The world's energy consumption is increasing due to the growth of the world's population and the economic growth of developing countries, and most of that energy is dependent on fossil fuels. To solve this energy problem, the introduction of renewable energy sources that do not affect the environment is being promoted. However, one of the major problems with renewable energy sources is the fluctuation of output power. It is necessary to establish an output fluctuation compensation function, which is independent of the commercial grid, to expand the introduction of renewable energy sources.

To compensate for output fluctuations of renewable energy sources, energy storage devices play an important role which can charge the surplus generated power and discharge it when the generated power is insufficient for demand. The energy storage devices must have high capacity and durability to provide stable power for a long time and must be able to respond quickly to compensate the rapid power fluctuations in renewable energy sources. High energy density is also required. However, there is no energy storage device that satisfies all these requirements. We have proposed a “Hybrid Energy Storage System (HESS)” as an energy storage system that consists of a “hydrogen system” with high capacity and high energy density and an “electric double layer capacitor (EDLC)” with superior responsiveness and durability.

To expand the introduction of renewable energy in the future, it is necessary to increase the capacity of the HESS. We proposed to introduce the SMES, which has the advantage of scale, to the power storage device of the larger-capacity HESS. However, it has been reported in previous studies that the fluctuation compensation performance of the SMES is lower than that of the EDLC and other devices. Thus, to apply the SMES to the large-capacity HESS for future expansion of renewable energy sources, this study investigated the application possibility of the SMES to the HESS by examining the system configuration and operation control method that can improve the fluctuation compensation performance of the SMES.

We first simulated that the SMES is installed in a power supply system with the rated power of photovoltaic power generation of 20 kW and investigated the effective system configuration and operation control method for improving the fluctuation compensation performance of the SMES. As a result, we identified an effective system configuration in which the SMES and a capacitor are connected in parallel, and a bidirectional DC-DC converter is installed between the DC bus and the SMES system. Furthermore, as an operation control method, we found that the passive voltage control method was effective, which controls the voltage at both ends of the capacitor by controlling the amount of input and output energy of the SMES. The bidirectional DC-DC converter controls the amount of input and output energy of the SMES system while controlling the DC bus with constant voltage. The fluctuation compensation performance was significantly improved compared to the conventional SMES system by applying the proposed system configuration and operation control method.

The effectiveness of the proposed system configuration and operation control method in the large-scale renewable energy power supply system was also investigated. However, in the large-scale system, high fluctuation compensation performance could not be achieved with the proposed system configuration and operation control method for the 20 kW test system. This was due to the poor response and small amount of stored energy of the superconducting coil. We proposed a SMES system with superconducting coils arranged in parallel as the system configuration method that can improve the response and stored energy of the superconducting coils. As a result, the SMES system with superconducting coils arranged in parallel achieved the same level of high fluctuation compensation performance as the EDLC. It is necessary to select the number of paralleled superconducting coils and the inductance of the superconducting coils considering the scale of the HTSS system within the cost constraint.

This study established a system configuration and operation control method of the SMES that is effective for high fluctuation compensation performance of large-scale renewable energy power supply systems for the future expansion of the introduction of renewable energy.