Japan's Largest Battery-equipped Solar Plant Almost Completed in Hokkaido
102MW of panels, 52MW storage battery set up on former ranch
Renewable energy fully promoted by town
"What is the only city, town or village that faces both the Pacific Ocean and the Sea of Japan?" The answer to this question, which is likely to be asked in a quiz show, is Yakumocho, Hokkaido, Japan.
This town is situated in the north of the Oshima Peninsula halfway between Hakodate and Muroran and is known for its dairy and beef farming as well as scallop cultivation and other fishery products.
Yakumocho is enthusiastic about renewable energy development and drafted its "Yakumocho Vision for Promoting Renewable Energy Adoption" in 2017. The town is also approaching geothermal and wind farm development, operating one mega- (large-scale) solar power plant with an output of 2MW in Yamazaki District and domestic animal waste-based biogas power plants with an output of 150kW each at four locations. Given the circumstances where grid connections with Hokkaido Electric Power Co Inc (Hokuden) are growing difficult, Yakumocho is considering promoting local energy production and consumption in the form of heat utilization and hydrogen manufacturing (Fig. 1).
Power generation more than 3 times town's demand
A symbolic project for such a town-wide renewable energy development is being constructed in Yamazaki and Hanaura districts along the coast of the Pacific Ocean. As you drive north on Route 5, through the window you will see solar panels orderly arrayed for about 3km on your right (Fig. 2).
This is the "SoftBank Yakumo Solar Park" with a solar panel capacity of 102.3MW and a grid capacity of 75MW. Construction began in March 2018, with panel and substation facility installation almost complete. Power transmission from Hokuden is slated to begin by the end of 2019, followed by test power transmission in April 2020 and commercial operation in October.
The power producer is "Hokkaido Yakumo Solar Park GK," a special purpose company (SPC) equally financed by SB Energy Corp (Minato-ku, Tokyo), engaged in the renewable energy business of the SoftBank Group, and Mitsubishi UFJ Lease & Finance Co Ltd (Chiyoda-ku, Tokyo).
The long and narrow project site of about 132ha is a former ranch that faces the Pacific Ocean and stretches in a north-south direction. The SPC rents the site and will run the power generation business (Fig. 3).
Annual power generation is expected to total approximately 10,682,000kWh. This corresponds to the power consumption of about 27,967 general households and about 3.3 times the overall power demand in Yakumocho.
Designed, constructed by Toshiba, TMEIC
In this project, a storage battery system with a capacity of 27MWh is installed in order to smooth short-time output fluctuations (short-period fluctuations) of the photovoltaic (PV) system. The rated output of the inverters for the battery system is 52.5MW, and Li-ion batteries are used.
The panel capacity of 100MW and the grid capacity of 75MW are the 10th largest among the initial capacities of the mega-solar plants in operation throughout Japan. Furthermore, as a "mega-solar plant equipped with a storage battery system," this plant may be the world's largest and is, needless to say, Japan's largest.
Toshiba Energy Systems & Solutions Corp and Toshiba Mitsubishi-Electric Industrial Systems Corp (TMEIC) jointly provide engineering, procurement and construction (EPC) services and will operate and maintain this power plant after completion. Toshiba Corp provided solar panels (72-cell, 335W/unit), and TMEIC supplied inverters for both solar panels and the storage battery system. TMEIC also built the battery system using storage batteries manufactured by LG Chem Ltd of South Korea.
In view of snow cover during winter, the panels are tilted by 30° and set up 1.3m from the ground (Fig. 4).
Storage battery system to meet requirement for '1% fluctuation per minute'
The plant installed a storage battery system in parallel with a PV system in order to meet the "technical requirements concerning measures to smooth output fluctuation of a solar power generation facility" announced by Hokuden in April 2015. Hokuden requires a "1% fluctuation per minute" that limits the range of fluctuations in mega-solar plant output combined with the battery's charge/discharge to 1% or less per minute compared with the rated output of the PV inverters. TMEIC's control system helps the inverters for the PV and battery systems coordinate their outputs so they can meet this requirement.
In regard to storage battery systems installed in parallel with a renewable energy system, the cost for adoption is partly subsidized by the national government in many cases; however, such a subsidy system has not been used this time.
In addition, this power plant was required to accept an "unlimited output control without compensation" as its application for grid connection was filed after applications had exceeded the "limit for 30-day output control" within Hokuden's service area. If renewable energy adoption in Hokkaido further advances, Hokuden might issue an output control directive exceeding 30 days per year, which will consequently increase risk in power generation business performance.
Despite the business risk of "unlimited output control" in addition to the "cost for the storage battery system," the SPC succeeded in structuring project finance this time.
Behind the success lies the cost reduction in the storage battery system as well as the improved evaluation and analysis of future output control. 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.
The adoption of the "TMEIC Battery Control System (TMBCS)," which controls mega-solar output and battery's charge/discharge in an integrated form, as well as the fact that the same manufacturer produces inverters for both the PV and storage battery systems, also significantly contributed to the success (Fig. 5).
'Yard for temporary assembly' to build units in advance
Despite the large scale exceeding output of 100MW, panels were set up in accordance with the terrain without large-scale land reclamation. As the ground in the site was almost flat, the constructor could easily boost the construction efficiency by designing and setting up the panels in the same array (unit of panel installation) composition pattern.
Each array basically consists of nine panels sideways in three rows (9 x 3 panels), tilted by 30° and set up 1.3m from the ground.
The constructor divided the mounting systems for these arrays into three units and consecutively assembled them in the "yard for temporary assembly" first. The assembled mounting systems were lifted by a crane, loaded onto trucks, carried to the designated block in the site and lifted by the crane again before installation (Fig. 6, 7 & 8).
Such a "temporary assembly method" is unusual in mega-solar construction. In the cases of a large-scale solar power plant connected with an extra-high-voltage power grid, it is generally important to prepare multiple array structures and adjust them at the site of installation, depending on the angle of the ground to set them up and the shape of each construction block.
In that respect, the 11,312 mounting systems were set up in no more than one pattern because this overall construction site was flat and almost square. As a result, it was reportedly efficient to consecutively assemble the units in advance, carry them to the designated construction block, install them and finally adjust them.
Soil improvement using shells of local specialty scallops
As the project site is peatland near the coast, however, it was not easy to construct the foundations. To supplement the soil bearing capacity, the plant adopted a method of driving pile foundations 4 to 6m into the ground and solidifying the surrounding area with concrete. For this purpose, the constructor created a jig to keep the pile in the middle of the hole, which was dug in advance, and made it possible to fix eight pile foundations at one time and efficiently deposit the concrete (Fig. 9).
In addition, the plant uses the scallops cultivated in Uchiura Bay (Volcano Bay) in front of Yakumocho as a soil conditioner by crushing their shells and scattering them on some parts of the ground. Calcium carbonate in the shells neutralizes the acid peatland and dries the soil at the same time (Fig. 10).
Although Yakumocho is a relatively warm region in Hokkaido, the deepest snow cover reaches about 80cm on average in one winter. In light of that, panels were tilted by 30° so the snow accumulated on the panels would easily fall off and the power generation loss would be minimized.
Even though banks of snow would be formed in front of the arrays, the snow banks are not likely to reach the arrays without snow removal during the winter if the snow cover is like that in an average year thanks to the lowest part of the panels being 1.3m above the ground (Fig. 11).