Sharp Contrast in Solar Power Generation Amounts on 2 Islands
Panels inspected on-site
Ukishima and Ohgishima in the corner of the Keihin Industrial Zone are both landfill islands facing the sea and belong to Kawasaki City, Kanagawa Prefecture, Japan. On these islands, Tokyo Electric Power Company Holdings Inc (hereinafter TEPCO HD) developed and runs two mega- (large-scale) solar power plants with a total output of 20.0MW: "Ukishima Solar Power Plant" and "Ohgishima Solar Power Plant" with an output of 7.0MW and 13.0MW, respectively (Fig. 1). It has been more than seven years since they began operation in August and December 2011, respectively.
The Japanese power suppliers formerly categorized as General Electric Utilities constructed mega-solar power plants in their own coverage areas fully subsidized by the Ministry of Economy, Trade and Industry just before the feed-in tariff (FIT) scheme was implemented. The Ukishima Solar Power Plant and the Ohgishima Solar Power Plant were the largest class among those constructed in that way and the first to begin operation among the extra-high-voltage-class solar plants.
The solar plant on Ukishima is adjacent to a PR and educational facility that focuses on three themes: global warming, renewable energy and resource circulation. It is a joint project unique to such an industrial city where power consumption and CO2 emission are high, and from that perspective, its location can be regarded as perfect. The plant is easily accessible from the metropolitan area and faces Haneda Airport across the sea. The mega-solar plant can be seen from an airplane immediately after taking off or just before landing (Fig. 2).
The Ukishima Solar Power Plant was constructed on landfill of refuse incineration ash. Kawasaki City proposed a mega-solar plant to Tokyo Electric Power Co Inc (present TEPCO HD) as an effective use of the site for the 20 years required to purify the site after landfill was completed, and as a result, it became a mega-solar site.
According to Kawasaki City, building construction is restricted on this site until it is purified under the Waste Disposal and Public Cleansing Act of Japan, so a solar power plant which does not correspond to a "building" was an ideal way to utilize the site (Related article: Solar Power Plant Designed for Coastal Industrial Zone).
The Ukishima Solar Power Plant became TEPCO HD's first mega-solar plant among its power supply facilities. It can also be referred to as a pioneer of so-called "waste disposal solar," which municipalities in various regions started developing on former waste disposal sites following the implementation of the FIT scheme.
When setting up the solar panels, measures against uneven ground settlement became important. As a measure, the number of solar panels on a mounting system was reduced to six with a view to limiting panels that would be affected if uneven ground settlement occurs (Fig. 3).
At the Ukishima Solar Power Plant, 18 single crystal silicon-type solar panels were connected in series per each DC 600V-supported circuit (string). Each of the 280 combiner boxes in the site receives power from seven to eight strings (unit of connected solar panels).
Toshiba Corp provided engineering, procurement and construction (EPC) services at the mega-solar plant on Ukishima. A total of 37,926 single crystal silicon-type solar panels (198W/unit) manufactured by Sharp Corp were arrayed across the site.
As for PV inverters, 28 units with a rated capacity of 250kW produced by Toshiba Mitsubishi-Electric Industrial Systems Corp (TMEIC) were adopted.
Meanwhile, Hitachi Ltd provided EPC services at the mega-solar plant on Ohgishima. A total of 63,792 polycrystalline silicon-type solar panels of Kyocera Corp were arrayed, while 26 units of Hitachi's PV inverters with a rated capacity of 500kW each were adopted.
Favorable power generation at Ukishima, serious downswing from estimate at Ohgishima
During the eight years since the operation started, the amounts of power generation have been significantly different between the mega-solar plants on Ukishima and Ohgishima. At the plant on Ukishima, the amount has always significantly outperformed the estimate in the planning phase since it began operation.
Compared with the expected annual power generation of about 7,400MWh, all the results marked a large upswing after the first year (about 5,340MWh), when the plant only operated for four months, followed by about 9,690MWh in fiscal 2012, about 9,510MWh in fiscal 2013, about 9,300MWh in fiscal 2014, about 8,920MWh in fiscal 2015, about 8,910MWh in fiscal 2016, about 9,360MWh in fiscal 2017 and about 9,210MWh in fiscal 2018.
At the Ohgishima Solar Power Plant, on the other hand, power generation has fallen far short of the estimate in the planning phase since its sixth year. The rate of decline has also expanded year by year.
Compared with the expected annual power generation of about 13,700MWh, the results after the first year (about 3,760MWh), when the plant only operated for one month, went above the estimate until its third year to 15,330MW in fiscal 2012 and about 15,350MWh in fiscal 2013. However, power generation was almost on par with the estimate at about 13,900MWh in fiscal 2014, 10 to 15% below the estimate at about 12,530MWh in fiscal 2015, 11,930MWh in fiscal 2016, about 11,630MWh in fiscal 2017 and even 24% below the estimate at about 10,420MWh in fiscal 2018.
Situated next to each other, both mega-solar plants' surrounding environment and weather conditions are considered to be almost the same. It is very interesting that, despite such circumstances, the contrast has grown so clear in the amounts of power generation at these two plants over the years since they began operation.
However, it is unknown why power generation has sharply decreased at the Ohgishima Solar Power Plant since its fourth year because TEPCO HD has refused to provide any further information other than the power generation amount it has revealed thus far. The company seems to be currently analyzing influencing factors.
Damaged panels tested by 'PV test car'
At the mega-solar plants on Ukishima and Ohgishima, solar panel conditions were recently investigated on-site. Targeting the solar panels which had been found to be damaged through a visual inspection, I-V (current-voltage) characteristics were measured and an electroluminescence (EL) test was also carried out (Fig. 4).
These two tests were contracted to Adler Solar Works Co Ltd (Yokohama City) of the Yokohama Kankyo Design Co Ltd (Yokohama City) Group. This company is engaged in operation and maintenance (O&M) services and technical due diligence of solar power plants.
The on-site I-V characteristic measurement and EL test were implemented using a "PV test car" made in Germany. To measure I-V characteristics, the car evaluates output characteristics by string. In the EL test, it assesses the presence and state of micro cracks in the cells (power generation elements) of solar panels.
The major purpose of this inspection at the mega-solar plants on Ukishima and Ohgishima was to confirm how the micro cracks in the cells were affecting power output. At the mega-solar plant on Ohgishima, how far the dirty cover glass of solar panels affected output was also verified. Output was measured before and after cleaning the solar panels.
TEPCO HD will leverage these inspection results as the data for optimal mega-solar operation from now.
In Adler Solar Works' service, the seriousness of the effect of the micro cracks in cells is evaluated and presented to clients in four levels from "A" to "D" based on the results of the I-V characteristic measurement and EL test (Fig. 5).
"A" is mostly a favorable level, "B" is inferior to A in the number of cracks and I-V characteristics, while "C" and "D" are levels corresponding to defects. Such evaluations are indicated in different colors such as green, yellow and red.
According to Adler Solar Works, requests to implement solar panel inspection on-site using the "PV test car" are gradually increasing.