Long-distance Wireless Power

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:

1. Microwaves, which are part of the electromagnetic
spectrum, reach the dipole antennae.
2. The antennae collect the microwave energy and transmit
it to the diodes.
3. 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.
4. 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.