and then adds the appropriatevalue to the measured voltage to eliminate these ‘parasitic’thermocouple effects but the error introduced by your CJCsensor compounds any error already existent in measure-ment [8]. Especially at long range remote temperaturemeasurement requires very long length thermocoupleextension wire. This disadvantage brings together high costand high noise amplitude during the measurement. When

Nomenclature

T temperature (K)a, b, c coefficients of Steinhart–Hart equationRNTC resistance of the thermistor (X)V1 input voltage (V)R1 resistance (10 kX)Vref reference voltage (2.5 V)R2 resistance (70 kX)Rx resistance of NTC (X)RF feedback resistance (12 kX)Vo output voltage (V)I current flowing through the circuit (A)IL light generated current (A)ID diode current (A)ISH leakage currents (A)IO PV module dark saturation current (A)

RS series resistance (X)RSH shunt resistance (X)a modified ideality factorV voltage across the load (V)NCS the number of series connected cellsn diode ideality factorkB Boltzmann constant (1.38 � 10�22 J K�1)TC PV cell/module temperature (K)q electron charge (1.6 � 10�19 C)Ls short cable length (m)Lm medium cable length (m)Ll long cable length (m)s timing parameter (ms)

1500 R. Eke et al. / Measurement 45 (2012) 1499–1509

measuring the temperature, you will see that this noisevoltage can be significant and should be considered. Insome situation high amplitude noise voltage modulatesthe thermocouple’s dc voltage. This drawback makes it anavoidable sensor at remote temperature measurement.Also the number of components in a circuit should be keptto a minimum, because each component that is added in-creases cost, circuit errors and complexity [9].

NTCs are thermally sensitive resistors used in a varietyof applications, including temperature measurement. NTCson the other hand do not require a CJC sensor since there isno thermo-electric voltage produced at the leads. Accord-ingly, no additional error is introduced. In general, NTC’sare considered ‘precision’ temperature sensors. NTC’s arealso typically more accurate than thermocouples and willmaintain that accuracy over a longer period of time. Thehigh resistivity of the NTC affords it a distinct measure-ment advantage [10]. The four wire resistance measure-ment may not be required [11]. For instance, a commonNTC value is 10 kX at 25 �C. With a typical temperaturecoefficient (TC) of 4%/�C, a measurement lead resistanceof 10 X produces only 0.05 �C error. This error is a factorof 500 times less than thermocouple.

Various instrumentation techniques are being utilizedto measure the temperature with NTC [12–22]. However,the data given in literature do not in general cover a multi-ple place of temperature measurement with appropriatesurface and in fact are lack of temperature distribution dis-playing especially at different photovoltaic modules. Oper-ating temperature strongly affects the efficiency of a solarcell. It thus became necessary to design and to constructalternative automated experimental set-up. The mainadvantages of such an experimental set-up for tempera-ture measurements should be its low cost, high-speedand high resolution yielding a full set of information onthe variation of temperature distribution on the any largesurface or photovoltaic module. One can also monitor all16 thermistors with developed circuitry installed in mea-surement unit. The circuitry are directly connected to PCvia RS-232 and LPT port, which acquires the analogue

signals from all 16 NTCs and stores the digitized readingson PC Hard disc without using external memory. Addi-tional interface board is no necessary. RS-232 and LPT portare readily available with every IBM compatible PC.

This article is reporting about a ceramic NTC sensorinterface circuits, which seemed to be useful for metrologystation and climate tracing system, and a signal processingfor converting the electric resistance of the sensor of whichorder is from 102 to 105 X to electric voltages which ismeasurable with ADC at satisfied accuracy.

2. Calibration procedure of the commercial NTC sensor

NTC thermistor devices are inherently nonlinear, multi-ple vendors supply thermistors that contain more than onedevice, designed for a linear change of resistance with tem-perature. Since these thermistors are precisely calibrated,they can also be replaced by a part of the same type andstill retain their accuracy – in other words, they are inter-changeable. The sensor selected for this application is aNTC with the part number ITC, manufactured by ITC. Ithas a resistance of 10 kX at 25 �C.

