19.5MW Solar Plant Arranges Panels on Slopes

Steel molds improve foundation accuracy, shorten construction time

2014/03/08 22:26
Shinichi Kato, Nikkei BP CleanTech Institute

The mega (large-scale) solar power plant with a total output of 19.5MW, which is being constructed by the Daiwa House Group on about 300,000m2 of land of the former Kaijima Coal Mine in Miyawaka City, Fukuoka Prefecture, is nearing full-fledged operation (Fig. 1). This mega-solar project consists of three large-scale solar power plants. The 1.5MW and 1.9MW output solar power plants started operation in October 2013 whereas the remaining 16.0MW mega-solar power plant is slated to start generating power in April 2014.

"The land, which once contributed to Japan's energy through coal, is now in the position to again make a contribution to the energy supply as a solar power plant." Ahead of the construction, Yoshiteru Adachi, New Energy Project Leader at the New Energy Department, Daiwa Energy Co Ltd, heard such opinions from the landowners of the former Kaijima Coal Mine, where the power plant was built. "It felt sobering knowing they rented us the land with those thoughts," Adachi recalled.

Toyota Motor Corp's Kyushu Plant is located nearby. Given this location, an industrial park was established with the aim of attracting plants from the automobile-related industry. The mega-solar power plant is situated in a corner of this industrial park. Time passed without finding a company to build a plant in the park as was expected at the beginning, until the Daiwa House Group came to construct the mega-solar power plant prior to the implementation of the feed-in-tariff (FIT) program for reusable energy.

Reasons why lease style was adopted

For this mega-solar power plant, Daiwa House Industry Co Ltd's subsidiary, Daiwa Energy, runs the power generation business and sells the power to Kyushu Electric Power Co Inc. The mega-solar power plant was constructed by Daiwa House Industry. As has thus far been described, the project appears to have been completed within the Daiwa House Group, but this is not so.

It is Mitsubishi UFJ Lease & Finance Co Ltd that purchased the mega-solar power plant from Daiwa House Industry and rents it to Daiwa Energy. And it is also Mitsubishi UFJ Lease & Finance that rents the land from the landowners.

There were various reasons why such a business structure was adopted. First, it was aimed at clarifying the roles of Daiwa Energy's power generation business and Daiwa House Industry's mega-solar power plant construction business. It was also difficult for Daiwa Energy alone to procure the roughly ¥6.5 billion (approx US$62.9 million) needed for construction of the mega-solar power plant. Mitsubishi UFJ Lease & Finance, however, can serve as a financing function.

For a 20MW-class mega-solar power plant like this, a special purpose company (SPC) is generally established to structure a loan as project finance. In the case of this mega-solar power plant, however, the cost to establish, manage and run an SPC would exceed the cost of leasing the property. As Mitsubishi UFJ Lease & Finance also manages the contract, salary transfer, etc, the benefit of complementing each other with a company that is skilled in risk management and maintenance operation is significant."

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The Daiwa House Group said it has almost established the optimum scheme for mega-solar power plant construction and the style of power generation business in accordance with different business scales and landowners, through various solar power plant approaches (See related article). From now, the group will appropriately leverage such a lease method as this.

Response according to land, surrounding environment

When installing the power generation system, efforts were made in accordance with the land and the surrounding environment. For example, the 1.5MW and 1.9MW output large-scale solar power plants that already started power generation are facing housing areas and roads (Fig. 2). Hence, trees were planted at the east and south ends of the sites so they can lower the risk that the solar panels' reflection might affect the housing areas and the cars driving on the roads (Fig. 3).

A regional reservoir for water discharge is included in the 1.5MW and 1.9MW capacity large-scale solar power plants (Fig. 4). By re-establishing this reservoir, Daiwa House Industry could install more solar panels in the area to the north of the reservoir. As a result, more solar panels with an output of about 200kW were set up (Fig. 5).

