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Space-based solar power (SBSP)
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ABSTRACT
SBSP would differ from current solar collection methods in that the means used to collect energy would reside on an orbiting satellite instead of on Earth's surface. Some projected benefits of such a system are:
• Higher collection rate: In space, transmission of solar energy is unaffected by the filtering effects of atmospheric gases. Consequently, collection in orbit is approximately 144% of the maximum attainable on Earth's surface.
• Longer collection period: Orbiting satellites can be exposed to a consistently high degree of solar radiation, generally for 24 hours per day, whereas surface panels can collect for 12 hours per day at most.[1]
• Elimination of weather concerns, since the collecting satellite would reside well outside of any atmospheric gasses, cloud cover, wind, and other weather events.
• Elimination of plant and wildlife interference.
• Redirectable power transmission: A collecting satellite could possibly direct power on demand to different surface locations based on geographical baseload or peak load power needs.
Besides the cost of implementing such a system, SBSP also introduces several new hurdles, primarily the problem of transmitting energy from orbit to Earth's surface for use. Since wires extending from Earth's surface to an orbiting satellite are neither practical nor feasible with current technology, SBSP designs generally include the use of some manner of wireless power transmission. The collecting satellite would convert solar energy into electrical energy on board, powering a microwave transmitter or laser emitter, and focus its beam toward a collector (rectenna) on the Earth's surface. Radiation and micrometeoroid damage could also become concerns for SBSP.
History
In 1941, science fiction writer Isaac Asimov published the science fiction short story "Reason", in which a space station transmits energy collected from the sun to various planets using microwave beams.
The SBSP concept, originally known as Satellite Solar Power System (SSPS), was first described in November 1968.[2] In 1973 Peter Glaser was granted U.S. patent number 3,781,647 for his method of transmitting power over long distances (e.g., from an SPS to Earth's surface) using microwaves from a very large antenna (up to one square kilometer) on the satellite to a much larger one, now known as a rectenna, on the ground.[3]
Glaser then was a vice president at Arthur D. Little, Inc. NASA signed a contract with ADL to lead four other companies in a broader study in 1974. They found that, while the concept had several major problems – chiefly the expense of putting the required materials in orbit and the lack of experience on projects of this scale in space – it showed enough promise to merit further investigation and research.[4]
SERT
In 1999, NASA's Space Solar Power Exploratory Research and Technology program (SERT) was initiated for the following purposes:
• Perform design studies of selected flight demonstration concepts.
• Evaluate studies of the general feasibility, design, and requirements.
• Create conceptual designs of subsystems that make use of advanced SSP technologies to benefit future space or terrestrial applications.
• Formulate a preliminary plan of action for the U.S. (working with international partners) to undertake an aggressive technology initiative.
• Construct technology development and demonstration roadmaps for critical Space Solar Power (SSP) elements.
SERT went about developing a solar power satellite (SPS) concept for a future gigawatt space power system, to provide electrical power by converting the Sun’s energy and beaming it to Earth's surface, and provided a conceptual development path that would utilize current technologies. SERT proposed an inflatable photovoltaic gossamer structure with concentrator lenses or solar heat engines to convert sunlight into electricity. The program looked both at systems in sun-synchronous orbit and geosynchronous orbit.
Advantages
The SBSP concept is attractive because space has several major advantages over the Earth's surface for the collection of solar power.
• There is no air in space, so the collecting surfaces could receive much more intense sunlight, unobstructed by weather. In space, transmission of solar energy is unaffected by the filtering effects of atmospheric gasses. Consequently, collection in orbit is approximately 144% of the maximum attainable on Earth's surface.
• A satellite could be illuminated over 99% of the time, and be in Earth's shadow on only 75 minutes per night at the spring and fall equinoxes.[29] Orbiting satellites can be exposed to a consistently high degree of solar radiation, generally for 24 hours per day, whereas surface panels can collect for 12 hours per day at most.[1]
• Relatively quick redirecting of power directly to areas that need it most. A collecting satellite could possibly direct power on demand to different surface locations based on geographical baseload or peak load power needs.
• Elimination of weather concerns, since the collecting satellite would reside well outside of any atmospheric gasses, cloud cover, wind, and other weather events.
• Elimination of plant and wildlife interference.
Disadvantages
The SBSP concept also has a number of problems.
• The space environment is hostile; panels suffer about 10 times the degradation they would on Earth.[30] System lifetimes on the order of a decade would be expected, which makes it difficult to produce enough power to be economical.
