Abandoned Farmland Serves as Solar Site

Amount of power generation boosted by overload, CIS solar cells

2013/12/07 23:31
Kenji Kaneko, Nikkei BP CleanTech Institute

Takasecho, Mitoyo City, Kagawa Prefecture, is well known as a tea-growing district. The completion ceremony of "Orix Mitoyo Mega-Solar Power Plant" took place there Oct 21, 2013.

"Amaterasu Omikami is the sun god. It must be fate that I can announce the completion of a solar power plant and pray for its future safety."

After the Shinto priest said a solemn prayer, Mayor Tadashi Yokoyama of Mitoyo City, President Koichi Sakaguchi of Asahigaoka Sangyo (Mitoyo City, Kagawa), who is also the chairman of Mitoyo City Council, Executive Officer Yuichi Nishigori of Orix Corp and Executive Managing Director Masato Nanba of Takenaka Corp cut the tape in front of the brand new large-scale solar power plant (Fig. 1).

Solar power plant built on former farm

Orix Mitoyo Mega-Solar Power Plant is a 30-minute drive from Kotohira Station, which is the nearest train station to Kotohiragu, on the JR Yosan Line and is situated in flatland surrounded by hills (Fig. 2). Green and yellow-green shrubs cover the surrounding mountains, with tea and other plantations scattered on the slopes. As you walk along the path leading to the solar power plant, you will find bamboo groves, which are often seen in wasteland, and "No dumping!" and other signboards warning against illegal dumping. According to a survey by the Agricultural Committee of Mitoyo City, abandoned arable land accounts for 20% of all fields that are intended for rice and other crops, or about 40% excluding rice fields.

In fact, the site of this power plant was previously a chicken farm run by Asahigaoka Sangyo until about ten years ago. The site partly became idle after the business closed. It was earmarked for use after Kagawa Prefecture had requested its municipalities to look for large areas of developed land because it had been asked to introduce appropriate sites by large-scale solar power plant operators following the implementation of the Feed-in Tariff (FIT) program.

Kagawa Prefecture introduced some solar power plant operators to Asahigaoka Sangyo, the landowner. After the bidding process, Orix was selected for a variety of reasons including rent terms, reliability as a business operator and measures to contribute to local society. Orix's subsidiary, OR Solar, rented the 3,400m2 land owned by Asahigaoka Sangyo, installed solar panels with an output of 2.346MW and sells the generated electricity to Shikoku Electric Power Co Ltd under the FIT program.

"We changed the category of the land containing the solar power plant from an agricultural site to a housing hybrid site after the chicken farm was closed," President Sakaguchi of Asahigaoka Sangyo said. "It was relatively easy to build the solar power plant since the land category was already converted from an agricultural site."

Asahigaoka Sangyo is considering constructing another 2MW solar power plant on the adjoining site. President Sakaguchi, who is also the chairman of Mitoyo City Council, is promoting large-scale solar power plant construction as a measure to effectively use abandoned farms, which are gradually increasing in number in the surrounding area.

To lease farming land to someone and set up solar power generation facilities there, the landowner has to apply for conversion of the land category; however, approval is generally difficult. Asahigaoka Sangyo was able to construct the solar power plant in a short period as it had already converted the site's land category.

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CIS solar cell, TMEIC PV inverter deployed

"Should the conversion of an agricultural site be approved, there would be good potential in leasing abandoned tea and tangerine plantations to solar power plant operators and making use of them," President Sakaguchi said.

Kiyoshi Kondo, chief of the Business Location Promotion Div, Commerce, Industry and Labor Dept, Kagawa Prefecture, commented on such a view.

"It costs a lot to develop the abandoned farms in Kagawa Prefecture, as most of them are located on mountain slopes," he said. "In addition, Kagawa has a number of challenges other than the conversion of land categories to satisfy the requirements for business profitability. For example, the plant operator has to negotiate with multiple landowners to start a large-scale power generation business, as many farmers are small-sized in Kagawa."

Orix Mitoyo Mega-Solar Power Plant, where a site as large as 34,000m2 had already been developed and held by one owner, might be one of the few optimum spots in Kagawa Prefecture, where only a few sites are suited to large-scale solar power plants.

Kagawa Prefecture is one of the sunniest regions in Japan and suited to solar power generation, yet has few sites to construct a 10MW-class solar power plant due to its mountainous terrain. The prefecture has thus far recognized 61 solar power plants with output of 500kW or larger, which totals approximately 80MW, primarily 1 to 2MW per plant.

The plant's EPC (engineering, procurement and construction) contractor was Takenaka Corp. This was Takenaka's first EPC contract in the Shikoku region. The plant adopted CIS solar cells manufactured by Solar Frontier KK (Fig. 3) and PV inverters manufactured by Toshiba Mitsubishi-Electric Industrial Systems Corp (TMEIC) (Fig. 4). Its reliability increased by forming a system with domestic facilities that have been broadly employed across Japan.

Given the firm, flat ground, the plant deployed a mounting system with concrete foundations that is wind resistant because of the weight of the solar panels and the concrete (Fig. 5). Partly reflecting a request by Mitoyo City, which was involved in the site selection, an observation deck to view the entire solar power plant was set up.

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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.