Anchors Coming Away Cause Fire at Floating Solar Plant in Chiba

2019/11/19 19:29
Kenji Kaneko & Shinichi Kato, Nikkei BP Intelligence Group, CleanTech Labo

77% of solar panels damaged

Japan's Ministry of Economy, Trade and Industry (METI) held a meeting of the Working Group (WG) for Accident Response for Renewable Energy-based Power Generation Facilities and Structural Strength Oct 28, 2019. At the meeting, Kyocera TCL Solar LLC, the power producer of the "Chiba Yamakura Floating Mega Solar Power Plant," reported the background and situation of the accident, the progress in investigating the cause and findings that have been confirmed so far, in respect to the plant damaged by Typhoon 15.

The Chiba Yamakura Floating Mega Solar Power Plant, which was constructed on the surface of "Yamakura Dam," a pond in Ichihara City, Chiba Prefecture, started operation in March 2018. The plant features an output of about 13.7MW and is one of the largest floating solar power plants in Japan. In the afternoon of September 9 after Typhoon 15 had passed through Chiba Prefecture, a fire broke out. Floating mounting systems were swept away and damaged by the strong winds of the typhoon, causing fires at multiple locations (Related article: Major Typhoon Damages Floating Mega Solar Plant).

The progress of the accident is as follows, according to Kyocera TLC Solar. At about 6:30 in the morning of September 9, a "DC leakage" alarms were activated at the monitoring center in Sakura City, Chiba Prefecture, and at a mobile terminal of the chief electrical engineer.

The engineer hurried to the site and arrived there at about 7:30 and confirmed the damage, stopping all PV inverters during the period from 7:50 to 8:30. Fire was detected at 13:00 and reported to the fire department. Firefighting started at 14:00 and the fire was extinguished at 17:20.

No injuries were reported, but about 77% of the 50,904 solar panels were swept away and damaged by the winds, with some of them burnt due to the fire (Fig. 1 & 2).

Fig. 1: More than 70% of panels pushed to northern shore (source: Nikkei BP)

Fig. 2: Panels overlapped and short circuited, causing fires (source: Nikkei BP)

Connected to 420 anchors by 828 mooring wires

At the mega solar power plant, one solar panel was mounted on one main resin float, using fixing brackets, and main floats were connected placing "second floats" between them. Resin pins were used to connect the main floats with second floats (Fig. 3).

Fig. 3: Components of solar panel island and mooring method (source: Kyocera TCL Solar)

The shape of the island made of solar panels connected by the above method is wider in width from east to west at the southern end, measuring 503.1m at the widest part. The length from south to north is longer on the western side, and the maximum south-north width is 487m (Fig. 4).

Fig. 4: Outline of power plant and size of island (source: Kyocera TCL Solar)

The outer circumference of the island is connected to anchors fixed to the pond bottom by 828 mooring wires to prevent it from being pushed away by the wind. Two mooring wires were generally connected to one anchor. Therefore, 420 anchors were used in total (112 locations on the northern end, 133 locations on the eastern end, 68 locations on the southern end and 107 locations on the western end) (Fig. 5).

Fig. 5: Float components and island mooring method (source: Kyocera TCL Solar)

Anchors coming away caused accident

According to the report on the damage situation, the large island was divided into three parts. The rectangular portion on the western side, which extends to the south, and several rows on the southern end of the eastern side remained in their original positions, but the remaining part on the northern side broke away and pushed toward the northern shore.

In the process of separation and drifting of the island, the floats on the southern end, which were broken and separated, rolled up, while some of the floats at the center of the northern part (inner part of the island), which were pushed away by winds, were pushed by floats behind them and overlapped, causing significant bulging. Floats on the northern end were pushed to the shore and pulled down from the surface toward the pond bottom by the anchors (Fig. 6).

Fig. 6: Outline of the accident and situation of the damage. The island was broken into three parts, separating at places indicated by yellow dotted lines. (source: Kyocera TCL Solar)

According to the investigation of the pond bottom after the accident, seven anchors at the center of the southern end, among 68 anchors on the southern end, had come away from the bottom. Because of the "removal" of the anchors, the investigation deduced that mooring wires at the center of the southern end lost their function and the wind load applied to the western side and eastern side of the southern end increased, damaging and breaking connecting pins used to link the floating mounting systems, resulting in the entire northern area of the island separating from the southern area and the northern area drifting further north (Fig. 7).

Fig. 7: Estimated cause of breakage of island and damage flow (source: Kyocera TCL Solar)

Resin floats eventually burned

Why did the anchors come away, resulting in damage to and separation of the island? According to the cause analysis in the report, the possibilities are: (1) The wind speed on the day possibly exceeded the design wind speed (41.53m/s), (2) The island was swung by winds and waves, generating unbalanced loads, (3) Loads exceeding the design load were applied to pins and joints because of the size of the island or (4) Stress was concentrated because of the shape of the island.

As for the fire, the parts that were actually burnt were in the water. They were pulled out of the water to investigate the cause; the investigation is still in progress, but the company believes that cables were ignited by DC leakage or sparks, eventually resulting in the fire spreading to resin floats.

Kyocera TCL Solar plans to submit the final report in February 2020 and work out recurrence prevention measures in March (Fig. 8).

Fig. 8: Investigation schedule for cause of accident (source: Kyocera TCL Solar)

At a meeting separate from the meeting of the WG, Toshihide Koyano, assistant director of the solar energy business headquarters at Kyocera Corp, said that the Chiba Yamakura Floating Mega Solar Power Plant will be repaired "in six months to one year." He said, "If it is determined that the problems identified by the future investigation cannot be solved without changing the initial design, a new design will be incorporated for the repair work."

Assistant director Koyano also said, "It is important to eliminate the concerns of the nearby residents about safety by repairing the facility so that an accident like this never occurs again."