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A maximum power point tracker for long term lagging of pv module performance
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A maximum power point tracker for long term lagging of pv module performance

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  • 1. IEEE JOURNAL OF PHOTOVOLTAICS, VOL. 2, NO. 1, JANUARY 2012 47A Maximum Power Point Tracker for Long-TermLogging of PV Module PerformanceUwe Zimmermann, Member, IEEE, and Marika EdoffAbstract—We present a monitoring system for the field test ofphotovoltaic modules. The system is designed for the monitoringof individual modules under maximum power point (MPP) con-ditions that allow the extraction of the energy yield of differentmodules under optimum conditions. It can also be used to monitorthe performance and long-term stability of modules under realisticfield conditions. The monitoring system consists of an individualMPP tracker attached to each module under test. The measure-ment data are transmitted to a central multichannel data logger bymeans of analog voltages proportional to the current at MPP, i.e.,Im p , and voltage at MPP, i.e., Vm p , of the modules.Index Terms—Embedded software, maximum-power-pointtracking, performance evaluation, photovoltaic cells, power MOS-FET.I. INTRODUCTIONMONITORING the performance of photovoltaic (PV)modules under field conditions is important for boththe testing of individual modules and the comparison of en-ergy yield for different types of solar cell modules at certaingeographic locations [1].Some past studies perform such testing using small-scale PVinstallations connected to commercial balance of system com-ponents, like grid-connected inverters or electric loads, whileothers use dedicated test equipment [2]–[4]. Under real-worldconditions, the electrical parameters of the maximum powerpoint (MPP) change constantly with changing irradiance, causedby the changing angle of incidence of the solar radiation, by at-mospheric perturbations and clouds and by local shading. Ded-icated test equipment should, therefore, consist of a maximumpower point tracker (MPPT) and an electronic load that dissi-pates the electric power generated by the PV modules [5], [6].In recent years, research has been done on improving the ca-pabilities of MPPT algorithms, partly driven by the availabilityof more powerful control circuitry (see, e.g., [7]–[13]). How-ever, even a quite simple hill-climbing or perturb-and-observealgorithm can give sufficient accuracy and response speed toassess the power output of PV modules to within a few percentof error margin over a wide range of irradiances [14].Manuscript received September 6, 2011; revised October 18, 2011; acceptedOctober 23, 2011. Date of publication December 8, 2011; date of current versionJanuary 30, 2012. This work was supported by the Swedish Electrical UtilitiesR&D Company—ELFORSK AB—with contributions from the Swedish EnergyAgency within the SolEl-programme (http://www.solelprogrammet.se/) underProject 40015.The authors are with the Division of Solid State Electronics, Departmentof Engineering Sciences, Uppsala University, 75121 Uppsala, Sweden (e-mail:uwe.zimmermann@angstrom.uu.se; marika.edoff@angstrom.uu.se).Color versions of one or more of the figures in this paper are available onlineat http://ieeexplore.ieee.org.Digital Object Identifier 10.1109/JPHOTOV.2011.2174031Fig. 1. Overview over the PV module monitoring system. The shaded arearepresents the MPPT unit, which is attached to the PV module under test. Thedata are captured with a central multichannel logger.The goal of the work presented in this paper is to develop amodular MPPT unit to be used in a field-test installation on theroof top of our institute. With this installation, we are currentlycomparing the annual energy yield of four different types ofcommercial PV modules in the Swedish climate. The MPPTunits were planned to be flexible enough to easily adapt to PVmodules of different sizes and, thus, power levels. Because ofthe inherent differences between, e.g., wafer-based and thin-filmPV modules, there was also a need to handle different levels ofmodule voltage and current, from Voc 20 V and Isc 5 Afor a typical 80-W crystalline silicon module to Voc 80 V andIsc 1.5 A for a typical 80-W thin-film module.The electronic load was realized using power MOSFETs ina constant-voltage feedback circuit. Power MOSFETs are reg-ularly used as the active element in electronic loads becauseof the convenient control of the load resistance by means of avaried gate potential. These devices are also characterized bytheir ruggedness and scalability [15]–[17].II. MEASUREMENT SYSTEMAn overview over the measurement system for a single mod-ule can be seen in Fig. 1. The modules are installed on the rooftop of the ˚Angstr¨om Laboratory in Uppsala, Sweden. Each PVmodule is connected to an individual MPPT unit, which keepsthe module under optimum working conditions independent ofthe actual irradiance.The connection of the module at the input of the MPPT unitis shown in more detail in Fig. 2, indicating the variable loadand the shunt resistor R41. The unit acquires the instantaneousvalues of the PV module voltage Vmod = Vin,+ − Vin,− and thevoltage drop over the shunt resistor caused by the module currentVin,− = ImodR41. The load itself is regulated by a control signalfrom the MPPT, closing a feedback loop. Within the MPPT unit,the module voltage and current are translated to analog voltage2156-3381/$26.00 © 2011 IEEE
