A low-cost Photovoltaic (PV) array Monitoring System - IEEE Xplore

2013 IEEE Conference on Clean Energy and Technology (CEAT)

A low-cost Photovoltaic (PV) array Monitoring System Ahmad Rivai*, and Nasrudin Abd. Rahim UM Power Energy Dedicated Advanced Centre (UMPEDAC), Level 4, Wisma R & D, Lembah Pantai, University of Malaya, 59990 Kuala Lumpur, Malaysia * Corresponding Author. Tel: (603) 22463246, Fax: (603) 22463257, E-mail: [email protected] characteristics of a PV module in real meteorological test conditions. The temperature and solar irradiance are the common parameters observed by monitoring systems [2, 4, 5, 7]. These are the parameters that affect the energy produced by a PV system. The deviation of temperature or solar irradiance on PV array changes all PV array parameters [2, 7]. The monitoring of PV array characteristics will enable an analysis of performance, degradation and failure of PV systems [4]. I-V characteristics are also monitored by monitoring systems [4, 7]. Monitoring the I-V characteristics can be used to investigate and compare the actual power produced by the modules under realistic operating conditions [4]. In this paper, a low cost monitoring tool to monitor PV array characteristics is developed. The system monitors temperature, solar radiation and I-V characteristic of PV array. A simple and low-cost resistive load I-V curve tracer is used for sweeps the I-V characteristic of PV array. The tool is also able to monitor the power produced by the PV array and draw the I-V characteristic of the PV array. The power and the curve are presented on a graphical data display. Hence, the monitoring tool can monitor whether the MPPT is working properly or not.

Abstract— The hardware and software design of a low-cost photovoltaic (PV) monitoring system are presented. The system was designed on ATmega8535 microcontroller and applied for PV array characteristics. The PV array characteristics monitored are solar irradiance, temperature, short-circuit current, open-circuit voltage, maximum power, voltage and current at maximum power, fill factor and also efficiency of PV array. The system uses a Pyranometer LI-200 for solar irradiance sensor, an LM35 for temperature sensor, a simple voltage divider for the voltage sensor, and a shunt resistor for the current sensor. The software was developed using Visual Basic (VB). Index Low-cost, Monitoring system, Microcontroller, Photovoltaic.

I. INTRODUCTION Monitoring system is important to maintain a system’s sustained operability, and for a user to understand glitches that occur while system is operating. In developing the PV system, information of photovoltaic characteristics is essential as well as the information on meteorological. Many monitoring systems have been developed in order to evaluate PV system performance. Several instruments using conventional electronics or based on microprocessor dataacquisition system (DAQS) are developed. It is used to collect, register, integrate and record meteorological data and also the electrical characteristic of PV system. [1-11] A wireless data acquisition system that helps to estimate solar energy potential considering the remote region’s energy requirement has been proposed in [2]. A system for the remote monitoring and control of complex stand-alone photovoltaic plants has been described in [3]. This system records and periodically reports the overall performances. In case of incorrect behavior, the system will immediately inform the operator. Some researchers also develop a monitoring system for current-voltage characteristic of PV module. For example a system in [4] that is capable of continuously monitoring the I-V characteristics of seven modules. Also [7] has developed a computer-based instrumentation system for PV module characterization that allows drawing of the I-V

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II. PV MONITORING SYSTEM DESCRIPTION The PV monitoring tool is developed to collect and display PV array characteristics. The tool is developed using ATMega8535 microcontroller. The ATMega8535 microcontroller plays a big role in collecting data to determine the PV characteristic. However, the main functions are: • Control switching combinations • Reading data from sensors • Display data on LCD • Transfer and receive data using serial communication. Fig. 1. shows the block diagram of the PV monitoring system. The computer is used to present data and control the tool operations. For this purpose the graphical user interface is developed using VB.Net, with features as the following:

