There are a large number of different proposed or operational systems where a power source is physically separated from the active or working components of a system. Because it is frequently difficult to store energy at high densities, and expensive or inconvenient to carry around large stores of energy, doing this can be convenient in any situation where active components must be mobile. Some examples of such situations include providing power for existing wireless electronics, providing power for the much larger range of potential wireless products that could exist if power storage were more convenient or unnecessary, devices internal to the body, and the launch of certain proposed or experimental spacecraft.
Upon occasion, wireless transmission of power has also been proposed for applications where placing extremely long wires or networks of wires would simply be inconvenient, as in the use of microwaves to transmit power from solar energy collecting satellites to receiving stations on Earth or in Nikola Tesla’s proposals for the use of wireless power transmission instead of an electric grid. Technically, the transformers that Tesla invented and which are a necessary component of all high voltage electrical systems involve wireless power, but the short range of transmission associated with transformers is not generally associated with the term.
The crystal radio sets of the 1920s and 1930s were probably the most commercially successful historical application for wireless power transmission. Other commercial applications have included the use of wireless transmission of power to artificial hearts and other forms of implant, and to RFID (radio frequency identification) tags. RFID tags are used in automated toll collection systems such as E-Z pass, in animal tracking, and in inventory management. They are widely expected to revolutionize inventory management and other aspects of product tracking over the next few years.
Wireless power transfer always involves the use of fairly low frequency, electromagnetic radiation. Usually, radio-frequency waves or microwaves are used, and power transfer is through induced electrical currents. In some proposed space transport systems, however, infrared lasers are used and the power transfer is via heating of a target, which then expands outward at a high speed and provides reaction mass for propulsion. Because the temperatures that substances can achieve as a result of being targeted by a high energy laser are much greater than those achievable through participation in chemical reactions, the amount of reaction mass can be greatly reduces, lightening the spacecraft and hopefully reducing its price.