Developing an educational kit for I-V curve measurement of photovoltaic devices

John Peter Raja David Raja, Loughborough University, Loughborough, Leics LE11 3TU

Abstract

This research main goal is to provide the kit to the students for understanding the basic concepts of solar energy and their performance characteristics to explore sustainable energy solutions. Developed educational kit provides the students with the tools needed to effectively investigate the performance of PV. There are so many factors which affects the performance of Solar PV like temperature, Dusts, Shading, Orientation and Tilt angle. So, periodic measurement of Solar PV performance is necessary. Electrical I-V characteristics of a solar cell determines the device output performance and efficiency of the solar cell. Since this kit is for teaching purpose a highly reliable method is needed to measure the I-V curve. So, Electronic Load method using MOSFET is used to measure I-V curve of PV. Using this curve, the students could understand the important terms such as short circuit current (Isc), open circuit voltage(Voc), Fill Factor(FF) and its connection with the efficiency, students can also analyse the temperature and irradiance effects on PV parameters for various conditions with both manual mode and automatic mode using educational kit. Arduino is used as a microcontroller to control the variable load for measuring voltage, current, temperature and irradiance. The resulting I-V curve for different temperature and irradiance obtained from the final circuit shows very low noise disturbance and the curve is relatively smooth same as keithley but with such a low cost of 40 pounds. From the analysis we see that use of this particular method for the purpose of tracing I-V curve is very suitable and convenient.

Keywords – I-V curve, Arduino, MOSFET, Educational

Introduction

AimTo develop a device for educational purpose that can trace I-V curves based on the outputs from photovoltaic modules.

Objectives 1.To build a device that can trace current and voltage from the photo-voltaic devices using Electronic Load(MOSFET)2. To display the output as a graph in Excel3. The Educational Kit should work in both Manual sweep mode and Automatic sweep mode.4. To sense temperature and irradiance using sensors and display the data along with the I-V curve.5.To test the educational kit in comparison to the currently used Keithley SMU with the post graduate students.

Explanation of the Topic

The Current-Voltage characteristic curve demonstrates the relationship between the current flowing through the electronic device and the applied voltage across its terminals. Graphing the obtained current and voltage data is referred to as I-V curve and this curve usually acts as the tool to determine and understand the basic parameters of a component or device. Similarly, in PV module the I-V curve determines the conversion capability from solar energy to electrical energy for a particular irradiance and temperature. The various parameters to characterise the solar cell are short circuit current (Isc), open circuit voltage(Voc), Fill Factor(FF) which are obtained from the curve. The efficiency of the solar cell can be analysed only from these parameters. So, it is an important measurement for understanding PV. [1]

Because of the benefits provided by Renewable Energy the study of renewables becomes important in both school and college level. For a complete understanding of solar power, the I-V curve should definitely be included in the solar education. Then only the students can analyse the temperature and irradiance effects on PV for various conditions through practical work. Only practical work connects two

different domains (domain of real objects and observable things and domain of ideas). While, doing the experiments practically it becomes interesting in the form of educational trip or real life projects and the difficult concepts retains in our mind forever [2].

The theory of learning

There are several possible methods to obtain I-V curve of the PV which are enlisted below

Variable resistor and Bipolar Power supply method - In 2002, Malik, Salmi used variable resistor and Bipolar power to obtain the I-V curve and examine the performance of PV. By varying the resistance, they obtain current and voltage data but from their findings they figured out that short-circuit current cannot be obtained using variable resistor. Then also by using BJT switch in Bipolar power they measured the current and voltage data from PV to obtain I-V curve. [3].