Measurements were performed inside an evacuatedclose cycle refrigerator system manufactured by AdvancedResearch System. The temperature of the NTC was mea-sured by a temperature controlled close cycle cryostat sys-tem. Temperature dependent resistance measurements ofNTC were performed in the temperature range 253–393 K. The accuracy of the temperature measurement isbetter than 0.1 K [23].

The NTC was connected to Keithley 6512 Electrometerunit for dc resistance measurement via equipped suitablecoaxial cable. All measurements were automated with apersonal computer. A delay of a few minute between sub-sequent measuring steps was built into the program to as-sure steady-state conditions for the measurement of theresistance at low temperatures. In addition, for each resis-tance value the average of up to thirty-two resistance

Fig. 1. Measured resistance–temperature response curves for 10 K com-mercial NTC.

R. Eke et al. / Measurement 45 (2012) 1499–1509 1501

measurements was taken to increase the signal-to-noiseratio and extend the accuracy of resistance measurement.

Results are presented in Fig. 1. It is seen that observedcurve is in a good agreement (R2 = 0.999) with traditionalSteinhart–Hart equation,

T ¼ 1

aþ bðInRNTCÞ þ cðInRNTCÞ3� 273 ð1Þ

The thermistor curve can be approximated with theSteinhart–Hart equation. Where T is the temperature in�C and RNTC is the resistance of the thermistor in X. Thecoefficients a, b, and c can be extracted by the curve fittingof resistance-versus-temperature data. Detail of the NTCcalibration procedure has been given elsewhere [8,24–26].

Fig. 3. Flowchart of the developed software.

3. Description of the measurement set-up

A schematic diagram of the temperature measurementset-up is given in Fig. 2. The circuitry consists of a 12-bitA/D, 16 channel multiplexer circuit and signal conditioneramplifier. The typical sampling rate of developed A/D cir-cuit is about 10 kHz and it works 0–5 V range with anapproximately 1.25 mV resolution. The circuit is controlled

Fig. 2. The schematic of the experimental

by a Qbasic routine on PC, which allows you to acquire 100data per second and the flowchart of the developed soft-

temperature measurement set-up.

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ware is given in Fig. 3. The arithmetic averages have alsobeen calculated and so that it can be easily logged thetemperature data as a function of time. The NTC is a singlecomponent including a resistive ceramic sensing element.The multiplexer circuit allows communicating betweenthe sensor and the amplifier with suitable runningsoftware.

4. Concept of the instrument

4.1. The sensor and amplifier

The applications of op-amps are converting and condi-tioning signals from transducers into signals that other de-vices, including analogue-to-digital converters (A/D). Asimple method of obtaining a voltage signal from a NTCis to measure its resistance by connecting a dc powersource through a resistor to the NTC [9,10,27,28]. The volt-age developed across the device will be its resistance timesthe current through the device (Ohm’s law). Therefore, thelevel of the voltage output signal will depend directly onthe thermistor resistance and magnitude of the currentor voltage excitation source. A higher level of voltage exci-tation will be used to produce a higher-level output signal.But, this can be very detrimental because the current heatsthe thermistor internally, which appears as an error in thetemperature measurement [9]. If thermistors have a smalldissipation constant, for minimizing the self-heating error,excitation current levels should have to minimized. Thus,there are two ways to minimize the effects of device heat-ing. These are;

1. To use the lowest current possible while still main-taining the desired voltage across the device beingtested. If the current cannot be reduced, use a nar-row current pulse and a fast responding A/D con-verter [9].

2. To use the multiplexer board one-shot trigger modeduring measurements. While in this mode, theinstrument will apply only a single, current pulseto the NTC during the measurement cycle, therebyminimizing errors caused by device heating [9].

