Solar Plant Designed to Minimize Damage to Roof

Spreading across logistics center roof, securing ease of maintenance

2014/02/24 12:12
Shinichi Kato, Nikkei BP CleanTech Institute

Logistics bases with large roofs are being focused on as prospective locations for on-roof-type large-scale solar power generation systems. The mega (large-scale) solar power plant with 2MW output located on the roof of "Hibiki International Logistics Center" in Kitakyushu City, Fukuoka Prefecture, is one such base. On the roof, which has a length of 290m, 8,244 solar panels are mounted (Fig. 1 & 2).

Daiwa House Industry Co Ltd owns and rents Hibiki International Logistics Center to tenant enterprises (Fig. 3). The mega-solar power generation system on the roof was installed by Daiwa House Industry, and its subsidiary Daiwa Energy Co Ltd sells the generated power to Kyushu Electric Power Co Inc as a power producer. Daiwa Energy plans roughly ¥80 million (approx US$780,107) power sales per annum, expecting a total of about ¥1.6 billion in power sales over the 20 years from October 2012, when it started selling electricity.

Daiwa House Industry is Japan's leading house manufacturer dealing not only with houses but also large-scale facilities. Given the nature of its business, the company is highly efficient at securing land and constructing buildings. In terms of solar power generation, Daiwa House Industry has more than 15 years of experience, focusing on panel installations on roofs. Furthermore, Daiwa Energy, which was established in 1999, has built up power generation business experience through wind power generation facilities that have been set up across Japan.

Compared with structures such as large buildings, the construction of a mega-solar power plant is easier and hence provides a good chance for market entry to various companies. Amid such circumstances, Daiwa House Industry said it can demonstrate its strengths by achieving both safety and low cost through, for example, its technologies to create the strongest structure possible with the least amount of steel by leveraging its knowledge and expertise accumulated through being involved in the construction of many structures.

As described above, the on-roof-type system at Hibiki International Logistics Center is the first large-scale solar power plant that the Daiwa House Group, which has abundant knowledge in both building structures and solar power generation, has ever handled.

To prevent negative impact on roof coating

When setting up a solar power generation system on a roof, the Daiwa House Group gives priority to "limiting the impact on the building as much as possible," said Ryoichi Nishimoto, manager of the Property Management Group at Daiwa House Industry.

This is because Daiwa House Industry wants to prevent the situation where setting up a solar power system has a negative impact on a building owned and rented by the company, which is a building construction expert.

"To reduce the load on the building as much as possible" extends further than just limiting the impact on the roof structure and coating. It includes having no negative impact on roof maintenance for the next 20 years as long as the power generation business continues.

For example, the building could be damaged by rust occurring on a solar panel frame and spreading to the roof. Should there be signs of such a problem, the solar panel needs to be replaced immediately, Daiwa House Industry said.

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In addition, given the service life of roof coating, it is sometimes necessary to repaint the waterproofing and other coatings every five to 10 years. The solar panels installed on the roof should not interfere with the recoating process.

In view of these circumstances, Daiwa House Industry arranged the distribution of panels so it is easy to confirm the state of the roof by sight and fixed panels without damaging the roof.

The logistics center's roof is metal and is called a folded-plate roof where V-shapes line up sideways; this is a type that is widely used at large-scale facilities. Daiwa House Industry installed the solar panels so that they would neither damage the top part of the folded-plate roof nor affect its coating while realizing a strength that would not let the solar panels be blown away by the wind.

For the clasps that would meet these goals, the products of Sakata Co Ltd and Nichiei Intec Co Ltd were deployed.

"The clasps use fewer bolts than other models and accordingly help constructors to install panels more easily with a smaller load," said Yoshiteru Adachi, New Energy Project leader, New Energy Division, Daiwa Energy. "As a result, construction errors would decrease, increasing the benefit of a shorter time required for installation. If it is easier to construct, the number of days to work on the roof will decrease. And that will lead to a cut in costs."