Capacitive Load method - Marwan M. Mahmoud (2005) inspected the PV performance using capacitive load and he claims that by using reasonable capacitor value to obtain I-V curve this method will be more efficient comparing to first method. [4]

Electronic Load method - As an alternative method, Yingying Kuai, Yuvarajan (2005) examined the PV module performance using MOSFET due to its fast variation of equivalent load resistance. He studied the performance of this particular method both theoretically and practically by connecting the MOSFET with PV to obtain different current and voltage data for graphing the curve. [5]

DC-DC Converter - Duran, Enrique, Bohorquez, Sidrach-de-Cardona, Carretero, Andujar (2005) found out that SEPIC converter can sweep a complete I-V curve comparing to buck and boost converter from their results. [6]. A best method can be applied to develop the educational kit only by comparing the advantages and disadvantages of all methods. So, the compared advantages and disadvantages of every method is given below:

Advantages and Disadvantages [7]

Method Advantage Disadvantage

Variable Resistor 1)Very cheap and easy to replace 2)Easiest method

1)Reliability and Response is low 2)Need to program in case of using programmable variable resistor.

Capacitive Load1)Excellent uses of their characteristic for conducting a varying voltage2) By charging the capacitor to negative voltage second quadrant can be obtained.

1)Relatively unreliable in circuits -For every new measurement the capacitor must be discharged2)Difficult to control the switches to operate in proper sequence

Bi-polar Power 1)Simple circuit 2)Dark current can also be measured using this method.

1)Switches(BJT) should be operated in three modes 2)Cannot be applied for large power systems

Electronic Load(MOSFET) 1)Highly reliable 2)Frequency of MOSFET is very high (very fast)

1) It has high impedance and low capacitance2)High voltages may destroy the MOSFET.

4 – Quadrant Power Supply

1)Direct display of output is possible in this method2)With this method second and third quadrant curves can also be obtained.

1)Cost is high2)Difficult to build due to higher number of switches. And cannot be used for large PV systems

DC-DC Converter 1)High efficiency 1)Complicated design with

2)Can handle a large output current ripples due to inductor2)Cost factor

Methodology

This kit is for education purpose to teach in the classroom. So, it should be highly reliable. Electronic load (MOSFET) method is the best method to use for obtaining the I-V curve of PV. MOSFET (Metal Oxide Semiconductor Field Effect Transistor) is used in this method for switching the signals i.e. to control the flow of voltage and current in

the circuit. The MOSFET used is IRFZ44N it is an n-channel MOSFET the reasons to choose an n-channel MOSFET for this circuit is it has high efficiency, low resistance and also it is easy to scale. It has three terminals Source, Gate and Drain. Usually MOSFET works in various modes here it operates in enhancement mode. The symbol 1 shows N-channel MOSFET. For safety measures in case of higher voltage or current flows through MOSFET there is an inbuilt diode in the MOSFET to protect it. SYMBOL1

The PWM (Pulse Width Modulation) is given to the gate of MOSFET to control the voltage and current flow in the circuit. It depends on the duty cycle, here the duty cycle is from zero percent to 100 percent to obtain the complete I-V curve. The complete Project flowchart is given below:

Project Steps

Project Flow chart

Voltage Measurement – The Voltage from the PV is measured by connecting voltage divider circuit with the PV input. For reducing the voltage magnitude from 22volts to 5volts two resistors of 4.621Kohm and 0.9958Kohm (four times difference) is connected in series across the input supply. The output from the voltage divider is connected to the microcontroller to measure the voltage.

Current Measurement – The Current from the PV can be measured both directly and indirectly. Resistors or Transistors can be used to measure directly, Hall effect coil and Rogowski coil can be used to measure indirectly [8]. Here, Current sense resistor of 10ohm is used to measure the current from PV [8].

Irradiance Measurement – A Pyranometer is used to measure the irradiance. But the output from the Pyranometer is low. So, an operational amplifier connected in non-inverting amplifier configuration is used to amplify the voltage to 5volts. The resistors used in the configuration are 0.746ohm and 200Kohm

Temperature Measurement – A temperature sensor (LM 35) is used to read the temperature and the microcontroller converts the analog data into digital data in Celsius.

Every measurement output is displayed in the serial monitor of Arduino and initially, it is copied to excel for graphing the I-V curve but later it is made automatic using visual basics.

