5MW Solar Plant Built on 26-degree Sloping Dam Wall

Slope specialists hired for construction

2016/06/21 15:21
Shinichi Kato, Nikkei BP CleanTech Institute

In February 2016, Hyogo Prefecture Public Enterprises Agency started generating power at a 5MW-output mega- (large-scale) solar power plant on the wall of Kotani Dam in Toyotomicho, Himeji City (Fig. 1).

Fig. 1: Mega-solar plant set up on the body of Kotani Dam. A total of 19,580 panels were arrayed. (source: top Hyogo Prefecture Public Enterprises Agency, bottom Nikkei BP)

Kotani Dam was completed in 2000 and supplies tap water. It is a pumped-storage dam that pumps up and stores water from a nearby river when the river's level is high, and supplies the stored water to purification plants in times of water shortages.

At Kotani Dam, a solar power plant that leveraged a former borrow pit from the time of the dam's construction started operation in November 2014 (Fig. 2). This time, another mega-solar plant that uses the dam's body started generating power.

Fig. 2: A solar power plant that utilized a former borrow pit from the time of the dam's construction is situated along a mountain trail leading to the body of Kotani Dam. (source: Hyogo Prefecture Public Enterprises Agency)

Panels were set up on the "dam body" that plays the role of damming the water (Fig. 3). The body of Kotani Dam is a "rockfill dam" that was built by using rocks, stones, earth and sand as infill.

Fig. 3: Panels were attached to slopes built by using rocks, stones, earth and sand as infill. The top is before panel installation, and the bottom is after installation. The panels can generate power in conditions almost facing south and tilting by 30 degrees. (source: top Novac, bottom Nikkei BP)

Considering the plant is suited for solar power generation with the dam body facing south, Hyogo Prefecture decided to make the best use of the site as a place for a mega-solar power plant based on the feed-in tariff (FIT) scheme for renewable energy-based power generation.

A joint venture (JV) formed by Sanwa Electrical Engineering Co Ltd (Osaka City) and Novac Co Ltd (Himeji City, Hyogo Prefecture) was entrusted with the development of this mega-solar power plant at Kotani Dam. As a result, the same methods and products as those used at Heiso Dam were adopted for the power generation system, foundations and mounting systems.

The same JV also provided engineering, procurement and construction (EPC) services. Sharp Corp's single crystal silicon-type solar panels and Toshiba Mitsubishi-Electric Industrial Systems Corp's (TMEIC) PV inverters were adopted.

The JV deployed a method of forming mortar foundations on the rocks that cover the dam body surface (Fig. 4). The rocks on the surface of Kotani Dam are large, weighing from 1 to 2t and measuring from 1 to 2m each. The JV sprayed mortar in the metal net molds placed on the rocks.

Fig. 4: Foundations were formed on the rocks that cover the dam body. Spraying mortar (top) and completed foundations (bottom) (source: top Novac, bottom Nikkei BP)

The JV also adopted aluminum mounting systems and buried them in the foundations for holding. A hole to put a supporting post of the mounting systems was made in the center when forming each mortar foundation. The supporting posts were secured by pouring a mixture of cement and water in the holes where they were inserted.

Slope specialists

The length of the dam body slope is about 150m (about 50m at both Gongen Dam and Heiso Dam). The height is about 80m (about 33m at Gongen Dam and 26m at Heiso Dam), and the average pitch is about 26° (about 22° on average at both Gongen Dam and Heiso Dam). Sloping more (Fig. 5) and being longer, it was more difficult to set up solar panels on this site.

Fig. 5: The average 26-degree slopes give the impression of slopes used by advanced-level skiers. (source: top Novac, bottom Nikkei BP)

As I walked around the body of Kotani Dam when I visited for an interview, I got the impression I was walking on slopes for advanced-level skiers.

Based on these conditions, construction staff called "slope workers," who are proficient in working on sides and other slopes, were hired (Fig. 6). During the peak construction period, approximately 100 workers were engaged in construction on the slopes each day.

Fig. 6: Construction staff trained for working on slopes were used. Novac also took all possible safety measures, for example by selecting the staff's shoes and keeping them from working while the slopes were wet. (source: Novac)

At Kotani Dam, where the length of the slopes is as long as about 150m, a monorail for construction work was used (Fig. 7). Laying tracks on completed foundations along the stairs that had originally been set up for dam body inspection and other purposes, the plant operated monorails from the top to the bottom of the dam body so they could transport heavy loads. Monorails were laid along the stairs that had been built on both sides of the dam body.

Fig. 7: Heavy loads were transported by a monorail for construction work. Because of the slopes being as long as about 150m, cranes could not be used for transporting heavy loads in some areas. (source: Novac)

Light-weight aluminum mounting systems alleviate work load significantly

One of the benefits of this construction was the adoption of aluminum mounting systems. With steel mounting systems, each of such components as rails and posts is heavy and requires two workers to transport them even on flat land. The adoption of aluminum mounting systems allowed workers to carry components alone, reducing their work load and time. According to Novac, this effect was significant.

The aluminum mounting systems were designed to hold their rails at 26°, exactly the same as the slopes, when assembled (Fig. 8). After the foundations were completed, arrays within a range of about 50m were set as one group, and the height of mounting system posts was adjusted using temporary clasps and staking (a string-based method to confirm the position of a structure). As a result, the plant realized a layout where solar panels are orderly arrayed while slight undulations on the dam body surface are absorbed by the mounting systems.

Fig. 8: Rails were designed to tilt by 26 degrees, the same as the slopes. Height adjustment of the mounting system posts. (source: Novac)

Facility overview