A solar power satellite, or SPS, is a proposed satellite built in high Earth orbit that uses microwave power transmission to beam solar power to a very large antenna on Earth where it can be used in place of conventional power sources. The advantage to placing the solar collectors in space is the unobstructed view of the Sun, unaffected by the day/night cycle, weather, or seasons. However, the costs of construction are very high, and SPS will not be able to compete with conventional sources unless there is a big reduction in the costs of launching equipment into space, or unless a space-based manufacturing industry develops and they can be built in orbit from off-Earth materials.
History
The SPS concept has been around since late 1968, but was considered impractical due to the lack of an efficient method of sending the power down to the Earth for use. Things changed in 1974 when Peter Glaser was granted patent number 3,781,647 for his method of transmitting the power to Earth using microwaves from a small antenna on the satellite to a much larger one on the ground, known as a rectenna.
Glaser's work took place at Arthur D. Little, Inc., who employed Glaser as a vice-president. NASA then became interested and granted them a contract to lead four other companies in a broader study in 1972. 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.
Most major aerospace companies then became briefly involved in some way, either under NASA grants or on their own money, to preserve a chance at the large contracts that would have been let out had the decision been made to go ahead with this concept. At the time the needs for electricity were booming, and there seemed to be no end in demand. When power use levelled off in the 1970s, the concept was shelved.
More recently the concept has again become interesting, generally due to increased energy demands and costs. At some price point the high construction costs of the SPS become favourable due to their low-cost delivery of power, but this price point remains far higher than current rates. Nevertheless continued advances in material science and space transport continue to whittle away at the startup cost of the SPS.
Description
The SPS essentially consists of three parts:
- a huge solar collector, typically made up of solar cells
- a microwave antenna on the satellite, aimed at Earth
- an antenna occupying a large area on Earth to collect the power
The SPS concept arose because space has several major advantages over earth for the collection of solar power. There is no air in space, so the satellites would receive somewhat more intense sunlight, unaffected by weather. In a geosynchronous orbit an SPS would be illuminated over 99% of the time. The SPS would be in Earth's shadow on only a few days at the spring and fall equinoxes; and even then for a maximum of an hour and a half late at night when power demands are at their lowest. This allows expensive storage facilities necessary to earth-based system to be avoided.
In most senses the SPS concept is simpler than most power systems here on Earth. This includes the structure needed to hold it together, which in orbit can be considerably lighter due to the lack of gravity. Some early studies looked at solar furnaces to drive conventional turbines, but as the efficiency of the solar cell improved this concept eventually became impractical. In either case another advantage of the design is that waste heat is re-radiated back into space, instead of warming the biosphere as with conventional sources.
The Earth-based receiver antenna (or rectenna) is also key to the concept. It consists of a series of short dipole antennas, connected with a diode. Microwaves broadcast from the SPS are received in the dipoles with about 85% efficiency. With a conventional microwave antenna the reception is even better, but the cost and complexity is considerably greater. Rectennas would be multiple kilometers across. Crops and farm animals may be raised underneath the rectenna, as the thin wires used only slightly reduce sunlight, so the rectennas are not as expensive in terms of land as might be supposed.
For best efficiency the satellite antenna must be between 1 and 1.5 kilometers in diameter and the ground rectenna around 14 kilometers by 10 kilometers. For the desired microwave intensity this allows transfer of between 5 and 10 gigawatts of power. To be cost effective it needs to operate at maximum capacity. To collect and convert that much power the satellite needs between 50 and 100 square kilometers of collector area using standard ~14% efficiency silicon polymer solar cells, state of the art and expensive triple junction gallium arsenide solar cells with a max efficiency of 28% could reduce the power area by ~50%, but in both case the solar stations structure would be several kilometers wide as most designs are based on a rectangular grid, making it much larger than most man-made structures here on Earth. While certainly not beyond current engineering capabilities, building structures of this size in orbit has never been attempted before.
Problems
Launch costs
Without a doubt, the most obvious problem for the SPS concept is the currently immense cost of space launches. Current rates on the Space Shuttle run between $3,000 and $5,000 per pound ($6,600/kg and $11,000/kg), depending on whose numbers are used. In either case the concept of building a structure some kilometres on a side is clearly out of the question. Launch costs of less than $450/kg are necessary.
