Visit to Plant

2.1MW Solar Plant Built on Roadside Slope

Construction materials conveyed by monorail, foundations created by 'spraying'

2015/08/11 15:32
Shinichi Kato, Nikkei BP CleanTech Institute
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As you look at the hills to the north of Nagasaki Fishing Port, you can see solar panels set up as if they are pasted on a steep slope. This is the 2.141MW-output "Sakuranosato Mega Power Plant" that uses a roadside slope owned by Nagasaki Prefecture (Fig. 1).

Sakuranosato Mega Power JV (Nagasaki City), a special purpose company (SPC) established by Toda Corp and TB Corp (Nagasaki City) of the Kyushu Gas Group (Isahaya City, Nagasaki Prefecture), developed and started constructing the power plant in February 2015. The construction is slated for completion in September 2015 followed by the distribution of power to start in October.

The project was selected as one of the "Proposals for a Solar Power Project Using a Roadside Slope," which were publicly offered by Nagasaki City. The JV set up a mega-solar power plant on the south-facing roadside slope (approximately 20,500m2).

Solar panels set up on a 30-degree slope without using heavy machinery

The mega-solar power plant in Sakuranosato was confronted with the challenge of setting up a solar power generation system on a slope as steep as 30° (Fig. 2).

"30 degrees" is the angle of steep slopes in medium to advanced courses at ski resorts. When a slope is this steep, almost no heavy machinery cannot be used in construction. As a result, the amount of the process that relies on human resources increases, with cranes being incapable of conveying materials in some areas. Working while constantly paying attention to the slope increases the stress for on-site workers, compared with working on normal flat areas.

On the other hand, there is a benefit: given that the land slopes by 30°, solar panels would naturally be tilted to 30°, which is considered the optimum angle in Japan, just by setting up the panels in parallel with the ground (Fig. 3).

Track to run a monorail on the slope

Heavy materials for construction were conveyed by a monorail on the slope. A track was set up from the bottom to the top of the slope to run the monorail (Fig. 4).

Heavy trucks and crane vehicles can drive on the east-west road that runs inside the mega-solar power plant from its entrance. Based on these circumstances, heavy facilities such as PV inverters and transformer were set up on this road. Construction materials were also temporarily stored on this road and then conveyed up and down the slope by the monorail.

In addition to the monorail, portable stairs and narrow lanes running east-west were also used (Fig. 5). The portable stairs were newly installed. The narrow lanes running east-west had originally been set up every 7m between the top and the bottom of the slope for slope management.

Foundations formed by spraying mortar

The foundations were formed by spraying mortar (Fig. 6). Such a construction method is often seen when reinforcing cliffs made by leveling mountains. Covering the surface of a slope with mortar has the effect of preventing rocks and dirt from falling off.

When setting up panels on a slope in accordance with the terrain, steel pile foundations are adopted in many cases with the aim of boosting construction efficiency on the slope and lowering construction cost. However, pile foundations need heavy machinery to be driven into the ground. Pile foundations could not be applied to this 30° slope, where the constructor could not use heavy machinery.

As a measure to fulfill the load bearing capacity required for the 30° slope and to lower the cost at the same time, according to the plant, a method of manually driving steel tube piles into the ground was initially considered.

The plant eventually adopted a method of burying steel beams in the four corners and in the center and spraying mortar to form a foundation using a net as a mold (Fig. 7). The small foundation measures 45 x 45 x 25cm.

As the solar panels get closer to the bottom of the slope, the trees shade the panels more on the east and west side of the site. The constructor avoided such areas and succeeded in arraying a total of 8,204 solar panels (about 2.1MW) manufactured by Mitsubishi Electric Corp. An array consists of 15 panels (3 panels x 5 rows). The solar panels were set 50cm from the ground.

The capacity of PV inverters is 1.99MW. They are products of Toshiba Mitsubishi-Electric Industrial Systems Corp (TMEIC).

As for drainage, facilities including the water ditch that had been equipped with the roadside slope were used without modifications. Kyushu Gas Co Ltd's subsidiary Kyushutechno Corp (Isahaya City, Nagasaki Prefecture) is responsible for the operation and maintenance services.