Jun 26, 2016

Defects in PV module created during installation

Defects in PV module created during installation

After the previous article ‘Can people step on the modules?’ was published, many people asked questions like ‘Can people stand on the frame?’, ‘Does the impact during the workers move the module damage the module?’, ‘Does the dropped tools result in cell cracks?’… Here I share a report from ISFH [1] and our internal experiment result to give you some idea what might be dangerous during installation.

During the installation of PV system, the following external forces are frequently seen:
Frequently seen external force acting on modules
Overturning from vertical onto glass side
Overturning of a pallet (vertical transport)
Poor pallet for modules (horizontal transport)
Horizontal drop out of carrying height
Vertical drop out of carrying height
Scratching module corner over another module´s back sheet during stacking
Pulling module rear side over ladder
Step on module frame
Crossing a generator area
Drop a tool onto module
Drop of one module side caused by stacking of the stacking corners
Ungentle lay down onto module substructure
Overhead handling (module backsheet
lays on helmet)

Item 16 are not allowed to happen, and they are usually taken as ‘accident’. Once it happens, the modules should be check. It is helpful to have module EL inspection before installation, after the modules are installed, the responsibility of the cracks is not easy to be clarified.

In order to understand the influence of these forces, IEA Task 13 had an investigation to get more understanding [1]. Here is the test plan:
Test Description

1. Charge the module corner with rising load.
2. Charge the long side with rising load.
3. Charge the module middle with rising load .

4. Drop down of skittles ball onto module middle/corner with rising drop height.

5. Drop down of one module edge with rising drop height, sunny side down.
6. Drop down of one module edge with rising drop height, sunny side up.

The test samples are all 60-cell modules, and the tests were performed under 25°C and -5°C. As a conclusion, the cell breakage at -5°C is more serious than at 25°C, because the encapsulant EVA is harder under low temperature, and the material can not absorb the external impact forces.

Influence of stepping on the module
The experiment is done with two module types, and the result is shown in the following charts:

Module type 1
Stepping test at 25°C (type 1)
Stepping test at -5°C (type 1)

Module type 2
Stepping test at 25°C (type 2)
Stepping test at -5°C (type 2)

1. Stepping on middleCell cracks in all conditions, two temperatures and two module types all have broken cells.
2. Stepping on cornerThere are no cracks at 25°C, but both module types have cracks at -5°C.
3. Stepping on middle of module edgeThere are no cracks at 25°C; but one module type has crack at -5°C.

As a summary, the highest risk is stepping on the middle, and stepping on corner is the next. Although the risk at high temperature is lower, we still suggest installers should not step on the corner of the module. If necessary, stepping on the fixing point has the least risk.

The above test results meets our own test result (Can people step on the modules?). We also tested stepping on the middle of module edge, and the result is the same (photos below). Under room temperature, stepping on the long side and on the short side did not have any cracks.
Stepping on the middle of the long edge

Stepping on the middle of the short edge

Stepping on the corner

One interesting finding in our experiment, there is a soldering problem in this module, when a person standing on the module, the contact between ribbon and busbar became better, so the EL image also became brighter. When the person left, the EL image of the cell became darker again (see GIF animation below). This is an insufficient soldering, and the contact is bad in most of the time. Only when it is pressed under external force, the contact temporarily becomes better. This kind of defect will reduce the module power, and there is also high risk of hotspot. The contact between ribbon and cell will be worse and worse after outdoor exposure, and the problem could be even worse in the future.
Insufficient soldering under external stress

Falling tool

This experiment used wood ball to simulate heavy tool falling on the module. The weight of the wood ball is 1515±30g, the diameter is 130±0.2 mm. The wood ball will hit the same position several times, every time the height is increased until the cell is broken. The module temperatures are 25°C and -5°CHere is the result:
Wood ball falling test
Module type 1
Module type 2

Conclusion of wood ball falling test:
1. At-5°C, serious cell breakage starts to happen when the height is below 50cm, and no matter it fell on the center or corner of the module.
2. At 25°C, there are minor cracks for height below 50cm.
3. In general, falling on the module edge is more serious than falling on the center of the module.

When the wood ball hit the module edge, there is more support from the frame at edge, so the module has less deflection and unable to absorb the impact energy, therefore the cell damage is more serious. When the wood ball hit the module center, the module have larger deflection and decrease the impact to the cells. This kind of defect is frequently seen during our on-site EL inspection, it can be the falling of tools (photo below).
EL image of impacted cell

Dropping of module

One end of the module is supported by a rod of 5-50cm, and subsequently remove the rod to let the module fall down. The experiment is done with module sunny side up as well as sunny side down. For the test of sunny side down, there is another module under the testing module, and there are plastic corner protect at four corners of the module. The test of sunny side up let the module fall on the pallet directly, because most packing design has the bottom module sunny side up.

Module type 1
Module type 2

1. -5°C is worse than 25°C, especially for module type 2.
2. Sunny side down has less influence, because there is another module under it, and the frame surrounded form an ‘air bag’ to reduce the influence of the impact. The module is safe up to 30cm without creating cracks.
3. Sunny side up falling has serious influence, the cells start to crack at height of 5cm.

Does the experiment make you feel the modules are weaker than you expected? Yes, it is, however if the installer handle modules carefully, these defects will not happen. On the other hand, systems owners should be careful and notice whether your installer do the work in the right way. If possible, an EL inspection for acceptance test is helpful to secure the investment in.

[1] Influence of PV Module Handling on Solar Cell Cracking, ISFH, Hanwha Qcells GmbH, IEA INTERNATIONAL ENERGY AGENCY Task 13 Workshop @ Intersolar 2015, Munich, Germany

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