Wireless Data Acquisition for PhotovoltaicPower System

Makbul Anwari', Arief Hidayat', M. Imran Hamid', and Taufik''Faculty of Electrical Engineering, Universiti Teknologi Malaysia, 81310 UTM Skudai, Johor, Malaysia

Electrical Engineering Department, Cal Poly State University, San Luis Obispo, USAE-mail : makbul@ieee.org

Abstract - This paper presents a wireless system formonitoring the input and output of the array in a photovoltaicgeneration plant. The system comprises of sensors, dataacquisition system, wireless access point and user computer thatenable the users to access the array parameter wirelessly.Description and function of set up equipment are presented aswell as the application program that supports the system.

1. INTRODUCTION

Recently, the number ofenergy in the world is reaching toconcern state. This is caused by the need of energy is growingvery fast. Due to concerns regarding global warming and airpollution, there has been an international movement in thepromotion of renewable energy technologies for electricitygeneration, green energy is one among proposed solutions forthese issues [I].

Solar energy is converted to electricity in a photovoltaicgeneration plant that contains photovoltaic array as solar­electricity conversion equipment, electrical power converter,power storage and other supporting equipment. According tooperation mode, photovoltaic generation plants are met inisolation mode, grid interconnected mode and plant that canoperate in both modes . For all of these modes, there are needsto acquire the input and output parameter of the photovoltaicarray as the generation equipment. The acquired parameters

are used for control action information, generation planning,energy forecasting, and performance observation ordocumentation need. Some of the parameters are irradiance,temperature and array's electrical output. A satisfied dataacquisition system is required for this need.

Related to acquisition system for photovoltaic performance,Benghanem et aI. have accomplished a research in which,several instruments are used to detect, integrate, and recordsolar energy measurement using both conventional electronicsas well as microprocessor data acquisition system [2]. Further,Machacek et aI., developed a system for measuring, collecting,analyzing, and displaying data for 100 W solar energyconverter, data acquisition is formed by NI-6023E plug-incard and feed the rough data to the control program built in aMATLAB script [3].

Data from acquisition system module are needed to produceuseful information. The speed of the process is the importantparameter. To accommodate this requirement, during the pastdecade, digital control has been widely used. Digital control,which is determined by application of microprocessors, makesthe sampling and computing process are faster than before.Implementation of such a system has been done on a dcvoltage monitoring and control system for a wind turbineinverter [4].

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Wireless Data Acquisition for PhotovoltaicPower System

Makbul AnwariI, AriefHidayatI, M. Itnran HamidI, and Taufik2

IFaculty of Electrical Engineering, Universiti Teknologi Malaysia, 81310 UTM Skudai, Johor, MalaysiaElectrical Engineering Department, Cal Poly State University, San Luis Obispo, USA

E-mail: makbul@ieee.org

Abstract - This paper presents a wireless system formonitoring the input and output of the array in a photovoltaicgeneration plant. The system comprises of sensors, dataacquisition system, wireless access point and user computer thatenable the users to access the array parameter wirelessly.Description and function of set up equipment are presented aswell as the application program that supports the system.

l. INTRODUCTION

Recently, the number ofenergy in the world is reaching toconcern state. This is caused by the need of energy is growingvery fast. Due to concerns regarding global warming and airpollution, there has been an international movement in thepromotion of renewable energy technologies for electricitygeneration, green energy is one among proposed solutions forthese issues [l].

Solar energy is converted to electricity in a photovoltaicgeneration plant that contains photovoltaic array as solar­electricity conversion equipment, electrical power converter,power storage and other supporting equipment. According tooperation mode, photovoltaic generation plants are met inisolation mode, grid interconnected mode and plant that canoperate in both modes. For all of these modes, there are needsto acquire the input and output parameter of the photovoltaicarray as the generation equipment. The acquired parameters

are used for control action information, generation planning,energy forecasting, and performance observation ordocumentation need. Some of the parameters are irradiance,temperature and array's electrical output. A satisfied dataacquisition system is required for this need.

