Wireless Energy Transfer
Wireless energy transfer is the transfer of significant amounts of electrical energy without a physical connection for the purpose of doing work. This is distinct from electromagnetic transmission for the purpose of transferring information (radio), where the amount of power is not as important as the integrity of the signal, though similar physics are usually involved.
Wireless energy transfer has been around for about a century, but as of yet has only found usage in things like electric toothbrush rechargers and specialty applications such as beaming solar power down from space.
A simple demonstration of it can be constructed at home with ordinary materials. A loop of wire short circuiting a battery will cause a compass needle placed near it to point, and by tapping one end of the wire on and off of its electrode you can cause the needle to twitch. The transverse magnetic field induced in the line acts on the compass, performing (although trivial) work without the use of a physical connection.
Another, more dramatic demonstration of wireless energy uses a radio transmitter generating more than a few watts, such as an amateur radio transmitter. An unpowered fluorescent bulb held near the antenna will glow when the transmitter is activated: for instance, relaying the abbreviation "SOS" by Morse code will cause the light to correspondingly blink "SOS" as electromagnetic induction by the transmitted signal causes the gas inside the bulb (acting as a natural receiver) to glow, not unlike the causal mechanism in the aurora borealis.
However widespread use of wireless energy transfer over room-sized or community-sized ranges has not proven feasible to date. Energy that is sufficient to power ordinary electrical devices would be deadly if broadcast in all directions at practical frequencies. With focused beams of energy, there are still serious health and safety risks, aligning devices to receive the energy beam is problematic, and broadcast transmission is in any case inefficient, with a great deal of power loss.
Induction
Transformers are probably the simplest example of wireless power transfer. The two circuits of a transformer are physically isolated, but transfer (and transform) power by magnetic coupling through induction. Induction cookers are a prime example of how this is used. In an induction cooker, energy is transferred directly and wirelessly into the pot or pan, where it is converted ohmically into heat for cooking. Toothbrush rechargers work in a similar way. The main drawback to induction, however, is the short range. The receiver must be very close (nearly direct contact) to the inductor unit in order to magnetically couple with it. Nevertheless, the company SplashPower sells charging units for small gadgets such as cell phones that work by placing them on an induction mat.
Electromagnetic transmission
Electromagnetic waves, commonly known as light, can also be used to transfer power wirelessly. By converting electricity into light, such as a laser beam, then firing this beam at a receiving target, such as a solar cell on a small aircraft, power can be beamed to a single target. This is generally known as "power beaming". There are several drawbacks to this, however. First, the conversion to light, such as a laser, is usually very inefficient (although quantum cascade lasers improve this), then, conversion back into electricity is also typically very inefficient, with the best solar cells achieving only 20% efficiency. Also, atmospheric absorption causes further losses. Finally, this method requires a direct line of sight with the target, and is unsuitable for transmitting to many targets or over a broad area.
Despite these drawbacks, there are systems which can circumvent some of these problems. When using longer wavelengths, typically in the microwave range, a rectenna may be used to convert the microwaves back into electricity. Conversion efficiencies exceeding 95% have been achieved in this manner. Power beaming using microwaves finds niche applications in transferring energy from orbital solar power satellites to earth and possible applications in beaming power to spacecraft leaving orbit.
Evanescent wave coupling
Researchers at MIT believe they have discovered a new way to wirelessly transfer power using non-radiative electromagnetic energy resonant tunneling. By sending electromagnetic waves, typically radio waves or microwaves, around in a highly angular waveguide, evanescent waves are produced which carry no energy. If a proper resonant waveguide is brought near the transmitter, the evanescent waves can allow the radio waves to tunnel (specifically evanescent wave coupling, the electromagnetic equivalent of tunneling) to the power drawing waveguide, where they can be rectified into DC power. Since the electromagnetic waves would tunnel, they would not propagate through the air to be absorbed or wasted, and would not disrupt electronic devices or cause physical injury like microwave or radio transmission. Researchers anticipate up to 5 meters of range for the initial device, and are currently working on a functional prototype.