  • 2. 48 IEEE JOURNAL OF PHOTOVOLTAICS, VOL. 2, NO. 1, JANUARY 2012Fig. 2. Module current Im od is measured as the voltage drop over the shuntresistor R41 , while the module voltage is measured as Vm od = Vin,+ − Vin,−.The electronic load is a constant-voltage current sink. The set point is derivedby means of a low-pass filter (LPF) from the PWM output of a microcontrollerand compared with the scaled measured voltage VVPV of the PV module.Fig. 3. Differential amplifier to measure the terminal voltage of the PV moduleconnected to the measurement system.signals which are remotely logged by an Agilent 34970A datalogger equipped with 34901A 20-channel armature multiplexersin our measurement lab. The signals from all modules, readoutsfrom temperature sensors at the backside of the modules, andfrom four independent irradiance sensors are scanned in 10-sintervals.A. Voltage MeasurementThe PV module voltage is measured at the connection ter-minal by means of a classic differential amplifier shown inFig. 3. With the resistor values R21 = R22 = 100 kΩ andR23 = R25 = 10 kΩ, the gain of the amplifier in the prototypeswas set to result in an output signal VVPVVVPV =R23,25R21,22(Vin,+ − Vin,−) =Vmod10. (1)Scaling the full-range input voltage of 0–100 V to a 0–10 Vsignal, the module voltage is transmitted to the central datalogger by means of a shielded twisted-pair cable. Changing theratio of the resistors allows us to easily adopt the unit to differentinput voltage ranges. The voltage signal VVPV formed by theresistive voltage divider R26 and R27 matches the input rangeof the analog-to-digital converter (ADC) of the microcontroller(μC).B. Current MeasurementThe load current is measured in the form of the voltage dropover the shunt resistor R41 = 0.1 Ω by an inverting amplifier,as shown in Fig. 4. With this value for the shunt resistor andthe resistors R31 = 10 kΩ and R32 = 100 kΩ, the gain of theFig. 4. Inverting amplifier to measure the load current of the PV moduleconnected to the measurement system.amplifier in the prototypes was set to giveVIPV = −R32R31Vin,− =R32R41R31Imod = Imod1VA(2)where VIPV stands for a voltage signal representing the mea-sured current.Scaling the full-range input current 0–10 A to a 0–10 Vsignal, the load current is transmitted to the central data loggerby means of a shielded twisted-pair cable. Changing the ratio ofthe resistors allows us to easily adopt the unit to different inputcurrent ranges. The signal VIPV is scaled to match the inputrange of the ADC.C. Maximum Power Point Tracker ControllerThe central part of the MPPT unit is a μC, which samples themeasured current IPV and voltage VPV of the PV module andcontrols the electronic load according to an MPPT algorithm.For the prototype units described here, the Atmel ATtiny45 run-ning on an internal clock frequency of 8 MHz at 5-V supplyvoltage was chosen as the μC [18]. Two of the four ADC chan-nels of this controller are used to sample the derived voltagesignals VIPV and VVPV with 10-bit resolution.In place of digital-to-analog converter (DAC) outputs, thecontroller features four independent pulsewidth modulation(PWM) channels with 8-bit resolution. One of these channelsis low-pass filtered and is used to control the electronic load,which is described in Section II-D. The low-pass filter is a third-order voltage-controlled voltage source or general Sallen–Keyfilter [19] with a cutoff frequency of 600 Hz and a dc voltagegain of 2, while the PWM signal has a repetition frequency of15 kHz. This gives a 0–10 V signal that is used as a set point forthe electronic load.The currently implemented algorithm of the MPPT controllertakes up only 14% of the available program memory of the AT-tiny45. The principle of the algorithm is shown as the exampleof the power–voltage characteristics of a PV module in Fig. 5and as a flow chart in Fig. 6. This algorithm is described asperturb-and-observe or hill-climbing in the literature [20]. Itis based on the measurement of the power output P1,2 of thePV module for two different set-point voltages V1 = Vset andV2 = Vset + 2ΔV , respectively. The relation between the powerlevels P1 and P2 determines the action of the MPPT.
