Japan's Largest Solar Plant Withstands Ash, Salt, Strong Wind

Widespread effort to realize 70MW output, finish construction in 14 months

2013/12/01 19:56
Shinichi Kato, Nikkei BP CleanTech Institute

"I feel very happy about what we have achieved starting from the poor power generation equipment of the time when we had just started our research into solar panels," said Kazuo Inamori, Kyocera Corp's chairman emeritus.

Japan's largest solar power plant to date, the "Kagoshima Nanatsujima Mega-Solar Power Plant" in Nanatsujima, Kagoshima City, built by Kagoshima Mega Solar Power Corp (Kagoshima City) led by the Kyocera Group, with about 70MW output has started generating power (Fig. 1).

The power producer, Kagoshima Mega Solar Power, is financed by seven firms: KDDI Corp, IHI Corp, Kyudenko Corp, the Kagoshima Bank Ltd, Bank of Kyoto Ltd, Takenaka Corp and Kyocera. The solar panels are produced by Kyocera while the PV inverters are produced by SMA Solar Technology of Germany. A construction consortium formed by Kyocera Solar Corp (Kyoto City), Kyudenko and Takenaka constructed the plant. And Kyocera Solar and Kyudenko operate and maintain the facilities. The total funding, about 27 billion yen, was raised by project finance managed by Mizuho Bank.

The plant is expected to generate approximately 78,800MWh per year, which corresponds to the amount of power consumed by 22,000 households. The generated power is entirely sold to Kyushu Electric Power Co Inc. The amount can provide 2.2% of the power demand in Kagoshima Prefecture. The plant was constructed on a landfill site facing Kagoshima Bay, which was previously used as an IHI plant. IHI will receive rent for the land for about the next 20 years while participating in the solar power business as a sponsor.

Approximately 290,000 solar panels in front of Sakurajima Island

The sight of 290,000 solar panels arrayed neatly in front of Sakurajima Island has a certain impact, as if you were viewing a sea of solar panels (Fig. 2).

The roughly 1,270,000m2 site is about 27 times larger than the Tokyo Dome. It takes about 1.5 hours to walk the 4.3km outer perimeter. It is so big that graders (construction vehicles used to flatten land) featuring GPS (global positioning system) were used to grade the site.

In the foundation work to arrange the 290,000 solar panels, 20,000m3 of concrete was used from 5,400 cement mixer trucks. The foundations to hold the mounting systems, the weight of the steel beams of the assembled mounting systems and the number of anchor bolts totaled 37,660 units, 4,840t and 150,000, respectively. To complete this construction in 14 months, a total of 78,000 workers from 208 Kyushu-based construction companies were employed on the site. At its peak, 450 workers per day were used.

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Solar panels tilted at 20 degrees to realize 70MW

Some skeptics might wonder if the location near Sakurajima is suited for solar power generation, remembering the thick carpet of volcanic ash in Kagoshima City seen on the TV news, etc.

Kagoshima Nanatsujima Mega-Solar Power Plant is located to the southwest of Sakurajima. Volcanic ash from Sakurajima does not fall frequently over the area to the southwest side where the solar plant is located since it is blown by the wind to the north side. Even if volcanic ash did land on the solar panels on one day, it would be almost completely blown away by sea wind by the following day or be washed away should it rain.

Of course, measures have been taken to prevent the accumulation of a thick layer of volcanic ash and dust. Three 8mm-wide slits are added to both sides and the middle of the bottom of the solar panel frames (Fig. 3). Volcanic ash and dust can be washed off through these slits by rain. If a conventional frame were used, some of the volcanic ash and dust would be left at the bottom of the frame.

If volcanic ash was still piled up and the amount of power generation went below expectation for six consecutive days, all 290,000 solar panels will be cleaned with trucks, moving between the panels, loaded with high-pressure washers and power generators. If this happens, 200t of well water prepared in the power plant will be used for washing.

The solar panels were tilted at 20 degrees when installed on the mounting system. Based on the sun's culmination altitude in Kagoshima Prefecture, the panels should be tilted at 26.3 degrees to secure the maximum amount of annual power generation.

However, if tilted at this angle, the solar panels to realize an output of 70MW could not be set up at a distance from each other that is enough for each panel not to shade the neighboring panel in the morning and late afternoon when shadows lengthen as the sun comes up or goes down. Based on this, the solar panels were tilted at 20 degrees, an angle at which they could be laid out to realize 70MW and a greater amount of annual power generation could be secured.