Related to acquisition system for photovoltaic performance,Benghanem et ai. have accomplished a research in which,several instruments are used to detect, integrate, and recordsolar energy measurement using both conventional electronicsas well as microprocessor data acquisition system [2]. Further,Machacek et aI., developed a system for measuring, collecting,analyzing, and displaying data for 100 W solar energyconverter, data acquisition is formed by NI-6023E plug-incard and feed the rough data to the control program built in aMATLAB script [3].

Data from acquisition system module are needed to produceuseful information. The speed of the process is the importantparameter. To accommodate this requirement, during the pastdecade, digital control has been widely used. Digital control,which is determined by application of microprocessors, makesthe sampling and computing process are faster than before.Implementation of such a system has been done on a dcvoltage monitoring and control system for a wind turbineinverter [4].

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Regarding with data transmission, Chen,et.al, have studiedcarrying the acquired signal from data acquisition moduleusing internet [1]. Java language is used for designing adynamic webpage to graphically display various real timewaveforms of the controlled system for multi-user at the sametime. The system is implemented on a small scale wind powergeneration system equipped with an EZDSP 2812 controller.An FPGA ECIO is implemented as a bidirectionalcommunication interface for coordinating the asynchronousdata transmission modes.

In this paper, we present a photovoltaic generationmonitoring system for a 5 kWp laboratory scale photovoltaicgeneration. Temperature, irradiance, voltage, and current ofthe array are acquired, processed and then transmitted suchthat can be used for reviewing the performance of thegeneration plant. Acquired data is transmitted by wirelessmethod using Wi-Fi signal (IEEE 802.11 standard), whilemicrocontroller of PIC 16F877A is used to control theacquisition system process and a Delphi application programis built to graphically display the acquired data. Figure 1show the simplified diagram of such system.

As shown in Figure 2, photovoltaic array characteristic (V-Icurve) shows the dependency of cell current and voltage toirradiance and temperature. Irradiance contributes to the cellcurrent, the higher irradiance the higher current draw by arrayphotovoltaic, while the temperature effects to the cell voltage,the higher temperature the lower voltage appears on the cellterminal. In Figure 2a, it is shown a set of photovoltaic cell 1-Vcurve under varying irradiance at a constant temperature,meanwhile figure 2(b) shows the one at the same irradiancevalues, but under varying temperature. Both figures are alsoshow the point where the multiplication of PV array voltageand current reaches the maximum value: maximum powerpoint (MPP), at which condition that the array operates withmaximum efficiency and produces maximum output power.

Variation of irradiance and temperature in photovoltaicmodule is characterized as short time fluctuation; follows thebehavior of atmospheric condition around plant during time.The effect of this variation is the unpredictable variation ofpower output, current and voltage of the plant. An acquisitionsystem for this condition should consider the phenomenon.

III. PV GENERATIONMONITORING SYSTEM

The PV generation and monitoring system shown in Figure1, is a diagram of a laboratory scale system that contains threeunits photovoltaic array produces three de voltages of 0 - 150range. These de voltages is then fed to three single phase PVinverter respectively to be inverted to ac power before sendingto the utility. Maximum current for each array are lOA de.Array power input in form of temperature and irradiance andthe de output of array are picked up as data acquired for themonitoring system.

Three ACS754 current sensors with maximum current 50Aand sensitivity of 37.8 mV/A are connected to de output ofeach solar array, while irradiance and temperature sensorplaced around the array. For voltage acquisition, 1k.o.-1M.o.voltage divider was used, which means 10mV for every onevolt de solar panel output. LM35 temperature sensor whichsensitivity of 10mV/oC is used for measure the ambienttemperature of solar panel. The irradiance is sensed usingLDR.

Data acquisition diagram is shown in Figure 3. In order todetermine the analog signal from the sensor to be passed toADC, which contains eight analog signal from sensors (3 forde current, 3 for de voltage, one for irradiance and theremaining for temperature) dual eight-channels analogmultiplexer DG407B are used. The analog multiplexer wascontrolled using microcontroller and passing the multiplexingsignal with their own voltage references.

