20MW Panels Photographed, Analyzed in 2 Days by Using Drone

The market for aerial solar panel inspection using drones (unmanned aerial vehicles) was established following aerial shooting of still and moving images. Such services are improving daily on a trial-and-error basis.

A drone improves the efficiency of shooting operations, which is an advantage of using a drone. The efficiency is also being improved for operations after using the drones. Energy Solutions Inc of Chiyoda Ward, Tokyo, is one of the companies that reflect the improved post-operations in actual services. Inspections performed by the company are introduced in this report.

The report covers inspections at a mega (large-scale) solar power plant constructed on the former site of a golf course in the Tohoku region. Inspections were performed by using a drone.

About 70,000 solar panels with an output of roughly 20MW are installed on the site that covers about 80ha. The company has been entrusted with inspection of solar panels using a drone at this mega solar power plant for three consecutive years since 2015.

Solar panel inspections by drones are started by shooting thermal distribution images. An infrared camera is mounted on a drone for aerial shooting to obtain these images. The process of obtaining thermal distribution images is significantly improved just by using a drone.

Conventionally, inspectors walked along arrays (units of installed panels) on the site, carrying a portable infrared camera, and shot images of solar panels one by one. If the site is large and has many solar panels, an enormous amount of time and effort is required. Inspectors have to walk a long distance and the physical burden is substantial.

The level of improvement in efficiency of thermal distribution image acquisition by use of drones is at issue. The time needed for aerial shooting of panels at a 2MW plant connected to high voltage transmission lines, which is popular in Japan, is 15 to 20 minutes, according to many companies that provide such services.

On the other hand, one or two days are required for shooting images of panels at a same-scale plant if they are shot by walking around using the conventional method. The difference in operating time increases in line with the increase in the plant scale. Therefore, the efficiency of aerial shooting improves more drastically when the plant scale is large.

The mega solar power plant on a former golf course site, which was inspected by Energy Solutions, features conditions that require knowhow on aerial shooting by drones. The ground on which the solar panels are installed is not flat and the levels of the panels arranged on the ground significantly differ when they are shot from the sky.

To identify locations of defective solar panels and types of expected defects based on thermal distribution images, a certain level of resolution is required of the images to make a judgment while the images need to allow for an understanding of thermal distribution at a certain level of accuracy.

The results are greatly affected by the distance and angle between solar panels and the infrared camera. If a drone is flown at a constant altitude when panels are installed on a site with valleys and undulations, the height of the camera above the panels and the angle of the camera in relation to the panels vary depending on the location, posing a risk of failure to obtain thermal distribution images of the required level.

It is ideal that a drone flies while maintaining a constant distance and angle between solar panels and an infrared camera. Energy Solutions realizes the above by setting the travel route in advance and performing minor adjustments on-site

The automatic flight route is set by reflecting the level difference against the takeoff/landing point and fine adjustments are made on-site according to the actual situation. A constant distance and angle are maintained between panels and the camera to obtain thermal distribution images of the required level. It is difficult to perform the above by manual radio control, according to the company.

The angle between an infrared camera and solar panels is set at about 45° for aerial shooting. The influence of light reflected by the cover glass of the panels is minimized by setting at this angle. Once the automatic flight route is determined, the route can be used repeatedly for aerial shooting of the same power plant. The battery (storage battery) can be used more effectively compared with a case where a drone is flown for testing and determining the route every time.

Solar panels at this mega solar power plant were installed on a site that extends from south to north, which was a locational characteristic. This situation is disadvantageous for aerial shooting by a drone because the battery tends to be consumed more quickly when the flying distance in the east/west direction is short and the drone is turned frequently. The flying time per charging cycle is extended when the flying distance in the east/west direction is long.

The company reports on the solar panel situation of each PV inverter so that the power producers to which it provides the services can easily compare the results with data on the power generation amount. Therefore, panels are shot in a PV inverter unit in aerial shooting. At the power plant, two PV inverters featuring 500kW of rated output are enclosed in one housing, and aerial shooting was performed for each unit.

During the inspections this time, three blocks could be shot by a single flight in some areas by devising the route setting. The aerial shooting was finished in a day and a half by flying 24 times. It will be difficult for other service providers to finish shooting a 20MW mega solar power plant that extends from south to north in a day and a half, the company emphasized.

For the inspections this time, the company was also entrusted with inspection of panels with a high possibility of defects based on the obtained thermal distribution images on the ground and analysis of defect types and situations, in addition to aerial shooting of solar panels using a drone.

The basic service contents for inspections on the ground include visual inspection, shooting by a visible light camera and an infrared camera and failure diagnosis using the “Solamente iS,” an inspection system manufactured by Ites Co Ltd of Yasu City, Shiga Prefecture.

Two inspectors were sent to the site and one of them operated the drone while the other analyzed the thermal distribution images shot from the sky and carried out inspection on the ground based on the analysis results. The aerial shooting was completed in a day and a half and the analysis and inspection on the ground, which were carried out in parallel with the aerial shooting, were completed in two days, finishing all operations in two days.

According to the original plan, the inspectors were expected to stay on the site for three days, including the time for preparation, but the period was shortened by one day. At this point, some of the panels with potential defects based on the thermal distribution images were identified manually.

The aerial image data is transmitted to the cloud and the thermal distribution images are superimposed on the layout drawing of solar panels at the plant. “Defect situations” classified into four types were input to positions of panels with potential defects while analyzing the images.

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