Micro-fractures, also known as micro-cracks, represent a form of solar cell degradation. The silicon used in the solar cells is very thin, and expands and contracts as a result of thermal cycling. During the day, the solar panels expand because of higher temperatures. Small imperfections in the silicon cell can lead to larger micro-cracks. The length of micro-cracks can vary; some span the whole cell, whereas others appear in only small sections of a cell. Micro-cracks can affect both energy output and the system lifetime of a solar photovoltaic (PV) system.
How do micro-cracks occur?
Cell fractures are a common issue faced by solar panel manufacturers and system owners alike, before and after installation.
Manufacturing defects can usually be attributed to poor quality or process control. The environmental conditions that can cause micro-cracks in solar PV systems include:
- Thermal cycling (variation of temperature between night and day)
- Humidity and freezing
- Cyclic (or dynamic) pressure loads and wind loading
- Heavy snowfall
Mechanical stress in the supply chain and logistics process can be caused by:
- Incorrect packaging
- Unsuitable transportation methods
- Incorrect handling techniques
During the installation of a solar PV system, mechanical stress can be caused by:
- Stepping on modules or resting other equipment on modules
- Bumping or dropping modules as they are lifted onto the roof
- Installation on a nonplanar surface, which may cause twisting of the mounting frame and place stress on the module
How to detect micro-cracks?
Cell and module manufacturers work to prevent micro-cracks in cells and modules during manufacturing and assembly. However, wafers and cells can chip, which can lead to microcracks. Manufacturers perform several quality testing methods to identify micro-cracks. Manufacturers perform incoming and outgoing inspection, such as electroluminescence (EL) or electroluminescence crack detection (ELCD) testing.
EL testing is a process that makes use of image analysis and measurement, which enables sight directly into the solar cells to locate inherent potential defects. EL testing can detect hidden defects that were not found by other testing methods, such as infrared imaging with thermal cameras, flash testing and V-A characteristic. The information gathered from these tests produces an image, which can provide evidence of any faults or variations within a cell, including micro-cracks.
Ideally, EL testing is carried out once before lamination to identify and replace the defective solar cells, and then repeated after lamination. These tests can be time-consuming and require extensive resources that some PV manufacturers are not willing to undertake, but it is necessary to produce quality solar panels.
With the help of an ELCD test, a PV manufacturer can evaluate the structural quality of solar cells and any other possible defects caused by improper handling of photovoltaic panels.
Nowadays, the majority of large solar panel manufacturers have integrated the ELCD test in their production lines. At the same time, many small and medium-size manufacturers do not invest in ELCD test equipment.
What defects can the ELCD test detect?
With the help of the ELCD test, a manufacturer can detect defects that are normally not visible. Defects that can be found with an ELCD test are:
- Broken cells and micro-cracks in the cells
- Detection of busbar contact defects
- Detection of missing or interrupted screen-printed fingers
- Detection of nonhomogeneity and foreign matter in the crystalline silicone
What are the consequences of micro-cracks?
According to research, micro-cracks have the potential to create an electrical separation, resulting in inactive cell parts. However, determining the power loss caused by these microcracks is difficult because micro-cracks can have varying to no effects. Some modules that have micro-cracks can still meet the warranted power over the module lifetime, so rejecting every module that contains a micro-cracks is not necessary but can create issues, especially when dealing with warranty. Some research determined that losses of up to 2.5% can be experienced in a module with a large number of cracks that do not isolate parts of the cell. Larger losses can be experienced for a module with micro-cracks that do isolate parts of the cell. Micro-cracks also have the potential to produce hot spots. These occur when the internal resistance of the damaged cell rises and causes an increase in cell temperature as the current passes through. Hot spots have been shown to cause further damage to a cell.
How to prevent micro-cracks
To effectively prevent solar panel micro-cracks, three key areas must be addressed: manufacturing, transportation/installation and environment (manufacturing construction). Selecting a solar panel manufacturer that acknowledges the prevention of micro-cracks is a critical part of the solution.
A reputable manufacturer and certified installer are part of the prevention of solar panel micro-cracks. Certified installers must purchase solar panels through authorized distribution channels. Such channels are often in place to uphold the integrity of the solar module from manufacture through installation.
For installers who are looking for a module supplier, the best way to avoid micro-cracks is to ensure that the supplier has the following:
- A well-defined supply chain
- A warranty program that guarantees consumer confidence
- A testing procedure that ensures each individual module receives EL testing
- A strong reputation
New customers should consider incorporating active monitoring into their system; this can be at a module level, at a multiple module level or at a string level. Module-level data can pinpoint a power loss to an individual module, whereas string-level data can identify a power loss to a whole string or section of panels. Although module-level active monitoring is more accurate, it is more expensive than string-level active monitoring. There are several solar monitoring services that can be used to ensure that the solar PV system is performing as expected. New customers should also select a Tier 1 accredited installer with a credible reputation as well as solar industry construction experience. This installer must follow all standards and manufacturer installation guidelines. For system owners with active monitoring who are concerned that their solar PV system is affected by micro-cracks, an analysis of the data will show if the system is underperforming. This data can be used as evidence to claim warranty on the module. At that point, owners can contact their installer or refer to their warranty documentation for more information. For system owners who are concerned that their system is being affected by micro-cracks and who do not have active monitoring installed, retrofitting active monitoring should be considered, so that data can be collected. Along with all other recommended operations and maintenance procedures, system owners should visually inspect their solar PV system regularly. System owners should also consider an operations and maintenance service plan, under which qualified personnel come and service the solar PV system.
It is becoming increasingly important to adapt to the new realities and manage changes brought by the hardening insurance marketplace in the renewables sector. Insurance companies are responding to micro-cracking by restricting coverage (see following page for an example) as well as establishing sublimits for micro-cracking. It is critical to get in front of our clients and prospects with a full understanding of the coverage and restrictions.