Shoken Co Ltd (Otsu City, Shiga Prefecture), a local general constructor centering on civil engineering in Shiga Prefecture, constructed two mega (large-scale) solar power plants: "Shoken Ishibe Solar Power Plant" (hereafter Ishibe Solar) in Konan City, Shiga Prefecture, and "Shoken Kashiwabara Solar Power Plant" (hereafter Kashiwabara Solar) in Maibara City, Shiga Prefecture. The capacity is approximately 1.811 and 1.995MW, respectively.
"By combining the two mega-solar power plants, the expected annual amount of power generation corresponds to about two-thirds the power our company consumes every year," Shoken President Mamoru Yamakawa said.
Shoken's mega-solar power plants have two major features. One is the entirely asphalted site (Fig. 1), and the other is that the solar power generation systems are built up with 1,000V-based components. These measures were adopted in an effort to lower both construction and operation costs.
It is unusual for a 2MW-class mega-solar power plant to adopt these specifications. Shoken Executive Director and Manager of Solar Business Tomoji Chihiro said, "We have always been positive about challenging moves as evidenced by the introduction of a plant that was twice as large as those of our industry peers and a cutting-edge manufacturing process in the asphalt business."
Shoken also kept its capital investment at roughly ¥600 million (approx US$5.9 millin) at "Ishibe Solar" and "Kashiwabara Solar," lower than other general mega-solar power plants, which were constructed at around ¥400 million per 1MW.
The procurement of business funds, which can reach hundreds of millions of yen, often forms a barrier to mega-solar power plant business by medium-sized enterprises, but Shoken said it provided all the funds itself. Partly because Shoken cut the initial investment per capacity, it is expected to recover the capital investment in about eight years.
Reason why fully asphalted ground can cut costs
The benefits of asphalting are the ease of solar power generation system construction and the reduction of construction costs. Moreover, it helps to prevent ground subsidence and reduces maintenance costs by preventing weeds after operation starts.
The ease of construction increases because asphalt makes the entire site flat, enabling stable construction even on rainy days and boosting the efficiency in conveyance and moving (Fig. 2).
A worker can transport secondary concrete foundations using a forklift and solar panels using a pallet jack to the positions to install them. At general unpaved mega-solar power plants, a team of two workers carry solar panels by hand. Compared to this, the carrying-in of related materials becomes easier because heavy machinery can be flexibly used.
The asphalt also enables tools and other items to be put on a trolley and installed one at a time, allowing workers to move around, as in a production line at a plant; this also leads to a shorter construction period and higher accuracy. By adopting Kyocera Solar Corp's inlay-style mounting system, a row of solar panels can be slid up to the designated spot like a sliding door and fixed by a bolt at both the top and bottom of the panels.
Thanks to the drainage ditch set up when asphalting the ground, the plant site is well drained even on rainy days. And this brings about a noticeable effect where construction efficiency can be kept almost equivalent to that on sunny days. At mega-solar power plants where mounting systems are set up directly on the ground, construction would generally stop for one to three days after rain.
Recycled asphalt lowers cost for paving
Shoken used its own material for asphalting, and Omi Doro Doboku (Koga City, Shiga Prefecture) surfaced the ground. A large portion of the material was "recycled asphalt" that Shoken collected when re-surfacing old roads. Recycled asphalt sufficiently meets the objectives of surfacing the ground for mega-solar power plants, where asphalt does not need to endure the friction caused by automobile tires like normal roads.
Recycled asphalt needs to be laid while it is heated after being collected and finely crushed. Accordingly, Ishibe Solar (Fig. 3) in the "Ishibe Ascon Plant," which has a recycled asphalt material plant, only used recycled asphalt because it could be immediately laid after being heated. Meanwhile, Kashiwabara Solar mixed the recycled material with new asphalt in a proportion of 1:1 because the heated material would cool down while being conveyed.
Panel temperature kept from rising even above asphalt
However, it was a concern that the surface temperature of the paved ground may rise more due to strong sunshine compared with ground such as soil and lower the amount of power generated by the crystal silicon solar panels. In light of this, Shoken minimized the asphalt area that was exposed to the sun by arranging the solar panel's installation angle and the space between the panels in rows next to each other, in an effort to limit the rise in the surface temperature of the solar panels. At Ishibe Solar, the solar panels are tilted at 10° while the rows of panels are spaced 60cm from each other.
1,000V-compatible system enables 3 PV inverters to form 2MW plant
Meanwhile, the adoption of a 1,000V-compatible solar power generation system also has the effect of lowering both initial investment and maintenance costs.