Circuit Diagram – To measure Voltage and current

Figure 1a – Circuit diagram to measure voltage and current

Components Specification/ModelResistors R1 and R3 4.621KohmResistors R2 and R4 0.9958ohm

Resistor R6 10ohmCurrent Sense Resistor R5 10ohm

Capacitors C1 and C2 100microFaradMOSFET n-Channel (IRFZ44N)

Power Supply 5voltsMicrocontroller Arduino UNO

Photo-Voltaic At Standard Test ConditionWp=5W, Vmp=17.5V, Imp=0.29A, Voc=22.0V, Isc= 0.32A, Operating Temperature = -40degree to +85degree. Max system voltage = 600v

Working Theory - The PV is connected to two voltage dividers with one resistor four time bigger than the other one. The 22 volts from PV is converted to 5volts and given as analog input to the Arduino in pins A3 and A5. Current Sense resistor is connected in between the two voltage dividers. From the two voltage values and current sense resistor value the current data can be obtained from ohm’s law (Ipv = (V1-V2)/10ohm). The PWM for the GATE terminal of MOSFET is given from the Arduino PWM pin 5 to obtain current and voltage data for duty cycle 0percent to 100percent. The ground of the circuit is connected to the Arduino ground pin. The capacitors are connected with the circuit to smooth the curve.

Circuit Diagram - To measure Temperature and irradiance

Figure 1b – Circuit diagram to measure temperature and irradiance

Working Theory - The input from the irradiance sensor (Pyranometer) is connected to the non-inverting amplifier configuration. Since, the pyranometer gives only 10microvolts/Wm -2 the amplifier is used to

amplify the voltage to 5volts [9] and given as analog input to the microcontroller (Arduino UNO). a power supply of 5volt is used to power the amplifier. Finally, output from the temperature sensor is also given as analog input to the Arduino.

Results – Steps of project progression

Voltage and Current Measurement

a) The Voltage Divider is connected directly across a voltage supply initially instead of PV, 4.621Kohm is connected to the positive side of the supply and 0.9958Kohm is connected to the negative side of the supply. When the input of 19volts is given from the voltage supply an output of 3.72volts is obtained using voltage divider and it is measured in the keithley. The main reason to convert the voltage is the Arduino UNO can read only maximum voltage of 5volts.

Figure 2a – Voltage Divider Circuit Figure 2b – Keithley Output

b) To measure the current, a current sense resistor of 10ohms is also connected with the voltage divider. As shown in the figure 2c, Keithley is used to read the current flow in the circuit.

Figure 2c – Current measurement Circuit

c) Since, PWM (pulse width modulation) using MOSFET acts as the load in the circuit. So, it is initially tested with LED. The Digital pin 3 acts as the PWM pin to control brightness of the LED. The LED with 1Kohm resistor is connected with drain terminal of MOSFET, the main reason to use a resistor with LED is to limit the flow of current to prevent damage and source terminal is connected to the ground. Finally, PWM pin from microcontroller is connected with gate terminal. When a duty cycle of 90percent is given to the MOSFET the LED is seen bright as shown in the figure 2d. And an oscilloscope is also connected with the MOSFET terminals to measure the duty cycle and voltage.

R1= 1Kohm R2 = 4.7Kohm

Current sense Resistor = 10ohms

MOSFET

LED1Kohm

Arduino UNO

Figure 2d – MOSFET connection to control LED Figure 2e – Oscilloscope Output

d) The MOSFET is connected with voltage divider and current sense resistor. But now an additional voltage divider is also added along with the present circuit to measure precise values. The new circuit is simulated using Falstad’s circuit simulator before being built in the breadboard. The two voltage values from the voltage dividers is connected to two Analog pins (A3, A5) in the Arduino UNO and the current from the circuit is calculated using ohm’s law Ipv = (V1 – V2)/current sense resistor value. The PWM pin 5 from Arduino UNO is connected to the gate of MOSFET. Figure 2f – Circuit Simulation

The current and Voltage values for duty cycle 0 percent to 100 percent is obtained in the serial monitor of Arduino UNO. The built simulated circuit is shown below in figure 2g. A voltage supply of 22volts is used as input in the circuit to check the measurements. Arduino is programmed to measure an average of 1000values for both current and voltage.

