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.
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.
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.
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.
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.
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.
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.
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.
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.