Recently, interest in wireless power transfer(WPT) is increasing rapidly because of the widespread use of electronic devices such as mobile phones, tablet PCs, smart watches, etc. WPT technologies can be divided into magnetic induction, microwave and magnetic resonance. The magnetic induction type has a high efficiency but the transmission distance is very short. The microwave type has a high efficiency and significant transmission distance. However, due to the extremely short wavelength range of GHz bands, the method is vulnerable to high terrain and surrounding objects. It also poses harm to the human body. The magnetic resonance type has a high efficiency and transmission distance. It poses no harm to humans because it uses low frequency bands(kHz to MHz), and it is less affected by environmental conditions. With these features, magnetic resonance appears to be the best technology for application in real life. However, the size of the antenna it requires is larger than that of the magnetic induction type. To resolve these drawbacks, magnetic resonance WPT is being used to miniaturize the antenna and improve efficiency as well as transmission distance.
In this paper, the variable capacitors and the superconducting relay antenna were applied to the superconducting WPT system to increase efficiency and the transmission distance of a magnetic resonance WPT system. Superconductivity possesses zero resistance characteristics below the critical temperature. When superconductivity is applied to antennas, energy loss from the antenna can be reduced. First, a high frequency structure simulation(HFSS) program was used to analyze the S-parameter of the superconducting WPT. Based on this setup, the superconducting WPT was configured. As a results, the superconducting WPT efficiency increased by about 10%, as compared to a normal WPT.
During transmission, resonance frequency is changed through mutual inductance, which occurs between the transmitting and receiving antennas. It is important to maintain constant resonance frequencies because the transition of resonance frequency can drastically reduce superconducting WPT efficiency. Consequently, variable capacitors were applied to the superconducting antennas and the changes in the resonance frequency of the superconducting WPT were analyzed. When variable capacitors are not applied to the superconducting WPT, the resonance frequencies changed to 6.78 6.84, 6.9, 6.94 MHz, respectively, according to the transmission distance. But when variable capacitors are applied to the superconducting WPT, resonance frequency was stable at 6.78 MHz. In addition, the transmission distance of the superconducting WPT increased by 30 cm.
Finally, a variable superconducting relay antenna was applied to increase the transmission distance and to maintain high magnetic coupling between the transmitting and receiving antennas. As a results, the superconducting WPT efficiency was about 70% at a transmission distance of 60 cm. This efficiency increased by 30% when the variable relay antenna is not applied at the same transmission distance. In addition, the superconducting WPT could increase transmission distance by more than 60 cm.
The efficiency and transmission distance of superconducting WPT systems can be increased based on the findings of this study. Superconducting WPT systems are deemed to have sufficient value for research not only for low power transmission applications, such as IT devices or mobile phones, but also in high power transmission use, such as in electric vehicles or electric railroads.