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Heavy Rain in Kyushu Damages Mega Solar Plant (1)

Causing collapse of ground, slopes

2019/08/05 08:28
Kenji Kaneko, Nikkei BP Intelligence Group, CleanTech Labo
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Mega solar power plant constructed on white sand layer

The southern Kyushu region of Japan was hit by record-setting heavy rains from June 28 to July 3, 2019. The total rainfall from the start of the rain on June 28 exceeded 1,000mm. The rains caused flooding of rivers and landslides, as well as power failures and suspension of railway services, in many areas of the southern region.

An evacuation order was issued to about one million residents of some 500,000 households in nine cities and two towns in Kagoshima Prefecture on July 3, when the heaviest rainfall was recorded, and hazards caused by landslides were observed at more than 40 locations.

Heavy rains in the southern Kyushu region tend to cause landslides partially because of white sand accumulated on the ground surface. The white sand is generated by accumulation of volcanic ash and pumice stones forced out by volcanic eruptions over many years. The sand features fine grains and low water retention property.

Because of such features, surface landslides are easily caused when rainwater on mountain slopes, and so forth, reaches levels deeper than tree roots. When the rainwater moves deeper and reaches the boundary between geological layers, the ground itself slides, resulting in large-scale collapse.

The "Kirishima Sunday Power Plant," a mega (large-scale) solar power plant with an output of about 41MW in Kirishima Town, Kirishima City, Kagoshima Prefecture, was constructed on white sand ground. The ground of the mega solar plant site was washed away by the heavy rain, causing the ground to sink and slopes to collapse (Fig. 1).

Fig. 1: Collapse of slope extended 20m wide (source: Nikkei BP)

The soil that slid down was completely retained by a retention basin inside the site and did not flow out of the project site. However, the collapse caused downstream clouding of river water, and people are concerned about its influence on agriculture.

The power plant was constructed on a former golf course development site and the operation was started on August 1, 2017. The output of the solar panels is approximately 41.3MW and the output to the grid is 34MW. The power plant is connected to an extra high-voltage transmission line. The area that was planned to be used as a fairway of the golf course was made into flat steps, and arrays (panel installation units) were fixed by using pile foundations.

The arrays basically consist of three rows of four panels installed horizontally. Depending on the area, three arrays consisting of nine rows of four panels are mounted on one mounting system.

A substation consisting of a PV inverter and a transformer is installed for approximately 1MW of solar panels, and the power is converted to AC and boosted for transmission to the grid substation.

A fund scheme is incorporated for operation of the plant. The power producer is Solariant Portfolio Two GK and designing, construction and O&M (operation and management) are handled by Tokyo Energy & Systems Inc (Fig. 2).

Fig. 2: "Kirishima Sunday Power Plant" with about 41MW of output from solar panels (source: Nikkei BP)