A Brief Overview of Wireless Power Transfer Techniques <s> Magnetic Resonance Assisted Wireless Power Transfer <s> The capacity of global electric power systems must be increased tenfold by the year 2050 to meet the energy needs of the 10 billion people assumed to populate the Earth by then. Few studies directly address this enormous challenge. Conventional terrestrial renewable, nuclear, and coal systems can not provide the power. Solar power collected on the moon can meet these needs. It would be collected by large area, thin-film photovoltaics and converted into thousands of low intensity microwave beams. These beams would be projected from shared, large diameter synthetic apertures on the moon to receivers located anywhere on Earth. Engineering and cost models indicate that the Lunar Power System (LPS) is economically robust and can be built at a faster rate than all other power systems. Internal rates of return in excess of 40% per year may be feasible. LPS uses understood technology. It can be environmentally supportive rather than simply benign or damaging. LPS implementation can immediately channel national and world R&D aerospace and electronics capabilities into completely peaceful directions and enable human prosperity. <s> BIB001 </s> A Brief Overview of Wireless Power Transfer Techniques <s> Magnetic Resonance Assisted Wireless Power Transfer <s> Radio waves can benefit the welfare of humanity through other purposes than communications. Microwave power transmission (MPT) is one of the new technological frontiers. Solar power satellites (SPS) will provide a clean and limitless energy resource from space through this technique. This article answers the fundamental question of why we need to develop SPS from the viewpoint of critical global issues for mankind. It then reviews SPS research, mainly in Japan, in the 1980s and 1990s. These efforts include two MPT rocket experiments; a microwave-driven airplane experiment; and MPT technology developments, concepts, and systems studies of SPS. A summary of recent research committee activities on SPS and a road map of future SPS research plans in Japan concludes the article. <s> BIB002 </s> A Brief Overview of Wireless Power Transfer Techniques <s> Magnetic Resonance Assisted Wireless Power Transfer <s> We investigate whether, and to what extent, the physical phenomenon of long-lifetime resonant electromagnetic states with localized slowly-evanescent field patterns can be used to transfer energy efficiently over non-negligible distances, even in the presence of extraneous environmental objects. Via detailed theoretical and numerical analyses of typical real-world model-situations and realistic material parameters, we establish that such a non-radiative scheme can lead to “strong coupling” between two medium-range distant such states and thus could indeed be practical for efficient medium-range wireless energy transfer. <s> BIB003 </s> A Brief Overview of Wireless Power Transfer Techniques <s> Magnetic Resonance Assisted Wireless Power Transfer <s> Using self-resonant coils in a strongly coupled regime, we experimentally demonstrated efficient nonradiative power transfer over distances up to 8 times the radius of the coils. We were able to transfer 60 watts with ∼40% efficiency over distances in excess of 2 meters. We present a quantitative model describing the power transfer, which matches the experimental results to within 5%. We discuss the practical applicability of this system and suggest directions for further study. <s> BIB004
From Maxwell's equations it can be seen that a time varying current passing through a coil produces a magnetic field around its surrounding space. As this time-varying magnetic field interacts with another coil placed in the vicinity of the coil producing the magnetic field, an induced current arises in the second coil BIB003 . This is called mutual induction between the two coils, and the frequency at which the coupling waves attain their maximum amplitude is called the resonant frequency. In this method, primary and secondary coils serve as transmitting and receiving antennas respectively. Some of the critical parameters affecting the performance of this system include the operating frequency, diameter, resistance and the number of turns of the coils BIB003 . As shown in figure 3 , inductive power transfer works by creating an alternating magnetic field (flux) in a transmitter coil and converting that flux into an electric current in the receiver coil. Depending on the distance between the transmitting and receiving coils, only a fraction of the magnetic flux generated by the transmitter coil penetrates the receiver coil and contributes to the power transmission  . The more flux reaches the receiver; the better the coils are coupled. In conventional inductive coupling, efficiency drops off rapidly as distance increases. Magnetically coupled resonators with sufficiently high quality factor (Q) coils exhibit a surprising behavior BIB004 . Maximum power transfer efficiency can be achieved over a broad range of distances and orientations. The transmit antenna consists of a LC based transmitting coil. As electricity travels through this coil, the coil begins to resonate. The oscillator generates a signal at a frequency where maximum power transfer takes place; the signal is then amplified through a power amplifier thus resulting in an oscillating magnetic field  . The magnetic waves resonating at a specific frequency are transmitted from the transmitter to a receiver BIB002 . These magnetic waves interact with a coil at the receiver to induce an electric current at the receiver. If the receiving coil is tuned so that its resonant frequency matches the frequency of the magnetic waves, the current it produces is amplified. The received power would have an alternating current, which is undesirable for powering a DC load, thus a rectifier would be needed to rectify the AC into a smooth DC. Each of the three WPT methods discussed in this paper has its own merits and demerits. The three WPT methods have been compared with respect to output power, transmitting distance range, economic feasibility, system efficiency, and biologic effects. Table 1 summarizes the results.  No harmful effects BIB002 Microwave 100 Kilowatts BIB001 Few Kilometers BIB001 Expensive Up to 54% BIB001 Damages living tissue BIB001 Laser Several hundred Kilowatts  Few Kilometers  Expensive Up to 30%  Damages living tissue  