For years
humanity has dreamed of a clean, inexhaustible energy source. This dream
has lead many people to do what, in retrospect, seems obvious, and look
upward toward nature's "fusion reactor", the sun. However, while
sunlight is clean and inexhaustible, it is also dilute and intermittent.
This led Peter Glaser of the Arthur D. Little Company to suggest in 1968
that solar collectors be placed in geostationary orbit. Such collectors
are known as solar power satellites (SPS). The solar energy collected by
an SPS would be converted into electricity, then into microwaves. The
microwaves would be beamed to the Earth's surface, where they would be
received and converted back into electricity by a large array of devices
known as a rectifying antenna, or rectenna. (Rectification is the
process by which alternating electrical current, such as that induced by
a microwave beam, is converted to direct current. This direct current
can then be converted to the "slower" 50 or 60 cycle alternating current
that is used by homes, offices, and factories.) At geostationary orbit
(36,000 kilometers or 22,000 miles high), the SPS would have a 24-hour
orbital period. It would therefore always hover over the same spot on
the equator and can keep its beam fixed on a position at a higher
latitude. Since the Earth's axis is tilted, an SPS orbiting over the
equator outlawing above or below the Earth's shadow during its daily
orbit. Sunlight would not be blocked, except for a period of about an
hour each night within a few weeks of the equinoxes.
It is
interesting to compare the availability of sunlight in space with that
on Earth. A solar panel facing the sun in near-Earth space receives
about 1400 watts of sunlight per square meter (130 watts per square
foot). (Of course, only a fraction of this is usable due to conversion
inefficiencies.) On Earth, the day-night cycle cuts this in half. The
oblique angle of the sun's rays with respect to the ground cuts this in
half again for a typical spot on the Earth. Solar panels on the ground
can be angled upward to circumvent this, but they must then be spread
out over more ground to avoid casting shadows on each other. Clouds and
atmospheric dust cut the available sunlight in half again. Thus,
sunlight is about eight times more abundant in geostationary orbit than
it is on the Earth. Although the microwave beam from an SPS would also
be dilute, it would be converted to electricity at a greater efficiency
than sunlight. However, the largest cost savings in SPS versus
terrestrial solar collectors may be the elimination of the need for
storage at night .
Spurred on
by the oil crises of the 1970's, the US Department of Energy and NASA
jointly studied the SPS during that decade. The result of this study was
a design for an SPS which consisted of a 5 x 10 kilometer rectangular
solar collector and a 1-kilometer-diameter circular transmitting antenna
array. The SPS would weigh 30,000 to 50,000 metric tons. The power would
be beamed to the Earth in the form of microwaves at a frequency of 2.45
GHz (2450 MHz), which can pass unimpeded through clouds and rain. This
frequency has been set aside for industrial, scientific, and medical
use, and is the same frequency used in microwave ovens. Equipment to
generate the microwaves is therefore inexpensive and readily available.
The rectenna array would be an ellipse 10 x 13 kilometers in size. It
could be designed to let light through, so that crops, or even solar
panels, could be placed underneath it. The amount of power available to
consumers from one such SPS is 5 billion watts. (A typical conventional
power plant supplies 500 million to 1 billion watts.)
Nevertheless,
even the peak of the beam is not exactly a death ray. Underneath the
rectenna, microwave levels are practically nil.
The reason
that the SPS must be so large has to do with the physics of power
beaming. The smaller the transmitter array, the larger the angle of
divergence of the transmitted beam. A highly divergent beam will spread
out over a great deal of land area, and may be too weak to activate the
rectenna. In order to obtain a sufficiently concentrated beam, a great
deal of power must be collected and fed into a large transmitter
array