Unique measures intended for solar power plants facing the sea were added to the mounting system, namely, materials and a structural design incorporating salt and wind resistance. The steel used in the mounting system is zinc-coated. In addition, the design reflected the results of wind-tunnel experiments using a 1/4-sized model. And it features resistance to winds of 60mps, which exceeds Kagoshima Prefecture's reference wind speed of 33.7m when designing structures.

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PV inverter selected focusing on protection against salt

Kagoshima Mega Solar Power explained that the major factor behind choosing SMA Solar Technology's PV inverter was its salt-resistance. To prevent salt damage, Japanese manufacturers' PV inverters for large-scale solar power plants had to be housed in a chassis that is covered by another salt-resistant chassis. This requires not only extra expense for the additional chassis but also running costs for the air-conditioning system necessary to keep the temperature inside the chassis from rising.

On the other hand, SMA Solar Technology's PV inverter has the benefit that it can be installed without such additional chassis and air-conditioning system because it features salt-resistance as a standard specification. It requires no additional chassis and air-conditioning system thanks to its simple structure based on natural cooling by fans and thermal exchange with the enclosed space.

"Although there were still some uncertain aspects including the support system, we took our chances based on SMA Solar Technology's global share of 30% and abundant experience in hostile regions such as salt damaged areas and deserts," said Michihiro Kita, Engineering Division, Kyocera Solar.

As it requires neither additional chassis nor air-conditioning system, SMA Solar Technology's PV inverter has the additional benefit of only taking up a smaller space. Just like the tilted angle of the solar panels, this compactness counted a lot in setting up the solar panels to realize 70MW output.

Also, SMA Solar Technology's string monitoring system was adopted for this plant. It measures the amount of power generation by each string consisting of multiple panels and detects malfunctions through embedding sensors with the connecting box (Fig. 4).

Japan's 1st lightning rods for solar panels

At Kagoshima Nanatsujima Mega-Solar Power Plant, 2MW of power generation facilities are allocated to each block of about 210 x 110m. There are 36 blocks. Each block is filled with solar panels arranged like a grid. And four 500kW PV inverters and transforming substations are positioned on the path located in the middle of each block (Fig. 5). These paths allow a 4t truck with a crane to pass and workers to promptly replace defective components, for example, if a PV inverter becomes out of order.

Placing PV inverters in the middle of each block helped to reduce the time and cost required to construct the plant, enabling the collection of electricity at the shortest distance from the connecting box that connects the solar panels in series and to linearly lay the grid underground between the PV inverters and the transformer substation to the northwest of the power plant. Kyushu Electric Power's Goino transforming station is located about 350m from the solar power plant and connected with the grid via the underground transmission wire.

Furthermore, 1,380 4.5m-high lightning rods were erected around the solar panels, PV inverters and transformer substations. This is said to be the only case in Japan in which solar panels are protected by lightning rods (Fig. 6). There was lots of lightning from Sept 1 to 3, 2013, when the plant was under construction, and the plant was reportedly struck. Along with the lightning rods, the power plant is also equipped with infrared sensors and security cameras.

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Foundation measures to shorten construction period

Construction was carried out in series from the seaside to the landside, dividing the 36 2MW blocks measuring about 210 x 110m each into six zones. Efforts were also made in this phase to shorten the construction period.

One of those efforts was the way the foundations were built. A "direct finish" method was deployed in which concrete was poured from cement mixer trucks into a mold and the foundations were completed without post processing. This method is suitable to reduce the time to build the foundations. Cement mixer trucks could navigate the 3.5m space between solar panels and directly pour cement into the mold.

Using metal molds, the foundation could be made in a shorter period of time without specialist workers (Fig. 7). There was a cost involved to create the molds, but the cost increase was minimized by making 37,660 foundations one by one using the same molds. In the conventional method, the foundations would be made by pouring concrete into wooden molds assembled on site by specialist workers.

This metal mold also eliminated the previous need for specialist workers to install reinforcing bars because it enabled the foundation to integrate reinforcing bars with concrete just by setting the bars in the foundation when it is made. Moreover, this mold can be set with a rain cover similar to a tent so the foundations can be laid even on rainy days.