• Space debris are a major hazard to large objects in space, and all large structures such as SBSP systems have been mentioned as potential sources of orbital debris.[31]
• The broadcast frequency of the microwave downlink (if used) would require isolating the SBSP systems away from other satellites. GEO space is already well used and it is considered unlikely the ITU would allow an SPS to be launched
Laser power beaming
Laser power beaming was envisioned by some at NASA as a stepping stone to further industrialization of space. In the 1980s, researchers at NASA worked on the potential use of lasers for space-to-space power beaming, focusing primarily on the development of a solar-powered laser. In 1989 it was suggested that power could also be usefully beamed by laser from Earth to space. In 1991 the SELENE project (SpacE Laser ENErgy) had begun, which included the study of laser power beaming for supplying power to a lunar base. The SELENE program was a two-year research effort, but the cost of taking the concept to operational status was too high, and the official project ended in 1993 before reaching a space-based demonstration.[40]
In 1988 the use of an Earth-based laser to power an electric thruster for space propulsion was proposed by Grant Logan, with technical details worked out in 1989. He proposed using diamond solar cells operating at 600 degrees to convert ultraviolet laser light.
Orbital location
The main advantage of locating a space power station in geostationary orbit is that the antenna geometry stays constant, and so keeping the antennas lined up is simpler. Another advantage is that nearly continuous power transmission is immediately available as soon as the first space power station is placed in orbit; other space-based power stations have much longer start-up times before they are producing nearly continuous power.
A collection of LEO (Low Earth Orbit) space power stations has been proposed as a precursor to GEO (Geostationary Orbit) space-based solar power.[41]
Earth-based receiver
The Earth-based rectenna would likely consist of many short dipole antennas connected via diodes. Microwaves broadcasts from the satellite would be received in the dipoles with about 85% efficiency.[42] With a conventional microwave antenna, the reception efficiency is better, but its cost and complexity is also considerably greater. Rectennas would likely be multiple kilometers across.
In space applications
A laser SBSP could also power a base or vehicles on the surface of the Moon or Mars, saving on mass costs to land the power source. A spacecraft or another satellite could also be powered by the same means.[43][44]
Dealing with launch costs
One problem for the SBSP concept is the cost of space launches and the amount of material that would need to be launched.
Reusable launch systems are predicted to provide lower launch costs to low Earth orbit (LEO).[45][46]
Much of the material launched need not be delivered to its eventual orbit immediately, which raises the possibility that high efficiency (but slower) engines could move SPS material from LEO to GEO at an acceptable cost. Examples include ion thrusters or nuclear propulsion.
Power beaming from geostationary orbit by microwaves carries the difficulty that the required 'optical aperture' sizes are very large. For example, the 1978 NASA SPS study required a 1-km diameter transmitting antenna, and a 10 km diameter receiving rectenna, for a microwave beam at 2.45 GHz. These sizes can be somewhat decreased by using shorter wavelengths, although they have increased atmospheric absorption and even potential beam blockage by rain or water droplets. Because of the thinned array curse, it is not possible to make a narrower beam by combining the beams of several smaller satellites. The large size of the transmitting and receiving antennas means that the minimum practical power level for an SPS will necessarily be high; small SPS systems will be possible, but uneconomic.
On the Moon
David Criswell suggests the Moon is the optimum location for solar power stations, and promotes lunar solar power.[53][54] The main advantage he envisions is construction largely from locally available lunar materials, using in-situ resource utilization, with a teleoperated mobile factory and crane to assemble the microwave reflectors, and rovers to assemble and pave solar cells,[55] which would significantly reduce launch costs compared to SBSP designs. Power relay satellites orbiting around earth and the Moon reflecting the microwave beam are also part of the project. A demo project of 1 GW starts at $50 billion.[56] The Shimizu Corporation use combination of lasers and microwave for the lunar ring concept, along with power relay satellites.[57][58]
From an asteroid
Asteroid mining has also been seriously considered. A NASA design study[59] evaluated a 10,000 ton mining vehicle (to be assembled in orbit) that would return a 500,000 ton asteroid fragment to geostationary orbit. Only about 3,000 tons of the mining ship would be traditional aerospace-grade payload. The rest would be reaction mass for the mass-driver engine, which could be arranged to be the spent rocket stages used to launch the payload. Assuming that 100% of the returned asteroid was useful, and that the asteroid miner itself couldn't be reused, that represents nearly a 95% reduction in launch costs. However, the true merits of such a method would depend on a thorough mineral survey of the candidate asteroids; thus far, we have only estimates of their composition.[60] One proposal is to capture the asteroid Apophis into earth orbit and convert it into 150 solar power satellites of 5 GW each.[61]