Extra-high-voltage Solar Plant Operating on North-facing Slope
Almost no weeds even after 2 yrs of operation
Enters 'on-ground solar plant business' following FIT scheme implementation
Hijimachi in Oita Prefecture, Japan, is located on the coast of Beppu Bay in the southern Kunisaki Peninsula. As its name in Japanese indicates, a beautiful sunrise from Beppu Bay can be viewed from the high hills in the town.
Hijimachi is the home town to Hijidenki Co Ltd, the area's representative medium-sized electric engineering company that logged annual sales of 3.8 billion yen (fiscal 2017) with 30 employees. Having approached, at an early stage, marketing and construction of all-electric houses and solar power generation systems for houses, the company embarked on designing and constructing on-ground-type solar power plants as well as running the power selling business in the wake of the feed-in tariff (FIT) scheme in Japan.
Hijidenki has provided design, procurement and construction (EPC) services to solar power plants with a total output of more than 120MW primarily in Oita Prefecture while undertaking operation and maintenance (O&M) services for solar plants equivalent to about 110MW. Of the 110MW, the projects that Hijidenki owns and runs for the power selling business account for roughly 50MW.
By scale, "commercial low-voltage solar plants" connected with low-voltage power grids, "high-voltage projects" connected with 6.6kV high-voltage power grids and "extra-high-voltage projects" connected with extra-high-voltage power grids total about 700, 40 and one, respectively.
The "Hijidenki Solar Park Fujiwara Yurino Power Plant" completed in the mountains of northern Hijimachi in February 2017 was the company's first "extra-high-voltage project." Hijidenki constructed and owns the plant by itself and sells the power (Fig. 1).
Panels set up in 22 separate areas
The plant's solar panel capacity is 14.42MW while its grid capacity is 10MW. The large-scale electric facility provided with an extra-high-voltage substation required comprehensive civil engineering and land reclamation; however, Hijidenki constructed it separately doing EPC services by itself and provides O&M services on its own after completion.
Solar panels manufactured by REC of Norway, PV inverters of Toshiba Mitsubishi-Electric Industrial Systems Corp (TMEIC), combiner boxes made by Weidmuller Interface GmbH & Co KG of Germany and Nisso Pronity Co Ltd's foundations and mounting systems were adopted.
The Fujiwara Yurino Power Plant was constructed by transforming the hillside slopes into steps like terraced fields and arraying solar panels in the leveled areas. The plant largely consists of the south site and the north site with a panel capacity of 9.5MW and 4.9MW, respectively.
Panels were further separated into 11 blocks in each of the south and north sites. There was a major reason for the plant design, where the panels have been separated into so many blocks and the number of isolated blocks has increased (Fig. 2).
Overloaded by 1.4 times by securing additional site areas
From the south end and the highest point of the power plant's south site, not only the town of Hijimachi and Beppu Bay, but also the industrial area on the coast of Oita City across the bay can be viewed. However, the solar panels were set up like steps on the northern slopes facing inland, instead of the southern slopes stretching from the top of the hills toward the sea. Generally speaking, such location is not suitable for solar power generation (Fig. 3).
"When designing the Fujiwara Yurino Power Plant, the major challenge was how to set up as many solar panels as possible on the northern slopes, which were not suitable for a mega-solar plant location," said Koji Watanabe, president of Hijidenki.
The plant's south site had originally been slated for a golf course. Having been put up for auction during the planning phase before construction began, its ownership transferred to a real estate company. Hijidenki purchased the site from this real estate company as a mega- (large-scale) solar plant site.
After the takeover, Hijidenki applied for prefectural permission to develop forestland under the Forest Act while considering civil engineering and reclamation as well as panel positioning layout. To increase the number of solar panels to be set up, the company made efforts to efficiently utilize space, such as even setting up panels in a reservoir to be constructed at a lower level, for example (Fig. 4).
Even so, it gradually turned out that the solar panel capacity would not reach 10MW, which was the grid capacity obtained when the facility was approved, if the company secures necessary reserve forest areas and a sufficient slope size focusing on safety.
Consequently, the company began to consider increasing the number of panels to be set up by leveraging the forest area on the adjoining north side in addition to the purchased site. Negotiating with multiple landowners and signing land lease contracts with them, Hijidenki eventually secured a new site where panels equivalent to a total output of 4.9MW could be set up in 11 blocks. As a result, the plant's panel capacity totaled 14.4MW, combining 9.5MW in the south site and the 4.9MW in the north site.
Panels even in reservoir
The huge reservoir in the south site was constructed around the projecting central slope in a U-shape and, if you approach the reservoir, you can find panels set up at about 1m from the paved ground. Combiner boxes were also set up slightly high on the assumption that the area under the panels would submerge (Fig. 5).
At the north end of this "panel area/reservoir," there are no panels but you can find another deeper reservoir and a rainwater exit hole in the middle. Rain on the south site initially flows into the deep reservoir at the bottom, starts submerging the reservoir under the panels only after the deep reservoir becomes full and is discharged downstream through the exit hole before the water reaches the panels (Fig. 6).
Flood control-related technology generally required in the Forestland Development Permission System is stipulated to "be able to keep the peak amount of water flow in the heaviest rain possible in 30 years within the downstream flowing capacity." In other words, no floods are supposed to occur in the downstream areas of the developed site even in the heaviest rain that would only strike once in 30 years.
The company reportedly designed the drainage and flood control facility in the south site of the Fujiwara Yurino Power Plant with due allowance, assuming a higher amount of rain compared with the technical requirements. In fact, water has never submerged the area under the panels since the plant began operation, even though it experienced a major typhoon last summer.
Slopes hardened by spraying concrete
According to Hijidenki, it struggled with the rocky ground during such large-scale reclamation. As for foundations to support mounting systems, the company used both pile foundations and concrete stationary foundations in accordance with the terrain. Examining the ground in advance, Hijidenki used stationary foundations in areas that contained a lot of rocks (Fig. 7).
Also, Hijidenki hardened the slopes by spraying concrete all over the surface in areas with many rocks near the surface with the aim of preventing rockfalls due to heavy rain and other factors.
"In civil engineering and reclamation, we paid almost excessive attention to design in preparation for natural disasters," Watanabe said (Fig. 8).
Weed barrier sheet covered with gravel
Hijidenki designs prioritizing safety and reliability in this manner based on its abundant experience in managing solar power plants equivalent to an output of more than 100MW reflecting its emphasis on O&M services as well as EPC services. The company considers boosting O&M efficiency will bring more business benefits over the long term even though its initial investment is greater to a certain degree.
Greatly appreciating their reliability, Hijidenki has also primarily adopted solar panels manufactured by REC of Norway and PV inverters of TMEIC at solar plants other than the Fujiwara Yurino Power Plant (Fig. 9).
Hijidenki also adopted a comprehensive measure against weeds at the Fujiwara Yurino Power Plant. Although it has been two years since the plant began operation, no areas with panels have been weeded so far. The secret can no longer be seen after construction, but the company laid unwoven fabric-type weed barrier sheets across the site and covered the sheet with gravel up to a thickness of about 10cm (Fig. 10).
If weed barrier sheets are spread but exposed on the ground, their weeding effect is generally said to last no longer than 10 years due to damage by wind, rain, UV rays, and so forth.
"By covering the weed barrier sheets with a thick layer of gravel, we expect the weeding effect to last more than 10 years," Watanabe said (Fig. 11).