Wireless power or wireless energy transmission is the transmission of electrical energy from a power source to an electrical load without man-made conductors. Wireless transmission is useful in cases where interconnecting wires are inconvenient, hazardous, or impossible. The problem of wireless power transmission differs from that of wireless telecommunications, such as radio. In the latter, the proportion of energy received becomes critical only if it is too low for the signal to be distinguished from the background noise.
With wireless power, efficiency is the more significant parameter. A large part of the energy sent out by the generating plant must arrive at the receiver or receivers to make the system economical. The most common form of wireless power transmission is carried out using direct induction followed by resonant magnetic induction. Other methods under consideration are electromagnetic radiation in the form of microwaves or lasers and electrical conduction through natural media.
This action of an electrical transformer is the simplest form of wireless power transmission. The primary and secondary circuits of a transformer are not directly connected. Energy transfer takes place through a process known as mutual induction. Mobile phone and electric toothbrush battery chargers, and electrical power distribution transformers are examples of how this principle is used. Induction cookers use this method. The main drawback to this basic form of wireless transmission is short range. The receiver must be directly adjacent to the transmitter or induction unit in order to efficiently couple with it.
The application of resonance increases the transmission range somewhat. When resonant coupling is used, the transmitter and receiver inductors are tuned to the same natural frequency. Common uses of resonance-enhanced electrodynamic induction are charging the batteries of portable devices such as laptop computers and cell phones, medical implants, and electric vehicles. Resonance is used in both the wireless charging pad (the transmitter circuit) and the receiver module (embedded in the load) to maximize energy transfer efficiency. This approach is suitable for universal wireless charging pads for portable electronics such as mobile phones. It has been adopted as part of the Qi wireless charging standard. It is also used for powering devices having no batteries, such as RFID patches and contactless smartcards.
Electrostatic induction or capacitive coupling is the passage of electrical energy through a dielectric (an electrical insulator that can be polarized by an applied electric field). The electric energy transmitted by means of electrostatic induction can be utilized by a receiving device, such as a wireless lamp. Nikola Tesla demonstrated the illumination of wireless lamps by energy that was coupled to them through an alternating electric field. He wrote: ‘Instead of depending on electrodynamic induction at a distance to light the tube . . . [the] ideal way of lighting a hall or room would . . . be to produce such a condition in it that an illuminating device could be moved and put anywhere, and that it is lighted, no matter where it is put and without being electrically connected to anything. I have been able to produce such a condition by creating in the room a powerful, rapidly alternating electrostatic field. For this purpose I suspend a sheet of metal a distance from the ceiling on insulating cords and connect it to one terminal of the induction coil, the other terminal being preferably connected to the ground. Or else I suspend two sheets . . . each sheet being connected with one of the terminals of the coil, and their size being carefully determined. An exhausted tube may then be carried in the hand anywhere between the sheets or placed anywhere, even a certain distance beyond them; it remains always luminous.’
Unlike near field technologies, far field methods achieve longer ranges, often multiple kilometer ranges, where the distance is much greater than the diameter of the device(s). The Rayleigh criterion dictates that any radio wave, microwave, or laser beam will spread and become weaker and diffuse over distance; the larger the transmitter antenna or laser aperture compared to the wavelength of radiation, the tighter the beam and the less it will spread as a function of distance (and vice versa). Microwave power beaming can be more efficient than lasers, and is less prone to atmospheric attenuation caused by dust or water vapor losing atmosphere to vaporize the water in contact. A rectenna may be used to convert the microwave energy back into electricity. Rectenna conversion efficiencies exceeding 95% have been realized. Power beaming using microwaves has been proposed for the transmission of energy from orbiting solar power satellites to Earth and the beaming of power to spacecraft leaving orbit has been considered.
Following World War II, which saw the development of high-power microwave emitters known as cavity magnetrons, the idea of using microwaves to transmit power was researched. By 1964 a miniature helicopter propelled by microwave power had been demonstrated. Japanese researcher Hidetsugu Yagi also investigated wireless energy transmission using a directional array antenna that he designed. In 1926, Yagi and Uda published their first paper on the tuned high-gain directional array now known as the Yagi antenna. While it did not prove to be particularly useful for power transmission, this beam antenna has been widely adopted throughout the broadcasting and wireless telecommunications industries due to its excellent performance characteristics.
The wireless transmission of alternating current electricity through the earth with an equivalent electrical displacement through the air above it achieves long ranges that are superior to the resonant electrical induction methods and favorably comparable to the electromagnetic radiation methods. The electrical displacement takes place predominantly by electrical conduction through the oceans, and metallic ore bodies and similar subsurface structures. The electrical displacement is also by means of electrostatic induction through the more dielectric regions such as quartz deposits and other non-conducting minerals. Receivers are energized by currents through the earth while an equivalent electric displacement occurs in the atmosphere. This energy transfer technique is suitable for transmission of electrical power in industrial quantities and also for wireless broadband telecommunications. The Wardenclyffe Tower project was an early commercial venture for trans-Atlantic wireless telephony and proof-of-concept demonstrations of global wireless power transmission using this method. The facility was not completed due to insufficient funding.
The atmospheric conduction method depends upon the passage of electrical current through the earth, and through the upper troposphere and the stratosphere. Current flow is induced by electrostatic induction up to an elevation of approximately 3 miles (4.8 km) above Earth’s surface. Electrical conduction and the flow of current through the upper atmospheric strata starting at a barometric pressure of approximately 130 millimeters of mercury is made possible by the creation of capacitively coupled discharge plasma through the process of atmospheric ionization. In this way electric lamps can be lit and electric motors turned at moderate distances. The transmitted energy can be detected at much greater distances. A global system for ‘the transmission of electrical energy without wires’ called the ‘World Wireless System,’ dependent upon the high electrical conductivity of plasma and the high electrical conductivity of the earth, was proposed as early as 1904.
The Daily Omnivore 
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