Yagai Tsuyoshi

Abstract We have demonstrated an advanced superconducting power conditioning system, in which a superconducting magnetic energy storage (SMES) device, a generator based on a fuel cell (FC), and an electrolyzer are used to compensate for electricity fluctuations over a wide frequency range, combined with a liquid hydrogen storage system to both cool the SMES and provide pure hydrogen gas to the FC and other gas-dependent systems. To manufacture the coils for the SMES, we used MgB₂, whose critical temperature is below the boiling temperature of hydrogen. We developed a 10 kJ SMES coil system indirectly cooled by liquid hydrogen using thermosyphon passive heat exchange to isolate the flammable hydrogen from the electrical components. We performed a successful demonstration of this system for both DC and AC currents ramped at different rates. In the present study, we use computer simulations involving heat balance equations to evaluate the stability of the system. The results obtained are expected to lead to the design of future large-capacity energy storage systems, such as the MJ class, which offer comparable performance to conventional NbTi SMES devices.

The superconducting Magnetic Energy Storage (SEMS) application still has a great potential to stabilize the utility grid when the uncontrollable power generation from renewable sources increases and power flows change rapidly due to the broad introduction of high-speed response semiconductor switching devices. Along with the development of liquid hydrogen supply chain, the SMES system using MgB₂ conductors also attracts great attention at this point. Although the MgB₂ wires which have critical temperature of around 39 K have been commercially available with more affordable prices, their bending strain sensitivity is an issue to be solved for fabricating large-scale conductors and coils. The experience of constructing a 10-kJ SMES system using Bi2223 tapes and the successful demonstration of compensating very fast electric power fluctuations in the previous project will help us to develop a larger-scale MgB₂ SMES system by investigating conductor and coil design while considering its bending strain sensitivity and mechanism of critical current deterioration to maximize its performance as one of the most promising energy storage devices, following the movement toward a CO₂-free environment.

Superconducting magnetic energy storage (SMES) devices of several tens of kJ class are generally suitable for voltage compensation for microgrids, which produce and distribute electric power to restricted areas. MgB2 material has been developed with superconducting properties by decreasing the production cost. Since hydrogen energy would be widely utilized to realize society with low carbon emission and stored in liquid state for reducing its volume, the power distribution system consisting of MgB2 SMES for compensation of voltage fluctuations cooled by the liquid hydrogen would be effective by synergy effect. However, the MgB2 introduction to large-scale devices is still not enough and under investigation. Our group carried out the investigations to develop MgB<sub>2</sub>cable and pancake coil for the SMES device with specific capacity. The bending strain-sensitive characteristic of MgB2 material forces us to design the twisted conductors and pancake coils with various parameters properly within its tolerable bending strains of both before/after heat treatment.