Solar Plant Leverages Lessons From Thunderbolt
Cost-cutting efforts include weeding measures
Daiichi Jitsugyo Co Ltd, a general machine trading company focusing on industrial machines, constructed the mega- (large-scale) solar power plant "Daiichi Jitsugyo Iida Solar Power Plant" with an output of about 1MW in Iida City, Nagano Prefecture, and started generating power in March 2014 (Fig. 1).
Daiichi Jitsugyo had built up its expertise in solar power generation for some time by selling solar power generation systems and operating a 20kW small-scale system at its own building in Kawaguchi City, Saitama Prefecture, subsidized by the Ministry of the Environment. However, the domestic market for solar power generation had focused on houses for years, and the market for industrial systems, which the company had been targeting, was limited.
Amid such circumstances, Daiichi Jitsugyo decided to approach the mega-solar power plant business by itself in light of the drastic changes in the business environment following the implementation of the feed-in tariff (FIT) program. Iida Solar Power Plant is Daiichi Jitsugyo's second mega-solar power plant, following the 1.5MW-output "Daiichi Jitsugyo Kasama Solar Power Plant" in Kasama City, Ibaraki Prefecture.
Mitsubishi Electric's panel uses locally manufactured solar cells
Daiichi Jitsugyo advocated a policy to contribute to energizing the local area where the mega-solar power plant is constructed. Mitsubishi Electric Corp's solar panel (Fig. 2), which was employed at this plant, uses the solar cells manufactured at Mitsubishi Electric's plant in Iida City.
Manager Yoichi Matsuda of the Petroleum & Petro Chemical Plant Dept, Plant & Energy Div, Daiichi Jitsugyo, stressed, "We could help vitalize local industry by using materials and facilities that had been manufactured locally as much as possible." Daiichi Jitsugyo also deployed Toshiba Mitsubishi-Electric Industrial Systems Corp's (TMEIC's) PV inverter considering the business compatibility with Mitsubishi Electric, among other factors.
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Daiichi Jitsugyo could construct the plant while having to barely develop the land. It only added some drainage because the site had originally been flattened and had even been provided with some drainage and a balancing reservoir (Fig. 3).
Changed way of using weeding sheets
In turn, Daiichi Jitsugyo also made some changes leveraging its experience at Kasama Solar Power Plant. The weeding method was one of those changes.
The company deployed a method that spreads crushed stones over the weeding sheets at Kasama Solar Power Plant. The crushed stones were expected to not only fix the weeding sheets in place but also protect them.
At Iida Solar Power Plant, on the other hand, Daiichi Jitsugyo lowered construction and other costs by using only weeding sheets without crushed stones on them. The sheets, which are simply placed on the ground, were fixed with screws, positioned at short intervals, into the ground (Fig. 4).
As the wind carried dirt and weed seeds that later started to grow between the crushed stones at Kasama Solar Power Plant, the company this time did not overlay the sheets with stones. The weeding sheet's effect would not be fully achieved if weeds grew from the upper side of the sheets.
Should the weeding sheets be laid without stones on them, dirt and weed seeds carried by the wind could be washed away from the sheets. Weeding sheets are made of plastic and might accordingly deteriorate during the twenty years of power generation business. Daiichi Jitsugyo, however, estimates the cost to replace the old sheets with new ones would be lower than the cost to overlay the sheets with crushed stones or remove weeds.
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The solar panels were installed with the lengthwise rows limited to two panels (Fig. 5). If the lengthwise rows were increased to three panels, a more-costly mounting system would be required due to a heavier load caused by the stronger wind pressure that the mounting system has to endure. In addition, the construction and maintenance costs would rise due to the higher edge of the panels. Furthermore, the construction cost was also lowered by the deployment of metal pile-type foundations, instead of concrete foundations, that support the mounting systems.
Stronger protection from thunderbolts in light of damage at Kasama
Daiichi Jitsugyo also enhanced the protection from thunderbolts. In May 2013, part of the solar power generation facility was damaged by thunderbolts around the area, and the amount of power generation sharply decreased for about a month at Kasama Solar Power Plant.
This was because part of the substrate embedded with the core components in the PV inverter was damaged by the high voltage that reached beyond the designed value and ran through the transmission and distribution cables following the thunderbolts.
Fortunately, the damaged area was limited, and the plant could continue to generate power even though the output declined. The amount of generated power recovered every time the substrate was replaced. Based on this experience, Daiichi Jitsugyo decided to add the surge protective devices (SPD) to all the cables and circuits connected with the PV inverters from the construction phase.