In the ADC block, each analog signal from multiplexer isdigitalized to eight bits digital signal. Eight-bit signal is usedcaused by the condition that communication betweenmicrocontroller and serial to Ethernet module uses eight bitdata. Thus, voltage reference Vrej for ADC isVrej = resolution x 28 , where the resolution is equal tosensitivity of each sensor. ADC operation is also controlled bythe microcontroller.

In this system, the microcontroller is employed to run the

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II. PV GENERATIONCHARACTERISTIC FOR MONITORING

SYSTEM

The acquisition system is aimed to detect and collect theparameters that indicate the electrical characteristic of thearray and the factors influencing them.

Fig. 2. V- I characteristic ofPV module, (a) characteristics on variousirradianceat a constanttemperature, (b) Characteristic on various

temperature at a constant irradiance

Regarding with data transmission, Chen,et.al, have studiedcarrying the acquired signal from data acquisition moduleusing internet [1]. Java language is used for designing adynamic webpage to graphically display various real timewaveforms of the controlled system for multi-user at the sametime. The system is implemented on a small scale wind powergeneration system equipped with an EZDSP 2812 controller.An FPGA ECIO is implemented as a bidirectionalcommunication interface for coordinating the asynchronousdata transmission modes.

In this paper, we present a photovoltaic generationmonitoring system for a 5 kWp laboratory scale photovoltaicgeneration. Temperature, irradiance, voltage, and current ofthe array are acquired, processed and then transmitted suchthat can be used for reviewing the performance of thegeneration plant. Acquired data is transmitted by wirelessmethod using Wi-Fi signal (IEEE 802.11 standard), whilemicrocontroller of PIC 16F877A is used to control theacquisition system process and a Delphi application programis built to graphically display the acquired data. Figure 1show the simplified diagram of such system.

As shown in Figure 2, photovoltaic array characteristic (V-Icurve) shows the dependency of cell current and voltage toirradiance and temperature. Irradiance contributes to the cellcurrent, the higher irradiance the higher current draw by arrayphotovoltaic, while the temperature effects to the cell voltage,the higher temperature the lower voltage appears on the cellterminal. In Figure 2a, it is shown a set ofphotovoltaic cell I- Vcurve under varying irradiance at a constant temperature,meanwhile figure 2(b) shows the one at the same irradiancevalues, but under varying temperature. Both figures are alsoshow the point where the multiplication of PV array voltageand current reaches the maximum value: maximum powerpoint (MPP), at which condition that the array operates withmaximum efficiency and produces maximum output power.

Variation of irradiance and temperature in photovoltaicmodule is characterized as short time fluctuation; follows thebehavior of atmospheric condition around plant during time.The effect of this variation is the unpredictable variation ofpower output, current and voltage of the plant. An acquisitionsystem for this condition should consider the phenomenon.

III. PV GENERATION MONITORING SYSTEM

II. PV GENERATION CHARACTERISTIC FOR MONITORING

SYSTEMThe PV generation and monitoring system shown in Figure

1, is a diagram of a laboratory scale system that contains threeunits photovoltaic array produces three dc voltages of 0 - 150range. These dc voltages is then fed to three single phase PVinverter respectively to be inverted to ac power before sendingto the utility. Maximum current for each array are lOA dc.Array power input in form of temperature and irradiance andthe dc output of array are picked up as data acquired for themonitoring system.

Three ACS754 current sensors with maximum current 50Aand sensitivity of 37.8 mVfA are connected to dc output ofeach solar array, while irradiance and temperature sensorplaced around the array. For voltage acquisition, lkQ-IMQvoltage divider was used, which means 10mV for every onevolt dc solar panel output. LM35 temperature sensor whichsensitivity of 10mVfOC is used for measure the ambienttemperature of solar panel. The irradiance is sensed usingLDR.

Data acquisition diagram is shown in Figure 3. In order todetermine the analog signal from the sensor to be passed toADC, which contains eight analog signal from sensors (3 fordc current, 3 for dc voltage, one for irradiance and theremaining for temperature) dual eight-channels analogmultiplexer DG407B are used. The analog multiplexer wascontrolled using microcontroller and passing the multiplexingsignal with their own voltage references.