  • 3. ZIMMERMANN AND EDOFF: MAXIMUM POWER POINT TRACKER FOR LONG-TERM LOGGING OF PV MODULE PERFORMANCE 49Fig. 5. MPPT process visualized on the power-voltage characteristics of a PVmodule. In region (I), the voltage set point is increased in steps of ΔV fromV1 to V1 per iteration, climbing up the power curve toward the MPP in region(III). In region (II), the algorithm lowers the voltage in steps of ΔV toward theMPP. The step size ΔV is exaggerated in this sketch—in the prototype devices,the step size is around 0.4–2% of Voc .1) P1 < P2: The set-point voltage is increased V1 = V1 +ΔV (see region (I) in Fig. 5 and label (A) in Fig. 6). Thetest at label (C) defines the upper limit of the set-pointvoltage.2) P1 > P2: The set-point voltage is decreased V1 = V1 −ΔV (see region (II) in Fig. 5 and label (B) in Fig. 6). Thetest at label (D) defines the lower limit of the set-pointvoltage.This results in overlapping voltage intervals moving towardthe MPP. At the MPP, the algorithm will either settle in anequilibrium position, where P1 = P2 as depicted in region (III)in Fig. 5, or continue to alternately step around the location ofthe MPP. Making the voltage step ΔV small enough will ensurethat the algorithm will come sufficiently close to the MPP, eventhough it will never rest at the MPP itself. In the prototypesystems, the width of the voltage step ΔV was in the range of0.4–2% of the Voc, depending on the actual Voc rating of themodule.As a consequence of sudden changes in the irradiance, Vocof the module can quickly drop below the set-point voltage. Inthis case, the measured voltage over the PV module will dropbelow the set-point voltage, and the algorithm would not beable to track the change in the MPP because P1 = P2 = 0 W.Additionally, leakage currents and noise present in the measuredsignals can guide the algorithm into the wrong direction also atlow irradiance levels. Therefore, the decision at label (E) inFig. 6 resets the algorithm to the starting point V1 = 0 V if sucha condition is detected.D. Electronic LoadThe electronic load shown in Fig. 2 is constructed as aconstant-voltage current sink, using an operational amplifieras error amplifier [15]. The error amplifier compares the voltagesignal VVPV , which is derived from the module voltage with theset-point value generated by the μC, and controls the load re-sistance accordingly in order to minimize the voltage error. The8-bit resolution of the PWM signal that is used to derive the set-point value limits the resolution of the set point to 0.4% of thefull-range value. The RC network C14 = 1 μF and R17 = 50 kΩimproves the stability of the closed feedback loop.Fig. 6. Flow chart of the utilized MPPT algorithm. The labels (A)–(E) aredescribed in the text.The output voltage of the error amplifier is applied as gate-to-source voltage VGS of the power MOSFET Q41. The drain–source channel resistance of the MOSFET in series with thecurrent-sensing shunt resistor R41 acts as a variable resistiveload for the PV module. The choice of the MOSFET dependson the voltage and power rating of the PV module that is to bemonitored. In commercial power MOSFETs, the drain contact isgenerally connected to the metal surfaces of the package; there-fore, the MOSFET needs to be mounted electrically isolatedonto an appropriate heat sink. This results in a higher thermalresistance from the junction to the heat sink which has to be
  • 4. 50 IEEE JOURNAL OF PHOTOVOLTAICS, VOL. 2, NO. 1, JANUARY 2012TABLE IPOWER MOSFET PARAMETERSFig. 7. Power-handling capacity of the MPPT unit can be increased by parallelconnection of several power MOSFETs.considered. In our setup, PV modules with output power from50 to 80 W at Voc values from 20 to 80 V were first connectedto MPPT units equipped with a single International RectifierIRFP3710 power MOSFET each. This MOSFET was chosenbecause of its power- and voltage-handling capabilities even athigher junction temperatures of up to 175 ◦C (see Table I) [21].The lowest obtainable module voltage Vmod is limited by thevoltage drop over the MOSFET and the shunt resistor R41 =0.1 ΩVmod,min ≥ R41ISC + RDS,min (3)where RDS,min is determined by the characteristics of the par-ticular MOSFET and the maximum applied gate voltage VGS.While this limits the ability to measure the full short-circuit cur-rent Isc of the PV module, with Vmod,min ≤ 1 V Voc, for ourprototype units, it does not affect the performance in determin-ing the MPP.Since the MOSFET in the electronic load is constantly operat-ing in a regime where drain–source resistance RDS is dominatedby the channel resistance of the device, the use of a special lin-ear power MOSFET would be preferred [23]. In these devices,the equal current spreading over the individual cells of the de-vice is improved by increasing the resistance of the individualsource contacts, while maintaining the general properties ofpower MOSFETs [16], [24]. However, in the prototype unitsdescribed here, we used easily available standard power MOS-FETs that are originally optimized for switch-mode operation.We could not detect any indication of instabilities that couldhave been caused by inhomogeneous current spreading withinthe individual MOSFETs.The power-handling capabilities of the electronic load canbe enhanced by using several power MOSFETs in parallel, asshown in Fig. 7. The advantage of the parallel connection ofseveral MOSFETs is not only the increased current-handlingcapacity but mostly the enhanced heat conduction to the heatsink. The individual