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In this work, PA3..PA0 are used as analog inputs. They are connected to the sensors. PA7, PA6, and Port B are used as digital output that connects to the LCD. PA7 is used as register select signal (RS), PA6 is used as enable signal (E), and Port B used as data bus signal (DB). Port C, PD7, and PD6 are used as the MOSFETs signals. Port C (PC7..PC0) are used as SW7..SW0 signal, PD7 is used as SWrtm signal, and PD6 is used as SWscanning signal. The MOSFETs signals are first connected to the gate drive. The gate drives are used to drive the MOSFETs gate. Pin PD0 (RXD) and PD1 (TXD) are used as serial communication between PC and microcontroller. The configuration of serial communication is set as follows: • Baud rate = 19200 • Parity = None • Data bits = 8 • Stop bits = 1 • Encoding = Default One of the peripheral features of ATMEGA8535 is programmable serial Universal Synchronous and Asynchronous serial Receiver and Transmitter (USART). USART is a highly flexible serial communication device. USART Baud Rate Register (UBRR) functions as a baud rate generator. Error in the serial communication can be minimized by using crystal 11.0592 MHz and baud rate of 19200 because the value of UBRR is equal to 35. The value of UBRR is an integer so this combination has an error percentage of 0.00%. Equation for calculating the UBRR value for asynchronous normal mode of operation using an internally generated clock source is described in Eq. 1.

• Graphical data display. • Able to save file data for analysis. The tool ability to trace I-V curve on high-power PV panel is one of its novelty (the maximum voltage and maximum current depends on the resistor and switching component of I-V curve tracer). This allows to measure PV panel characteristics directly at the PV system place of assembly. It is also able to analyse the MPPT performance of power converter. In this work, its capability is tested using a grid-connected inverter and PV Module.

ATmega8535 TX RX

Port B, PA6, PA7

I-V curve scanner

2X16 LCD

PC ADC

DATA

PD6, PD7

Adjustable resistive load

Gate Drive SW

SW scaning

Voltage sensor

Current sensor

SW rtm

+

-

+

-

Inverter LM35 Pyranometer LI-200

PV Array

Power Converter

Grid

UBRR =

Fig. 1. The PV monitoring system block diagram.

(1)

Where, UBRR : Contents of UBRR Register (0–4095). fosc : System oscillator clock frequency (Hz). BAUD : Baud rate (bps).

A. I-V Curve Tracer In order to determine the I-V characteristics, the power generated by the PV panel must be extracted. The low-cost resistive I-V curve tracer is used to serve this purpose. It acts as an adjustable resistive load, which consists of resistors and MOSFETs. The MOSFET’s maximum ratings on this prototype are VDS = 200V, and ID=18A so, the tool is capable to monitor PV array with voltage open circuit less than 200V and Short circuit current less than 18A.

C. Sensor A sensor is needed to measure a physical quantity and convert it into a signal which can be read by the microcontroller. The microcontroller board was designed to read analogue signal between 0 until 2.56V, so the sensor circuit output is designed within that range. The sensors used are low-cost sensors; simple voltage divider for sensing voltage, simple shunt resistor for sensing current, pyranometer LI-200 for sensing the solar irradiance , and LM35 for sensing temperature. The solar irradiation sensor circuit is shown in Fig. 2. and the temperature sensor circuit is shown in Fig. 3.

B. Microcontroller The ATmega8535 provides 32 general purpose I/O lines, which are Port A (PA7..PA0), Port B (PB7..PB0), Port C (PC7..PC0), and Port D (PD7..PD0). They are 8-bit bidirectional I/O port. Port A also serves as the analog inputs to the ADC. The ATmega8535 has a 10-bit successive approximation ADC. The ADC is connected to 8-channels analog multiplexer which allows eight single-ended voltage inputs constructed from the pins of Port A.

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f osc − 1. 16 ⋅ BAUD

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Fig. 2. Solar irradiation sensor circuit.

Fig. 3. Temperature sensor circuit.