In the ADC block, each analog signal from multiplexer isdigitalized to eight bits digital signal. Eight-bit signal is usedcaused by the condition that communication betweenmicrocontroller and serial to Ethernet module uses eight bitdata. Thus, voltage reference Vrej for ADC isVrej = resolution x 28 , where the resolution is equal tosensitivity of each sensor. ADC operation is also controlled bythe microcontroller.

In this system, the microcontroller is employed to run the

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temperature at a constant irradiance

The acquisition system is aimed to detect and collect theparameters that indicate the electrical characteristic of thearray and the factors influencing them.

function: controlling the multiplexer fordetermining the analog signal from the sensor to be passed;controlling the operation of ADC, and as communicationprotocol between Ethernet and the acquisition system. Toaccommodate these functions, the low-power consumptionPIC 16F877A microcontroller is used. This unit is built withineight channels 10 bit ADC and an UART connection for serialcommunication.

Microcontroller serial connection is connected to WiznetEGSR7150 in order to convert format data from serial toEthernet data, conversion process is reversal. Further, theacquainted data is sent to the access point to be sent in form ofWi-Fi signal to connected user computer.

To accommodate communication between computer andacquisition system and to display the result, a computerapplication program is required.

There are 5 charts that show the measured and calculatedparameters. Voltage, current, and calculated PV array powerof whole channel is displayed in one chart respectively. EachPV array power output is obtained by multiplying the voltageand current of respective array. The array temperature andirradiance are displayed separately in others charts. Nominalvalues of these parameters are also displayed. To start dataacquisition, user must connect the application to acquisitionmodule by pressing the "Connect to network" button. Timersetting button is provided to determine measurement intervalof the acquisition data.

Prototype of the photovoltaic acquisition system is shown inFigure 6. The left side picture shows layer for power supply,voltage divider and current sensor, this layer is placed in thebottom of acquisition compartment. The right side picturesshows layer for the microcontroller, multiplexer, Vref boardand data converter.

For this need, an application program, written in Delphilanguage is developed. This program is built to allow the usercan interact and control the process steps in these two sub­systems (microcontroller in acquisition system and displayingprocess in computer). Communication between computer andacquisition system is done wirelessly.

Communication process involves two application programs,one is in the computer side and the other is in microcontrollerside. Flowchart of communication between these twoapplication programs is shown in Figure 4, which shows twomain blocks, indicates the process flow in each side.

First of all, the programs will self-initialize when they areactivated. User can decide whether to acquire data or not. Ifacquisition data will be done, program will send a querycommand to the acquisition program on microcontrollercontains which data to be acquired. Microcontroller inacquisition system -based on the command query- orders themultiplexer to by-pass the intended analog signal from thesensors and digitalized them.

Analog signal from the sensors contains high noise, whichcan degrade the measurement accuracy. To avoid this, analogsignal is picked up and converted three times; their average iscomputed and become data to be sent back as acquisition datato user computer. In user computer, acquisition data isreceived by Ethernet port. Application program then processesthe data to be displayed and to be saved in the virtual memory.

The displaying application program is shown in Figure 5.

NoReceivequery? :>----......

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wlrelessaccesspoint?No

Fig. 4. PC and microcontroller software flowchart

Serial to EthernetConverter

SerialPIC 16F877A Connection Serial Microcontroller Ethernet

Microcontroller~~~

Fig. 3. Data acquisition system diagram

Vref

SensorSystem

following function: controlling the multiplexer fordetermining the analog signal from the sensor to be passed;controlling the operation of ADC, and as communicationprotocol between Ethernet and the acquisition system. Toaccommodate these functions, the low-power consumptionPIC 16F877A microcontroller is used. This unit is built withineight channels 10 bit ADC and an UART connection for serialcommunication.

Microcontroller serial connection is connected to WiznetEGSR7150 in order to convert format data from serial toEthernet data, conversion process is reversal. Further, theacquainted data is sent to the access point to be sent in form ofWi-Fi signal to connected user computer.