gate resistors labeled R18,[a,b...] preventdetrimental high-frequency oscillations caused by the gate ca-pacitances [25]. Thermally controlled current sharing betweenthe devices cannot be relied on in the case of linear operationof MOSFETs [26]. Instead, additional source resistors labeledR40,[a,b...] need to be added to compensate for the wide variationin gate threshold voltage Vth(GS) normally found in commercialdevices. Otherwise, the transistor with the lowest threshold volt-age will carry the major part of the load current and, hence, stilldissipates most of the power of the electronic load. In two outof four prototype units, the single IRFP3710 MOSFET has laterbeen replaced with three parallel IRF520I MOSFETs in orderto counteract problems caused by overheating of the MOSFET.The source resistors were set to R40a , b , c= 1 Ω and the gateresistors to R18a , b , c= 1 kΩ.III. RESULTS AND DISCUSSIONA. Circuit SimulationsComputer simulations of the electronic load were performedusing LTspice [27]. A spice model of a 36-cell 80-W crystallinesilicon PV module (Voc = 21.5 V, Isc = 5.1 A) was connectedto a model of the electronic load with a MOSFET of the typeIRFP3710 dissipating the electric power of the module, accord-ing to the circuit diagram in Fig. 2. The simulation results arevisualized in Fig. 8, where the set-point voltage of the electronicload was swept from 0 to 25 V, with the Voc of the simulatedPV module at 21.5 V.The plot in Fig. 8(a) shows the voltage of the PV moduleVmod which follows the set-point voltage (red, dotted line) inthe range0.65 V = Vmod,min ≤ Vmod ≤ Voc = 21.5 V.The corresponding drain current IDS through the electronicload is plotted in Fig. 8(b) as a function of the set-point voltage.Above Voc, the leakage currents through the MOSFET and theresidual input currents into the operational amplifier (opamp)circuits are negligible.Fig. 8(c) displays the power delivered by the PV module(black, upper curve) and the power dissipated in the MOS-FET (red, lower curve). The difference between these twocurves corresponds to the power dissipated in the shunt resistorPshunt ≤ R41Isc = 0.5 W.Finally, Fig. 8(d) and (e) shows the gate–source voltage, i.e.,VGS, applied by the feedback circuit and the resulting resistanceof the electronic load, respectively. For close-to-short-circuitconditions, the maximum VGS is limited by the supply volt-age and the drive capability of the opamp, giving a total load
  • 5. ZIMMERMANN AND EDOFF: MAXIMUM POWER POINT TRACKER FOR LONG-TERM LOGGING OF PV MODULE PERFORMANCE 51Fig. 8. Spice simulation result of the constant-voltage load using a singleMOSFET. The load is connected to an 80-W PV module, and the set point forthe module voltage is swept from 0 to 25 V.resistance of about Rload = 0.12 Ω. With an increasing set-pointvoltage, the applied VGS decreases toward the V(th),GS = 3 Vof the MOSFET used in the simulation. The achievable dynamicrange of load resistances in this setup covers more than five or-ders of magnitude, 0.12 Ω ≤ Rload ≤ 50 kΩ for a gate voltagerange of 4 V ≥ VGS ≥ 3 V.In order to study the possibility of increasing the power-handling capability of the electronic load by using several dis-crete MOSFETs connected in parallel, simulations with twoMOSFETs of the type IRFP3710 dissipating the electric powerof the module were performed in the connection shown in Fig. 7.In order to simulate the influence of the variation in commercialMOSFET parameters, the Vth(GS) of the two MOSFETs wereset to different values, i.e., Q41a : V(th)GS = 2.7 V and Q41b:V(th)GS = 3.3 V, which is well within the expected spread ofthreshold voltages from the datasheet and measurements on ac-tual devices [21]. The two sets of graphs in Fig. 9 and the datain Table II show the simulation results for two different valuesFig. 9. Spice simulation results of the constant-voltage load with two powerMOSFETs with slightly different V(th)G S connected in parallel. The load isconnected to an 80-W PV module, and the set point for the module voltage isswept from 0 to 25 V. (a) Source resistors were set to R40a = R40b = 1 Ω.(b) R40a,b = 1 μΩ were practically omitted.TABLE IISIMULATION RESULTS FOR PARALLEL-CONNECTED MOSFETS IN THEELECTRONIC LOADof individual source resistors for the parallel-connected MOS-FETs.Setting the value of the source resistors to R40a,b = 1 Ωcauses a stabilizing negative feedback of the actual drain cur-rent onto the VGS of the individual MOSFETs. The results of thecorresponding Spice simulation are displayed in Fig. 9(b). Theupper graph shows the individual VGS of the two MOSFETsQ41a,b. For close-to-short-circuit conditions, at low set-pointvoltages, the attainable maximum value of VGS is now addition-ally limited by the voltage drop over the source resistors; fortwo MOSFETs in parallel, we getVGS,max ≤ VDD − ΔVop − R40ISC2∼= 8 V. (4)
