General Contractor Uses Drone Technologies for Solar Panel Inspection (2)

2019/03/12 21:20
Shinichi Kato, Nikkei BP Intelligence Group, CleanTech Labo

Continued from General Contractor Uses Drone Technologies for Solar Panel Inspection (1)

Finds 4 failure types

Four types of failures are mainly found by drones. They are classified into "damage," "string defect," "dirt" and "cluster failure." "Damage" refers to cracking of solar panel cover glass (Fig. 4). Overheating is observed at multiple locations inside a panel in many cases.

Fig. 4: Examples of "damage" (source: Shoken)

The company believes that "cracking of glass causes water intrusion into the panel, generating abnormalities at various locations inside the panel and resulting in overheating." "String defect" refers to a state where power generation stops on a string unit basis (Fig. 5).

Fig. 5: Example of "string defect" (source: Shoken)

In this state, power is not transmitted from the defective string when power is generated by the solar panels, due to a fuse blowing inside a combiner box, disconnection or other reasons. The defect is detected easily from temperature distribution images shot by drones.

"Dirt" refers to dirt on solar panel surfaces such as bird droppings, causing the amount of power generated by the cells (power generation elements) to decrease under such dirt (Fig. 6).

Fig. 6: Example of "dirt" (source: Shoken)

"Cluster failure" refers to a cessation of power generation from a cluster, which consists of multiple cells and forms part of a panel such as 1/3 of a panel, resulting from activation of a bypass diode caused by a decline in the output of cells due to a problem of some kind (Fig. 7).

Fig. 7: Example of "cluster failure" (source: Shoken)

Cluster failure is known as a defect easily discovered from temperature distribution images shot by drones. In an inspection performed by the company, cluster failure was discovered in 60 of about 1,600 panels at a power plant operated by an external company and uses panels made by foreign manufacturers, for example. On the other hand, no cluster failure was discovered in panels made by two domestic manufacturers, according to the company.

Inspects 'CIS' solar panels that drone inspection companies avoid

CIS compound solar panels are used in some of the solar power plants of external companies where the company shot aerial images.

CIS panels are generally excluded from the targets of solar panel inspection services by drones, because there are almost no CIS panel failure modes that can be discovered from aerial temperature distribution images at present. However, such an undertaking is easy for Shoken because the company has yet to commercialize the service and is not paid for the service. The company, with the cooperation of power producers, shoots aerial photos aiming to validate the technologies.

At present, "cracks in the glass" of CIS solar panels are almost certain to be discovered (Fig. 8).

Fig. 8: Example of cracks in glass of CIS panels (source: Shoken)

CIS panels are black on the surface. When the cover glass is cracked and the cracks extend like a spider's web, many spots on the panel become white. It is easy to identify panels with broken glass because of the contrast. Interestingly, temperature distribution is also abnormal in four panels that are in the same row as the panel with broken glass.

The company shoots aerial videos in black and white. The aerial videos are checked in real time by a mobile terminal on the ground. When abnormal temperature distribution is discovered, still images of the area are shot from the ground.

The images can be colored and the temperatures can be indicated by using software for analysis. When a defective panel is enclosed by a rectangle on the screen displayed by the software, using the temperature measurement function of the software, the area with the highest temperature inside the box is indicated by a red triangle and the area with the lowest temperature is indicated by a blue triangle. The lowest temperature, average temperature and highest temperature in the box are displayed in the top left of the screen.

CIS panels with broken glass and panels in the same row as such panels with abnormal temperature distribution are higher in temperature than other panels, and this can be confirmed by using the software function.

CIS panels are connected in series and in parallel to form a string. Five panels are connected in series and four circuits of connected panels are connected in parallel in many cases. The array normally consists of four rows, with five panels connected in series in one row, and the panels in multiple rows are frequently connected in parallel.

Judging from such a situation, it is estimated that power is not transmitted to four panels that are connected in series with a CIS panel with broken glass, stopping power transmission from the panels.