To accommodate communication between computer andacquisition system and to display the result, a computerapplication program is required.

There are 5 charts that show the measured and calculatedparameters. Voltage, current, and calculated PV array powerof whole channel is displayed in one chart respectively. EachPV array power output is obtained by multiplying the voltageand current of respective array. The array temperature andirradiance are displayed separately in others charts. Nominalvalues of these parameters are also displayed. To start dataacquisition, user must connect the application to acquisitionmodule by pressing the "Connect to network" button. Timersetting button is provided to determine measurement intervalof the acquisition data.

Prototype of the photovoltaic acquisition system is shown inFigure 6. The left side picture shows layer for power supply,voltage divider and current sensor, this layer is placed in thebottom of acquisition compartment. The right side picturesshows layer for the microcontroller, multiplexer, Vref boardand data converter.

For this need, an application program, written in Delphilanguage is developed. This program is built to allow the usercan interact and control the process steps in these two sub­systems (microcontroller in acquisition system and displayingprocess in computer). Communication between computer andacquisition system is done wirelessly.

Communication process involves two application programs,one is in the computer side and the other is in microcontrollerside. Flowchart of communication between these twoapplication programs is shown in Figure 4, which shows twomain blocks, indicates the process flow in each side.

First of all, the programs will self-initialize when they areactivated. User can decide whether to acquire data or not. Ifacquisition data will be done, program will send a querycommand to the acquisition program on microcontrollercontains which data to be acquired. Microcontroller inacquisition system -based on the command query- orders themultiplexer to by-pass the intended analog signal from thesensors and digitalized them.

Analog signal from the sensors contains high noise, whichcan degrade the measurement accuracy. To avoid this, analogsignal is picked up and converted three times; their average iscomputed and become data to be sent back as acquisition datato user computer. In user computer, acquisition data isreceived by Ethernet port. Application program then processesthe data to be displayed and to be saved in the virtual memory.

The displaying application program is shown in Figure 5.

Receive query? >---No-......

Receive query from PCInotebook trough

wlrelessaccess point ?No

Fig. 4. PC and microcontroller software flowchart

Serial to EthernetConverter

SerialPIC 16F877A Connection Serial Microcontroller Ethernet

Microcontroller~~~

Fig. 3. Data acquisition system diagram

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The authors thank the Malaysian Government, Ministry ofScience, Technology and Innovation (MOST!) for the ScienceFund Grant, Project No. 01-01-06-SF0205.

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VI. REFERENCES

[I] Po-Yen Chen; Se-Kang Ho; Wei-Jen Lee; Chia-Chi Chu; Ching-TsaPan, "An Internet Based Embedded Network Monitoring System forRenewable Energy Systems", Proc. The 7th International Conference onPower Electronics, pp.225-228 , October 22-26,2007.

[2] M.Benghanem; A. Maafi, "Data Acquisition System for PhotovoltaicSystems Performance Monitoring", IEEE Trans. on Instrumentation andMeasurement, Vo1.47, No.1, pp.30-33, February 2008.

[3] J. Machacek; z. Prochazka; 1. Drapela, "System for Measuring andCollecting Data from Solar-cell Systems", Proc.9th InternationalConference Electrical Power Quality and Utilization, Barcelona, 9-11October 2007.

[4] Z. Wang; L. Chang, "A DC voltage Monitoring and Control Method forThree Phase Grid-Connected Wind Turbine Inverters", IEEE Trans. OnPower Electronics, Vol. 23, No.3, pp. 1118-1125, May 2008.

[5] T. Yuki; H. Yoshikiro; K. Kosuke, "Temperature and IrradianceDependence of the I-V Curves of Various Kinds of Solar Cells"International Photovoltaic Science & Engineering Conference,Shanghai, China 2005

[6] A. Moein, M. Pouladian, "WIH-Based IEEE 802.11 ECG MonitoringImplementation", Proceedings of the 29th Annual InternationalConference of the IEEE EMBS Cite Internationale, Lyon, France August23-26,2007.

