Abstract Body

We have been researching and developing high-temperature superconducting (HTS) generators that use the cold heat of liquid hydrogen, eliminating the need for a refrigerator ¹⁾. Winding the generator's field coils with REBCO wire results in high efficiency, which is difficult to achieve with existing generators. However, one of the challenges to the practical application of large, high-speed rotating HTS field coils is the mechanical fragility of the REBCO wires and coils. The compressive stress is estimated to be 50 MPa, which is generated by the expected centrifugal force of 8,000 g (rotation radius 0.5 m, rotation speed 3,600 rpm), but previous studies have reported that the I-V characteristics of the REBCO coils decreases from about 10 MPa²⁾. It is necessary to develop a technology to produce high-strength coils that are at least five times stronger than conventional coils. To achieve this, it is essential to investigate the cause of the reversible degradation in the I-V characteristics of the REBCO coils reported in previous studies (the I-V characteristic deteriorates when a load is applied, but returns to its original state when the load is removed).

First, a numerical model of the test coil in the previous study was built to analyze the stress and strain generated inside the superconducting wire when the coil was compressed. Even the internal structures of the REBCO wire, such as the superconducting layer and the silver layer, were modeled in detail. From the results of this simulation, even when a compressive stress of about 10 MPa was applied to the coil, no strain was observed that would lower the Ic of the REBCO wire. It was estimated that the Ic was lowered by a phenomenon other than mechanical.

Next, a test coil was made by winding a double pancake coil (inner diameter 85 mm, outer diameter 95 m) with 4 mm wide REBCO wires and reinforcing it with a SUS case. As a result of the coil compression test, it was confirmed that the I-V characteristic of the coil deteriorated even when a light compressive load was applied, as in the previous report, and that it returned after the load was removed. However, the I-V characteristic slowly deteriorated with time after the start of compression, and it was assumed that there was a reason for the coil temperature to rise during compression. After reconsidering the test configuration, it was found that the reversible decrease in I-V characteristics no longer occurred by providing gas vent paths from the inner circumferential space of the coil. It was determined that the cause of the reversible phenomenon was the increase in pressure and temperature caused by the evaporation of liquid nitrogen in the inner space of the coil when a load was applied. Compression tests were performed with this configuration and the I-V characteristics did not degrade even when the target compression load of 50 MPa was applied, indicating that a high-strength coil design was in sight.

References

[1] M. Ohya et al., “Mechanical simulation and energizing tests of HTS coils for 10 kW generator cooled by liquid hydrogen,” IEEE Trans. Appl. Supercond., Vol. 34, No. 3 (2024) 5201507.

[2] Y. Nagasaki et al., “Axial compressive stress dependence of critical current of REBCO double-pancake coil,” IEEE Trans. Appl. Supercond., Vol. 31, No. 5 (2021) 8400405.

Acknowledgment

This presentation is based on results obtained from a project, JPNP14004, subsidized by the New Energy and Industrial Technology Development Organization (NEDO).

Figure 1. Compression test results (a) for the initial configuration, (b) after providing the gas vent paths.

Keywords: High-temperature superconducting coil, REBCO, Compressive stress