Rechargeable Batteries Help Stabilize Solar Plant Output (page 2)
Results by smoothing output fluctuations, scheduled operation
Fig. 3: PV inverters and secondary batteries are all housed in buildings to prevent salt damage. The PV inverters are products of Toshiba Mitsubishi-Electric Industrial Systems (TMEIC) and the secondary batteries are products of NDK Insulators (NAS cells) and Toshiba (Li-ion cells).
100 standard homes, 4 large volume customers simulated
Focusing on Sharp Corp’s polycrystalline silicon solar panels, Kyocera Corp’s polycrystalline silicon type and Kaneka Corp’s amorphous (non-crystal) type were installed. As for the PV inverters, Toshiba Mitsubishi-Electric Industrial Systems Corp’s (TMEIC) product was adopted. NGK Insulators Ltd’s and Toshiba Corp’s secondary batteries with outputs as much as 4MW (sodium-sulfur/NAS) and 100kW (Li-ion), respectively, were installed (Fig. 4).
In addition to these power generation and storage facilities, the verification facility features a system that can simulate a power transmission grid connected with 100 standard homes and four large volume customers such as schools (Fig. 5). Of the 4MW solar power generation facility, 1MW is connected with a grid (6.6kV) under this simulated load while the remaining 3MW is connected with a transmission grid (22kV). As for the secondary batteries, the 4MW NAS and 100kW Li-ion cells are connected with the transmission grid and the distribution grid, respectively.
Frequency fluctuations cut by half
Four major subjects are being verified using these facilities. Subject 1 is the verification of a function to smooth the drastic fluctuations in solar power output using secondary batteries. The output of solar power generation varies depending on the amount of sunshine (solar irradiation). It typically shows when there are clouds on a sunny day because the output varies a lot in a short time as the clouds pass over the solar panels. In the verification, secondary batteries were used to smooth the drastic fluctuations by charging and discharging so they could offset the fluctuations in photovoltaic (PV) output.
An example effect is shown in Fig. 6. The photovoltaic output was smoothed with the secondary batteries (NAS cells/red line) charging and discharging so they could offset changes in the photovoltaic output (PV/blue line). As a result, the frequency fluctuations were also alleviated.
Subject 2 is the verification of a function that reduces the grid’s frequency fluctuations by the secondary battery control. Apart from the new 4MW solar power generation facility, the island’s power grid includes a wind power generation facility with a total output of 4.2MW. And it could also cause the frequency to fluctuate as the balance between supply and demand could be disrupted by drastic changes in output.
By monitoring frequency fluctuations and charging and discharging the secondary batteries to offset the fluctuations, an attempt is being made to ease the frequency fluctuations stemming from wind and solar power generation.
Okinawa Electric Power Co Inc targets a deviation within 0.3Hz based on a frequency of 60Hz. The frequency rises as supply surpasses demand while it drops as supply falls short of demand. The causes that disrupt the balance between supply and demand are demand variations and drastic changes in the wind and solar power output.
In Miyakojima, the balance between supply and demand is maintained by adjusting the output of diesel engines. However, should the output of wind and solar power generation change too much for the diesel engines to follow, the frequency would also vary. For example, when the width of fluctuation (the ratio of fluctuation against the output of diesel engines) reached about 10% in wind and solar power, the frequency sometimes fluctuated by nearly 0.3Hz. However, after smoothing the frequency fluctuations using the 4MW secondary batteries as in the verification project, the frequency deviation was cut almost by half to 0.15Mz.
While solar power output was monitored to smooth the output fluctuations in Subject 1, the frequency fluctuations in the power grid were monitored and reduced in Subject 2. After the verification, it was confirmed that both control methods had the effect of stabilizing the frequency.
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