Mega-solar Power Plant Plagued by Vines

Picket-based panel installation changed to foundation-based method

2014/04/06 12:22
Shinichi Kato, Nikkei BP CleanTech Institute

"Sun Factory Hiyama Hitachiota Solar Power Plant", located at an industrial site in Hitachiota City, Ibaraki Prefecture, which started operation in April 2013, is a mega (large-scale) solar power plant built by Nippon Comsys Corp (Fig. 1). Each of its two blocks, dubbed A site and B site, which is across the road, has a 990kW power generation system. The power producer is Comsys Create Corp (Shinagawa-ku, Tokyo), which is aiming at creating new businesses in the Comsys Holdings Group.

Since it was known as Nippon Telecommunications Construction Co Ltd, Nippon Comsys has been establishing communication infrastructures, laying on telecom cables and constructing switching facilities and radio relay base stations targeting the NTT Group. It has been working on the contracted construction of solar power generation systems since around 2003.

Power producer Comsys Create was established in May 2012 with the aim of creating new businesses by making the most of technologies and expertise accumulated within the Comsys Group. Currently, its mainstay is the solar power selling business based on the feed-in tariff (FIT) scheme for renewable energy; however, Comsys Create is still exploring entry into other markets apart from energy-related ones.

Comsys Group's 1st asset-based business

The mega-solar power plant business has advanced with Comsys Create developing the project as a power producer and Nippon Comsys providing the EPC (engineering, procurement and construction), operation and maintenance services. Starting from Hitachiota Solar Power Plant, the 2MW "Showa Sennennomori Solar Power Plant" in Showamura, Tone Gun, Gunma Prefecture, and another 2MW "Tsu Solar Power Plant" in Tsu City, Mie Prefecture, started operation in November 2013. The Comsys Group also started constructing another mega-solar power plant in Kitaibaraki City, Ibaraki Prefecture (See related article) in February 2014, with the operation slated to start in November of the same year.

The construction of all these mega-solar power plants has been financed by loans within the group. The Comsys Group said it is planning to procure the funding by loans within the group for the time being though some banks and lease companies have come up with loan and lease proposals.

Case abandoned because of Agricultural Land Act

Hitachiota Solar Power Plant was constructed on idle land leased by Hiyama Industry Co Ltd (Hitachi City, Ibaraki Prefecture), a company that processes films and metals as well as manufactures coating products. The project started when Hiyama Industry was introduced while the Comsys Group was visiting another site.

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"When selecting a candidate construction site for a mega-solar power plant, we have maintained a system in which we promptly propose the project to the head of the owner company immediately after understanding solar radiation conditions and other factors important for solar power generation and researching whether the site has a mortgage or not as soon as we find a desirable site," said Director Minoru Kubo, Comsys Create.

The company once abandoned the construction of a mega-solar power plant despite having reached an agreement with landowners. The reason was because of the barrier of land use categories.

"It was land that neither the landowner nor the agricultural committee of the local city recognized as farmland," said Tsugunori Kondo, manager of the Development Business Div, Comsys Group Power Generation Business Dept, Comsys Create. "Only after we had announced our construction project and started to investigate the land and proceed with construction did we have to abandon the project when the land was recognized as farmland just because some leaseholder of the land once farmed there. There must be quite a few power producers and landowners who are confused by the ambiguity of the Agricultural Land Act that can easily be affected by such judgments by municipalities and agricultural committees."

Foundation varied by ground condition

At Hitachiota Solar Power Plant, A site is in a position down from B site (Fig. 2). At each of these sites, 1,392kW-equivalent solar panels were set up despite PV inverters with a rated output of 990kW. Installing solar panels with an output (nominal maximum output) exceeding the rated output of PV inverters is called "overload."

In solar power systems, the amount of power sold always falls short of the total value of the nominal maximum output of the panels even on a sunny day due to system loss. Furthermore, panel output further drops on cloudy days or in the morning and late afternoon. Under these circumstances, installing panels with an output exceeding that of PV inverters is effective as it can boost the utilization rate of the PV inverters as much as the amount of the power generation rises, increasing the amount of power sold at the same time.

Keeping the installation angle of the solar panels as small as 15°, Comsys Create arrayed 4,800 panels with 1,392kW output in each of the two sites.

To maximize the amount of power generation per panel, the optimum installation angle in Japan is considered to be 30°. Solar panels tilted at 30°, however, would require a broader space for each row as the impact of shadows over the panels in the rear row would grow. As a result, the number of solar panels that can be set up in the same size area would decline.

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When solar panels are tilted at 15°, on the other hand, the shadow's impact would decrease, and more panels can be set up. To maximize the amount of power generation in a limited size of area, it is generally considered more beneficial to set up more panels by reducing the installation angle.

Both "overload" and the method of setting up more panels by narrowing the installation angle cause the initial investment to grow. However, at the current stage where a relatively high purchase price is applied while the panel price has been decreasing, a power producer can boost the internal rate of return (IRR) by pumping up the amount of power generation by installing more solar panels. This is one of the reasons why overload and a smaller installation angle were adopted at Hitachiota Solar Power Plant and many other mega-solar power plants across Japan.

China's Suntech Power Co Ltd's solar panels and Toshiba Mitsubishi-Electric Industrial Systems Corp's (TMEIC) PV inverter were installed. Nippon Comsys said it chose Suntech Power's solar panel based on performance and pricing benefits while it selected TMEIC's PV inverter considering its performance and reliability in view of the use for the next 20 years. The employment of a proprietary PV inverter temperature control system and the burial of connecting cables (Fig. 3) are also rarely seen at other mega-solar power plants.

Different methods were used to hold the mounting system at A site and B site. At B site, which was constructed first, mounting systems are supported by pickets without creating foundations (Fig. 4). This was changed to a method that holds the mounting system with concrete foundations at A site, which was built later.

In general, the total construction cost can be lowered with the picket-based method, which requires no land leveling and foundation work, compared with the method that uses concrete foundations. However, the picket-based method is not strong enough when the ground is soft, and it requires special heavy machinery for construction when the ground is too hard.

One of the challenges the picket-based method faces also became clear at B site. The picket-hammering work took a long time due to part of the ground containing rock. As a result, for A site, it was changed to the method that uses concrete foundations. To minimize the cost increase, Nippon Comsys filled the inside of the U-shaped concrete ditch, which was procured for a drain channel set along the plant's perimeter, and used it as the foundation (Fig. 5).

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Unexpected strength of vines

As one year has passed since power generation started in April 2013, some challenges started to surface. For example, the vines outside the mega-solar plant site grew over the fence around the perimeter. Having been neglected, the vines grew as fat and strong as wood and climbed over the metal net fences, turning them into sheets or walls of vines (Fig. 6 & 7). A typhoon then struck, and the fences leaned after being directly hit by the strong wind with little room left for the wind to escape.

Learning from this experience, Nippon Comsys decided to entrust weeding from the plant's environs to the slopes twice a year to a local affiliated enterprise. Each time, it costs several million yen.

"They may not seriously affect the amount of power generation itself, but they could cause danger to the surrounding area if the fences are broken," Comsys Create said. "The force of the weeds was beyond our imagination."