String inverter supplier Solis has underlined three important factors that could affect the performance of crystalline silicon photovoltaic (PV) modules – potential-induced degradation (PID) effect, micro-cracks, and hot spots.

Of these, PID effects and hot spots usually appear a while after PV modules are installed and have been in operation. While micro-cracks are a common problem associated with solar PV modules, they are difficult to detect with bare eyes.

Given these potentially hidden problems, the mechanism to identify and rectify the defects becomes important.

This article discusses the causes of cell micro-cracks, ways to identify them, and how these can be prevented.

Micro-cracks and their possible causes 

Micro-cracks are a relatively common defect of crystalline silicon PV modules, which mainly refers to some small cracks that are not easily detectable by the human eye. Due to the characteristics of their crystal structure, crystalline silicon modules are easily prone to cracking.

In the course of the crystalline silicon module production flow, many parts of the process could cause the cell to crack. But the root cause is usually one of the three types:

The influence of micro-cracks on the performance of PV 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 leads to partial or even entire cell failure, and may also cause fragments, hot spots, etc. At the same time, it could cause power reduction of the components.

How to identify micro-crack 

EL equipment

Electroluminescence (EL) equipment is a solar cell or module internal defect detection equipment, which uses the EL 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 PV modules, such as expensive equipment, long detection time (usually several weeks or even months for large systems), and the need for professional personnel to be on-site, increasing the labor cost.

I-V curve scanning method

For PV modules 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 damaged cracks or blocked current.

Advantages

• This solution can identify a variety of PV failures.
• It has a fast response time. The investigation can be completed in only 5 minutes.
• It does not need professional equipment and/or personnel intervention, as a result, saving costs.
• It allows one-click scanning, which is cost-effective.

Example of applications utilizing I-V curve scanning

This case is the application of I-V curve scanning in a commercial and industrial 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 PV curve shown as follows:

Through this precise analysis, the team could quickly identify the PV modules with cracks in the field, ultimately improving the operation and maintenance efficiency of the system and lowering costs.

The micro-crack problem has a great impact on the power output of the PV module and, ultimately, the entire system, and it cannot be recovered. While module manufacturers go to great lengths to prevent micro-cracks during production, great care must be 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 changes.

Connecting any large-scale PV system to a monitoring platform such as SolisCloud will ensure that many possible issues can be dealt with quickly and efficiently, thereby managing the levelized cost of energy of the overall system.

This article is sponsored by Ginlong Technologies, solar inverter manufacturers.

 

Image credit: Ginlong Technologies