|Fig. 1: Solar Power Satellite. Mirrors concentrate solar radiation, which is beamed back to earth via a microwave beam, and subsequently collected and diverted to an external circuit. (Source: Wikimedia Commons)|
The theory surrounding Spaced-Based Solar Power, the idea that solar energy can be collected in space, has developed significantly since its introduction in the 1970s. Contemporary solar power technology is comprised of panels that reside on earth, and convert solar radiation into electricity by absorbing light in a semiconducting silicon, separating opposite charge carriers, and extracting charge into a circuit. Such technology could theoretically be adapted for use in space, where solar radiation is much more intense in the absence of atmospheric gas. The use of solar cells to harness energy has increased exponentially in the last 15 years among developed countries, and many on the frontier of the solar industry argue that Space-Based solar power (SSP) represents the next breakthrough in solar technology.
The most realistic methods of harnessing solar radiation in space fall into two categories: the Solar Power Satellite (SPS), and the Solar Tower. The conceptual SPS would be placed in geostationary orbit to ensure constant antenna geometry, and collect solar radiation through amplifying mirrors and currently-available solar cells, and subsequently beam the stored energy back to earth through an electromagnetic beam. On earth, a ground segment comprised of a large photovoltaic array would capture the microwave beam, convert it into electricity, and distribute it to the local grid. (See Fig. 1.) Alternative methods of capturing energy from solar rays in space include the "Sun Tower," a tethered array of solar concentrators in low orbit channeling solar energy to an electromagnetic beam transmitter. (See Fig. 2.) Subsequently, the microwaves would be sent back to earth to be collected by a terrestrial passive array of photovoltaic cells. 
|Fig. 2: Frontal view of the architecture of the sun tower, including solar concentrators and "backbone." (Courtesy of NASA. Source: Wikimedia Commons)|
The tethering system would be made of a superconducting "backbone" used to transport energy from the solar concentrators to the beam transmitter. Predictions of a sample Sun Tower placed at 6,000 km at 30 degree inclination orbits would produce an average of 250 MW, producing the same amount of energy as the maximum capacity of the California Valley Solar Ranch, the United States' largest passive photovoltaic cell array. 
The advantages of Space Solar Power are many. Whereas passive arrays on earth can only absorb sunlight for a maximum of 12 hours a day, solar satellites and sun towers could absorb solar radiation nearly continuously, with a maximum of 72 minutes per day obstructed from the sun. Furthermore, the absence of gravity would allow for the use of light-weight materials coupled with large surface area, which would not be feasible on earth.  By operating on geostationary orbits, space-based solar mechanisms would reduce energy infrastructure on earth, as the cosmic ray reception stations could be placed near power distribution centers, eliminating the need for wasteful transfer over land. In addition, the use of solar space satellites is environmentally feasible, as all waste heat accompanying solar conversion into the microwave beam would be dissipated into space. Lastly, the use of space-based solar devices would make transportation of energy across multiple surface locations easier by adjusting its orbit, helping to reduce peak-load problems and deliver energy to the most deficient areas.  With respect to profitability of solar satellites, researchers have estimated that the net present value of the future cash flows generated by a typical SPS is over $8.5Billion, controlling for many factors, including rising energy prices and variability of input costs. As such, solar power satellites are economically viable investments. 
While the benefits of space-based solar power are numerous, there are many drawbacks that make the initial investment in solar projects difficult. First, many cash flow analyses of space solar ventures estimate incredibly high up-front costs, followed by steadily increasing profits. As such, the barrier to entry from the private sector is high, and the availability of funding from the public space is scarce due to high early costs. Furthermore, while the estimated need for maintenance is low due to the lack of weather-related issues in space, any miscellaneous maintenance expenses are high due to the costs associated with sending astronauts to repair the satellite. Moreover, current photovoltaic cells have not been adequately tested for the high levels of solar radiation that exist in space, so using contemporary cells could pose unforeseen problems when used for solar satellites.
As the global demand for clean, renewable energy continues to grow in the coming years, large infrastructural projects will need to be undertaken to meet such demand. While the difficulties associated with space-based solar power stem from political, financial, and technical constraints, contemporary technology holds the building blocks necessary for early space-based solar devices. However, rising efficiency rates of current photovoltaic cells, and the long-term profitability of solar satellites make space-based solar an attractive venture in expanding the frontier of renewable energy. With advances in space transportation, thermal management, and energy distribution, solar satellites and sun towers could soon become viable options in meeting global energy demand. 
© Scott White. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
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