To overcome these errors, we have carefully read self-heating specifications (according to dissipation constant)of commercial thermistors and determine the excitationcurrent (27 lA to 0.25 mA). A regulated low voltage(2.5 V) is supplied through a resistor (10 kX). A resistor(R3) pulls the NTC up to a reference voltage. The NTC/resis-tor combination makes a voltage divider, and the varyingNTC resistance results in a varying small voltage at thejunction (I = 2.5 V/(10 kX + RNTC)). The accuracy of this cir-cuit depends on the NTC tolerance, resistor tolerance, andreference accuracy. STMicroelectronics LM336 is chosenfor this circuit. This monolithic IC voltage references oper-ate as a low temperature-coefficient 2.5 V zener with 0.2 Xdynamic impedance. A third terminal on the LM336 allowsthe reference voltage and temperature coefficient to betrimmed easily. The device’s 2.5 V precision is useful as alow voltage reference for op-amp circuitry. The 2.5 V make

it convenient to obtain a stable reference from 5 V logicsupplies. This device is rated for operation over 0–70 �Ctemperature range.

The measurement circuit having an analogue outputusually incorporates a very stable dc bias supplied by aprecision regulated voltage, whose signal is passed througha voltage divider circuit consisting of the sensor and anoutput amplifier. The restrictions, which need to be takeninto account, are the low dc resistance of the sensor at hightemperature (about 102 X) and the dynamic range of sen-sor’s resistance which varies with temperature variationfrom 30 to 120 �C. In principle, the gain must be fixed forwhole range of temperature. This is achieved by differen-tial amplifier. It is suitable to connect the sensor with asmall output current into operational amplifier based onlow offset voltage operational amplifier [28–30]. Suchamplifiers are known to be used successfully in such a cir-cuitry. The OP07 is a low cost, high speed bipolar-input-transistor operational amplifier. The device requires alow supply current and yet maintains a large gain band-width product and a fast slew rate. In addition, the true dif-ferential input, with a wide input voltage range andoutstanding common-mode rejection, provides maximumflexibility and performance in high-noise environmentsand in non inverting applications. Low bias currents andextremely high input impedances are maintained overthe entire temperature range. One of the examples of suchamplifiers is OP07, which is manufactured by Analogue De-vices. The OP07 is unsurpassed of low-noise, high-accuracyamplification of very low-level signals. There are severalmethods, which allow the use of operational amplifierregarding the sensor interface. The use of such circuitryis determined by the nature of the application. Fig. 4 pre-sents the connection of a voltage divider with NTC to anoperational amplifier. Operational amplifier of the OP07is provided with both inverting and non inverting inputterminals, and can readily be used as differential amplifi-ers. Differential amplifier give an output that is propor-tional to the difference between two inputs signals, i.e.,to the value of one input minus the other, and such circuitsare thus capable of carrying out the function of subtraction[28–30].

The circuit shown in Fig. 4 can, if required, be made togive voltage gain by suitably selecting the divider resistorvalues. The resistor can be given any values on conditionthat the ratio of RF to R2 is the same as that of R3, in whichcase the output voltage is equal to

Vo ¼ V1 1þ RF

R1

� �þ ðV1 � Vref Þ

RF

R2; V1 ¼

Vref

R3 þ Rx

� �Rx ð2Þ

The ambient temperature dependent dc resistance ofthe ceramic sensor, Rx in Fig. 4 can be given in terms of Vref,V1 and R3 as:

Rx ¼ R3V1

Vref � V1

� �ð3Þ

What follows from this fact that the dependence of theoutput voltage on V1 is measurable in 0–5 V voltage rangeand so the circuit proposed can be easily used for detectingthe variations on the ceramic sensor’s resistance? In order

Fig. 4. Linearization circuitry.

R. Eke et al. / Measurement 45 (2012) 1499–1509 1503

to get more accurate data operational amplifier gain as afunction of input voltage need to be examined. Hence,12-bit D/A is used to send input voltage to amplifier. Thetheoretical value of the output voltage of the amplifier isgiven in Eq. (4). In general, the characteristic was foundto be in good agreement with theoretically obtainedvalues.

