Performance of redox flow battery systems in Japan

Shibata Toshikazu，Kumamoto Takahiro，Nagaoko Yoshiyuki，Kawase Kazunori，Yano Keiji

(Sumitomo Electric Industries, Ltd.，1-1-3, Shimaya，Konohana-ku，Oska 554-0024，Japan)

1 Introduction

Further introduction of renewable energies, such as wind and solar power, is promoted from the standpoints of resource depletion preservation and energy self-sufficiency. In order to utilize such new power sources effectively, energy storage systems will be key technologies in a next generation power network. Taking such situation into consideration, Sumitomo Electric industries Ltd. has conducted some demonstrations to check the effectiveness of a redox flow battery system used for the stable supply of renewable energy[1-2]. Some performance results are described below.

2 Experimental results

2.1 Demonstration for a wind farm

The demonstration test for stabilizing the short-period fluctuations of a wind farm’s power output using a 6MW⋅h redox flow battery system was carried out over the period from January 2005 to February 2008 at the Tomamae Winvilla power plant (output power： 30600 kW, wind turbines： 19units) of Electric Power Development Co., Ltd. (JPower). This demonstration is performed in the “Development of Technologies for Stabilization of Wind Power in Power Systems”[2-5] project promoted by New Energy and Industrial Technology Development Organization (NEDO). The major specifications and equipment configuration, equipment layout, and circuit configuration of the redox flow battery system are shown in Table 1, Figure 1 and Figure 2.

Table 1 The specification and equipment configuration of 6MW·h redox flow battery system

Fig.1 The layout of 6MW·h redox flow battery system

Fig.2 The circuit configuration of the system

The battery system comprised four banks. The number of battery banks to be operated was changed automatically in response to the wind farm’s power output to minimize the power losses in the auxiliary equipment. The rated output of the redox flow batteries installed in each battery bank was 1000 kW. On the other hand for the power conditioner, a 1500 kW AC/DC inverter was used for each battery bank, since instantaneous high-output characteristics allowed the battery system to charge and discharge[6] AC power of up to 1.5 times the rated output of the batteries.

Figure 3 shows an example of the wind farm’s power output stabilizing at a time constant of 30 minutes. The battery system would not be able to follow drastic fluctuations in the wind farm’s power generation output, since the maximum capacity of the storage batteries (6MW) was smaller than the wind farm’s rated power generation capacity. To cope with this situation, this system was controlled to reduce the stabilizing time constant temporarily when the wind farm’s power output fluctuated intensively, and thus to limit the output required of the battery system. As Figure 3 shows, the redox flow battery system was confirmed to stabilize output stably without its storage capacity collapsing.

Fig.3 Example of wind farm power output stabilization

2.2 Demonstration for solar power generation

With the aim of checking the effectiveness of various control schemes using redox flow batteries, a demonstration system (a megawatt-class electric power generation/storage system[2,7-8]) was installed on the premises of Sumitomo Electric’s Yokohama Works. This demonstration system consists of a 100 kW photovoltaic system and a 5 MW·h redox flow battery system. An external view of the system is shown in Figure 4. The main purpose of the demonstration is to confirm the effectiveness of redox flow batteries for output power stabilizing and the planned operation of the photovoltaic system. Another purpose is to test the factory energy management system (FEMS) by optimally combining the operations of the demonstration test system and the existing gas engine power plant.

The redox flow battery system comprises three banks of batteries： 500 kW, 250 kW and 250 kW units. For the photovoltaic system, the concentrator type system[9] developed by Sumitomo Electric is applied. The existing gas engine power plant consists of six generators, and their rated power output is 3.6 MW in total. The configuration of the demonstration system is shown schematically in Figure 5. Table 2 presents the configuration of the equipments.

Fig.4 Outlook of a megawatt-class storage system

Fig.5 The circuit configuration of the system

Table 2 The equipment configuration of the system

The redox flow battery system comprises a new type of cell stacks. An improved structural design and the use of newly-developed component materials have increased the charge/discharge power density and also extended their service life. Figure 6 shows the new cell stack and the external appearance of the battery cubicle on which the forur cell stacks are mounted.

The demonstration system has been in the process of testing since July 2012. Some of the test results are given below.

Fig.6 The new cell stacks and the battery cubicle

（1）Stabilizing output of the photovoltaic system. Figure 7 shows an example of the output power waveform measured when the output fluctuations of the photovoltaic system were stabilized at a time constant of 60 minutes. As this figure shows, the redox flow battery system absorbed the short-period output fluctuations, abrupt increase (at around 9：00 a.m.), and abrupt decrease (at around 3：40 p.m.) of the output and thus smoothed the total output.

Fig.7 Stabilizing solar power output operation

（2）Planned operation of the solar power output. In the planned operation, the system is operated to achieve the total power output according to a preliminarily established schedule. The difference between the planned and the momentary output of the photovoltaic system at each point in time is compensated by charging/discharging power of the redox flow battery system. The results of a planned operation are shown in Figure 8.

For this test, the output was planned to 50 kW of between 7：00 and 10：00, 100 kW between 10：00 and 15：00, and 50 kW between 15：00 to 18：00. Since it was cloudy on the test day, the amount of solar radiation fluctuated wildly, making the output of photovoltaic system fluctuate dramatically. Figure 8 confirms that the redox flow battery system can absorbs the fluctuations in the output and thus ensure the planned total output.

Fig.8 Planned operation of the solar power

（3）Peak-cut operation. Separately from stabilizing the supply of renewable energy, the demonstration system was combined with the existing gas engine power plant to cut the peak demand for power from the commercial power grid. An example of the test results is shown in Figure 9. Supplying power from the demonstration system during the peak power demand hours from 9：00 to 17：00 cut the Yokohama Works’ peak demand for power supply from the commercial power grid by 40%.

Fig.9 Peak-cut operation

3 Conclusion

This paper outlined two demonstration tests using redox flow battery systems to allow the renewable energy to be used efficiently. Stabilizing the short-period fluctuations in the output power of wind farms and photovoltaic systems was anticipated to help increase the amount of renewable energy that can be supplied to commercial power grids. The use of storage batteries will enable planned renewable power plants, thereby making it possible to operate them systematically, like thermal power plants. Thus, redox flow batteries are expected to enhance the practical value of renewable energy and contribute to accelerating the introduction of renewable energy. Owing to its high responsiveness and short-term high output power characteristics, the redox flow battery is also expected to be an effective tool for controlling demand/supply of commercial power network. We will continue to check the long-term reliability of redox flow batteries and develop marketable lower-cost batteries, thereby ensuring that redox flow battery systems will contribute to the effective use of renewable energy.

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