However, economies of scale on expendable vehicles could give rather large reductions in launch cost for this kind of launched mass. Thousands of rocket launches could very well reduce the costs by ten to twenty times using standard costing models. This puts the economics into the range where this system could be conceivably attempted. [1] Reusable vehicles could quite conceivably attack the launch problem as well; but are not a well developed technology.
To give an idea of the scale of the problem, assuming a typical solar panel mass of 20kg per kW, and without considering the mass of the support structure, antenna or significant mass reduction of focusing mirrors, a 4GW power station would weigh ~80,000 metric tons. This would require 1600 space shuttle launches in order to move the materials to lower earth orbit (how to get it to high earth orbit is another question). Very lightweight designs could achieve 1kg per kW[2], or 4000 metric ton 4GW station, or about 80 space shuttle launches. With the extremely light weight design it would cost roughly 40 billion dollars to launch to low orbit and a variable percentage more to manufacture the structure, move to high earth/GEO orbit and assemble.
Gerard O'Neill, noting the problem of high launch costs in the early 1970s, came up with the idea of building the SPS's in orbit with materials from the Moon. Launch costs from the Moon are about 100 times lower than from Earth, due to the lower gravity. This concept only works if the number of satellites to be built is on the order of several hundred; otherwise, the cost of setting up the production lines in space and mining facilities on the Moon are just as huge as launching from Earth in the first place. O'Neill was probably more interested in coming up with a justification for his space habitat designs than any particular interest in the SPS concept on its own.
Asteroid mining has also been seriously considered. A NASA design study produced a 10,000ton mining vehicle to be assembled in orbit that would return a 500,000 ton asteroid 'fragment' to geostationary orbit. Only about 3000tons of the minding ship would constitute traditional aerospace-grade payload. The rest would be reaction mass for the mass-driver engine; which could easily consist of the spent rocket stages used to launch the payload. Assuming that 100% of the returned asteroid was useful, and that the asteroid miner couldn't be reused, that represents nearly a 95% reduction in launch costs. The true merits of such a method would depend on a through mineral survey of the candidate asteriods. Once built, NASA's CEV should be capable of beginning such a survey, Congressional money and imagination permitting.
More recently the SPS concept has been suggested as a use for a space elevator. The elevator would make construction of an SPS considerably less expensive, possibly making them competitive with conventional sources. However it appears unlikely that even recent advances in materials science, namely carbon nanotubes, can reduce the price of construction of the elevator enough in the short term.
Safety
The use of microwave transmission of power has been the most controversial item concerning SPS development, but the incineration of anything which strays into the beam's path is an extreme misconception. The beam's most intense section (the center) is far below the lethal levels of concentration even for an exposure which has been prolonged indefinitely. Furthermore, the possibility of exposure to the intense center of the beam can easily be controlled on the ground and an airplane flying through the beam surrounds its passengers with a protective layer of metal, which will intercept the microwaves. Over 95% of the beam will fall on the rectenna. The remaining microwaves will be dispersed to low concentrations well within standards currently imposed upon microwave emissions around the world. However, most people agree that further research needs to be done on the effects of these stray microwaves upon the environment. Likewise, more research upon the effects of microwave transmission has upon the atmosphere needs to be carried out extensively.
The intensity of microwaves at ground level that would be used in the center of the beam can be designed into the system, but is likely to be comparable to that used by mobile phones. The microwaves must not be too intense to avoid injury to wildlife, particularly birds. Experiments with deliberate irradiation with microwaves at reasonable levels have failed to show any negative effects even over multiple generations.
Some have suggested locating rectennas offshore, but this presents problems of its own.
A commonly proposed approach to ensuring fail-safe beam targeting is to use a retrodirective phased array antenna/rectenna. A "pilot" microwave beam is emitted from the center of the rectenna on the ground to establish a phase front at the transmitting antenna, where circuits in each of the antenna's subarrays compare the pilot beam's phase front with an internal clock phase to use as a reference to control the phase of the outgoing signal. This allows the transmitted beam to be centered precisely on the rectenna and to have a high degree of phase uniformity, but if the pilot beam is lost for any reason (if the transmitting antenna is turned away from the rectenna, for example) the phase control system fails and the microwave power beam is automatically defocused. Such a system would be physically incapable of focusing its power beam anywhere that did not have a pilot beam transmitter.