Vo ¼ V1 1þ RF

R1þ RF

R2

� �� Vref

RF

R2ð4Þ

Putting the all the values of the components into Eq. (4)we find;

Vo � ½2:37�V1 � 0:43 ð5Þ

and the experimental analysis in Fig. 5 indicates that theamplifier gain may be given as;

Vo � ½2:36�V1 � 0:42 ð6Þ

Now the dc resistance of the sensor as a function oftemperature can be calculated using output voltage values.The inverse function of amplifier gain may then be ob-tained as;

V1 � ½0:42�Vo þ 0:18 ð7Þ

Fig. 5. Experimental amplifier gain.

Using Eqs. (3) and (7), we can obtain the sensor resistanceas a function of output voltage of the differential amplifieras;

Rx ¼ 10 kXVo þ 0:435:52� Vo

� �ð8Þ

Using the above equation we can easily convert voltagevalues into sensor resistance by using suitable software.This allows us to measure sensor resistance as a functionof temperature.

4.2. Serial port controlled 12-bit A/D converter

The circuit shown in Fig. 6 performs to acquire voltageoutput from the bridge amplifier and logs to disc: that ofdriving a 12-bit A/D converter from the RS-232 port of aPC. A RS-232 port was used as an interface rather thanthe transmitter/receiver lines of a UART. Whilst the port’sof request-to-send (RTS) line provides a chip-select signal,the data-terminal-ready (DTR) line yields a synchronous-clock signal. A single-supply RS-232 interface chip(HIN232) converts these signals from RS-232 levels toCMOS-logic levels. The conversion data appears on thedataset ready (CTS) line. MAX189 is an 8-pin DIP which in-cludes a 12-bit ADC, voltage reference, track/hold, serialinterface, clock generator, 3-wire digital interface consist-ing of chip select (CSbar), serial clock (SCLK) and data out(DOUT). Conversions are started by a high-to-low transi-tion on CS-bar with an elapsed time less than 8.5 ms. Theend of conversion, indicated by a high level on DOUT,leaves the 12-bit result stored in the converter’s outputshift register. The PC reads this result by clocking DTRwhile sampling CTS 12 times.

4.3. NTC multiplexer circuit

The multiplexer circuit, when configured for thermis-tors, operates as a 16-channel scanner. In this mode, thecurrent is multiplexed sequentially to each thermistor,synchronized with the multiplexing of the thermistor

Fig. 6. Serial port controlled 12-bit A/D converter.

Fig. 7. LPT port controlled multiplexer board.

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input voltage. Therefore, at any one time, the currentsource is applied only to the thermistor being measured.Otherwise, the single current source does not have thepower to drive 16 thermistors at the same time. This cir-cuit designed as external multiplexer board, to expandthe number of input channels that you can connect to anop-amp board. Multiplexing board which is described inFig. 7 sequentially switches multiple channels to a single-input channel of the op-amp board. The NTC multiplexer

circuit was controlled by the PC parallel port outputthrough its interface. The parallel port can be programmedto control 4-bit data information. This control bit informa-tion is protected with a 74LS244 chip before driving the74HC/HCT4067 (16-channel analogue multiplexer/demul-tiplexer) multiplexer circuit. IC is a 16-channel analoguemultiplexer which runs as analogue switches. The BCDoutput controls the address input lines of IC2, and selectsswitches one out of 16 channels by turning on the appro-

Fig. 8. Complete circuit diagram of the developed electronic units with the connections of the in Figs. 2 and 4.

R. Eke et al. / Measurement 45 (2012) 1499–1509 1505

priate analogue switch. The active high outputs D0, D1, D2and D3 of the LPT are used for controlling the 16-channelanalogue multiplexer/demultiplexer in the 74HC/HCT4067 IC. To avoid high voltage on switch resistancewe have applied maximum limiting supply voltage value(15 V) to the 74HC/HCT4067 IC. One hundred and eightyX switch on contact resistance has been measured be-tween the multiplexer contact point. This value has beenverified by the 74HC/HCT4067 data sheet. To overcomethis parasitic resistance we have subtracted 180 X frommeasured resistance value simultaneously by writtenappropriate command line in Qbasic routine.

