Coal Mining Area Shaped Into Solar Town

Output lowered due to restriction by 'bank reverse power flow'

2013/11/25 13:47
Kenji Kaneko, Nikkei BP CleanTech Institute

Koyagimachi, Nagasaki City, is situated in the mountains facing a cove about 10km from central Nagasaki City. Since being connected in 2011 by a bridge with Iojima Island, which has spa facilities and bathing beaches, the town has become more bustling. Shortly after driving into Koyagimachi on the way from central Nagasaki City to Iojima, solar panels start coming into view just beside the road, with the sea behind.

This is the "SoftBank Nagasaki Koyagi Solar Park" with 1.75MW output and being run by SB Energy Corp (Minato Ward, Tokyo). The hill on the opposite side of the road is a residential area, and you can overlook the large-scale solar power plant consisting of about 10,000 neatly arrayed solar panels from halfway up the hill from the path between the houses.

Unused 160,000m2 site in former coal mine

Koyagimachi is known as the location of Mitsubishi Heavy Industries (MHI) Ltd’s Nagasaki Shipyard & Machinery Works, where large tankers are built. With an expanse of evergreen trees remaining in the mountains, this is an area where manufacturing and nature co-exist.

What originally developed the region was, however, a coal mine. After Koyagi Coal Mine started operation in 1874, rows of houses for miners were built and a lot of miners lived in the town. The site of "SoftBank Nagasaki Solar Park" was previously a landfill for coal waste, previously called "bota," which is produced during the coal mining process.

After the war, Koyagi Coal Mine came under the management of Tetsugen Corp (Chiyoda Ward, Tokyo). However, the mine, where coal was mined from a tunnel extending under the sea, gradually became less cost competitive and closed in 1964. Tetsugen, which owned the coal mine facilities, has since played a key role as a major landlord in Koyagimachi in rebuilding the town after the mine closed. Partly thanks to the location close to central Nagasaki City, the town has successfully avoided a rural exodus as the houses for the miners changed to rental apartments and the hillside land was developed as residential areas.

However, an area of land approximately 165,000m2 (Takesaki District) along the coast, which was developed by landfilling a cove with coal waste, had been virtually unused and vacant for nearly 50 years partly because it was designated as an urbanization control area.

"In urbanization control areas, under the City Planning Act, no houses or other buildings can be built except those related to agriculture and fishery," said Katsunori Goto, director of Nagasaki Office, Kyushu Development Unit, Tetsugen. "We started discussions after the Urban Renaissance Agency suggested building a large-scale solar power plant. We had been consulting with the Agency for some time about development of the land."

At first, it was unclear whether the building in which the PV inverters were to be housed corresponded to the "buildings" prohibited under the City Planning Act; however, the Ministry of Land, Infrastructure, Transport and Tourism later judged it did not, which enabled the construction of the solar power plant.

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Two meetings for local residents in summer, winter

SB Energy, which had been in talks with Tetsugen from a relatively early stage, rented part (about 35,000m2) of the land in Takesaki District from Tetsugen and constructed the large-scale solar power plant. When choosing an EPC (engineering, procurement and construction) contractor, SB Energy examined proposals from several companies before finally choosing Mitsubishi Electric Corp.

"Mitsubishi was most attractive from an all-round perspective including not only cost but also facility performance," said Ippei Yuki, general manager, Engineering Department, SB Energy.

Deploying a single crystal silicon solar panel manufactured by Mitsubishi Electric and a PV inverter manufactured by Toshiba Mitsubishi-Electric Industrial Systems Corp (TMEIC), its proposal was highly reliable. Operation and maintenance after completion is also carried out by Mitsubishi Electric in cooperation with Mitsubishi Electric Plant Engineering Corp.

Mitsubishi Electric also constructed the mounting systems. It intentionally did not make the slightly rough land surface even, setting the height of the mounted panels horizontal instead by arranging the length of the mounting system’s legs. There was also concern about corrosion caused by salty wind along the shore.

