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| Fig. 1: Artist's concept of a space-based solar power system. (Source: Wikimedia Commons) |
The Sun's rays contain approximately one kilowatt per square meter of energy when they reach the Earth's surface. This number varies by latitude and altitude; the Equator receives the most direct sunlight, where the poles receive very little, and a solar panel at sea level will produce measurably less energy than one positioned atop a nearby mountain. Thus, an optimally positioned photovoltaic cell would be located several thousand feet about sea level in the tropics. Yet no matter how high a solar panel is placed, large improvements in power output still remain impossible by conventional means. The Earth's atmosphere diverts or absorbs the vast majority of photons and energy dispatched by our Sun, severely handicapping any terrestrial solar energy collection method.
Numerous scientists, engineers, and environmentalists have proposed space-based solar power as an alternative to traditional terrestrial photovoltaic collection. See Fig. 1 for a basic schematic; the concept involves a large solar array in geostationary orbit, beaming energy to the ground via microwaves to a collector "rectenna." A pilot beam is also included to assist in the alignment of the satellite and collector. By placing the collector above the atmosphere, losses in solar energy due to atmospheric interference can be avoided. Moreover, the satellite's daily solar exposure time depends heavily on the orbit in which it is placed; a 24-hour daily exposure time is possible with a satellite placed far enough from Earth, though this complicates transmission. Yet a space-based solar system also presents a massive array of challenges, in the areas of configuration, cost, and physics. We will explore all three categories.
Like any satellite, a space-based solar array could be placed in multiple orbits. Low earth orbit, commonly abbreviated LEO, is much easier to reach than other orbit types. A standard LEO altitude would be around 500 km. At this altitude, a satellite zips by at 7 km/s relative to observers on the ground, appearing for only six minutes over the horizon. This makes it difficult to transfer to the ground whatever energy the satellite may have collected. Moreover, a satellite in LEO still spends a great deal of time in the Earth's shadow, nullifying a critical advantage of space-based power. Our 500 km orbit still spends about 38% of its time collecting no power, only a slight improvement over a terrestrial system. [1]
We may also consider a geostationary orbit. Geostationary orbits are significantly higher than LEO, about 42,000 km above Earth's surface, about 6.5 times Earth's radius. Such orbits are harder to achieve, and a rocket than can carry some amount of payload to LEO is only capable of carrying a fraction of that to geostationary orbit. They have the significant advantage of being over a single point at all times, making energy transfer more simple, and their distance from the Earth means that they spend only about 1% of the time in shadow. [2] It seems that the difficulty of access would be justified by the decreased technical complexity and additional power output.
The key barrier to implementation of space-based solar is the literally sky-high cost of launch. Unfortunately, information on the size and configuration of payload space on common rockets is not publicly available, as it is competition-sensitive. However, we can assume that establishing any space-based solar platform would require numerous launches of moderate-sized rockets; these range from approximately $60 million per launch for a SpaceX Falcon 9 to $200 million for a United Launch Alliance Delta IV. Present electricity rates are on the order of $0.10 per kWh; the provider would have to sell between 0.6-2 gigawatt-hours of energy just to recoup the price of launch. This is prohibitive.
An estimation of the price of space-based solar energy relies on a better understanding of the physics of light transmission through an atmosphere and the performance of terrestrial solar panels. Typical panels are collecting light about 29% of the time, due to day and night cycles, positioning, and weather. As noted above, a collection satellite in GEO is illuminated 99% of the time; our current factor of improvement is therefore 3.4. Moreover, with no atmosphere to interfere with light transmission from the sun, light intensity in orbit is 144% that available on Earth. Considering both duration and intensity, our total factor of improvement for light received per unit area is 4.92. [2] In other words, five square meters of a terrestrial panel produce the same energy as one in orbit. Given the costs, difficulty, and hazards of placing these panels in space, it is difficult to imagine widespread adoption given that the improvement is a factor of five - even before we consider losses from transmission to ground.
The factors outlined above make it clear that space-based solar power is not an immediate solution to our energy challenges. High costs and unproven technology (the microwave transmission system) stand in the way, and the increase in power generation is not nearly enough to justify the effort. However, if these challenges were miraculously overcome, we should consider that it took several Shuttle missions to launch the acre of panels that make up the International Space Station's ~100 kW generation capacity. [1] If we continue to assume $0.10/kWh and no transmission losses, this power is valued at $10,000 every hour. Four SpaceX missions, probably enough to launch the array (with the caveat that the ISS is in LEO, not GEO), cost on the order of $240 million, which could be recouped in 24,000 hours of operation - only three years to break even on launch costs, which likely dominate the cost curve. Although space-based solar presents numerous challenges for the present, it is not so far-fetched as some might believe.
Other applications of space-based power are discussed by White and Nagasawa. [3,4]
© Evan Long. 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] A. M. Delleur and T. W. Propp, "Space Station Power Generation in Support of the Beta Gimbal Anomaly Resolution," U.S. National Aeronautics and Space Administration, NASA/TM-2003-221012, January 2003.
[2] A. Atul, "A Study on Space-based Solar Power System," IOSR Journal of Environmental Science, Toxicology and Food Technology, Special Issue: Swami Shri Swaroopanand Swarsati Mahavidyalya Hudco Bhilai, Volume 1, Issue 5, Sep 2015.
[3] S. White, "Space-Based Solar Power," Physics 240, Stanford University, Fall 2013.
[4] D. Nagasawa, "Space-Based Power Arrays," Physics 240, Stanford University, Fall 2011.