5. Experiments and measurements

A prototype of the developed circuitry was built up andtested. The complete diagram of the setup is shown in

Fig. 8. The setup consists of the amplifier, analogue to dig-ital converter, sensor multiplexer and IBM-compatible PCas shown in Fig. 2. The amplifier was implemented as pre-sented in Fig. 4 with one OP07 (Analog Devices, USA). Theanalogue to digital converter MAX189 (Dallas Semiconduc-tor, USA) with 12 bit resolution and a PC were used. Thedetail information about the ADC speed and accuracy canbe found chip from Manufacture Company [31].

In Fig. 9 the built up prototype is shown when the mea-surement is in progress.

The proposed interface circuit for resistive sensors uti-lizing digital multiplexer has several advantages over theconfigurations presented earlier. One such advantage isflexibility of sensor types and resistances. The size of thesystem can be reduced greatly to be comparable to thefully surface mounted type of IC. Furthermore, the systemis adaptable to utilize modern algorithms to speed up the

Fig. 9. Prototype of the developed electronic units.

Fig. 10. The temperature response characteristics of the NTC sensorscorresponding to (a) 10 m cable length short point, (b) 32 m cable lengthmedium point, and (c) 64 m cable length very long point.

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operation. The system could be improved by the additionof a different sensor type. The software was written forthe PC using Qbasic. The measure feature was imple-mented using a simple algorithm.

6. Results and discussion

In order to determine the stability of the system, threedifferent acquisition methods have been implemented inthe system. The choice between the three methodsstrongly depends on the remote temperature to be sensed.The experiments were carried out on selected days during17th to 30th of July. Figs. 7 and 8 show that 16 NTC con-nected to developed circuitry were used to record temper-ature in the room at the 16 different points. Sixteenth NTCwas only connected for measuring noise contribution ofthe shielded cable.

The first implementation consists of shielded cablelength depended temperature acquisition at fluctuatednormally around room temperature (e.g. 33 �C), is basedon three consecutive readings, actuated at short cablelength (Ls = 10 cm), medium cable length (Lm = 32 m) andlong cable length (Ll = 64 m), respectively. The sensor resis-tance is acquired and averaged during the measurement.Time has been discretised in 20 ms intervals and timingparameters (s1, s2, s3. . .. and s16) are programmable. UsingEq. (7) we can convert the signal output voltage to sensorresistance for each time values using the written routinein the software. The calculated resistance data was thenconverted to the temperature value by using the Stein-hart–Hart equation. The obtained temperature data wasthen plotted as a function of time for different cablelengths in Fig. 10a–c. NTC sensor showed a minor fluctua-tion according to the initial measurement (10 cm cablelength, Fig. 10a). It was observed that all the characteristicsare very similar to each other. Each sensor’s temperaturewas found to be fluctuating about 0.1 �C when the cablelength was carried to 32 m (Fig. 10b). When the cablelength was carried to 64 m (Fig. 10c) the measured tem-perature difference between the sensors are not exceeding0.15 �C. The response of the NTC sensors was found to befluctuating about 0.15 �C whilst 16-sensor displayed somedegree of offset fluctuations. It can be clearly seen that thiscable length has not worthy noise also measurement erroroccurs in a negligible level.

Before going through the long term temperature mea-surement, the temperature gradient of the sensor at six-teen different points (NTC, from ITC) has to be checked.For this purpose we have drawn soldering iron near thecentre of NTC array in atmosphere (in Fig. 11a and b). After10 s we put them to normal atmosphere conditions andthen have started to acquire the data to the computer.Fig. 11a and b shows the temperature response character-istics of the sensors corresponding to the case of tempera-ture gradient process. It appears from the figure that theNTC sensor has high sensitivity and good selectivity inthe given range of temperature. The response of the 8th

Fig. 11. Soldering iron proximity depended temperature acquisition: (a)as a function of time, and (b) as a function of sensor order.

