Seafront Solar Plant Withstands Wind, Wave, Ground Subsidence

'Overload' helps PV inverters work at maximum efficiency around noon

2014/04/28 13:40
Shinichi Kato, Nikkei BP CleanTech Institute

Mitsui Engineering & Shipbuilding (MES) Co Ltd's Oita Works boasts a huge site of about 1.7km2 and started operation in 1981 as a plant where large structures such as bridges and cranes are manufactured. Throughout the site are a number of steel components as large as 10m or longer and huge dock cranes being assembled. Their massiveness and impact are overwhelming.

"Mitsui Engineering & Shipbuilding/Mitsui Fudosan Oita Solar Power Plant" is located in the corner of this vast plant (Fig. 1). The 14MW output mega (large-scale) solar power plant started selling electricity to Kyushu Electric Power Co Inc Dec 1, 2013. MES and Mitsui Fudosan Co Ltd are running the plant as a joint business.

Golf course, mega-solar power plant instead of huge shipbuilding yard

The location of MES's Oita Works (Fig. 2) is an area that Oita Prefecture established as an industrial site, where the operations of leading companies in the steel, automobile, chemical and other heavy industries are concentrated.

MES, which purchased the vast site from Oita Prefecture, initially planned to build an enormous shipbuilding yard that would be Japan's largest. However, hit by the shipbuilding recession following the Oil Shock, the company built a plant for producing bridges and other large steel products, postponing the construction of the shipbuilding yard.

As large structures are still much smaller than huge cargo ships, the area needed for manufacturing shrank to only one-third the acquired site, with the remaining west and north sides left idle. MES then opened the "Hiyoshibaru Country Club" golf course on the west side in 1990.

What made the company start considering the construction of a mega-solar power plant on the north side of the land followed a proposal by Mitsui Fudosan in the fall of 2011, when the implementation of the feed-in tariff (FIT) scheme for reusable energy was being discussed. The proposal was approved by the board of directors in August 2012 after the details of the FIT program were announced in June that year.

'Joint venture' with Mitsui Fudosan rather than SPC

Many of the mega-solar power plants with an output of 10MW or more establish a special purpose company (SPC) and structure project financing funded by financial institutions.

However, "Mitsui Engineering & Shipbuilding/Mitsui Fudosan Oita Solar Power Plant" established the joint venture "Mitsui Engineering & Shipbuilding/Mitsui Fudosan Joint Venture," instead of using project financing, with MES and Mitsui Fudosan providing business funding of about ¥4 billion (approx US$39.2 million), contributing 51% and 49%, respectively. The joint venture rents the mega-solar power plant site from MES.

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This is a common method in the building construction and real estate industries. In terms of this scale, the total return on investment is reportedly more efficient with the funds raised through each company's credit as a joint venture rather than structuring project financing.

In project financing, the cash in advance, known as the "upfront fee," the commission paid when a loan is made, is said to reach hundreds of millions of yen. Considering this cash payment in advance, legal and other payments as well as the cost for SPC operation, the return on investment is lowered unless the project financing extends to a quite large scale.

Connection with special high voltage grid within Oita Works

Compared with other areas in Kyushu, where many mega-solar power plants have been constructed, the concentration is exceptionally high in Oita Prefecture. As a result, during grid connection talks, some electric power companies have requested a restricted amount of power connection and cost provision.

At this mega-solar power plant, however, Mitsui said there were few such restrictions. This is because Kyushu Electric Power had established a large-volume grid at a special high voltage (66,000V) for the industrial site aimed at concentrating heavy industries that use a considerable amount of electricity.

Given the special high voltage grid that runs across the site (Fig. 3 & 4) in front of MES Oita Works, the capital investment for grid connection was reportedly no more than about ¥15 million.

When connecting a mega-solar power plant, "power factor control" to prevent the grid voltage from rising is almost always requested in grid connection talks. However, there was no power factor control set at "Mitsui Engineering & Shipbuilding/Mitsui Fudosan Oita Solar Power Plant."

Attention paid to impact of moisture on amount of power generation

Toko Electrical Construction Co Ltd provided the engineering, procurement and construction (EPC) services as well as operation and maintenance. Mitsubishi Electric Corp's solar panel and Toshiba Mitsubishi-Electric Industrial Systems Corp's (TMEIC) PV inverter were adopted.

A total of 65,058 261W model solar panels (17MW-equivalent), roughly 1.2 times the PV inverters' rated output of 14MW, were installed.

Setting up solar panels with more output than the PV inverters' rated output is a method called "overload." It has the benefit that it can increase the amount of power generated by adding the output of solar panels in consideration of the decline in the amount of power generation due to overall system losses and less sunshine on a cloudy day or in the morning and late afternoon. The method increases uptime rate of PV inverters.

The overload ratio of 1.2x is the typical ratio used at mega-solar power plants that have plenty of room for panels. Considering system losses, the generated power is said to hardly outperform the rated output of PV inverters.

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However, the panels already generated more power than the rated output of PV inverters and PV inverters limited their output between 11:00 and 14:00 most days around March. As April approached, the time when the amount of power generation reached the PV inverters' rated output gradually became earlier.

In light of the amount of power generated in spring, the amount of output control by PV inverters seems to further increase in summer, when the sunshine gets stronger. However, it does not have to be that way, according to Mitsui.

Solar panel power generation is related to factors including the kind of solar panels, temperature, the amount of sunshine and the amount of moisture in the air. In summer, the temperature rises as the amount of sunshine increases. With crystalline silicon solar panels, the decline in power generation efficiency caused by the cell temperature rise could be more significant than the increased amount of sunshine.

MES is also focusing on what impact the rise in the amount of moisture in the air could bring. The amount of moisture increases as summer approaches. Then, the increased amount of sunshine sometimes does not contribute so much to the rise in the amount of power generation due to the diffusion of near-infrared wavelength light that facilitates solar cells to generate more power. In addition, the amount of power generation could rise more in spring because the temperature is lower and the amount of moisture in the air is less.

Measures against wind, wave overtopping

The solar panels were tilted at 10°. This was a measure against strong winds from the north, along with the aim of setting up as many panels as possible. Facing the sea, the north side of the site is exposed to strong winds. The higher the angle at which the panels are tilted, the stronger the mounting systems for them have to be, and that could cause the construction cost to rise.

As the plant faces the sea, Mitsui also considered measures against wind and wave overtopping over the seawall. By enhancing the approximately 2m-high enclosing embankment built about 60m from the seafront, the plant prevented strong north winds from directly hitting the underside of the panels (Fig. 5). However, since the plant is located in an area where the reference wind speed is set at 32m, the plant was designed with a larger-than-necessary margin of safety excluding the effect of the enclosing embankment.

As for the foundation, Toko Electrical Construction's concrete molding, "PC (precast) Pile," was assembled like a curb (Fig. 6). Given the location in a landfill site, the mounting system could twist and be damaged if it were unable to adapt to tremors caused by earthquakes or irregular subsidence if long concrete foundations or pickets were used. As PC Pile features sufficient rigidity when built like a curb, it is said to be easier to prevent the mounting systems from being damaged even if the panel angle varies.

In the outer rows where the wind is stronger, the number of piles to be built like a curb was increased from 2 to 3, and transverse piles were added in an effort to boost toughness (Fig. 7).

On the south side where the PV inverters and transformers were installed, a large amount of sand and equipment are prepared as measures to stop oil from spreading if it leaks. Oita Works acquired ISO14001 authentication for environmental management systems. The facility management of the mega-solar power plant also complies with the same criteria, and management aimed at lowering environmental load is being sought at the plant.