Compared with more widely used 600V systems, the number of panels in a string that connects solar panels in series can be increased, and, as a result, the number of strings in an overall power generation system, cables and connecting boxes would decrease.
Kyocera Corp's 1,000V-compatible solar panel was deployed.
"Ishibe Solar and Kashiwabara Solar were one of the mega-solar power plants that introduced the panel ahead of others in Japan," Senior Managing Director Iwao Fuchigami of Kyocera Solar said.
In particular, as the panel that Ishibe Solar adopted was a 322W model that increased the number of solar cells connected in series from the usual 60 to 80, the number of foundations, mounting systems and cables in the overall power plant could be cut further.
The two plants also use fewer PV inverters compared with other plants. Many 2MW mega-solar power plants using 600V equipment have introduced four 500kW-class PV inverters. On the other hand, Ishibe Solar and Kashiwabara Solar reduced the number and installed three units of 1,000V-compatible 630kW and 665kW PV inverters, respectively. Both were products of Toshiba Mitsubishi-Electric Industrial Systems Corp (TMEIC).
Panels feature antidazzle treatment to prevent reflection on JR Kusatsu Line
Ishibe Solar is surrounded by Route 1 to the north, Shoken's asphalt plant to the east, West Japan Railway Co's (JR West) JR Kusatsu Line to the south and the Miyagawa River to the west (Fig. 4). Running across the middle of the site is the Route 1 bypass "Ritto Minakuchi Road", connecting Iwane, Konan City, Shiga Prefecture, and Kamitoyama, Ritto City.
Under these locational circumstances, Shoken was requested by JR West to come up with a measure against solar panel reflection on the JR Kusatsu Line, and adopted an anti-reflection coating for the solar panels (Fig. 5).
Ishibe Solar could be connected with the Kansai Electric Power Co Inc's transmission grid in a neighboring area along Route 1 at a cost of only about ¥600,000. No power ratio restriction was set. Since it started selling power in February 2013, the amount of power generation produced a large upswing from the expected 1,700,000kWh per year to approximately 2,210,000kWh per year.
There were also some problems. Due to rain caused by typhoon No. 18 that landed in September 2013, the level of the Miyagawa River rose and flooded, nearly to the upper part of the concrete foundations. It did not affect the power generation itself. After this flooding, the Ministry of Land, Infrastructure, Transportation and Tourism (MLIT) constructed a pool under the bypass to prevent water from flooding under the bypass, and now water runs into the pool when flooding occurs, as it did in September 2013.
In winter, there was unexpected snow cover. Since this is not a snow zone, no full-scale measure against snow had been taken at Ishibe Solar. Hit twice by snow cover of about 20cm during this winter, the plant did not generate power for one whole day with all the panel surfaces covered with snow.
Additional measures against snow
Kashiwabara Solar, on the other hand, started selling power in April 2014 (Fig. 6). Next to the site to the east is a mountain (Fig. 7). Kinden Corp analyzed in advance the impact of this mountain's shadow using a photovoltaic power generation analysis system dubbed "Solar Navi" and "designed the system so that the impact could be limited to 3% in the morning," Kinden Executive Officer Toshiaki Iwata said. He said Kinden is now verifying the impact on the actual amount of power generation.
Its major difference from Ishibe Solar was the need for measures against snow given its location in a snow zone (Fig. 8). The panels are tilted at 15° at Kashiwabara Solar whereas they are tilted at 10° at Ishibe Solar. The mounting height of the panel's ground side was raised from 60cm to 80cm from the ground. The mounting system was changed from aluminum to iron. The solar panel was also varied from the 322W product to a normal-size 245W product. Shoken explained all these changes were aimed at corresponding to the expected snow cover and vertical loads under snow cover.
As an effort to boost construction efficiency, Shoken also changed the concrete foundation to another type that allows builders to transport two foundations at a time without using any screws and boost the efficiency in transportation and installation accuracy.
As a result, the number of concrete foundations that can be installed in a day soared to a maximum of 300 in Kashiwabara, compared with 60 in Ishibe. The installation of foundations and mounting systems finished in about one and a half months, followed by about two weeks of solar panel installation. It generally takes three months per 1MW, or six months for 2MW, to set up the infrastructure.
In addition, the size of the connection cable was changed from 22 x 22mm to 38 x 38mm. According to Shoken, it chose a thicker cable because the loss in transmitted power was larger than expected due to the large resistance inside the 22 x 22mm cable when transmitting 1,000V power for a maximum distance of 300m at Ishibe Solar.