Multi-level Hill Transformed Into Slope at Island Solar Plant
Innovative measures for environment, soil in remote island
Fukuejima, the largest among the Goto Islands, is located at the southwest end of the Goto Islands in western Nagasaki Prefecture, Japan (Fig. 1). It has an area of about 326km2 and a population of over 30,000 people. The island can be accessed from Nagasaki and Fukuoka by air and sea and functions as the hub to connect the Goto Islands to each other by sea.
The "Sun Farm Nakamura Goto," a mega- (large-scale) solar power plant with a solar panel capacity of about 1.25MW and a grid capacity of 1MW, is located almost in the heart of the island (Fig. 2). Approximately four years have passed since the plant began power generation in March 2016. Generated power is sold at 36 yen/kWh (excluding tax) to Kyushu Electric Power Co Inc based on the feed-in tariff (FIT) scheme.
Nippon Comsys Corp Group, which primarily deals with telecom infrastructure, developed and operates this mega-solar power plant. Nippon Comsys provided engineering, procurement and construction (EPC) services, and operates and maintains the power plant. The power producer is Comsys Create Corp (Shinagawa-ku, Tokyo), a subsidiary that develops new business for Nippon Comsys.
The mega-solar plant on Fukuejima adopted polycrystalline silicon-type solar panels of Hanwha Q Cells Co Ltd and PV inverters manufactured by Toshiba Mitsubishi-Electric Industrial Systems Corp (TMEIC).
Nippon Comsys has adopted crystal silicon-type solar panels of various manufacturers at all of its power plants excluding CIS chemical compound-type panels adopted at the 1.8MW-output project in Azumino City, Nagano Prefecture, and TMEIC PV inverters at 17 of its projects, excluding two projects with an output of about 12.7MW and 2MW in Rifucho, Miyagi Prefecture, and Ebetsu City, Hokkaido, respectively.
The mega-solar plant on Fukuejima has some unique features that cannot be seen at the other solar power plants developed and being run by the Nippon Comsys Group. First, the solar panels are arrayed on a slope (Fig. 3). At the other plants, solar panels are basically set up on relatively flat areas.
Previously, there was a pig farm on the power plant site in Fukuejima. At that time, the site had been developed into three levels. Comsys Create decided to commercialize a solar power plant on this site, overloading solar panels equivalent to about 1.26MW against a grid capacity of 1MW.
The site is on the border between a sloping hill and flat land, with mountain forest covering the hill on the upper side and rice paddies spreading across the adjacent site to the bottom in the surrounding area.
As the company started designing a solar power plant on this site, it found that not as many solar panels could be arrayed as planned if the site was kept as it was with three levels. This was because flat areas were limited, and the side slopes between the levels were too steep. In addition, due to factors including the effect of shade, it was difficult to set up as many panels and achieve as much power generation as planned on-site in the original state.
As a result, the company decided to reclaim the site eliminating the side slopes between the levels and making the entire site sloping. As the slope was made gentle enough to set up solar panels on it, Comsys Create could set up panels across the site and realize a layout to generate as much power as expected, setting up as many panels as planned.
The ground appearance gradually changes as it gets lower from the top (Fig. 4). The top area is relatively flat and covered by crushed stones. PV inverters and step-up transformers were set up there. The lower you down the slope, the steeper it gets, the amount of crushed stones decreases and the area of red soil increases.
This red soil is one of Fukuejima's characteristics. It differs from ground soil at most solar power plants in Honshu and mainland Kyushu. Red soil in Fukuejima and other Goto Islands is easily eroded and washed away by rainwater, etc because its particles are less adhesive. The company said its design also kept red soil from running out of the site.
The crushed stones around the top area not only helped with the delivery of PV inverters and other heavy facilities and protecting them but also limiting red soil erosion by rainwater. The company also built "water blocks" by solidifying crushed stones to keep red soil from being washed away following a sharp rise in rainwater running down the slopes including the mountain forests outside the site (Fig. 5).
Remote islands in Kyushu are mostly hit by heavy rain when a fierce typhoon strikes. What are such circumstances like in reality?
Given the different circumstances from those in Honshu and mainland Kyushu, it is difficult to imagine heavy rain and strong winds on a remote island in a typhoon without experiencing them. Therefore, Comsys Create developed countermeasures from the design phase, gaining knowledge from the local community.
The company requests the landlord to inspect the site after a typhoon or heavy rain and to restore things to their original state if something unusual is found.
There was a concern that, if rainwater starts running down the slope before pouring into the drainage ditches following heavy rain, soil on the surface might be removed or water pathways might be formed. The water blocks were built to counter such risks, and partly thanks to them, soil has never been largely washed away thus far.
In summer 2019, however, the slope adjacent to the bottom fences collapsed following heavy rain during a typhoon. This consequently caused the fences and side slopes that supported the fences to collapse. When restoring this damage, Comsys Create added stone walls to further enhance the resistance, making it difficult for similar damage to take place again (Fig. 6).
Some solar panels were also detached from the mounting systems and blown away by strong winds in a typhoon. This typhoon caused wind-related and flood-related damage throughout the Goto Islands. Although the company had taken preparatory measures, some panels were blown off from the mounting systems by winds stronger than expected at the time of design.
However, the detached panels stayed within the site and did not scatter across the surrounding area. Using reserve panels as replacements, Comsys Create immediately restored operation.
There is an unusual point about the mega-solar plant in Fukuejima. Its grid connection point is situated at the bottom next to rice paddies (Fig. 7). The PV inverters and step-up transformers were, however, installed furthest from that point at the top of the hill.
According to Comsys Create, it would naturally have installed the PV inverters and step-up transformers at the bottom of the hill near the grid connection point; however, the area was adjacent to rice paddies. A farm road between the rice paddies extended to the front of the power plant. This road was, however, too narrow for a 10t vehicle to pass through to deliver PV inverters and so forth.
This is why the company gave up on setting them up at the bottom near the grid connection point and installed them at the top instead.
And yet, it was not easy to install the facilities at the top of the hill. The company still had to carry them along a narrow road in the mountains, and this road was also too narrow for a 10t vehicle to pass through in some areas. Accordingly, Comsys Create solved this problem through such efforts as trimming trees in the surrounding area and reloading the equipment to a smaller vehicle during delivery.