III. THE MONITORING AND COLLECTING DATA PROCEDURES For monitoring and collecting data, the microcontroller and computer performs many procedures for the PV monitoring system tool. However they can be subdivided into three main procedures: standby, scanning, and monitoring. Standby is a procedure where the microcontroller will wait for a command from the computer. In this procedure, all switches in the I-V curve tracer are disconnected, and LCD displays “standby”. Scanning is a procedure where I-V tracer finds out the IV characteristics of the PV arrays. In this procedure, first the computer sends a scanning command to the microcontroller. The LCD will displays “scanning” in second line and then the microcontroller will send data of open-circuit voltage, temperature, and solar radiation to the computer. After that the SWscanning is connected and SWrtm is disconnected, then SW0...SW7 start connecting sequential while microcontroller sending current, and voltage data to the computer. After this scanning procedure, the PV monitoring system tool will automatically go back to standby procedure and the computer will display this data in the GUI. The third procedure is monitoring procedure. This procedure is where the PV monitoring system tool will monitor all data (voltage, current, solar radiation and temperature) in real time mode. In this procedure, first the computer will send a monitoring command to the microcontroller. Then SW0..SW7 and SWscanning are disconnected and SWrtm is connected. The microcontroller sends data to the computer continuously. The data is calculated by the computer to get the nominal value. The calculated result will be displayed in the GUI and also sent to the microcontroller to be displayed on LCD. When the microcontroller receives the exit command from the computer, the monitoring procedure stops and automatically revert to standby procedure. The procedures described above are clearly presented in flowchart as shown in Fig. 4.

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Fig. 4. Flowchart of the proposed PV monitoring system.

The communication between the microcontroller and the computer is designed using master/slave model. In order to execute the subroutine in the microcontroller (slave), the computer (master) sends some command to the microcontroller. The command in ASCII code is sent using the serial port. The commands that are received by microcontroller will save in a “case” registry (r22 register of ATmega8535). Besides saving the command, the case register will also save data from the computer to be displayed on the LCD. Besides sending command to the microcontroller, calculation of the data is another task performed by the computer. The computer calculates data from the microcontroller to convert this data to physical nominal value. The conversion of the value is done by multiplying the ADC reading value with a constant. The constant is obtained from the comparisons of the ADC reading value with commercial measurement that has been calibrated. In the scanning procedure, the computer will also calculate the power for each combination of switching, the

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TABLE I. ELECTRICAL CHARACTERISTIC OF THE SOLAR MODULES AT

peak power (Pmax), the peak power voltage (VPmax), the peak power current (IPmax), the fill factor (FF), and the PV array efficiency. The computer calculates those parameters after receiving “stp” from microcontroller. The calculation results are displayed in the GUI and LCD. These data also can be saved in Microsoft excel file. To monitor PV array characteristics in a long time, the PV monitoring system tool is equipped with automatic scan mode. This mode will automatically execute the scanning procedure periodically according to the time chosen by the user. The I-V curve is drawn every period in Real Time Data tab. Peak power, fill factor, voltage open circuit, short circuit current, solar irradiation, and solar temperature are drawn in daily data tab. From these curves, users can see the effect of temperature and solar irradiation toward short-circuit current, open-circuit voltage and the peak power. These monitoring data can also be saved in Microsoft excel file. If S is the actual solar cell area, user can calculate the efficiency of PV array (ȘPV) from the peak power (Pmax) and the solar irradiation (G) data. It is calculated using the Eq. 2.

ηPV =

Pmpp G ⋅ S.

STANDARD TEST CONDITIONS (STC)

Parameter Peak power Peak power voltage Peak power current Open circuit voltage Short circuit current

Ppc Pmax .

Isc

Value 75W 17V 4.4A 21.7V 4.8A

STC: irradiance level 1000W/m2, spectrum AM 1.5 and cell temperature 25oC.

In this test four PV solar modules are connected in series. The solar radiation sensor is located at upper side of the PV module and the temperature sensor is located at under side of the PV solar module. The experiment setup is shown on Fig. 5.