Figure 2g – MOSFET connected with current and voltage measurement Circuit

e) Now a potentiometer is connected with circuit to measure the voltage and current values manually for different duty cycles. The first terminal of Potentiometer is connected to 5volts, second terminal is connected to the Arduino analog pin A1 and third terminal is connected to the ground.

Figure 2h – Manual Mode Circuit

Voltage Supply Voltage DividersCurrent Sense Resistor

Arduino UNO

Arduino UNO

MOSFET

Potentiometer

Current Sense Resistor

Voltage Dividers

The current and voltage of the circuit is measured using Oscilloscope and Keithley before drawing the curve for ten different duty cycles the current and voltage values are obtained and I-V curve is drawn using Excel. Which is shown below:

Figure 2i – Oscilloscope and Keithley output for manual mode

The oscilloscope shows the voltage of 5.5 volts and keithley measures the current for voltages from 0 to 5.7volts. Then the serial monitor is used as display to read the output current and voltage values from the circuit for ten different duty cycles. Instead of voltage supply as input, the PV is connected with circuit as input

Figure 2j – PV connected with manual mode circuit Figure 2k – manual mode – I-V curve

f) Instead of manual mode, the current and voltage values are obtained automatically from the circuit and Arduino is programmed to automatically calculate and display current and voltage values for duty cycle from 0percent to 100percent. An average of 100000samples and 1000samples are taken separately and the I-V curve is drawn for both program.

PV

Manual Mode Circuit

Output in serial monitor

Figure 2l – I-V curve for 100000 samples Figure 2M- I-V curve for 1000 samples in automatic mode

g) For 100000samples the I-V curve looks beneficial for understanding but for 1000 samples it is still not beneficial. So, additional to the circuit a capacitor between voltage divider and current sense resistor is added for limiting the ripples to get a smooth output. An oscilloscope is used to see the limited ripples. But still there are ripples in the output voltage. So, as the final circuit a capacitor is added in the input end across PV and another capacitor is added between voltage divider and MOSFET with a resistor between source terminal of MOSFET and ground. And a 5volt power supply is connected along with PV as a compensate for the voltage drop across the current sense and MOSFET stabilising resistors.

Figure 2N – Final circuit to measure I and V Figure 2O – I-V curve for the circuit

h) Temperature and Irradiance Measurement - To measure I-V curve for different temperature and irradiance. The irradiance sensor is connected to the pin 3 of operational amplifier. The resistors 220Kohm and 1Kohm are connected with Pin 1and 2 of the amplifier. The output from the pin1 of amplifier is connected to analog pin A4. Finally output from the temperature sensor is connected to the Analog pin A0. A separate voltage supply is connected to pin 8 for powering amplifier.

Figure 2P – Circuit to measure temperature and irradiance

Capacitor C1

Capacitor C2

Resistor R6

5 volt supply

Voltage Supply – to power amplifier

Operational amplifier

Irradiance Sensor

Temperature Sensor

Voltage and Current Measurement Circuit

Figure 2Q – IV curve for different temperature

Discussion

The resulting I-V curve obtained from the final circuit shows very low noise disturbance and the curve is relatively smooth. From the I-V curve of the final circuit, Short circuit current Isc of the PV is read as 0.223amps and open circuit voltage of the PV is 16.5 volts. Since no MPP (Maximum Power Point) Algorithm is used in the circuit precise values of Vmpp and Impp is difficult to read. But approximately the fill factor and efficiency of the PV can be calculated. Using this curve, the students could understand the important terms in the solar power to analyse the efficiency of the PV. There is a breakdown voltage in the curve due to the additional 5volt supply to compensate loss. So, the students can also understand the concept and causes of breakdown voltage.

