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Solar power satellite

A solar power satellite, or SPS, is a satellite built in high-orbit over the Earth that uses microwaves 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, uneffected 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.

Table of contents

History

This 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 granded 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 leveled 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:

  1. a huge solar collector, typically made up of solar cells
  2. a microwave antenna on the satellite, aimed at Earth
  3. 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 somwhat more intense sunlight, uneffected by weather. In a geosynchronous orbit an SPS would be illumininated 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 effeciency 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% effeciency. With a conventional microwave antenna the reception is even better, but the cost and complexity is considerably greater. Rectennas would be about 5km 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 5,000 and 10,000 megawatts (MW) 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 10km 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.

Problems

Without a doubt, the biggest problem for the SPS concept is the currency emense costs of all space launches. Current rates on the Space Shuttle run between $3500 and $5000 per pound, depending on who's numbers you choose to select. In either case the concept of building a structure some kilometres on a side is clearly out of the question. Development of an large expendable rocket to launch 100 ton loads at less than $400.00/kilogram are 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 it's 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, competing with other "green" sources such as nuclear power. 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. 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 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.

SPS's Economic Feasibility

Current prices for electricity on the grid fluctuate depending on time of day, but typical household delivery is about 5 cents per kilowatt hour in North America. If the lifetime of an SPS is 20 years and it delivers 5 Gigawatts at the grid, the commercial value of that power is $8,766,000,000, much less than the cost of the first unit even discounting interest on the capital costs.

In order to be competitive, the SPS has to surmount some seemingly impossible barriers. Either it must cost almost nothing to build, or it must operate for tremendous lengths of time. Many proponents have suggested that the lifetime is effectively infinite, but normal maintenance and replacement due to meteorite impacts makes this unlikely.

Perhaps the best argument against the SPS concept is that the same amount of power could be generated by building a slighyly larger array here on Earth in a sunlit area, the Sahara Desert for instance. Such a system would cost considerably less to construct, and require no advances in anything. Yet the very fact that such a project is not being carried out demonstrates the limited economic feasibility of solar power at this point in time.

Moreover any advance in construction techniques that make the SPS concept more economical are almost certain to effect a ground-based system 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.


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

Presentation of relevant technical background with diagrams: http://www.spacefuture.com/archive/conceptual_study_of_a_solar_power_satellite_sps_2000.shtml



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