  • 6. 52 IEEE JOURNAL OF PHOTOVOLTAICS, VOL. 2, NO. 1, JANUARY 2012Together with the increased voltage drop over the electronicload that makes it impossible to obtain measurements at lowmodule voltages Vmod, for the values in this simulation, we getVmod,min ≥ R41ISC + (RDS,min + R40)ISC2= 3.2 V (5)where R41 = 0.1 Ω is the shunt resistor for the measurement ofthe PV module current. However, under no practical conditions,the MPP can be located at such a low module voltage for thetypes of PV modules that are considered here.As can be seen in the upper graph of Fig. 9(b), the feedbackprovided by the individual source resistors causes the VGS forthe two MOSFETs to separate when the main feedback of theelectronic load circuit reduces the common gate voltage at set-point voltages above VPV,min. The middle graph again showsthe individual drain current ID of the two MOSFETs, as wellas the sum of both currents, corresponding to the current IPVdelivered by the module. Although not exactly equally shared,the difference between the ID of the two MOSFETs is in theorder of ±10%. MOSFET Q41,a with the lower V(th)GS carriesthe higher ID .The lower graph of Fig. 9(b) depicts the power delivered bythe PV module and the power dissipation in the MOSFETs.Following the unequal current sharing, Q41,a also dissipatesmore power than Q41,b, but this imbalance also stays within±10%. The sum of the power dissipated in both MOSFET isshown by the dashed line in this graph, as well as the powerdelivered by the PV module. The significant difference of up to17 W between these two curves represents the power dissipatedin the two additional source resistors R40a,b. Therefore, theseresistors have to be designed accordingly.In Fig. 9(a), the source resistors were set to a value ofR40a,b = 1 μΩ equivalent to omitting any source resistor asrecommended in [15] and [28]. This results in the exact sameVGS being applied to both MOSFETs shown in the upper graphof Fig. 9(a). For close-to-short-circuit conditions, the electronicload drives the gate voltage to VGS = 10.5 V as limited by thesupply voltage of VDD = 12 V and the voltage drop in the out-put stage of the opamp TL074. This voltage is sufficiently highabove the threshold voltage to drive both MOSFET into the con-ductive state. The lowest obtainable module voltage Vmod nowbecomes [also see (3)]Vmod,min ≥ R41ISC + RDS,minISC2= 0.6 V. (6)Increasing the set-point voltage, the feedback loop tends toreduce the VGS of both MOSFET simultaneously in order toincrease the RDS of the channel and thus increase the voltageover the load. As can be seen from the graph in the middle ofFig. 9(a), the current is evenly shared by the two MOSFET forthe short-circuit condition, where the sharing ratio only dependson possible variations in the R(on)DS which were neglected inthis simulation.However, with increasing set-point voltage, the current shar-ing becomes unbalanced because of the variation in V(th)GSand MOSFET Q41a with the lower V(th)GS carries practicallythe full-load current. This is, consequently, also reflected in thepower dissipation in the two MOSFETs shown in the lowergraph. The full-load power is dissipated in the single transistorQ41a . The resulting self-heating of Q41a will, in practice, leadto a shift of its characteristics, but for power MOSFETs like theIRFP3710 considered here, the transfer characteristics ID (VGS)shows a positive temperature coefficient for VGS < 6 V [21].This will, therefore, not counteract the imbalance between thetwo MOSFETs, unlike the case for switched applications withsignificantly higher VGS and negative temperature coefficientsof ID (VGS) [24]. In the lower graph, the electrical power deliv-ered by the PV module is shown as well. The difference betweenthe module power and the sum of the power dissipated in eachof the two MOSFETs is the power which is dissipated in thecurrent-sensing resistor R41.This comparison shows that it is important to use sufficientlyhigh values for the source resistors in the parallel connection ofpower MOSFETs used as resistive loads. The drawback of theuse of source resistors is the decreased voltage range limitedby the minimum achievable value of the load resistance and thepower dissipation in the source resistors, both of which have tobe considered in the design process of the electronic load.B. Field TestAt the time of writing, the first MPPT units have been inoperation on the roof of our laboratory for a full year. The unitsare contained in individual IP65-rated aluminum boxes that areequipped with aluminum heat sinks for the power MOSFETs.Each unit is mounted in the shade behind the PV module towhich it is connected. Measurement data have been collectedin 10-s intervals for a comparative study on energy yield fordifferent types of PV modules.The co-occurrence of particular high irradiances and poweroutput from the attached PV modules and of less efficient cool-ing of the power MOSFETs because of high air temperaturesand low wind speeds caused an overheating of the IRFP3710MOSFETs in two of the prototype units after half a year of op-eration. In these units, the single MOSFETs were then replacedby three parallel-connected IRF520I MOSFETs, as described inSection II-D. Apart from a higher reliability of these units, thechange of the MOSFETs did not affect the performance of themeasurement systems.Fig. 10 shows typical measurement data of an 80-W moduleon May 10, 2011, which was a particularly sunny day in Uppsala.The irradiance in the top part of this figure is measured with aKipp&Zonen CMP11 pyranometer in the tilted plane of themodule. The module itself is tilted at a slope of 42◦, orientedsouth. Around solar noon, the glass roof of another buildingreflects sunlight into the pyranometer, causing the spikes inthe irradiation data, marked in the graph with the letter A (seethe photo in Fig. 11 for the appearance of the reflections onthe wall beneath the test site). However, the spots of increasedirradiance are not covering the entire size of the PV modules,which is responsible for the significantly smaller response tothese spikes seen in the power output of the modules, shown inthe middle part of Fig. 10.
