Solar Plant in Toyama Endures Heavy Snow Without Removing Snow

2019/07/31 12:00
Shinichi Kato, Nikkei BP Intelligence Group, CleanTech Labo

The "Toyama Shinko Solar Power Plant," a mega- (large-scale) solar power plant with a solar panel capacity of about 5.251MW and a grid capacity of 4.5MW, is located in Ariso, Imizu City on the coast of Toyama Prefecture, Japan (Fig. 1).

Fig. 1: Equipped with visitor facility (source: top 2 photos Toyama Prefectural Government Enterprise Bureau, others Nikkei BP)

The power producer is Toyama Prefectural Government Enterprise Bureau, and power selling began in March 2016.

The mega-solar plant in Toyama Shinko was developed by utilizing a former waste disposal site undergoing its purification period. It is situated in the Toyama Shinko Coastal Industrial Site (Fig. 2).

Fig. 2: Coal ash landfill from thermal power generation. Toyama Shinko Thermal Power Station's chimneys center back of bottom photo. (source: Nikkei BP)

In Toyama Prefecture, solar power plants face certain conditions that do not exist in warm regions on the Pacific Ocean side. These conditions necessitate measures against snow cover (Fig. 3).

Fig. 3: Toyama Shinko Solar Power Plant in snow season (source: Toyama Prefectural Government Enterprise Bureau)

At mega-solar plants, there are two major measures against snow cover. One is to keep snow that has fallen from the panels and piled up from reaching the lowest part of the panels by raising the height of solar panel installation. The other is to make it easier for snow on the panels to slip off by increasing the panel installation angle.

However, both measures increase the load on foundations and mounting systems. This could result in even more difficulty in designing foundations to be used in sites being purified, where the load per unit area and the height of foundations are restricted.

At the mega-solar plant in Toyama Shinko, Toyama Prefecture adopted a stationary concrete foundation shaped like a broad-rectangular, 20cm-high, thin-planar block that covers most of the area under an array (unit of solar panels to be supported by a mounting system) (Fig. 4).

Fig. 4: Thin and rectangular foundations (source: Nikkei BP)

This foundation was also adopted at other solar power plants with similar environments in Fukui and Fukushima prefectures, and Toyama Prefecture reportedly referred to them when designing the mega-solar plant in Toyama Shinko.

When constructing the plant, the snowy season is avoided as much as possible. Therefore, the start of construction and operation were scheduled in May 2015 and March 2016, respectively.

The ground surface was improved by two methods. Being unable to dig the ground, both methods were limited to just improving the ground surface. One was the method of forming three layers on the ground surface in order to prevent weeds from growing on the surface (Fig. 5). This method would keep weeds from growing even if seeds and earth were blown and scattered on the ground by the wind from the surrounding area.

Fig. 5: Improved ground surface (source: Nikkei BP)

By making the three-layer ground surface exhibit its permeable nature to quickly absorb water as soon as it rains, the method keeps weeds from rooting and growing. This also has the benefit of keeping water from pooling on the ground surface. On a sunny day, this method also has the advantage of curbing the temperature rise through vaporization heat and reducing power generation loss owing to high temperatures. Ground surface improvement by this method was used in the perimeter of the site.

The other method was used in the area where solar panels were arrayed. Crushed stones were spread over the earth cover (Fig. 6). This method was also aimed at inhibiting the growth of weeds; however, compared with the three-layer ground surface in the perimeter, it is relatively easier for weeds to root and grow.

Fig. 6: Crushed stones spread in areas with solar panels (source: Nikkei BP)

Having been exposed to severe weather for years, the ground surface in the site was no longer flat and had grown uneven in some areas. It was also overgrown with weeds.

When constructing the mega-solar plant, the constructor reclaimed the site so the uneven surface could be leveled, measuring the depth of earth cover on the ground surface by area to maintain an optimum depth. At the same time, the constructor temporarily removed earth on the ground surface to remove overgrown weeds, separated the weeds from the earth, returned the earth and disposed of the weeds outside the site.

Solar panels manufactured by NextEnergy & Resources Co Ltd (Komagane, Nagano Prefecture) were adopted. A total of 20,592 polycrystalline silicon-type panels (255W/unit) were arrayed.

As for PV inverters, a DC 1,000V-supported product manufactured by Toshiba Mitsubishi-Electric Industrial Systems Corp (TMEIC) was adopted. Six units with a rated capacity of 750kW each were introduced (Fig. 7).

Fig. 7: Composed of six PV inverters (source: Nikkei BP)

Annual power generation is expected to total about 4,800,000kWh, which is equivalent to the consumption of about 1,300 general households. Generated power is sold at 32 yen/kWh (excluding tax) under the feed-in tariff (FIT) scheme to Hokuriku Electric Power Co Inc.

The plant has been hit by heavier-than-expected snow cover since it began operation. In the winter from 2017 to 2018, heavy snow dubbed "Gosetsu 2018" continued across the coast along the Sea of Japan. In this heavy snow, many solar power plants suffered damage such as collapsed mounting systems. Amid such circumstances, there was no damage at the mega-solar plant in Toyama Shinko. Toyama Prefecture considers the design that primarily focused on the snow cover load proved effective.

However, accumulated snow on the ground grew high enough to reach the lowest part of the solar panels following heavier-than-expected snowfalls that continued all night twice in a row. At that time, Toyama Prefectural Government Enterprise Bureau continued to monitor the situation without removing the snow. This was because the prefecture wanted to know what would happen if the situation was left as it was. As a result, while the amount of power generation was affected, neither arrays nor solar panels were damaged.

The enterprise bureau was worried that some panels in the lower rows would be broken by downward pulling strength because of the increased load when snow slips off the panels. However, no panels broke.

Partly because the sea is near, the amount of snow cover is usually less compared with other areas in Toyama Prefecture. The plant has also experienced operations stopping because of power outage in the power transmission cables supposedly due to winter lightning, which is unique to the Hokuriku region. But, according to Toyama Prefecture, operation did not stop so many times as to have a serious impact on the power selling business.

Winter lightning is unique to the Sea of Japan side and strikes along a cold front during winter. Compared with summer lightning, its electric energy is sometimes more than 100 times higher. The Ministry of Economy, Trade and Industry requires wind farms in the corresponding areas to take special measures against winter lightning.

Regarding the damage believed to have been caused by winter lightning, the cover glass of solar panels was broken and significantly warped at a mega-solar plant in Yatsuomachi, Toyama City (See related article).

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