[7] H. Zhao, X. Chen, K.H. Chon, "A Portable, Low-cost, Battery-poweredWireless Monitoring System for Obtaining Varying PhysiologicParameters from Multiple Subjects" , Proceedings of the 28th IEEEEMBS Annual International Conference, New York City, USA, Aug 30­Sept 3, 2006

[8] A. Perujo; R. Kaiser; D.U Sauer; H. Wenzl; I. Baring-Gould; N.Wilmot;F. Mattera; S. Tselepis; F

[9] H. Zhiqiang; Z. Wenxian; L. Jianke, "Research on High-Speed DataAcquisition and Processing Technique", The Eighth InternationalConference on Electronic Measurement and Instruments, ICEMI'2007

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Fig. 6. Prototype of the photovoltaic acquisition system, (a) upper layer, (b)lower layer

IV. CONCLUSION

A wireless data acquisition system for photovoltaicgeneration system that uses PICI6F877A microcontroller asthe main control has been presented. Implementation of theEGSR7150 Ethernet to serial module and the access point asWi-Fi communication tool between acquisition equipmentsand user computer for graphically displaying the acquiredparameter has work properly as intended. Implementation ofsuch a system on a laboratory scale photovoltaic generationshows the practical simplicity, efficient and low cost.

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VI. REFERENCES

[I] Po-Yen Chen; Se-Kang Ho; Wei-Jen Lee; Chia-ehi Chu; Ching-TsaPan, "An Internet Based Embedded Network Monitoring System forRenewable Energy Systems", Proc. The 7th IntenuJtional Conference onPower Electronics, pp.225-228, October 22-26,2007.

[2] M.Benghanem; A. Maafi, "Data Acquisition System for PhotovoltaicSystems Performance Monitoring", IEEE Trans. on Instrumentation andMeasurement, Vo1.47, No.1, pp.30-33, February 2008.

[3] J. Machacek; z. Prochazka; 1. Drapela, "System for Measuring andCollecting Data from Solar-cell Systems", Proc.9th InternationalConference Electrical Power Quality and Utilization, Barcelona, 9-11October 2007.

[4] Z. Wang; L. Chang, "A DC voltage Monitoring and Control Method forThree Phase Grid-Connected Wind Turbine Inverters", IEEE Trans. OnPower Electronics, Vol. 23, No.3, pp. 1118-1125, May 2008.

[5] T. Yuki; H. Yoshikiro; K. Kosuke, "Temperature and IrradianceDependence of the I-V Curves of Various Kinds of Solar Cells"International Photovoltaic Science & Engineering Conference,Shanghai, China 2005

[6] A. Moein, M. Pouladian, "WIH-Based IEEE 802.1 1 ECG MonitoringImplementation", Proceedings of the 29th Annual InternationalConference of the IEEE EMBS Cite Internationale, Lyon, France August23-26, 2007.

[7] H. Zhao, X. Chen, K.H. Chon, "A Portable, Low-cost, Battery-poweredWireless Monitoring System for Obtaining Varying PhysiologicParameters from Multiple Subjects" , Proceedings of the 28th IEEEEMBS Annual International Conference, New York City, USA, Aug 30­Sept3,2006

[8] A. Perujo; R. Kaiser; D.U Sauer; H. Wenzl; I. Baring-Gould; NWilmot;F. Mattera; S. Tselepis; F

[9] H. Zhiqiang; Z. Wenxian; L. Jianke, "Research on High-Speed DataAcquisition and Processing Technique", The Eighth InternationalConference on Electronic Measurement and Instruments, ICEMI'2007

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IV. CONCLUSION

A wireless data acquisition system for photovoltaicgeneration system that uses PICI6F877A microcontroller asthe main control has been presented. Implementation of theEGSR7150 Ethernet to serial module and the access point asWi-Fi communication tool between acquisition equipmentsand user computer for graphically displaying the acquiredparameter has work properly as intended. Implementation ofsuch a system on a laboratory scale photovoltaic generationshows the practical simplicity, efficient and low cost.