Fig. 12. Layout of the sensors at the back side of the test module.

Fig. 13. Equivalent circuit of a solar cell or a PV module.

R. Eke et al. / Measurement 45 (2012) 1499–1509 1507

and 9th NTC sensor was found to measure higher temper-ature whilst other NTC sensors displayed a few degree oftemperature gradient. It can be clearly seen that this typeof NTC sensors are cheap and a good choice for display intemperature gradient at any surface.

The system has been debugged and eventually tested inaggressive climate condition for monitoring the solar cell/module temperature. Fig. 12 shows the layout of 16 NTCsfor temperature measurement at the back side of themodule. All data were recorded for 1 min interval duringthe test period.

6.1. Photovoltaic model and methodology

An assessment of the operation of solar cells and the de-sign of power systems based on solar cells must be basedon the electrical characteristics, that is, the voltage–currentrelationships of the cells under various levels of irradiationand at various cell temperatures. Many cell models have beendeveloped, ranging from simple idealized models to detailedmodels that reflect the details of the physical processesoccurring in the cells. Fig. 13 is an equivalent circuit thatcan be used for an individual cell, a module consisting ofseveral cells, or an array consisting of several modules [24].

At a fixed temperature and solar radiation, the follow-ing equation may be written for the current flowingthrough the circuit at any chosen point according to cur-rent rule of Kirschhoff. Wherein, IL represents the light gen-erated current by in the solar cell or a PV module, ID

represents diode current and ISH represents leakagecurrents.

I ¼ IL � ID � ISH ð9Þ

when ISH and ID is inserted in Eq. (8) the current–voltagerelationship at a fixed cell/module temperature and irradi-ation for the circuit will be expressed as:

I ¼ IL � IO expðV þ IRSÞ

a

� �� 1

� �� V þ IRS

RSHð10Þ

where V is the voltage across the load. Dark saturation cur-rent (IO) depends on band gap and temperature of thematerial used in manufacturing the solar cell [25].

Single diode (single exponential) model is considered asthe equivalent PV model. Five parameters must be knownin order to determine the current and voltage, and thus thepower delivered to the load. These parameters are IL, thelight generated current or photovoltaic current, IO, darksaturation current, RS, series resistance, RSH or RP, shuntor parallel resistance and a, is a coefficient called as modi-fied ideality factor. Herein, the modified ideality factor iscalculated as a = NCSnkBTC/q; herein, NCS represents numberof the solar cells or PV modules, n represents diode idealityor quality factor, kB represents Boltzmann constant, q

Fig. 14. Current–voltage characteristics of the tested PV module withthree different irradiation levels in 15th of January.

Fig. 15. Time–temperature relationship of the test PV module in 21st ofJuly: (a) according to the sensors, and (b) 2 dimensional temperaturedistribution.

1508 R. Eke et al. / Measurement 45 (2012) 1499–1509

represents electron charge and TC represents temperatureof the solar cell or PV module in Kelvin [32].

The determination of solar cell/module model parame-ters from experimental data is important in design andevaluation of solar cells/modules [33,34]. There are severalmethods for extracting solar cell/module parameters of thesingle-diode lumped model. It is impossible to find ananalytical solution for the parameters in Eq. (9). Someassumptions are needed to solve it analytically or it canbe solved iteratively which is called as 5-point method.

The five important parameters for a solar cell and PVmodule can be calculated by using a single current–voltagecurve. A value is assigned for series resistance (RS) as initialvalue. And the other parameters: parallel resistance value(RSH), modified ideality factor (a), current generated bylight (IL), RS and the dark saturation current (IO) are calcu-lated using the experimental current–voltage characteris-tics [35].

