Simulation based on data obtained in Wakkanai
The capacity of the storage battery was set at 6.75MWh, which was "obtained by simulation to satisfy the requirement of 1% or less output fluctuation per minute," said Hidemi Hashimoto, assistant director at the System Engineering Headquarters, the Industrial Battery/Power Source Business Unit, GS Yuasa Corp.
The simulation was done by using the data disclosed by the "Wakkanai Mega Solar" validation project, which was implemented from 2006 to 2010 by HEPCO and Wakkanai City and entrusted by the New Energy and Industrial Technology Development Organization (NEDO). A control technology for smoothing the output by using a 1.5MW NAS battery was developed by the project responding to the issue of output fluctuation caused by the introduction of a 5.02MW mega solar power plant.
The grid connection capacity was set at 14.5MW, and the solar panel capacity was set at 17.9MW, which is overloading. The rated output of the inverter for the storage battery was set at 10MW. Because the solar power output of 14.5MW is controlled by the 10MW storage battery, the fluctuation cannot be covered by the storage battery in case it reaches 14.5MW. But it was determined by the simulation that it is within the allowable range.
As for charge/discharge characteristics, "charge/discharge characteristics up to 1.5C were required," in terms of C (Capacity) rate because charge/discharge is done at a high speed responding to output fluctuation, Hashimoto said. C rate is a value that indicates the level of charge/discharge capacity against the nominal capacity. 2C indicates that two times the nominal capacity can be charged/discharged. The company incorporated the "LIM50EN" (Fig. 4), a high-energy-type lithium-ion battery that can perform 6C discharge (Fig. 2) and 2.5C charge (Fig. 3) at maximum, providing a sufficient margin.