(2)

In order to monitor the MPPT performance of a power conditioning unit, the current and voltage sensors are connected as shown in Fig. 1. These connections make it possible to monitor voltage and current of the PV panel when it is connected to the power converter or I-V curve tracer. The power generated by the PV array is extracted by the power converter and monitored using the Real Time Mode (RTM). This mode will execute the monitoring procedure and display the real time data in LCD and GUI. The data displayed on the LCD is chosen by clicking the VI or Power button. The tool draws the I-V curve and then monitors the power of PV panel extracted by the power converter. The GUI shows the power extracted by power converter climbing the I-V curve. Data that are being displayed in the GUI are Voc, Isc, VPmax, IPmax, and Pmax of PV panel and also the power converter voltage (Vpc), the power converter current (Ipc), and the power converter power (Ppc). The users can also calculate the MPPT efficiency of power converter (ȘPmppt) using Eq. 3.

ηPmppt =

Symbol Pmax Vpmax Ipmax Voc

Fig. 5. The automatic scan testing experiment setup.

Fig. 6. shows result of scanning process at 12:56:03PM and the data as follow: • Peak power (Pmax) = 206.6W. • Peak power voltage (Vpmax) = 59.44V. • Peak power current (Ipmax) = 3.48. • Open-circuit voltage (Voc) = 76.48V. • Current short-circuit (Isc) = 4.10A. • Solar irradiance level (G) = 995.50 W/m2. • Temperature of solar panel (T) = 44.29ºC. • Fill factor (FF) = 0.66.

(3)

IV. RESULTS AND DISCUSSION The PV monitoring system was tested using shell SP75 module. The module consist of 36 series connected 125 x 125 mm mono-crystalline silicon solar cells. The electrical characteristic of the module at Standard Test Conditions (STC) is shown in TABLE I.

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Fig. 6. I-V curve and P-V curve of the scanning process.

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The peak power of PV panel is 276.036 watt and the inverter power is 224.146 watt. That is mean efficiency of MPPT is 81.20%.

Fig. 7. and Fig. 8. are the monitoring data from 12:10:08 PM until 1:23:01 PM. Fig. 7. shows graph of Pmax (Watt), G (Wm-2) and Isc (A). Fig. 8. shows graph of T (ºC) and Voc (V).

Fig. 7. Graph of peak power (Pmax), solar irradiance level (G) and shortcircuit current (Isc). Fig. 9. MPPT Performance monitoring experiment setup.

Fig. 8. Graph of solar panel temperature (T) and open-circuit voltage (Voc).

Fig. 10. I-V curve, P-V curve, and the inverter power.

The capability in monitoring MPPT performance of power converter has been verified using a 300 watt grid connected inverter. This inverter uses constant voltage method for MPPT. The inverter input is six PV solar modules (Shell SP75) put in series. Fig. 9. shows block diagram of experiment setup for monitoring MPPT performance. First the monitoring system scan the characteristic of the PV panel then it sets the PV monitoring system in Real Time Mode. A node is shown in GUI at power-voltage curve. The node shows the power produce by PV panel. Fig. 10. shows the result of experiment that was conducted at 2:51:21 PM. The results are as following: • Peak power (Pmax) = 276.036W. • Peak power voltage (Vpmax) = 77.612V. • Peak power current (Ipmax) = 3.556A. • Inverter power (Ipc) = 2.405W. • Inverter voltage (Vpc) = 93.2V. • Inverter power (Ppc) = 224.146V. • Fill factor (FF) = 0.578.

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V. CONCLUSION A photovoltaic (PV) monitoring system with a low cost sensors and resistive I-V curve tracer has been developed. It is capable of monitoring the environmental parameters (solar irradiance and temperature) and PV current-voltage (I-V) characteristic (I-V curve, Pmax, Vpmax, Ipmax, Voc, Isc, and FF). It is also capable of monitoring the efficiency of PV array and the performance of the MPPT algorithm. REFERENCES [1]

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