  • 7. ZIMMERMANN AND EDOFF: MAXIMUM POWER POINT TRACKER FOR LONG-TERM LOGGING OF PV MODULE PERFORMANCE 53Fig. 10. Measurement data of an 80-W PV module on a clear day. The globalirradiance at the top is measured in the plane of the module. The graph inthe middle shows the electrical power output of the module, while the lowergraph shows the corresponding load resistance constituted by the MPPT unit;markers represent individual measurement points every 120 s. The spikes A inthe irradiance are caused by reflections from another building, while the dip Bin the power output is caused by shading from a chimney.Fig. 11. Reflections of a nearby glass roof and shading of a chimney influencethe irradiance measurements of our test site.Later in the afternoon, a chimney on the roof casts a narrowshadow over the modules which does not extend to the posi-tion of the pyranometer. The effect of this shadow can be seenat the dip, marked B in the middle part of Fig. 10, showing theelectrical power output of the module as measured by the MPPTunit. The lower part of Fig. 10 illustrates the effective load resis-tance Rload = RDS + Rshunt calculated from the voltage VPVand current IPV logged by the measurement system. For thisparticular module, the load resistance is dynamically adjustedin the range from 4 to 4 kΩ between sunrise and sunset.Fig. 12 shows the effect of this shadowing event on anothermodule in more detail. It takes about 2 min for the shadow ofthe chimney to reach the maximum coverage on the module,while the module remains in the shadow for about 40 min.Fig. 12. Closeup of the dynamic reaction of the MPPT unit to the shading ofa module. The shading is caused by a nearby chimney, and the measurementdata are collected in 10-s intervals.During the time of the maximum shadowing, the module stilldelivers about 5 W of electrical power generated by diffuseillumination. The MPPT unit adjusts the load resistance from8 Ω before the shadow hits the module to up to 60 Ω during thetime of shadowing.In order to observe the reaction of the MPPT units to suddenchanges in irradiance of the attached PV module, a shadowingtest was conducted. The result of this test with a crystallinesilicon PV module is displayed in Fig. 13. Out of the 36 siliconsolar cells of the module, two were quickly and simultaneouslycovered with a sheet of translucent paper (80 g/m2). Duringthe test, the sampling rate of the measurement signals was setto a faster rate of one sample every 5 s. The irradiance duringthe experiment was stable at 1015 W/m2. Within one sampleinterval of 5 s, a new stable operating point is found, and theload resistance has changed from 3.2 Ω under full illumination to19 Ω under partial shadowed conditions. At the same time, Vmphas increased from 16 V under full illumination to 20 V underpartial shadowed conditions. This corresponds to the location ofa local maximum in the power output, which is marked as 2b inthe simulated characteristics of a shadowed silicon PV moduleof similar construction in Fig. 14. According to the simulatedcharacteristics, the global maximum, which is labeled 2a inFig. 14, is found at a significantly lower voltage of Vmp,2a10 V with an output power of Pmp,2a 40 W, twice as highas the power in point 2b. The reason for the tendency of theMPPT algorithm during this test to track the local maximum atthe higher voltage lies in the positive slope dP/dV of the powercurve at the voltage of the original MPP before the shadowing,which is marked as point 1 in Fig. 14. However, this is onlythe case for very abrupt changes in the irradiance of a few of amodule’s cells. For practical purposes, the algorithm has provento be sufficiently fast in adapting to natural changes in irradiance,as shown in the case of the shading by the chimney in Fig. 12.Furthermore, a different algorithm could be implemented in the
