% title ISSDC: Solar Power in Space

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<H2>Solar Power in Space</H2>

<p>Although solar energy is abundant in the inner solar system,
collecting enough of it to provide electricity for a large population
of humans is a non-trivial matter.

<p>The first comprehensive studies of large-scale solar power
generation in space were conducted over three decades ago. Legend has
it that the concept of Solar Power Satellites was first envisioned
by Dr. Peter Glaser as he sat in an early-1970's gas line. At the
time there was an Arab oil embargo, an energy crisis, and global
concern about increasing use and decreasing availability of energy.
People sometimes waited hours in lines at the few stations that had
not run out of gas, sitting in cars that got 15 miles per gallon
and had a range around 200 miles. There was plenty of time to think.

<p>The idea of using solar cells to generate electricity in space
was nothing new. Communications satellites had been doing that for
years. Indeed, the most distinguishing characteristics of most
Earth-orbiting satellites, even today, are their arrangements of
solar cells. A common configuration is a cylindrical shape with the
entire exterior covered in purplish-blue solar cells. Non-cylindrical
satellites have large "wings" covered with solar panels. The crewed
laboratories Skylab, Mir, and International Space Station all had
or have large solar cell arrays that generate power for the satellites'
use.

<p>The difference between existing satellites and Solar Power
Satellites (SPS) is that an SPS would generate more power--much
more power--than it requires for its own operations. Studies in the
1970's by Glaser, NASA, and major corporations produced a myriad
of design concepts. Their single most distinguishing characteristic
is that they were huge-with up to 60 square miles of surfaces covered
with solar cells. The figure shows an example of a concept developed
during that time. A common goal of designers was to put enough solar
cells on a structure in space to generate 10 gigawatts, approximately
equal to the output of ten nuclear power plants. The idea was not
entirely far-fetched; advantages over Earth-based solar power
facilities were that the GEO locations typically proposed for SPS
were almost always in sunlight and only rarely eclipsed, and the
amount of energy available to a unit area of solar panels is 6 to
15 times greater than for the same area of solar panels on Earth,
because sunlight in space is not filtered by atmosphere.

<p>Once having generated electricity in space, however, it is
necessary to get the power to where it is needed on Earth's surface.
The solution selected in the 1970's, and still valid today, was to
convert power into microwave energy that could be beamed to Earth's
surface. Microwaves pass through atmosphere, clouds, and precipitation
with no loss of energy. Experiments on Earth with transmission and
reception of energy converted to microwaves proved the concept. The
antennas designed to transmit the huge amounts of SPS power were,
however, huge (although dwarfed by the sizes of the solar panel
arrays). Typical designs were a half mile or kilometer across;
examples can be seen near the ends of the design shown in the figure.

<p>Antenna sizes were probably dictated not so much by constraints
of materials or technology as by concern for safety. A highly
concentrated power beam would be a tough sell for people concerned
about airplanes being zapped out of the sky or entire migratory
flocks of birds being cooked en route. Large antennas in geosynchronous
orbit, combined with the physics of an expanding microwave beam,
resulted in receiving antenna (rectenna) designs six to eight miles
across (10 to 13 kilometers), with maximum intensity at the center
of the microwave beam less than five times greater than standards
for kitchen emissions from a microwave oven. These facilities would
convert the microwaves back to energy, and contribute their power
to the energy grid in the same manner as a hydroelectric dam,
coal-fired plant, nuclear reactor, ground-based solar facility,
geothermal plant, or field of wind turbines. The benign radiation
environments under these widely-dispersed beams would enable air
traffic, radio, TV, and birds to continue their normal activities
with no impediments. Even so, safety concerns (and the importance
of not wasting power by beaming it away from the rectenna) dictated
that the microwave beam was kept centered on the target rectenna
by a "guide beam" reflected back to the SPS. Because the rectenna
structures would be ten to twelve feet off the ground and designed
to capture all of the microwave energy in the beam, the land under
the rectenna would be available for agriculture. It was speculated
that birds would avoid the rectennas during Summer months and
congregate in them during Winter months, because they would experience
a slight warming sensation. Dr. Peter Glaser himself offered a
standing bet that he would provide fine wine and salad to the person
who would eat the first fowl to venture into the microwave beam;
his point was that the bird would be very much alive and unwilling
to be eaten.

<p>Dr. Glaser hasn't yet had an opportunity to pay off on his wager,
because no SPS was ever built. As oil production increased, cars
used less of it, and prices came down, the interest in SPS waned.
There was a low-level continuing interest in energy-dependent
countries like Japan, but the concept faded from the public vision.

<p>Until recently. Occasional threats to energy supplies, projections
that coal and oil reserves will eventually be depleted, and concerns
that burning hydrocarbons contributes to environmental damage are
providing inspiration for new interest in Solar Power Satellites.
New technologies have, however, changed some of the parameters
involved in constructing a viable SPS. Solar cells can now convert
sunlight to power much more efficiently than when the first designs
were envisioned, resulting in new designs about half the size of
the originals for the same amount of power generation. Even so, any
viable SPS of the future will still be huge.

<p>The implication for a space settlement is that the need for
power--assuming it is provided from a solar source--will be a
significant factor in the settlement's configuration and a major
feature of its design. The need to orient solar panels toward the
sun or a rectenna toward its power source will determine how the
entire settlement is positioned in space. The Space Settlement
Design Competition organizers anticipate that future non-industrial
human communities will require about 10 megawatts per 5000 people.
Industrial communities will require more. The physics of microwave
transmission have not changed. Whether a space settlement is designed
with its own solar panels or with a rectenna for receiving power
generated elsewhere in space, the equipment for providing power
will be a major part of what is seen when the settlement is viewed
by approaching spacecraft. <BR>
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