24MW Solar Plant Overcomes Barriers of 'Farmland,' 'Output Fluctuation'
'Seikan Tunnel' entrance located nearby
The town of Shiriuchicho in the southwest of Oshima Peninsula, Hokkaido, Japan, across which spreads flat land, faces the Tsugaru Strait to the east and mountains in the other three directions. With the entrance to the "Seikan Tunnel" located in the town, the passengers on the Hokkaido Shinkansen (bullet train) will see the abundant nature of Shiriuchicho as soon as the train comes out of the tunnel running under the strait (Fig. 1 & 2).
On August 1, 2019, the "Shiriuchi 20M Mega Solar Power Plant," a mega- (large-scale) solar power plant with an output of 24MW, began operation in Yunosato District of the town where the entrance to the Seikan Tunnel is located. The completion ceremony took place on August 29 with many participants from the power producer, constructor and local communities including Shiriuchicho.
The power producer is GK Hayate Solar, a special purpose company (SPC) established and financed 60% by Orix Corp and 40% by Solar Frontier KK. This SPC rents the project site owned by Shiriuchicho and runs the power generation business. The solar power and PV inverter (grid) capacities reach about 24MW and 17.5MW, respectively. In addition, a storage battery system with an output of 12.5MW and a capacity of 7.2MWh is also installed in this plant.
Toshiba Plant Systems & Services Corp provided engineering, procurement and construction (EPC) services and adopted solar panels of Solar Frontier and inverters manufactured by Toshiba Mitsubishi-Electric Industrial Systems Corp (TMEIC) for both the photovoltaic (PV) power generation and battery systems (Fig. 3).
The Shiriuchi 20M Mega Solar Power Plant faced two major barriers before it was finally completed. First, it needed to convert the project site from Category 1 farmland in order to run the power generation business on it. Second, the plant had to install a storage battery system as a measure against short-period fluctuations ahead of connection with the grid of Hokkaido Electric Power Co Inc (Hokuden).
Converting Category 1 farmland to run a solar power generation business is not normally accepted. Furthermore, even under the feed-in tariff (FIT) scheme, setting up a storage battery system in parallel with a PV system increases initial investment and makes it difficult to secure a certain return on investment.
How did Shiriuchicho and the power producer overcome these tough barriers?
'Renewable Energy Act' serves as breakthrough
The project site was originally a publicly owned joint farm registered as Category 1 farmland under the Agricultural Land Act. The farm was shut down 17 years ago as a result of integrating public farms in line with the decreasing number of livestock farmers, after which the site's effective use became a problem. Although inviting a mega-solar project was considered in the wake of the FIT scheme implementation, it was difficult to convert the land category in order to run a solar power business when it came to Category 1 farmland.
A breakthrough came with the "Act on Promoting Generation of Electricity from Renewable Energy Sources Harmonized with Sound Development of Agriculture, Forestry and Fisheries" formulated in November 2013. It was only after adopting this act's scheme did it become possible to utilize Category 1 farmland for a renewable energy project (Fig. 4).
Subsidy leveraged for storage battery adoption
It was to meet the technical requirement Hokuden announced in April 2015 that the Shiriuchi 20M Mega Solar Power Plant installed a storage battery system in parallel with the PV system. Hokuden required a "1% fluctuation per minute" that limits the range of fluctuations in grid output of a mega-solar plant combined with the battery's charge-discharge control to 1% or less per minute compared with the rated output of the PV inverters. This is aimed at smoothing short-period fluctuations to limit the impact of sharp fluctuations in output from PV power generation on the grid frequency (Fig. 5).
Taking into consideration the cost for a storage battery system at that time, however, even if the FIT-based unit price was 40 yen/kWh for the project, it was difficult to secure a return on investment generally required by enterprises while setting up a battery system to meet this requirement in parallel with a PV system.
Amid such circumstances, a subsidy project announced in March 2015 served as a tail wind. It was the "Subsidy as an Emergency Response to the Suspension of the Grid Connection of Renewable Energy-based Power Generation Facilities" (Project for Helping Renewable Energy-based Power Producers Introduce Power Storage Systems), for which the Ministry of Economy, Trade and Industry (METI) called for public proposals through the Sustainable open Innovation Initiative (SII).
The project in Shiriuchicho applied for this subsidy project and secured a subsidy, which resulted in a sharp rise in the return on investment.
Integrated control of mega-solar facility, storage battery
In the project in Shiriuchicho, to achieve the requirement for a "1% fluctuation per minute," a storage battery with an output of 12.5MW and a capacity of 7.2MWh was set up for the PV inverters with a total output of 17.5MW for the whole mega-solar plant. The circuit was designed in the "AC link" method, in which PV inverters are separately set up for each PV and storage battery systems and the output power is transmitted to the power grid after being converted from direct current (DC) to alternating current (AC) and combined with generated power (Fig. 6).
The "TMEIC Battery Control System (TMBCS)" manufactured by TMEIC was adopted for charge-discharge control in conjunction with the mega-solar output. With the main site controller (MSC) that collectively controls multiple PV inverters set up in the mega-solar site also working in conjunction with the inverters for the storage battery, the TMBCS controls battery charge-discharge so sharp fluctuations in PV output can be smoothed while monitoring the amount of power generation at the grid point of the overall site in real time.
Given that the PV inverter's output is larger than the storage battery's maximum output, however, it is possible that the TMBCS cannot completely smooth sharp fluctuations in PV output and fail to keep a "1% fluctuation per minute," and in such case, the PV output should consequently be limited.
"If the capacity of a storage battery is further boosted, the output restriction amount will decrease while initial investment increases accordingly," Orix said. "We decided on the optimum battery capacity from the viewpoint of return on investment."
According to TMEIC, the TMBCS enables smoothing of sharp fluctuations in output of a mega-solar plant even with a relatively small battery capacity. To achieve the grid requirement for a "1% fluctuation per minute," a storage battery with a capacity equivalent to about 80% of the PV inverter output is usually set up while the capacity was only about 70% compared with the PV inverters this time.
Supporting posts' added against snow cover
As Shiriuchicho faces the Tsugaru Strait through which a warm current flows, the town is considered to be one of the easiest places to live in Hokkaido, with the highest temperature in summer reaching no more than around 30°C and the lowest temperature in winter falling no lower than about -10°C. In terms of snow cover, the highest snow cover reaches about 50 to 60cm, which is relatively low in the north.
Accordingly, sufficient consideration was given to measures against snow cover when designing the mounting systems and arrays (unit of solar panel installation) at the mega-solar plant. Because it is difficult to remove snow that accumulates on the panels at a power plant as large as one with 24MW output, the panel installation height and angle were increased so the snow would easily slip off the panels.
Each large-area array basically consisting of eight panels in four rows (eight panels sideways, four panels lengthwise) was tilted by 30° so the snow on the panels would easily drop off. As a result, however, snow would pile up in front of the arrays more easily.
If the banks of snow under the arrays grow high enough to reach the panels, the snow on the panels would no longer fall off and could cause damage to the panels and mounting systems because of the gravity that pulls melting snow on the panels downward. Given these circumstances, the lowest parts of the arrays were positioned 1.5m above the ground to prevent the snow banks from reaching the panels.
In preparation for the risk of snowbanks reaching the panels due to more snowfall than usual, supporting posts for reinforcement were set up in the front row of the arrays. They help the panels and mounting systems withstand the downward pulling force caused by the weight of snow and melting snow (Fig. 7).