From the I-V curves for different temperatures and irradiance. It is noted that as the temperature of the PV decreases it has little effect on short circuit current but it has a high effect in open circuit voltage. It is completely reverse for irradiance as the irradiance decreases it has little effect on open circuit voltage but it has high effect on short circuit current. Using this curve, the students could understand the effects of temperature and irradiance on PV.

Now the voltage, current, temperature and irradiance values are manually copied from serial monitor of Arduino to Excel for obtaining the I-V curve of PV. But it is little bit difficult and consumes some time so, using visual basics the excel is programmed to measure the current, voltage, irradiance and temperature values directly from the Arduino UNO via serial port

3. When the button e is pressed a particular set of values is obtained. This developed excel sheet can be send to the students via e-mail or can be downloaded from the online website.

Figure 3a – Excel Layout

As the final setup both circuits are designed in a strip board with a potentiometer for manual mode and a push button for Automatic mode with a toggle switch between them to change the mode. And an enclosure is used to cover the setup.

Manual Mode

Automatic Mode

Toggle Switch

IV Tracer

Figure 3b – Circuits in strip board Figure 3c – Educational Kit

Since there are no undergraduate students in the university in this period. The kit is tested with my classmate. As a result, he can understand three different aspects such as the way to measure voltage, current, temperature and irradiance, functions of important electronic components like microcontroller, resistors, capacitors, MOSFET and operational amplifier. Finally, he can understand the terms of I-V curve easily comparing to Keithley which he used in his first semester of Solar Power 1 lab.

The Educational Kit comes with PV and IV tracer. The cost of PV is 15 pounds and the cost of IV tracer is (Arduino UNO – 21.66 pounds, resistors 5pound, Mosfet – 1.24 pound, 25pence, operational amplifier – one pound,)30 pounds. So, all together the cost of educational kit is 45 pounds. Whereas the cost of keithley is 4,700 pounds. With such a low cost kit the students will able to understand the terms and conditions to measure the IV curve with same as keithley.

Figure 3d – Complete Setup

Conclusion

In conclusion, from the above analysis, we see that use of Electronic Load method to trace IV curve is very reliable and convenient method to educate the students about Solar Power, apart from irradiance sensor

the circuit is also cost effective (35pounds) comparing to use of Keithley which is a high cost component . The students can work in two different modes by using this kit. In-case if they want to learn the duty cycles connection with the curve they can use manual mode to graph the IV curve and in-case they want to learn terms in the IV curve to obtain the efficiency then they can move directly to automatic mode. A little ripple which was from the circuit is also eliminated by adding capacitors. The next improvement which can be done to the kit was to add a LCD display directly to the kit for graphing the IV curve instead of using excel in the laptop. Not only this kit can be used for undergraduate students it can also use by next year MSc students for their understanding in solar power 1 lab with keithley. In the long term this kit circuits can be modified with high power components to measure IV curve of large solar field.

Acknowledgement

I am very grateful to my supervisor Dr. Tom Betts, for patiently correcting my mistakes in the project. Who cared and encouraged me in every steps on my stairway to knowledge heaven. I have found these manifestations in him as a teacher, adviser, friend and a human being without whom it would never have seen the light of day!

PV

I-V tracer Laptop

Appendices

References

[1] University, Loughborough. PhotoVoltaic Characterisation, Laboratory Notes for Solar 1 Module. 2015.

[2] R. Millar, "The role and purpose of practical work in the teaching and learning of science (first draft)," 2012.

[3] A. Q. Malik and Salmi Jan Bin Haji Damit, "Outdoor testing of single crystal silicon solar cells," Renewable Energy, vol. 28, no. 9, pp. 1433–1445, 2003.

[4] M. M. Mahmoud, "Transient analysis of a PV power generator charging a capacitor for measurement of the characteristics,"Renewable Energy, vol. 31, no. 13, pp. 2198–2206, 2005.