A PV module with 40 series connected solar cells is usedfor the temperature test of the developed temperaturemeasurement system. Thus, all required parameters aredetermined for the tested PV module. Experimental cur-rent–voltage characteristics of the module is collected in15th of January in a clear sky winter day with 50 �C maxi-mum operating temperature (Fig. 14).

Current–voltage characteristics are taken at three dif-ferent irradiation levels 597 W/m2, 803 W/m2 and 985 W/m2 respectively. Irradiation measurements are taken withKipp&Zonnen CM11 type pyranometer on the array plane.The measured outside temperature is about 15 �C wherethe operating temperature of the photovoltaic module isabout 50 �C at 12.26. The calculated photovoltaic moduleparameters from the current–voltage curves shown inFig. 13 are given in Table 1. In summer season the operat-ing temperature of the module increases up to 80 �C withthe increasing ambient temperature as shown in Fig. 15.

Table 1Performance parameters of a 40-cell photovoltaic module.

G (W/m2) RS (X) RP (X) n IO (lA) IL (A

596.68 0.24 1059.08 1.68 2.21 1.79802.96 0.34 892.86 1.62 3.61 2.30985.30 0.36 764.51 1.55 5.49 2.60

Tested module is oriented south with an inclination of15�. Fig. 15a shows measured temperature during 21st ofJuly.

It shows that measurement of the temperature duringdifferent periods of the time provides different values ofsurface temperature gradient of the module. In the morn-ing, the module temperature increases rapidly withincreasing solar radiation until reaching the maximum val-ues at noon. There are fluctuations in the temperature ofthe module during noon time. After 14.00 the temperatureof the module decreases with the decreasing solar radia-tion and due to some shading effects. According to avail-able sensor alignment coordinate we have obtainedtemperature distribution.

Fig. 15b shows the backside temperature distributionmap of the module. It is obviously seen that module’s oper-ating temperature have maximum values at 11.30 and13.30 respectively. And also around 12.30 module’s operat-ing temperature decreases about 15 �C because of theclouds appeared on the sky.

) Isc (A) Voc (V) Pmax (W) FF (%) g (%)

1.79 23.52 30.50 72.31 12.052.29 22.95 37.40 71.02 10.972.60 22.53 41.13 70.18 9.84

R. Eke et al. / Measurement 45 (2012) 1499–1509 1509

7. Conclusion

The design and the implementation of a computer-con-trolled low cost temperature measurement setup have suc-cessfully been demonstrated and discussed. The advantageof such a low cost experimental setup for temperaturemeasurements is that it performs very well in a wide rangeof sensor’s dc resistance varying with radiant flux densityof the sun and also the ease of converting of such signalsinto temperature values between �20 and 120 �C. Usingsuch a proposed instrumentation the values of dc resis-tance of a NTC sensor as a function of temperature weresuccessfully obtained via 16-channel an automated multi-plexer. The introduction of a calibration procedure allowedus to reduce the errors due to the device tolerances. Fur-thermore, by exploiting the built-in RS-232 and LPT inter-face it was possible to control the system and to visualizethe sensor resistance using a simple PC. The data obtainedwere found to be reproducible and reliable. The RS-232 andLPT ports are readily available at old fashion computer. Soit is worth that this circuitry not requires any interfacecard or external memory to store data. This reported sizevalue quite smaller than compare to whole size of the harddisc (2 Gb). And also this type of design is low cost due tominimal component configuration.

The system was used to monitor the operating temper-ature distribution of a 40-cell mono crystalline silicon pho-tovoltaic module under realistic outdoor conditions duringa summer and a winter day. It is seen that photovoltaicmodule operating temperature reaches up to 80 �C closeto noon hours in summer and the measured temperatureon the backside of the module is not homogenous. Ten de-grees celsius difference was measured between the 16points behind the tested module.

Acknowledgement

Some part of this work was presented as a poster inTFD24 (Turkish Physical Society Congress in Malatya-Turkey).

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