Maximizing Efficiency of Electromagnetic Resonance Wireless Power Transmission Systems with Adaptive Circuits

Wireless power transmission (WPT) is a cutting-edge technology that signifies a new era for electricity without a bunch of wires. Wireless power or wi-power is increasingly becoming the main interest of many R&D firms to eliminate the “last cable” after the wide public exposure of Wi-Fi lately. Even though the first idea was devised from Nikola Tesla in the early 20th century (Tesla, 1919), there was never strong demand for it due to the lack of portable electronic devices. In recent years, with the advent of a booming development in cell-phones and mobile devices, the interest of wireless energy has been re-emerged. WPT offers the possibility to supplying power for electronic devices without having to plug them into AC socket. Until now, there have been many efforts to be made to improve this technology as well as its applications. These efforts include medium-range transmission based on electromagnetic resonance and long-range transmission using microwaves (Greene et al., 2007; Brown & Eves, 1992). Although the investigation of long-range power delivery via far-field techniques was carried out with endeavors, the efficiency or power delivery is still quite low that is not sufficient to fully charge typical electronic gadgets overnight. Therefore, increasing the transmitting power is necessary to provide energy enough to consistent DC supply of gadgets. However, the system would be harmful to human according to IEEE standard for radio frequency electromagnetic fields (IEEE, 1999). The other way is to utilize many transmitters simultaneously, but the implementation seems to be impractical. Additionally, the existence of an uninterruptible line of sight (LoS) is mandatory for microwave-based power transmissions and in a case of mobile objects requiring a complicated tracking system. In general, such power transmissions are relatively suited to very low power applications unless they are used in military or space explorations which are less regulated environments. On the other hand, medium-range WPT covering up to 30 feet is a growing research area that finds wide applications. In order to implement a viable WPT system, Q factors of coils and an impedance matching issue are critical and sensitive to achieve a high efficiency. In reality, due to a resonant coupling nature of the system, for the most efficient power transmission, there is an optimum range between a power coil and a transmitting coil for a fixed distance between the transmitting and receiving coils. This effect may not be clarified by a conventional magnetic induction theory. In this chapter, a simple equivalent circuit model for a WPT system via electromagnetic