  • 8. 54 IEEE JOURNAL OF PHOTOVOLTAICS, VOL. 2, NO. 1, JANUARY 2012Fig. 13. Dynamic reaction of the MPPT unit to the shading of a module. In acrystalline silicon PV module, two out of 36 cells were shaded with a piece ofpaper, and the measurement data are collected in 5-s intervals.Fig. 14. Simulation result of the partial shading of a PVmodule. In a crystallinesilicon PV module, the irradiance was lowered from 1000 to 200 W/m2 fortwo out of 36 cells, simulating the shading with a piece of translucent paper.The dashed curve shows the unaffected IV curve, while the solid curve showsthe partially shaded case with two distinct local maxima in the power output(marked as 2a and 2b).software to avoid the locking to local maxima, as described inthe relevant literature (see, e.g., [20]).IV. CONCLUSIONOutdoor testing and monitoring of PV modules is an im-portant part in the evaluation of the economic and technolog-ical viability of different types of modules at different loca-tions. It can also give valuable feedback to the manufacturer ofPV modules regarding long-term reliability under certain testconditions.Using a small μC, a simple MPPT algorithm, and a powerMOSFET as variable electronic load, we have constructed a toolto monitor the outdoor performance of individual PV modules.In spite of its simplicity, the measurement unit has proven itslong-term stability and reliability during one year of operationin the form of prototype units at ambient temperatures between−25 ◦C and +30 ◦C.The use of a constant-voltage electronic load linearizes thecontrol problem and circumvents the variability of the thresholdvoltage of power MOSFETs. This allows the utilization of thefull range of the output signal of the DAC and gives the bestpossible resolution along the voltage axis of the characteristicsof the PV module.The presented circuit is fast enough to track quick changesin irradiance and power output caused by partial shading of theattached PV modules. However, because of the implementedMPPT algorithm, only a local maximum in the power outputof the shaded PV module is found, but this could be taken careof by implementing a different control strategy in the software.The dynamic range of the measurement signal is sufficient toreasonably cover daily and seasonal variations in power output.The adaptation to the current and voltage characteristics of dif-ferent modules is easily accomplished by the dimensioning oftwo resistive voltage dividers.For a reliable construction of the electronic load, it is essen-tial to take the worst case of ambient temperature and poweroutput of the connected PV module into consideration. In orderto accomplish the effective removal of the generated heat fromthe junction of the power MOSFET to the heat sink, the parallelconnection of several smaller MOSFETs has proven to be ad-vantageous over a single MOSFET of higher power rating. Inorder to compensate for variances in the threshold voltage ofthese devices, the use of separate source resistors is necessary.ACKNOWLEDGMENTThe authors would like to thank A. Girard for his valuablehelp in the assembly of the MPPT units.REFERENCES[1] A. Virtuani and H. M¨ullejans, E. Dunlop, “Comparison of indoor and out-door performance measurements of recent commercially available solarmodules,” Prog. Photovoltaics: Res. Appl., vol. 19, pp. 11–20, 2011.[2] E. van Dyk, T. Strand, and R. Hansen, “Technical evaluation of two6-kW mono-Si photovoltaic systems at the National Renewable EnergyLaboratory,” in Proc. 25th IEEE Conf. Rec. Photovoltaic Spec. Conf., May1996, pp. 1533–1536.[3] S. Pietruszko, B. Fetlinski, and M. Bialecki, “Analysis of the performanceof grid connected photovoltaic system,” in Proc. 34th IEEE PhotovoltaicSpec. Conf., Jun. 2009, pp. 48–51.[4] E. Koutroulis and K. Kalaitzakis, “Development of an integrated data-acquisition system for renewable energy sources systems monitoring,”Renewable Energy, vol. 28, no. 1, pp. 139–152, Jan. 2003.[5] K. Emery, “The rating of photovoltaic performance,” IEEE Trans. Elec-tron Devices, vol. 46, no. 10, pp. 1928–1931, Oct. 1999.
