PID effect, micro-cracks, and hot spots are three important factors that can affect the performance of crystalline silicon photovoltaic modules. Among them, PID effect and hot spots usually appear after installation and operation of PV panels for a period of time. Micro-cracks are a common problem associated with solar photovoltaic modules and they are difficult to detect with the eyes. In view of these potentially hidden problems, how we identify and rectify them is important. In the last Solis Seminar, we introduced the problem of PID effect and its solutions. In this seminar, we will share with you the causes of cell micro-cracks, how to identify them and ultimately prevent them.
What is a “Micro-Crack” and Possible Causes
Micro-cracks are a relatively common defect of crystalline silicon photovoltaic modules, which mainly refers to some small cracks that are not easily detectable by the human eye. Due to the characteristics of its own crystal structure, crystalline silicon modules are very prone to cracking.
In the process flow of crystalline silicon panel production, many parts of the process could cause the cell to crack. But the root cause is usually one of three types:
- Mechanical Stress: This is when the cell is exposed to external forces such as welding, lamination, framing, installation, construction, etc., and when the parameters are set incorrectly, equipment fails or there has been improper operation which has caused micro-cracks.
- Thermal Stress: Mainly caused when the cell swells and ruptures after being exposed to sudden high temperature, such as the welding or lamination temperature are too high, and other parameter settings are not correct.
- Raw Material Defects: The sourcing of poor raw materials or materials that are not properly handled will result in material defects.
The Influence of “micro-crack” on the Performance of Photovoltaic Modules
The current of the cell is mainly collected and led out by the main grid lines and the thin grid lines whose surfaces are perpendicular to each other. Therefore, when the micro-cracks (mostly parallel to the bus bars) cause the thin grid lines to break, the current will not be effectively transported to the bus bars, which will lead to partial and even the entire cell failure, and may also cause fragments, Hot spots, etc., at the same time cause power attenuation of the components.
How to identify “micro-crack”
EL (Electroluminescence) equipment is solar cell or panel internal defect detection equipment which uses the electroluminescence principle of crystalline silicon to capture near-infrared images of components through high-resolution infrared cameras. This equipment obtains and determines component defects.
However, this method has several drawbacks in micro-crack recognition of installed photovoltaic panels:
1. The equipment is expensive
2. Long detection time, usually several weeks or even months for large systems
3. Professional personnel are required for on-site operation, and the labour costs is high
I-V Curve Scanning Method
For PV panels that have been installed and connected to an intelligent monitoring platform, the
I-V curve scanning function can be used to quickly scan and categorize the PV panels with micro-cracks.
If the scanning results show two types of curves as ① or ② in Figure 6, it indicates that the output current of the PV module is abnormal. The cause is likely to be damage cracks or blocked current.
1. This solution can identify a variety of PV failures
2. Fast response time, the investigation can be completed in only 5 minutes
3. No need for professional equipment and/or personnel, saving costs
4. One-click scanning, cost-effective
Example Applications Utilizing I-V Curve Scanning
This case is the application of I-V curve scanning in a C&I project. All the inverters on site were scanned remotely through SolisCloud, and it was found that one of the inverter strings showed the characteristics of curve ① in Figure 6, with the I-V and P-V curve shown as follows:
Through this precise analysis function, we could quickly identify the PV panels with cracks in the field, ultimately improving the O&M efficiency of the system and lowering costs.
The micro-crack problem has a great impact on the power of the PV module and ultimately the entire system, and it cannot be recovered. Whilst module manufacturers go to great lengths to prevent micro-cracks during production, it is vital that great care is taken in the subsequent storage, transportation, and installation, to avoid damage to the cells. Careful consideration should be given to the storage environment and to avoid sudden extreme temperature change.
Connecting any large-scale PV system to a monitoring platform such as SolisCloud will ensure that a vast number of potential issues can be dealt with quickly and efficiently thereby protecting the LCOE of the overall system.