Abandoned Farmland Serves as Solar Site (page 3)
Amount of power generation boosted by overload, CIS solar cells
Fig. 1: Completion ceremony that took place Oct 21, 2013. Mayor Tadashi Yokoyama of Mitoyo City, President Koichi Sakaguchi of Asahigaoka Sangyo, who is also the chairman of Mitoyo City Council, Executive Officer Yuichi Nishigori of Orix and Executive Managing Director Masato Nanbu of Takenaka cut the tape. (source: Nikkei BP)
Fig. 5: In light of the firm, flat ground, a mounting system with concrete foundations that is wind resistant because of the weight of the solar panels and the concrete was adopted. (source: Nikkei BP)
String monitoring, clover to prevent weeds
Orix Mitoyo Mega-Solar Power Plant's technical features include the employment of CIS solar cells, the installation of solar panels with 2.346MW output, which largely exceeds the 1.9MW of the PV inverters, and "string monitoring" to remotely monitor the amount of power generation by panels connected via direct current.
Viewed from the observation deck on a sunny day, the power plant draws a sharp contrast with the surrounding green mountains, as its black solar panels spread across the site like a gigantic mirror that reflects the sky. CIS solar cells are characterized by their black panels, whereas current mainstay silicon crystal solar cells are blue.
The power generation properties are also different. According to the conversion rates in the catalog value, many CIS solar cells are inferior to silicon crystal solar cells by approximately several points. But they are not that different when the actual amounts of power generation are compared.
That can be attributed to their temperature properties. The higher the temperature gets, the more the conversion rate of solar cells lowers due to a larger loss. The loss is smaller in CIS solar cells than in silicon crystal cells. The conversion rate of a solar panel indicates the value measured at 25°C, but the temperature of a panel reaches around 60°C on sunny days in summer. During the hours where the largest amount of power is generated, a decline in the amount of power generation is small in CIS solar cells.
Installing solar panels with an output exceeding the PV inverters' is called "overload," and such a system design in itself is not unusual. As the amount of electricity to be sent back to the grid and sold is determined by the PV inverters' capacity, the power generated more than the amount for sale would be wasted on a sunny day.
However, when the panel output is less than the rated amount in the morning and late afternoon or on a cloudy day, the method boosts the amount of power generation and the utilization rate of PV inverters thanks to the overloaded solar panels.
At Orix Mitoyo Mega-Solar Power Plant, the overloaded amount is larger than that at other general plants. There seem to be the following reasons for that. First, there is space large enough for a large-scale solar power plant with a smaller output than 2MW, which can be connected with a high-voltage transmission line. Second, it would not cost the operator so much to develop the site even if the number of installed panels is raised as the land was flat from the beginning.
The plant is also characterized by the introduction of string monitoring. Some solar panels generating direct current are first connected in series to boost voltage to a certain level. Then, the panels are connected in parallel by concentration boxes that transmit electricity to PV inverters and collectively convert direct current to alternate current.
String indicates the group of panels connected via direct current. If remotely monitoring only the output of PV inverters, a defect on a panel would be hardly detected. If monitored by string, however, abnormal panel output can be easily noticed.
At first, there were few cases of introducing string monitoring to solar power generation systems in Japan. However, the importance of remote monitoring by string has recently been recognized when establishing an operation and management system for a large-scale solar power plant. And some plants have started to adopt it.
As for the removal of weeds, which is a major issue in operation and maintenance, Orix is considering a measure to prevent tall weeds from growing by planting clover across the area under the mounting systems.