Rechargeable Batteries Help Stabilize Solar Plant Output
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).
Miyakojima Island is located about 300km to the southwest of Okinawa’s main island. Its size, 204.5km2, is slightly smaller than Osaka City. Its population is approximately 55,000, most of whom live in the Hirara urban district. Sugar cane fields spread across the flat landscape, and the island is crowded throughout the year with tourists who come to view the beautiful coastline and swim and dive in the sea endowed with coral reefs.
4MW solar panels combined with 4MW secondary cells
The island’s power demand peaks at about 50MW. Along with the core power supply, which is diesel engines with a total output of 59MW, gas turbine power generators with a total output of 15MW and wind power generators with a total output of 4.2MW are supporting the power demand of the island.
Adding to these power supplies, a mega-solar (large-scale solar) power plant with 4MW output started operation in October 2010 (Fig. 1). It was constructed using the "FY2009 subsidy for the project to verify stand-alone systems for new energy demonstration projects on remote islands" granted by the Ministry of Economy, Trade and Industry (METI).
The project was aimed at understanding what impacts the widespread introduction of solar power generation, whose output fluctuates widely, would have on the power transmission and distribution network (grid) and verifying the control function that benefits grid stabilization using secondary batteries. For this purpose, secondary batteries with a total output of 4.1MW were also installed.
Now the verification project has entered its final phase, having helped stabilize the power quality by smoothing the mega-solar power plant’s output using secondary batteries and generating power in accordance with the schedule.
The plant is located a 30-minute drive to the southeast of Miyako Airport. As you drive along with the steep coast on your right, you will see two wind turbines on your left and solar panels neatly arrayed along the road for about 1km from the foot of the wind turbines. That is the site of the verification project.
Since the place is frequently hit by typhoons, the solar panels have been tilted as low as 5 degrees (Fig. 2). The mounting system is reinforced in consideration of a reference wind velocity of 46m/s and a maximum instantaneous wind velocity of 73m/s. In addition, high-salt-resistant solar panels and mounting systems were deployed while the electric facilities including PV inverters and secondary batteries were all housed in the buildings (Fig. 3).
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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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Output load achieved as predicted the previous day
Subject 3 is the verification of "scheduled operation" of solar power generation. Scheduled operation indicates predicting and planning the output load (hourly changes) in solar power generation on the previous day based on weather forecasts and realizing the predicted output load by filling in the gaps with the actual output through secondary battery charging and discharging. If this could be realized, solar power generation, which is a variable power supply, can be positioned as a stable, programmed power supply like a diesel engine.
An example of the scheduled operation can be found in Fig. 7. As shown in this figure, the programs perfectly met the results with the gaps between the solar power generation programs on the previous day and the results of the day offset by the secondary battery charging and discharging.
Subject 4 is the verification of optimum secondary battery control using a simulated grid; this verification is now under way. Making the most of the load equipment to simulate 100 houses, this verification project’s key feature, cases of offsetting the fluctuations in solar power output using the secondary batteries installed in each home and substation are being compared and verified to determine the optimal installation spots for the secondary batteries when a large volume of solar power generation facilities are set up in a grid.
While collaborating with Okinawa Electric Power’s verification project, Miyakojima City announced "Declaration of Eco Island Miyakojima" and established goals to decrease its resource/energy dependency outside the island and cut its CO2 emissions by 70% by 2050. The pillars of this declaration include the introduction of large-volume reusable energy, energy saving and demand management.
As one of these specific approaches, the "Islandwide EMS (energy management system)" project was launched in alliance with Toshiba Corp and others in October 2013. Visualizing the energy use status at 200 homes and 25 offices, Miyakojima City will explore the possibility of energy-saving services and approach power use in accordance with the amount of solar and wind power supply.
With the verification project to control the demand side added to the secondary battery-based verification project by Okinawa Electric Power, expertise on maintaining power quality by quickly balancing supply and demand is likely to be acquired.