  • 9. ZIMMERMANN AND EDOFF: MAXIMUM POWER POINT TRACKER FOR LONG-TERM LOGGING OF PV MODULE PERFORMANCE 55[6] O.-M. Midtgard, T. Saetre, G. Yordanov, A. Imenes, and C. L. Nge,“A qualitative examination of performance and energy yield of photo-voltaic modules in Southern Norway,” Renewable Energy, vol. 35, no. 6,pp. 1266–1274, Jun. 2010.[7] V. Agarwal and S. Jain, “New current control based MPPT techniquefor single stage grid connected PV systems,” Energy Convers. Manage.,vol. 48, no. 2, pp. 625–644, Feb. 2007.[8] M. F. Ansari, S. Chatterji, and A. Iqbal, “A fuzzy logic control scheme fora solar photovoltaic system for a maximum power point tracker,” Int. J.Sustainable Energy, vol. 29, no. 4, pp. 245–255, 2010.[9] S. Brunton, C. Rowley, S. Kulkarni, and C. Clarkson, “Maximum powerpoint tracking for photovoltaic optimization using ripple-based extremumseeking control,” IEEE Trans. Power Electron., vol. 25, no. 10, pp.2531–2540, Oct. 2010.[10] L.-R. Chen, C.-H. Tsai, Y.-L. Lin, and Y.-S. Lai, “A biological swarmchasing algorithm for tracking the PV maximum power point,” IEEETrans. Energy Convers., vol. 25, no. 2, pp. 484–493, Jun. 2010.[11] N. Dasgupta, A. Pandey, and A. Mukerjee, “Voltage-sensing-based pho-tovoltaic MPPT with improved tracking and drift avoidance capabilities,”Solar Energy Mater. Solar Cells, vol. 92, no. 12, pp. 1552–1558, Dec.2008.[12] O. Lopez Lapena, M. Penella, and M. Gasulla, “A new MPPT method forlow-power solar energy harvesting,” IEEE Trans. Ind. Electron., vol. 57,no. 9, pp. 3129–3138, Sep. 2010.[13] G. Yu, Y. Jung, J. Choi, and G. Kim, “A novel two-mode MPPT controlalgorithm based on comparative study of existing algorithms,” SolarEnergy, vol. 76, no. 4, pp. 455–463, 2004.[14] T. Esram and P. Chapman, “Comparison of photovoltaic array maximumpower point tracking techniques,” IEEE Trans. Energy Convers., vol. 22,no. 2, pp. 439–449, Jun. 2007.[15] A. Garrig´os and J. M. Blanes, “Power MOSFET is core of regulated-dcelectronic load,” EDN Mag., pp. 92–93, 2005.[16] A. Sattar and V. T. V. Tsukanov, “MOSFETs withstand stress of linear-mode operation,” Power Electron. Technol., vol. 33, pp. 34–39, Apr.2007.[17] R. Severns, in MOSPOWER Applications Handbook, J. Armijos, Ed.Santa Clara, CA: Siliconix, 1985.[18] Datasheet ATtiny24/45/85, Atmel Corp, San Jose, CA, 2011, rev. 2586N-AVR-04/11. [Online]. Available: http://www.atmel.com[19] D. Lancaster, Active Filter Cookbook, 2nd ed. ed. Amsterdam, TheNetherlands: Elsevier, 1995.[20] H. Desai and H. Patel, “Maximum power point algorithm in PV generation:An overview,” in Proc. 7th Int. Conf. Power Electron. Drive Syst., Nov.2007, pp. 624–630.[21] Datasheet and Spice Model of IRFP3710 (rev. 2011). Int. Rectifier, ElSegundo, CA [Online]. Available: http://www.irf.com[22] Datasheet of IRF520, SGS-Thomson Microelectronics (1993). [Online].Available: http://www.st.com[23] Y. Kuai and S. Yuvarajan, “An electronic load for testing photovoltaicpanels,” J. Power Sources, vol. 154, no. 1, pp. 308–313, 2006.[24] B. J. Baliga, Power Semiconductor Devices. Boston, MA: PWS, 1996.[25] N. Mohan, T. M. Undeland, and W. P. Robbins, Power Electronics, 3rded. New York: Wiley, 2002.[26] J. M. Jacob, Power Electronics: Principles and Applications, 1st ed.Albany, NY: Delmar, 2001.[27] LTspice IV. Linear Technol. Inc. (2011). [Online]. Available: http://www.linear.com/designtools/software/[28] V. Leite and F. Chenlo, “An Improved Electronic Circuit for Tracing theI-V Characteristics of Photovoltaic Modules and Strings,” presented atthe Proc. Int. Conf. Renewable Energies Power Quality, Granada, Spain,2010.Uwe Zimmermann (M’06) received the M.Sc. de-gree in solid-state physics from the University of Kiel,Kiel, Germany, in 1997 and the Ph.D. degree in solid-state electronics from the Royal Institute of Technol-ogy, Stockholm, Sweden, in 2003.During Ph.D. studies, he was involved in researchon high-voltage silicon carbide diodes. Since 2003, hehas been a Researcher of Cu(in,Ga)Se2 based thin-film solar cells with Uppsala University, Uppsala,Sweden, where he is involved in teaching and R&Dactivities in photovoltaics and electronics. Between2005 and 2008, he was part-time employed by the thin-film solar cell manufac-turer Solibro.Marika Edoff received the M.Sc. degree in electri-cal engineering and the Ph.D. degree in solid-stateelectronics from the Royal Institute of Technology,Stockholm, Sweden, in 1990 and 1997, respectively.Since 2003, she has been leading research ac-tivities on solid-state thin-film solar cells with Upp-sala University, Uppsala, Sweden. She was one ofthe four founders of Solibro, where she has alsobeen part-time employed since 2005. Her researchinterests include Cu(In,Ga)Se2 based thin-film solarcells with a focus on material synthesis and devicecharacterization.