A new kind of superconducting energy storage/convertor with great application potential

Superconductors are promising candidates for energy conversion and storage-related applications owing to their distinctive zero resistivity properties. Among the available motors, superconducting motors exhibit higher energy density, higher efficiency and low energy loss during electrical energy to mechanical energy conversion. Similarly, superconducting induction heaters, superconducting generators and superconducting magnetic energy storage devices are superior to their conventional counterparts regarding performance, efficiency and output energy density.

The recently proposed energy storage/convertor based on permanent magnet and closed superconducting coil presented a key milestone in the efforts to develop high-density energy conversion/storage devices. This device can complete energy storing-releasing cycle of mechanical – electromagnetic – mechanical with high efficiency and minimal losses without requiring either a power supply or generator/motor. Such energy conversion/storage devices would be market competitive in some potential applications. Therefore, more research effort in this direction is necessary.

Energy capacity is a crucial parameter for energy storage/convertor. For instance, the operation capacity of the above device is highly dependent on the properties of the two main components: the superconducting coil and the permanent magnet. Thus, optimizing the selection of magnet and configuration is important in developing devices with high operating capacity. Consequently, optimized configuration (optima selection of magnets and their polarity arrangement) has been associated with enhanced storage capacity and energy density.

Inspired by their previous findings, Professor Ying Xin with his graduate students Wenxin Li, Tianhui Yang, and Gengyao Li from Tianjin University investigated the application prospects of a new superconducting energy storage/convertor device. Briefly, the authors commenced their investigation by first conducting a theoretical analysis to confirm the optimized configuration. Next, new experiments on a scaled-up testing platform were further conducted to validate the effectiveness and feasibility of the configuration. The application of such devices was demonstrated by building and testing a new prototype consisting of three closed superconducting coils and a large permanent magnet. Their work is currently published in the Journal of Energy Storage.

The research team demonstrated that a combination of configuration optimization and an enlarged magnet and superconductor coil could greatly enhance the energy-converting capacity. Consequently, the performance cost/ratio of the device could be increased by increasing the number of magnets because the cost of permanent magnets is much lower than that of superconducting materials. The advantages of the present energy storage/convertor included its simple structure, smaller size, high conversion efficiency, lightweight and low operation loss compared with that based on flywheel and supercapacitor. Moreover, larger superconductor coils consisting of a larger number of turns exhibited larger capacity.

The attenuation characteristics of the current in the coils were also evaluated for two hours at a stable energy storage state. It was observed that the attenuation was generally practically tolerable with a charging-discharging cycle below a few hours. It was worth noting that the efficiency values recorded by the prototypes are not the ceiling for such kinds of devices. The main internal loss associated with such devices is the joule loss on the superconductor coil joint, which is fixed at a certain current. Interestingly, it was reported that scaling up the device would result in higher efficiency and smaller losses.

In summary, the experiments of a newly developed superconducting energy storage/convertor device were reported. The results demonstrated the practical feasibility of the proposed superconducting energy storage/convertor. In a statement to Advances in Engineering, Professor Ying Xin noted that the proposed device has numerous advantages and commercial viability, making it a potential candidate for short-term energy storage in applications involving mechanical–electromagnetic–mechanical conversion such as regenerative braking of urban rail transportations.