"Having undertaken in-depth discussions, we applied measures against salt damage," Yuki said.

As for the solar panels, a highly weatherproof type was selected. TMEIC’s PV inverters are highly resistant to salt damage because they are housed in a tightly sealed chassis and air-conditioned to block outside air. However, part of the peripheral facilities including voltage inverters are exposed to the outside air. So, they are all housed in filters to prevent salt damage.

SB Energy provided explanatory meetings for the local residents while working out the details of the project. Located near a residential area, Takesaki District, which runs alongside a road, can be seen by the residents. Based on these circumstances, SB Energy first explained the project outline during the Koyagimachi residents’ association meeting in summer 2012. In winter, when the details of the project were determined, SB Energy held another meeting directly calling for participation of the residents, instead of exploiting a community meeting.

"Other large-scale solar power plants also have meetings for local residents in tandem with local government and residents’ associations," said Shunichi Ojima, general manager, Regional Liaison Department, Domestic Business Division, SB Energy. "In the case of Koyagimachi, after having discussions with the residents’ association, we decided to hold two meetings so more residents could join."

At the meetings, the company explained that the solar panels would not reflect the sun onto the houses as they would be installed to face the sea and that the company would regularly undertake weeding, instead of using herbicide, to prevent weeds. According to SB Energy, no objections were raised by the participants.

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Output lowered due to unexpected 'bank reverse power flow issue'

Meanwhile, an unexpected issue came up in the discussions on grid interconnection with the electric power company. If connected with a 6.6kV grid, less than 2MW output can be connected and flown back to the grid under the rules imposed on grid interconnection. In light of the 6.6kV grid in the Takesaki District, SB Energy, at first, was planning a maximum reverse power flow of nearly 2MW. However, after discussions with the electric power company, it turned out that reverse power flow is only allowed up to 1.75MW. This is an issue called "bank reverse power flow."

"Bank" indicates the grid between the distribution transformer stations and the utility customers. Bank reverse power flow is a phenomenon where reverse power flow occurs in distribution transformers at distributing substations due to the overall grid voltage boosted by the integrated power flown back to the grid because of the increased deployment of solar power generation. And it could cause problems with electric power security.

An assessment by the electric power company revealed that the risk of bank reverse power flow would increase if more than 1.75MW flew back to the grid in Koyagimachi. As a result, the maximum output of the PV inverters had to be reduced to 1.75MW despite the solar panels’ maximum output of 2.6MW.

Installing solar panels with an output exceeding the PV inverters’ output capacity is called "overload," which in itself is not unusual. Should the solar panels generate power at their maximum capacity on a sunny day, the output that exceeds the PV inverters’ capacity would be wasted. However, when the panels’ output lowers in the morning and late afternoon or on a cloudy day, overload has the benefit that it could improve the utilization rate of PV inverters thanks to the panels’ output capacity, which outperforms that of the PV inverters.

Partly because a sufficient amount of land can be procured in Takesaki District, a certain degree of overload had been assumed from the beginning. Yet the gap between the outputs of panels and PV inverters grew wider than anticipated because of the output being restricted to 1.75MW due to the issue of bank reverse power flow.

The bank reverse power flow issue is moving toward resolution after the Ministry of Economy, Trade and Industry revised its order after the completion of "SoftBank Nagasaki Koyagi Solar Park." It has been confirmed that efforts such as adding a protection system to the distributing substations can make power flow back to distributing transformers. However, the additional cost to install such systems should be borne by the power producer that connects with the grid and makes generated power flow back to the grid.

About four months after completion, the plant is working smoothly with "the amount of power generation showing an upswing compared with the initial estimate," Yuki said. As the solar power plant can be easily observed from the road, not only the residents of Koyagimachi but also people from outside the region come to look at the plant.

"We are planning to plant different kinds of azaleas along the fence surrounding the solar power plant in early spring 2014," Ojima said.