04 - Serious post title, attempt number 1: Measurement of the transparency loss of the semi-transparent layer

Hi guys,

I want to reassure everyone that all my samples are yet on my roof (proof in Figure 1).

Figure 1 - Proof

  
For sure you are wondering: why did you put your samples there? Well, the reason is that I’m studying the effect of the weather changes on my semi-transparent layer and in particular I want to investigate the reduction of transparency because of the aging of the polyurethane glue.

For this purpose, with the help of the technicians of IFSTTAR, I realized a low cost device for the measurement of the power reduction of the solar cell because of the presence of the top layer. The idea is to simulate the sunlight in a controlled environment and, as a good engineer, I made some simplifications.

The sun is replaced by an halogen lamp which is able to generate an irradiance power of about 300-400 W/m²_. The lamp is placed in a dark box (basically is an old bedside table with sliding doors) and the distance between the sample for the test and the lamp is about 60 cm (Figure 2).

Figure 2 - Very low cost device for the measurement of the power reduction of the solar cell

The electrical instrumentation is composed by:

• a solar cell having a peak power of 15W, an open circuit voltage Voc of 19.5V and a short circuit current Isc of 0.97A (these values are measured in the standard conditions of 1000W/m² of irradiance and 25°C and they are useful for discriminating the different characteristics of the solar cells in the market);
•  a solarimeter to measure the radiation of the lamp and to verify that it is always constant during all the test;
•  a set of resistances between 2 and 5000 Ω;
• two mutlimeters in order to measure at the same time the intensity and  the  voltage of the solar cell for each value of resistance;
•  a thermometer to check the temperature and verify that the box is not becoming a pizza oven;
•  a plate for the connection of all the devices.

Once all the equipment is installed, the test can be performed. The idea is to measure simultaneously the current intensity and the voltage of the solar cell for different values of resistances.

The data can be plotted in a graph having for abscissa the voltage and for ordinate the current intensity. With a miserable effort of fantasy we will call this plot intensity-voltage curve.

The goal is to find the highest power output of the solar panel (P max), which is given by the product of the intensity and the voltage (Figure 3).

Now the question is: what happens to the Maximum Power Point when the solar cell is covered by the semi-transparent layer? As you can expect, the maximum power point decreases and the reduction depends on the phenomena of absorption, reflection and diffraction between the solar radiation and the sample.

In order to well-quantify the power reduction, we are going to introduce a novel term called ‘’performance loss’’ (please, appreciate again the originality of the name).

 

  where:

-          P_max,unc is the maximum power point of the solar cell;

-          P_max,c  is the maximum power point of the solar cell covered by the semi-transparent layer.

Figure 3 - Example of Intensity-Voltage curve

The performance loss ranges theoretically between 0% (sample perfectly transparent) and 100% (sample totally black). My first samples had a value around 70%, but after the optimization of the mixture, I reached a value of 46%.

In terms of aging effect due the exposition to the weather changes, I observed an average increase of the performance loss of about 10%.

Recently I also studied the effect of each variable (thickness, glue content, grading curve) on the transparency and I quantified their impact through the factorial design, an experimental method commonly used in chemistry able to measure the effect of a variable on the outcome. Regarding this, I will probably dedicate a new post because it’s a very interesting topic and also because my blog will be more attractive with some matrices and formulas!

Bye for now and see you soon!

Domenico

 

“The research presented in this report/paper/deliverable was carried out as part of the H2020-MSCA-ETN-2016. This project has received funding from the European Union’s H2020 Programme for research, technological development and demonstration under grant agreement number 721493”