|Fig. 1: 2000 Concept of a Space Solar Power Array "SunTower" utilizing a large solar panel array to collect energy and a transmission system to beam the energy down to a ground based receiving antenna which then contributes collected energy to the electric grid.  (Courtesy of NASA)|
Solar power technology has been utilized as a source of power for space installations since the first space station was launched in 1971. Since then, most instrumentation and almost all installations in space rely on solar power in one form or another. However, the same cannot be said of the world at large; in 2008, the United States relied on solar technology for only 0.1% of its power generation. 
One idea that has been entertained since the 1970s is the idea of placing solar power stations in orbit and somehow transmitting the energy collected down to the surface for use in power grids. When this idea was conceived, such an idea seemed beyond the capabilities of the technology of the time, but in 2007, a report from the National Security Space Office revisited the idea and found it to be not only plausible but favorable given the lower availability of conventional fuels.  The question then becomes how to actually build and utilize such a system that could become a significant source of power on the ground.
There are several advantages for a large-scale space solar power array that is not achievable on the ground. The first is the elimination of atmospheric dispersion of light energy, allowing a higher amount of sunlight to go into power generation than for ground-based arrays; this would allow for more power to be generated with the same sized photovoltaic cell. Also, due to the absence of atmosphere, there is no concern for inclement weather blocking solar panels, a problem faced by ground-based units but inconsequential for space-based units.
In addition, space-based systems, due to their high altitude, have a much longer exposure to sunlight than do ground-based systems, which are entirely limited by the amount of time their locations are exposed to sunlight. Specific orbits can allow the exposure of these arrays greater than the 12 hours maximum possible on the ground.
Finally, space-based platforms are not as limited in their size as their ground-based counterparts. Ground-based systems require a significant amount of land for their construction and the placing of their arrays; size is a significant limiting factor in the output of these facilities. Space-based systems have no such limitation provided that the material for their construction can be placed into orbit. The only ground facilities necessary are the launch platforms and the receiving station for the power transmitted (in some form) to the ground.
Advantages such as these make the utilization of space solar power arrays a highly appealing prospect. Such advantages could yield up to 5 times more power than a ground based system. 
There are many competing designs for a space solar power array, but the basic idea remains the same in all cases. A large solar power collection facility is constructed in orbit with a means of wirelessly transmitting the collected energy to the ground to a receiving station which then converts the energy into useable electricity to be utilized in the grid.
Specific designs for the orbiting array itself include the "SunTower" design, which has the potential to contribute 100-400MW on its own (with additional orbiting arrays to keep that power output constant) to the "SunDisc" design, which has the potential to contribute 1GW utilizing a single array/receiver pair.  Estimated cost-to-first-power for these systems (in 1997 dollars) were $8 billion for a 250MW "SunTower" and $30-50 billion for a 5GW "SunDisc". Methods of transmission vary in their method from laser technology to microwave beams. NASA in 2009 awarded a $900,000 prize for a laser-powered system, and the year before had a company that transmitted power using RF frequency 90 miles away. 
Current efforts at building such as system include the EADS-backed system Astrium which would beam power in the infrared.  Astrium is currently being designed for the 10-20kW range, but it is primarily being designed as a proof of concept project with the expectation that others will produce larger systems.
There are, however, drawbacks to a space solar power array. The first is the prohibitive cost of such systems. Launch costs for systems of this size would be an extremely limiting factor in the construction and deployment of a space-based solar power array. This will continue to be the case until a solution to the high cost of launching material into orbit is discovered and implemented.
Another concern is the large amount of space material, i.e. space junk and micrometeoroids damaging the arrays. Ground-based system experience a similar problem with sand and the corrosion of their mirrors or photovoltaic cells, but due to the cost of launching materials into orbit, the maintenance cost of large facility in orbit would be exorbitant to say the least.
Despite the problems faced by the implementation of such systems, space solar power arrays are a solution worth considering in the present. The growing demand for clean, sustainable power and the lowering supply of fossil fuels will be a considerable force pushing in its favor in the coming years. Of course, this requires further research into solar technology, aerospace technology, and space technology in general. One day, this technology may play a role in the energy needs of the future.
© Daniel Nagasawa. The author grants permission attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
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