|Fig. 1: Basic schematic for space based solar power.|
As we head further into the 21st century, the question of how the world's rising energy needs will be satisfied in the face of the rapid consumption of our fossil fuel supplies, remains to be answered and becomes more pressing every year. The solution(s) to the energy problem will be one of the great achievements of human kind, but so far it has proven elusive. The difficult nature of this problem and the pressing urgency to solve it force us to consider every option, no matter how "crazy" they may seem. One intriguing solution that has been thought about for a long time, but seems to be gaining more traction recently is space-based solar power (SBSP).
An SBSP system is in principle a very simple and very neat idea. An array of mirrors and/or photovoltaic cells are assembled in geo-synchronous orbit (GSO). Solar energy collected by this array is reflected down to the earth's surface or in the more typically proposed scheme converted by a power amplifier into radio waves which can easily penetrate the atmosphere. The radio waves are then collected by a large terrestrial rectenna array and converted to electricity. This scheme is illustrated in Fig. 1. 
The U.S. government has shown a moderate amount of interest in SBSP over the past few decades and has spent $80M studying the subject. Most recently in 2007 the government commissioned a study by the National Security Space Office (NSSO) titled "Space-Based Solar Power As an Opportunity for Strategic Security".  There has also been some interest from the private sector recently. Last year Pacific Gas & Electric (PG&E) announced a deal to purchase 200 megawatts of SBSP based electricity over a 15-year period from Solaren Corporation. While this is a significant step, the private sector's commitment to SBSP still seems rather weak. PG&E is required by California Law to expand their renewable energy portfolio, and they've stated that the deal they've signed with Solera is a no-risk contract. 
|Fig. 2: SBSP exposure to sunlight while in geosynchronous orbit.|
There are several primary advantages to using a SBSP system instead of terrestrial solar technology. Due to the large radius of a geo-synchronous orbit, SBSP systems are exposed to the sun almost all the time, and therefore have an operating capacity of 95%+. In Fig. 2 the approximate shadow from earth is calculated in the worst case scenario, which is a satellite orbiting the equator, and only 18° of its orbit is shadowed. The nearly continuous exposure to sunlight differentiates SBSP systems from terrestrial based solar systems in that it is able to provide baseload power instead of peaking power.  SBSP systems are also theoretically able to use less land for an equal amount of solar energy than terrestrial solar systems. One significant downside to terrestrial systems is they shade the ground, which precludes usage of land for farming purposes. On the contrary rectenna arrays used in an SBSP system allow greater than 90% of ambient light to pass through. 
Generally people are skeptical or fearful of SBSP systems because of concerns for safety and weaponization issues. The NSSO studied these issues and concluded that they can easily be mitigated and hence avoid any significant national or international legal issues as long as care is taken in designing the SBSP system. They also observed that there is significant global interest in promoting the peaceful use of space and that a number of other large countries (India, China, Russia) have already expressed interest in SBSP. Weaponization of an SBSP system would be extremely difficult, because the geostationary orbit radius is so large that RF beams diverge and dramatically decrease in power concentration by the time they reach earth. The systems can also be opened to international inspection and can be easily checked that they are not weaponizable. The fundamental limit on how large the peak intensity I of a simple antenna can be is
where D is the antenna diameter, λ is the transmission wavelength, A is the area of the transmitting antenna, and P is the total power of the transmitting antenna. Note that the intensity scales inversely with the separation squared. This intensity may be exceeded using a phased array antenna, however the antennas could easily be checked during international inspections for this capability. The NSSO also observed that because the conversion of microwave beams are high (50%-70%) they can be beamed at densities lower than sunlight and still deliver sufficient energy per area of land, which effectively removes any safety issues. [1,4]
By far the most significant issues blocking the development of SBSP, as with most sustainable energy technologies, is the cost. It's very difficult to nail down the precise cost, but we can use some numbers provided by Solaren to show that the extreme lower limit estimate of cost today is still very high. Solaren's goal is to install a 200 megawatt system. Before we even look at the massive engineering costs associated with building the largest structure in space, all we need to do is look at the cost to launch such a system into geosynchronous orbit. Solaren's director for energy services, Cal Boerman, has stated that five heavy lift launches, at 25 tons a piece, would be required to build the SBSP system.  The Center for Strategic and Budgetary Assessments released a report showing that the approximate cost to launch a satellite into GSO is about $10,000 $/pound.  This means that the launch cost alone for Solaren's project will be $2.5 billion or $12.5/watt. The engineering, material, and fabrication cost for the system itself are also likely to be very expensive and increase this amount considerably. In contrast terrestrial photovoltaics today cost only approximately $2-3/watt. The NSSO predicts that SBSP systems will not be commercially viable until a launch cost of $200/pound is achieved which is a massive reduction in launch costs. They also outlined what they feel is an aggressive 10-year plan to build a 10 megawatt demonstration system and it would cost $10 billion, or $1000/watt! 
SBSP systems are a very interesting and futuristic concept. They would be a monumental and inspiring achievement if one were actually built, but I don't see it happening any time soon. Solaren's goal of launching a 200 megawatt system into space by 2016 and actually providing electricity to PG&E without going bankrupt seems borderline crazy. They're rather tight lipped about the details of their plans though, so it's always possible they have an ace up their sleeve. Also, while I'm confident that all national and international legal and social issues can be overcome, I wouldn't be surprised if it takes much longer than 6 years considering the magnitude of this project.
© 2010 Daniel C. Riley. 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.
 "Space-Based Solar Power As an Opportunity for Strategic Security," Report to the Director, National Security Space Office, Interim Assessment, October 2007.
 A. Boyle, "PG&E Makes Deal for Space Solar Power," MSNBC, 13 Apr 09.
 J. O. McSpadden and J. C. Mankins, "Space Solar Power Program and Microwave Wireless Power Transmission Technology," IEEE Microwave Magazine 3, No. 4, 46 (2002).
 W. C. Brown, "Beamed Microwave Power Transmission and Its Application to Space," IEEE Trans. Microwave Theory and Techniques 40, 1239 (1992).
 B. D. Watts, "The Military Use of Space: A Diagnostic Assessment," Center for Strategic and Budgetary Assessments, February 2001.