Solar cell curve tracer with 0.2% reproducibility

Introduction

In 2013, I started with my Maxun One solar bike project. For the development of very lightweight solar cell encapsulation materials, I needed to measure the wear due to atmospheric influence. This requires a measurement system with high long-term reproducibility. By using error corrections, the long-term reproducibility of the solar cell curve tracer is 0.2%. 

Arduino curvetracer with slide projector

Solar cell curve tracer dark box

Solar cell I-V curve

Solar cell I-V curve

YouTube

Matching solar cells

With this curve tracer you can measure the characteristic curves of a single solar cell. Because the reproducibility is high, the curve tracer can be used very well for matching solar cells. The generated power of the solar cell, at the MPP, can be measured accurately by averaging a large number of samples:

Solar cell MPPT measurement in mW

Solar cell curve tracer circuit

The curve tracer is based on an Arduino Uno microcontroller board. The solar cell is loaded with a power MOSFET BUZ10 which is used as a variable resistance. The gate is driven slowly by an increasing voltage so that the drain current increases from zero to the maximum current that the solar cell delivers. During this process the voltage and current are measured and printed to the screen. We need a 12V adaptor for the supply.

Circuit Solar cell curve tracer circuit

Slide projector illumination and tilt

To do reproducible measurements independent of the varying sunlight, a high-power light source is needed. In 2013, LEDs with sufficient power had not yet been invented. That is why I used a slide projector, which light intensity is about 1/3 sun.

The sun is always tilted at a certain angle to the PV panels. Therefore, the reflection of the encapsulation materials has to be measured also. Because, in the Netherlands the average sun angle in the summer is 30 degrees we take this tilt for the measurements.

Solar cell curve tracer

Long-term reproducibility

AFMAKEN.................

Measuring wear and different encapsulation materials require high long-term reproducibility. By using error corrections, the long-term reproducibility of the recent version of the solar cell curve tracer is 0.2%.

The high reproducibility is obtained by

1. 1. The temperature of the solar cell is measured with an LM335.
2. 2. The light is measured with a photocell.
3. 3. Measurements are done in a dark matte black painted box

Solar cell curve tracer compensation

Mechanical construction

Solar cell curve tracer

Solar cell curve tracer

Solar cell curve tracer back side

Solar cell curve tracer hinge joint

Solar cell curve tracer light beamLight tight boxSlide projector foot
Maxeon solar cell tests for Maxun One solarbike

Software

You can download the code at GitHub. Several libraries had changed since 2013, which I have fixed. But I haven't tested the program again, so I don't know if everything works 100%.

The hardware must be calibrated first, see below.

• With the Arduino Software run the project.
• Set the 4 preferences in void loop():

Create the graph in Excel

• Start the serial monitor and wait until the values are printed to the screen.
• When ready, copy the results from the screen with ctrl c. Select the numbers and the row "mV mA mW".
• Paste the results in Excel with ctrl v.
• Immediately look below the Excel table and select “use wizard”.
• Use “finish” to exit the wizard.
• Insert a XY graph from the first two columns.

Calibration

The hardware must be calibrated first. Required instruments:

• Laboratory power supply which can deliver a constant current of about 5 A.
• Accurate multimeter, minimum 5 A.

The calibration procedure is included in Curvetracer.pde. Use this Excel file for automatic calculation of the values.

Illumination sources

The sun

Sun

Doing reproducible measurements with the sun is difficult and we have to wait for the summer to do tests. The measurements have to be corrected for the current sun irradiation [W/m2]. This can be done with a solar irradiance meter.

Solar cell testing light source

LS 1000

Slide projector

The light intensity is about 1/3 sun. This not the recent version with the long-term reproducibility of 0.2%.