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| Fig. 1: SunTower Design Concept. (Courtesy of NASA. Source: Wikimedia Commons) |
The concept of space based solar power is the idea of collecting solar energy in space and wirelessly transmitting it back to Earth for distribution. The concept of space solar power is not a new one. NASA and the US Department of Energy engaged in a space solar study project in the 70s with the goal of enabling future development of large megawatt systems for wireless power transmission for both government missions as well as commercial distribution. [1] In 1979, NASA had established three working theoretical models, all of which used microwave power transmission to wirelessly transmit energy collected in space to earth for use. Ultimately, the conclusion of the study was that although it was feasible, it was expensive and therefore not economic. [2]
There are four primary reasons that Space-based Solar Power is being reconsidered today. First, the cost of solar power technology has significantly decreased since 1980. Second, the projected global market growth for electricity is increasing rapidly. Third, in the face of global climate change with carbon intensive alternatives, solar power both space and terrestrial is incredibly attractive as a cheap source of renewable power. [1,2] Finally, technological advancements in wireless power transmission, intelligent robotics, improved power management with robust distribution and control have all contributed to making a project like SSP more feasible. [3]
There are three primary components to successful wireless power transmission: signal transmitters, microwave beam control, and rectifying antennas.
Transmitters: The transmitters need to do two things: they need to efficiently convert DC power from the solar collectors into RF power and then be able to radiate it with minimal losses and maximal control. In the case of space solar power, the wireless power transmission would be large scale, requiring a phased array antenna in order to distribute the radio frequency power across the aperture. The efficiency of the transmitter is not only important for end-to-end WPT efficiency but also for temperature management which is very difficult in space. [3]
Beam Control: A hugely important aspect of the safety of wireless power transmission is the control of the power beam. The most preferred method has been to use retro-directive beam control to achieve accurate pointing. [3]
Rectennas: Rectifying antennas are receivers used to transmit electromagnetic energy into DC current. During the 1979 study it was estimated that the rectifying antenna would be approximately 4 kilometers in diameter. [1] That number in today's estimates has increased to about 7.5 kilometers with a transmitter of about 500 meters due to increased technological capability. [3] It would also be connected directly to a power substation that can convert the DC current to AC current using the same methods that are used for terrestrial solar power and distributed throughout the grid.
With approximated sizes of 500 meter transmitter aperture and 7.5 kilometer receiver, the total end-to-end efficiency of the power transmission over 36,000 km is estimated to be around 45%. [3]
SunTower: The SunTower concept is designed like a long tower configuration with RF- transmitting solar power collectors on either side, resembling the appearance of a stem with leaves or petals on either side; see Fig 1. The bottom of the tower closest to earth contains the microwave transmitter necessary for transmitting the RF frequency to Earth. This configuration would be sun-synchronous, initially deployed into lower Earth orbit and then subsequently migrate into an elliptical Earth orbit. [1,2]
SolarDisc: The SolarDisc concept is designed as one large disc containing many RF solar collectors, approximately 3-6 kilometers in diameter. The disc will be sun-synchronous and will require in space infrastructure for system deployment. The disc contains the microwave transmitter necessary for transmitting the RF frequency to earth at its center. [2]
There are a number of potential advantages of the deployment of space-based solar power, most importantly of which is the opportunity to collect more direct solar power. This is due to the ability to collect 24 hours a day and that the sunlight does not have to diffuse through atmospheric particles in space, which currently accounts for a significant portion of solar energy that we lose on Earth. While there is significant potential, there are a number of technological hurdles that need to be addressed. In particular, the feasibility and safety of large wireless transmission needs to be further studied. However, if space-based solar power were feasible, it could have a large environmental impact. As global energy demand continues to rise, we need to search for as much renewable energy adoption as possible. While the land usage for the ground receivers may be large, the land and water consumed by terrestrial solar power is much higher while producing less energy. SSP could provide a renewable carbon free solution that provides large amounts of power with low land and water use.
© Eva Wallack. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. 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.
[1] H. F. Feingold and C. Carrington, "Evaluation and Comparison of Space Solar Concepts," Acta Astronaut. 53, 547 (2003).
[2] J. C. Mankins, "A Fresh Look at Space Solar Power: New Architectures, Concepts and Technologies," Acta Astronaut. 41, 347 (1997).
[3] J. O. McSpadden and J. C. Mankins, "Space Solar Programs and Wireless Power Transmission Technology," IEEE Microwave Magazine 3, No. 4, 46 (2002).