A variety of instruments are utilized for management and maintenance with water faucets and surveillance cameras set up as well as solar radiation sensors attached in two positions in each plant. Of the instruments, the surveillance cameras were set up by Sohgo Security Services Co Ltd (ALSOK) and the Daiwa House Group. Daiwa House Industry jointly built a security system for the mega-solar power plant with ALSOK, which it partners in the area of security systems for its integrated housing business.

Concrete foundation easy to evenly set up on slopes

What was unique about the construction of this mega-solar power plant was the concrete foundation that was evenly set up in accordance with the sloping ground. At the 16.0MW output mega-solar power plant under construction, in particular, the land slopes down toward the east (Fig. 6).

Daiwa House Industry was required to build concrete foundations with high accuracy and low cost on such land. To line the solar panels at the same height on land sloping down from the east toward the west, concrete bases to determine the height were constructed first in the ground at the east and west ends and some points between them in each row of the solar panels (Fig. 7).

As the concrete bases are solidified, elongated concrete foundations are formed on them in an east-west direction. Given the even height of the concrete bases in the ground, elongated concrete foundations stretching east to west, whose vertical heights are parallel, are completed by pouring concrete into metal molds placed like a bridge over the concrete bases (Fig. 8).

Generally speaking, most concrete foundations at mega-solar power plants stretch south to north and are combined with a post that holds the upper parts and the reverse-sides of solar panels. This is because accuracy can be more easily secured for various reasons including the short length of the concrete foundation.

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At this mega-solar power plant, however, the land gets lower as it approaches east. To make elongated concrete foundations stretching south to north and keep the same height in the same row stretching east to west, the problem of maintaining the same height for each foundation would increase.

Based on these circumstances, Daiwa House Industry boosted the accuracy of vertical heights by making concrete bases for even panel heights like bridge beams at both the east and west ends in advance, and forming elongated concrete foundations that align east to west using metal molds that bridge those bases at both ends. The dimensions of the concrete foundations vary according to the wind pressure that the solar panels would be subject to in the mega-solar power plant (Fig. 9).

Furthermore, metal molds were adopted for the creation of these concrete foundations (Fig. 10). Wooden frames are generally used when pouring concrete. The conventional method, in which craftsmen assemble the wooden frames and place concrete, faces the challenges of maintaining accuracy and shortening the construction period because the wooden frames would be mostly handmade one by one. On the other hand, the benefits of using metal molds include the fact that they do not require mold and rebar experts to handle them and that they can be constructed in a shorter period of time due to higher dimension accuracy.

As the molds included some that use high-priced steel called H-shaped steel (Fig. 10 left), the material cost rose. Daiwa House Industry said, however, it succeeded in absorbing the cost increase throughout the entire plant, boosting construction efficiency by leveraging the large scale (where a large amount of solar panels, that is, as much as 22.9MW, was to be arrayed) and making the foundations in series block by block.

As to weeding, weed control sheets will be adopted at the 16.0MW output mega-solar power plant in accordance with the verification of several weed control measures at the 1.6MW and 1.9MW output power plants that were completed earlier. In addition, to help make the view of inorganic solar panels spreading like the sea give the impression of blending in with the surrounding environment as much as possible, clover, which could function as a weed control measure, will be planted across the 16.0MW mega-solar power plant.

Korean LG Electronics Inc's solar panel and Toshiba Mitsubishi-Electric Industrial Systems Corp's (TMEIC) PV inverter were adopted. The total output of the three power plants is 19.5MW, but about 22.9MW of solar panels were installed.

At the 16.0MW output mega-solar power plant, in particular, the space between the solar panels was narrowed to increase the number of panels that could be installed. As a result, for a few hours during the day, some panels are shaded by their adjoining panels on their south side. Daiwa House Industry explained that it adopted this design, finding out that the effect would be larger in the amount of power generation increase by setting up more solar panels, compared with the amount of decrease by the shadow over the panels.