Riverside Solar Plant Facilities Survive Flood of River

Remote monitoring of PV inverters added after automatic recovery failure

2019/10/29 14:56
Shinichi Kato, Nikkei BP Intelligence Group, CleanTech Labo

The "Daiko Mibu Solar Power Plant," a mega- (large-scale) solar power plant with a solar panel capacity of 2.43MW and a grid capacity of 1.99MW, is located beside the Omoigawa River in Mibumachi, Tochigi Prefecture, Japan (Fig. 1).

Fig. 1: Daiko Mibu Solar Power Plant (source: Nikkei BP)

This mega-solar plant is in what was previously a quarry. The power producer, KK Daiko (Sano City, Tochigi Prefecture), originally ran a quarry business. To efficiently use the former quarry site, the company, having sold the quarry business as it finished quarrying the site, developed and now runs this mega-solar power plant.

NEC Networks & System Integration Corp (NESIC) proposed this mega-solar development to Daiko and commercialized it (See related article).

In early October 2019, the solar panels at the Daiko Mibu Solar Power Plant were inspected using a drone (unmanned flying vehicle) (Fig. 2). The drone was flown above the power plant and shot the panels from above using an infrared camera. Using the thermographic images obtained in this way, any panels likely to be malfunctioning were discovered.

Fig. 2: Panel inspection using drone (source: Nikkei BP)

As a result of this drone inspection at the Daiko Mibu Solar Power Plant, only three panels were found to have possible malfunctions (Fig. 3).

Fig. 3: Malfunction in cells (top), bird droppings (bottom) (source: NESIC)

In two of the above three photos, bird droppings covered some power generation elements (cells) and curtailed power generation. If the bird droppings were washed off, power generation would return to normal. In the other photo, however, the solar panel itself had a problem stemming from its cells.

Despite monthly fluctuations, the power generation amount has been almost flat throughout the year. In every year, the result reportedly outperformed the power generation estimate in the initial business plan by about 20%.

Solar panels of Toshiba Corp and PV inverters of Toshiba Mitsubishi-Electric Industrial Systems Corp (TMEIC) were adopted.

Power generation facilities not submerged despite flooding in site

The solar plant has been struck by heavy rain, which exceeded initial estimates, putting the mega-solar plant's rainwater treatment and drainage function to the test.

The mega-solar plant said it had primarily undertaken two procedures on the ground of its site to improve maintenance efficiency and enable proper treatment and drainage of rainwater. Each procedure was designed to realize an appropriate balance between different elements.

The plant improved maintenance efficiency by asphalting near the perimeter of the site (Fig. 4). Compared with conventional mega-solar plants that are entirely covered by earth, it became easier to set up related facilities and equipment, while inspection and replacement efficiency also increased.

Fig. 4: Perimeter area asphalted (source Nikkei BP)

The power plant only asphalted the perimeter area in consideration of cost and the impact on power generation. If the whole site was asphalted, panel temperatures might rise due to heat reflection from the asphalted ground and result in a further decline in power generation during high summer temperatures.

Given these circumstances, the ground under the panels was roller-compacted and covered with crushed stones instead of asphalt (Fig. 5).

Fig. 5: Surface compaction over crushed stones (source: Nikkei BP)

From the design phase, the plant took measures to discharge rainwater out of the site. First, it built a new reservoir at the south end in addition to the existing reservoir that had been used since the site was a quarry. Furthermore, the plant developed the site so it would slope from north to south. In this design, rainwater would run through the drainage ditch southbound from the north and eventually pour into the reservoirs.

In addition, the ground under the panels was dug out slightly. Because of the clayish soil, the ground does not drain well. Accordingly, the plant made water pooling easier by further digging the ground under the panels.

In case of heavy rain, this design allows rainwater to pool not only in the two reservoirs through the drainage ditch, but also under the panels so the solar panel area can also play the role of a reservoir. The solar panels, combiner boxes and cables between them were fixed at a height taking this pooling effect into consideration (Fig. 6).

Fig. 6: Combiner boxes, electric cables supported a little higher (source: Nikkei BP)

Such a design proved effective when Typhoon No. 18 struck in September 2015.

At that time, record-breaking heavy rain continued to fall across the northern Kanto region while a broken dike and flooding caused serious damage in the Kinugawa River basin in Joso City, Ibaraki Prefecture.

The Omoigawa River next to the Daiko Mibu Solar Power Plant runs almost in parallel about 10km to the west of the Kinugawa River. It is located upstream from where the Kinugawa River flooded. Despite different discharge rates and other river conditions, the situation regarding the heavy rain seemed to be similar for both rivers.

At that time, a lot of water increasingly pooled under the panels in addition to the two reservoirs at the Daiko Mibu Solar Power Plant. Although the foundations and mounting systems were submerged on the south side where the ground had been lowered and the water level increased further, the power generation facilities fixed on the mounting systems survived the typhoon without being submerged even at the south end. The power plant believes this proves its preliminary design had been sufficient.

That said, power selling stopped at the power plant. An instantaneous drop on the grid side caused the safety feature of the PV inverters to be activated and stop operation. Such an operational stop due to an instantaneous drop can usually be restored automatically. This time, however, the operation was not restored due to a problem.

The power plant noticed the failure of automatic operation recovery about four days later. It did not immediately notice it because a remote monitoring system was not adopted for the PV inverters. Following this failure, the plant added a remote monitoring system for the PV inverters.

Panels annually washed with groundwater

Weeds have been prevented from growing just as planned (Fig. 7). Almost no weeds have grown in the asphalted perimeter area. In addition, herbicide has also been used.

Fig. 7: Well-controlled weeds (source: Nikkei BP)

It is also difficult for weeds to grow in the solar panel area where crushed stones were spread and roller-compacted. The solar panels are washed with water once a year. Although the mega-solar plant is located on a former quarry, the production of crushed stones continues in an adjacent site, from which dirt is frequently blown and accumulates on the solar panels (Fig. 8).

Fig. 8: Dirt annually washed off with groundwater (source: Nikkei BP)

The dirt on the panels is mostly washed off by rain. However, dirt tends to remain near the stepped edge between the bottom of the panels and the frames. The annual cleaning is carried out with the aim of washing out such dirt around the frames. A well was dug so groundwater can be used for this cleaning.

Eaves have been attached to the combiner boxes (Fig. 9).

Fig. 9: Eaves additionally set up (source: Nikkei BP)

The breaker blew once as a result of the temperature rising inside the combiner box on a hot summer day, and the eaves were set up after that as a measure against high temperatures. They are more effective than expected because the same phenomenon has never occurred since the eaves were attached.

This solar plant is also characterized by the mousetraps attached to the mounting systems (Fig. 10). The mousetraps were adopted during construction when the number of mice sharply increased in the rice and vegetable fields near the power plant. Farmers in the neighborhood came and asked the power plant to take countermeasures, believing the mice that had lived in and around the mega-solar site relocated to their fields. The mousetraps seem to be effective, as they sometimes catch the rodents.

Fig. 10: Mousetraps (source: Nikkei BP)

Facility overview