Solar power satellite
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 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, so it is unlikely the SPS will be able to compete with conventional sources unless some form of dramatically less expensive space transport system is constructed first.
The SPS concept has been floating 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.
Glasser's work took place at Author 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.
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
- a large antenna 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 "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 about 5 km across, and receive enough microwaves to be a concern. Some have suggested locating them offshore, but this presents problems of its own.
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 150 square kilometers of collector area thus leading to huge satellites.
"Huge" is by no means an understatement. Most designs are based on a rectangular grid some 10 km on a side, 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.
Without a doubt, the biggest problem for the SPS concept is the currently immense cost of all space launches. Current rates on the Space Shuttle run between $3,500 and $5,000 per pound ($8,000/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. Development of a vehicle that can launch 100 ton loads at less than $400/kg is likely to be necessary.
Gerrald O'Neill noted this problem in the early 1970s, and came up with the idea of building the SPS's in orbit with materials from the Moon. The costs of launch from the Moon are about 100 times lower than from Earth, due to the lower gravity. However 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. However it appears that O'Neill was more interested in coming up with a justification for his space habitat designs than any particular interest in the SPS concept on its own.
More recently the SPS concept has again been used to justify the construction of a space elevator. The elevator would make construction of an SPS considerably less expensive, possibly making them competitiv 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.
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.
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.
The SPS would also occupy very valuable geosynchronous orbit space. Only in geosynchronous orbit can a satellite remain over one spot on Earth permanently, making it able to broadcast to a fixed rectenna on the surface. However, one could mount communication equipment on the SPS itself to fill the role of any communication satellite that it displaces - with vastly greater power available for it, as well.
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.
In order to be competitive, the SPS must surmount some extremely formidable barriers. Either it must cost far 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 due to meteorite impacts makes this unlikely.
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. It 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. Night time at a terrestrial solar station reduces the amount of electricity produced by more than 50%, since no power at all is generated during the night and the Sun's angle is low in the sky during much of the day. 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 could prove to 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 not significant demand (Alternately, the power could be used on-site to produce chemical fuels for transportation and storage).
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 cheap to compete.
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.
Presentation of relevant technical background with diagrams: http://www.spacefuture.com/archive/conceptual_study_of_a_solar_power_satellite_sps_2000.shtml