Panels fixed avoiding slits for drainage

The solar panels were set up by attaching clasps at the top of the folded-plate roof. A total of six clasps are used to fix each solar panel, with three pieces set on each long side of the panel (Fig. 4).

Each solar panel was mounted across the three tops of the folded-plate roof. And, basically, 28 panels were installed on three folds. As a circuit called a "string", which connects 14 solar panels as one unit on direct current, is formed, two strings are allotted to every three folds and connected with a connecting box. There are, however, some exceptions. This is because the panels are installed to avoid the exhaust ports and soft parts of the structure (Fig. 5).

When attaching the clasps, issues arose stemming from the adopted solar panel design. Kyocera Corp's solar panels have three slits for drainage on both sides and in the middle, and it turned out that the middle part of the panel could not be fixed with the clasps because of the slit. For this reason, Daiwa House Industry modified the panel positions at tops of the folded-plate roof so that the clasps could be used to hold a panel at positions slightly away from the panel center (Fig. 6).

Schemes to realize ease of maintenance, short construction time

Also, in an effort to cut the construction time on a roof as long as 290m, Daiwa House Industry conveyed the related materials by a trolley on a track set up on the roof. Such a method could never be applied without being familiar with every part of the roof including the load capacity.

In addition to this, there were various others schemes and considerations. First, you will recognize the solid-looking stairs and platforms on the roof (Fig. 7). They are designed for a number of people to easily go up and down or walk on, aimed at securing safety and ease of inspection and maintenance.

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The connecting boxes that are set upright, instead of being laid down, also reflect considerations for ease of operation and maintenance (Fig. 8). As resistance to wind pressure was required because the boxes are standing, the metal fittings used to install the connecting boxes are noticeably large.

As this plant is Daiwa House Industry's first installation of an on-roof-type large-scale solar power plant and the project has an element of verification, a variety of instruments needed to operate the power generation system such as power supplies, sill cocks, solar radiation sensors, temperature sensors and remote monitoring cameras are installed and leveraged for maintenance (Fig. 9). Daiwa House Industry even redesigned some instruments, considering they were excessive, to make them simpler for future on-roof large-scale solar power generation systems.

As to the solar power generation system, the plant adopted Kyocera's solar panel and Toshiba Mitsubishi-Electric Industrial Systems Corp's (TMEIC) PV inverter. Kyocera's solar panel was adopted for its performance at the nearby "Hibikinada Solar Power Station" by Electric Power Development Co Ltd (J-Power) in consideration of the impact from salt damage owing to the plant's location by the sea. TMEIC's PV inverter was adopted because of the generally high praise it has received.

Roof preservation, heat shield effects

Solar power generation systems installed on the roof of a large structure have other benefits in addition to power generation. It is "the contribution to roof preservation and boosting the heat shield effect of the roof brought about by the solar panels that work as a roof cover," said Megumu Sonobe, manager of the Management Planning Department, Daiwa House Industry.

In terms of roof preservation, the panels can protect the roof without damaging it while slowing its aging. In fact, on one occasion, a radio-controlled helicopter fell and damaged the solar panels at this solar power plant. Had the panels not been mounted, the roof would have been seriously damaged. As for slowing its aging, the effects show in areas that are not directly affected by sunlight.

The heat shield effect of the roof contributes to, for example, reducing air-conditioning costs inside the building. Daiwa House Industry is verifying this effect with the 816kW capacity solar power generation system installed on the roof of Building No. 2 at its Okayama Plant. The temperatures of the roof surface and inside the building are compared to assess the effect of the panels. Such a benefit is likely to grow to an advantage regarding the future penetration of on-roof-type solar power generation systems.

Using Okayama Plant's solar power generation system, various solar panels and clasps are being compared and verified. As for solar panels, Korean LG Electronics Inc's single-crystal silicon type, Sharp Corp's and Suntech Power Japan Corp's polycrystalline silicon type and Solar Frontier K K's thin-film type were introduced with an output of about 207kW, 202kW, 205kW and 202kW, respectively. And about 100kW of panels of each manufacturer were set up facing south (and the other 100kW facing north) to measure the power generation efficiency and the amount of power generation.