It is important for the system that as much of the microwave radiation as possible is focused on the rectenna as that increases the transmission efficiency. Outside of the rectenna the microwave levels rapidly decrease, nearby towns or cities should be completely unaffected.
The long term effects of beaming power through the ionosphere in the form of microwaves has yet to be studied.
SPS's economic feasibility
Current prices for electricity on the grid fluctuate depending on time of day, but typical household delivery costs about 5 cents per kilowatt hour in North America. If the lifetime of an SPS is 20 years and it delivers 5 gigawatts to the grid, the commercial value of that power is 5,000,000,000 / 1000 = 5,000,000 kilowatt hours, which multiplied by $.05 per kWh gives $250,000 revenue per hour. $250,000 × 24 hours × 365 days × 20 years = $43,800,000,000. By contrast, in England (Oct 2005) electricity can cost 9-22 cents per kilowatt hour. Which would translate to a lifetime output of $77-$193 billion. In addition, in the case of England, the country is rather further north than even most inhabited parts of Canada, and hence receives little insolation over much of the year, so conventional solar power is not terribly competitive.
In order to be competitive, the SPS must cost no more than existing suppliers; this may be difficult especially if deployed to North America. Either it must cost less to deploy, or it must operate for a very long period of time. Many proponents have suggested that the lifetime is effectively infinite, but normal maintenance and replacement of less durable components makes this unlikely. It is certain in any case that the SPS concept is only profitable over a period of some years, as with any power generation technology.
A potentially useful concept to contrast SPS with is the constructing a ground-based solar power system that generates an equivalent amount of power. Such a system would require a large solar array built in a well-sunlit area, the Sahara Desert for instance. However, an SPS also requires a large ground structure -- the rectenna on the ground is much larger than the area of the solar panels in space. The ground-only solar array would have the advantages of costing considerably less to construct, and would require no significant technological advances.
However, such a system has a number of significant disadvantages as well. A terrestrial solar station loses out to two thirds of the solar energy, since no power at all is generated during the night and less strikes the panels when the Sun's angle is low in the sky. Some form of energy storage would be required continue providing power through the night, such as pumped storage hydroelectricity. This is both expensive and inefficient. Weather conditions would also interfere greatly with power collection, and can cause much greater wear and tear on the solar collectors than the environment of Earth orbit; a sandstorm could cause devastating damage, for example. Beamed microwave power allows one to send the power to where it is needed, while a solar generating station in the Sahara would primarily provide power to the surrounding area where there is no current significant demand. (Alternately, the power could be used on-site to produce chemical fuels for transportation and storage, for example a hydrogen economy).
Many advances in construction techniques that make the SPS concept more economical could make a ground-based system more economical as well. For instance, many of the SPS plans are based on building the framework with automated machinery supplied with raw materials, typically aluminium. Such a system could just as easily be used on Earth, no shipping required. However, it should be noted that Earth-based construction already has access to extremely cheap human labor that would not be available in space, so such construction techniques would have to be extremely competitive.
Current work
Summary of NASA's "Fresh Look" study in 2000
NASDA (Japan's national space agency) has been researching in this area steadily for the last few years. In 2001 plans were announced to perform additional research and prototyping by launching an experimental satellite of capacity between 10 kilowatts and 1 megawatt of power.
Source: http://www.space.com/businesstechnology/technology/nasda_solar_sats_011029.html
Presentation of relevant technical background with diagrams: http://www.spacefuture.com/archive/conceptual_study_of_a_solar_power_satellite_sps_2000.shtml
References
Glaser, Peter E.: Power from the Sun, Its Future, Science, vol. 162, no.3856, Nov. 22, 1968, pp. 857-861.
Solar Power Satellites (Hardback) Peter E. Glaser, Frank P. Davidson and Katinka Csigi, 654 pgs, 1998, John Wiley & Sons ISBN 047196817 X
External link
- The World Needs Energy from Space Space-based solar technology is the key to the world's energy and environmental future, writes Peter E. Glaser, a pioneer of the technology.
- Space-Based Solar Power Efforts A synopsis of proposals and feasibility of lunar and high-orbital solar power stations, including assessments of cost.
- Japan's plans for a Solar Power Station in Space - the Japanese government hopes to assemble a space-based solar array by 2040.