Whether or not it incorporates resonance, induction generally sends power over relatively short distances. But some plans for wireless power involve moving electricity over a span of miles. A few proposals even involve sending power to the Earth from space.
In the 1980s, Canada's Communications Research Centre created a small airplane that could run off power beamed from the Earth. The unmanned plane, called the Stationary High Altitude Relay Platform (SHARP), was designed as a communications relay. Rather flying from point to point, the SHARP could fly in circles two kilometers in diameter at an altitude of about 13 miles (21 kilometers). Most importantly, the aircraft could fly for months at a time.
The secret to the SHARP's long flight time was a large, ground-based microwave transmitter. The SHARP's circular flight path kept it in range of this transmitter. A large, disc-shaped rectifying antenna, or rectenna, just behind the plane's wings changed the microwave energy from the transmitter into direct-current (DC) electricity. Because of the microwaves' interaction with the rectenna, the SHARP had a constant power supply as long as it was in range of a functioning microwave array.
Rectifying antennae are central to many wireless power transmission theories. They are usually made an array of dipole antennae, which have positive and negative poles. These antennae connect to semiconductor diodes. Here's what happens:
- Microwaves, which are part of the electromagnetic spectrum, reach the dipole antennae.
- The antennae collect the microwave energy and transmit it to the diodes.
- The diodes act like switches that are open or closed as well as turnstiles that let electrons flow in only one direction. They direct the electrons to the rectenna's circuitry.
- The circuitry routes the electrons to the parts and systems that need them.
Other, longer-range power transmission ideas also rely on rectennae. David Criswell of the University of Houston has proposed the use of microwaves to transmit electricity to Earth from solar power stations on the moon. Tens of thousands of receivers on Earth would capture this energy, and rectennae would convert it to electricity.
Microwaves pass through the atmosphere easily, and rectennae rectify microwaves into electricity very efficiently. In addition, Earth-based rectennae could be constructed with a mesh-like framework, allowing the sun and rain to reach the ground underneath and minimizing the environmental impact. Such a setup could provide a clean source of power. However, it does have some drawbacks:
- The solar power stations on the moon would require supervision and maintenance. In other words, the project would require sustainable, manned moon bases.
- Only part of the earth has a direct line of sight to the moon at any given time. To make sure the whole planet had a steady power supply, a network of satellites would have to re-direct the microwave energy.
- Many people would resist the idea of being constantly bathed in microwaves from space, even if the risk were relatively low.
While scientists have built working prototypes of aircraft that run on wireless power, larger-scale applications, like power stations on the moon, are still theoretical. As the Earth's population continues to grow, however, the demand for electricity could outpace the ability to produce it and move it around. Eventually, wireless power may become a necessity rather than just an interesting idea.
Read on for lots more information about electricity, wireless power and related topics.