[5] Y. Kuai and S. Yuvarajan, "An electronic load for testing photovoltaic panels,"  Journal of Power Sources, vol. 154, no. 1, pp. 308–313, Mar. 2006.

[6] E. Duran, J. M. Enrique, M. A. Bohorquez, M. Sidrach-de-Cardona, J. E. Carretero, and J. M. Andujar, "A new application of the coupled-inductors SEPIC converter to obtain I-V and P-V curves of photovoltaic modules," p. 10, Sep. 2010.

[7] E. Duran, M. Piliougine, M. Sidrach-de-Cardona, J. Galan, and J. M. Andujar, "Different methods to obtain the I–V curve of PV modules: A review," pp. 1–6, May 2016.

[8] [5] T. Gamblin, "Voltage divider circuits: Divider circuits and Kirchhoff’s laws - electronics textbook,". [Online]. Available: http://www.allaboutcircuits.com/textbook/direct-current/chpt-6/voltage-divider-circuits/. Accessed: Aug. 3, 2016

[9] B. Yarborough, "Components and methods for current measurement," 2012. [Online]. Available: http://powerelectronics.com/power-electronics-systems/components-and-methods-current-measurement. Accessed: Aug. 7, 2016.

[10] electronics +radio, "Non-Inverting operational amplifier circuit," 2016. [Online]. Available: http://www.electronics-radio.com/articles/analogue_circuits/operational-amplifier-op-amp/non-inverting-amplifier.php. Accessed: Aug. 25, 2016.

Arduino Program int sensorPin1 = A5;int sensorPin2 = A3;int irradiancePin = A4;int Temperaturepin = A0;int fadePin = 5;int ledPin = 13;int PWM_duty;float autoduty = 0;long voltagesensorValue = 0;long currentsensorValue = 0;long irradiancesensorValue = 0;long temperaturesensorValue = 0;float Voltage;float Current;float Irradiance;

float Temperature;float resistor1 = 1000;float resistor2 = 4700;float currentsenseresistor = 10;float PWM_duty_display;String SerialReadString;char SerialReadChar;

void setup() { TCCR0B = (TCCR0B & 0b11111000) | 0x02; pinMode(ledPin, OUTPUT); pinMode(fadePin, OUTPUT); Serial.begin(9600);}

void loop() {// Serial.println("Voltage, Current, Irradiance, Temperature"); for(int x=0;x<=100;x++) { autoduty=int(255*float(x)/100); analogWrite(fadePin, autoduty);

SerialReadString = ""; while(SerialReadString != "e"){ if (Serial.available() > 0) { SerialReadChar = Serial.read(); SerialReadString = SerialReadChar; }// delay(1000); } // Measure voltage, current, irradiance and temperature (average of 1000 samples each) voltagesensorValue = 0; currentsensorValue = 0; irradiancesensorValue = 0; temperaturesensorValue = 0; for(int i=1;i<=1000;i++){ voltagesensorValue = voltagesensorValue + analogRead(sensorPin2); currentsensorValue = currentsensorValue + analogRead(sensorPin1); irradiancesensorValue = irradiancesensorValue + analogRead(irradiancePin); temperaturesensorValue = temperaturesensorValue + analogRead(Temperaturepin); } voltagesensorValue = voltagesensorValue/1000; Voltage = ((5./1023.)*voltagesensorValue)/(resistor1/(resistor1+resistor2)); currentsensorValue = currentsensorValue/1000; Current = ((5./1023.)*currentsensorValue)/(resistor1/(resistor1+resistor2)); Current = (Voltage-Current)/currentsenseresistor;

irradiancesensorValue = irradiancesensorValue/1000; Irradiance = ((5./1023.)*irradiancesensorValue*450);

temperaturesensorValue = temperaturesensorValue/1000; Temperature = (temperaturesensorValue/1024.0) * 5000 / 10; Serial.print(Voltage,3);

Serial.print(", "); Serial.print(Current,3); Serial.print(", "); Serial.print(Irradiance,1); Serial.print(", "); Serial.println(Temperature,1); } Serial.println();}