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|1. Excerpts from 'Beam it Down', by Martin I Hoffert and Seth D Potter; in SPAN, March/April 1998 issue; page 25.|
Satellites can capture vast amounts of solar energy and transfer it to Earth on uninterrupted beams of microwaves. Thanks to fleets of low orbiting communications satellites to be launched in the coming decade, the authors say, solar power may be the energy source of the 21st century.
In outer space, the Sun always shines brightly. No clouds block solar rays, and there is no nighttime. Solar collectors mounted on an orbiting satellite would thus generate power 24 hours per day, 365 days per year. If this power could be relayed to Earth, then the world's energy problems might be solved forever.
Satellite power satellites (SPS) were originally proposed as a solution to the oil crises of the 1970s by Czech American engineer Peter Glaser, then at Arthur D Little Inc., at Cambridge, Massachusetts. Glaser imagined 50 sq. km. arrays of solar cells deployed on satellites orbiting 36,000 kilometers above fixed points along the equator. A satellite at the "geosynchronous" altitude takes 24 hours to orbit Earth and thus remains fixed over the same point on Earth all the time.
The idea was elegant. Photovoltaic cells on a satellite would convert sunlight into electrical current, which would in turn, power an onboard microwave generator. The microwave beam would travel through space and the atmosphere. On the ground, an array of rectifying antennas, or "rectennas", would collect these microwaves and extract electrical power, either for local use or for distribution through conventional utility grids.
The technology, as originally envisioned, posed daunting technical hurdles. Transferring electrical power efficiently from a satellite in geosynchronous orbit would require a transmitting antenna on the ground about 10 kilometers in diameter. A project of this scale boggles the mind; government funding agencies shied away from investing immense sums in a project whose viability was so unclear. The National Aeronautics and Space Administration (NASA) and the US Department of Energy, which had sponsored preliminary design studies, lost interest in the late 1970s.
In the last few years, however, the communications industry has announce satellite projects that suggest the time has come to revisit the SPS idea. By early in the next century, swarms of communications satellites will be orbiting the Earth at low altitude, relaying voice, video and data to the most remote spots on Earth. These satellites will relay communication signals to Earth on beams of microwaves. The transmission of electrical power with a beam of microwaves was demonstrated as early as 1963, and power and data along the same microwave beam is well within the state of the art. Why not use the same beam to carry electrical power?
The new communications satellites will orbit at an altitude of only few hundred miles. Instead of hovering above on a spot on the Equator, low orbiting satellites zip around the globe in as little as 90 minutes, tracing paths that oscillate about the Equator, rising and dipping as many as 86 degrees of latitude. Because they are closer to the Earth's surface, the solar collectors on the satellite can be a few hundred meters across rather than 10 km. And because the microwave beams they generate would spread out much less than those rom geosynchronous satellites, the ground rectennas could be correspondingly smaller and less expensive as well. By piggybacking onto these fleets of communications satellites - and taking advantage of their microwave transmitters and receivers, ground stations and control systems - solar power technology can become economically viable.
Low earth orbit poses its
own difficulties, though. Because they whip around the planet so quickly,
low orbiting satellites must possess sophisticated computer-controlled
systems for adjusting the aim of the microwave beam so that it lands at
the receiving station. These satellites will have to use sophisticated
electronic systems, called phased arrays, to continuously retarget the
Energy for Development
Dependence on fossil fuels is not the answer because burning coal, oil and gas will pour carbon monoxide into the atmosphere, raising the risk of global climate change. (And of course these resources will not last forever.) Nuclear Fission reactors avoid the greenhouse problem but introduce the so-far intractable problem of disposing of nuclear waste. Controlled nuclear fusion might someday provide an inexhaustible supply of clean energy - but after 40 years of continuous funding, a practical fusion reactor is still not in sight.
That leaves the menu of renewable energy sources. But terrestrial renewables pose environmental problems because their relatively large land requirements. Hydropower, the most exploited renewable thus far, has significantly disrupted ecosystems and human habitats. Solar, biomass, and wind farms would similarly compete with people, agriculture and natural ecosystems for land were they the basis of a global energy system.
Moreover, ground-based renewable energy systems, such as terrestrial photovoltaics and biomass fuels, generate fewer than 10 watts of electricity per sq. meter, on a continuous basis. To generate enough electricity to meet demand could require developing countries either to divert land from agricultural use, and thus diminish the supply of food, or to destroy natural ecosystems, a move that could hasten the onset of global warming.
Solar power satellites would require far less land to generate electricity. Each sq. meter of land devoted to the task could yield as much as 100 watts of electricity. And the power-receiving rectenna arrays - a fine metallic mesh - would be visually transparent, so their presence would not interfere with crop growth or cattle grazing.
And the flow of power from terrestrial renewables is intermittent. Clouds blot out the Sun, the wind stops blowing; lack of rainfall nullifies a hydro generator. Because these technologies do not deliver power continuously, they require some means of storing energy, adding to overall cost and complexity. A network of solar power satellites in low Earth orbit could provide power to any spot on Earth on virtually continuous basis because at least one satellite will always be in "view" of the receiving station ...
Looking for a cheap launch
Launch costs are likely to drop, however, as the demand increase for hoisting large volumes of material into space on a regular basis: the more frequently a launch system is used, the lower the cost per use. Moreover, NASA is seeking a new generation of reusable launch vehicles. The agency recently sponsored a competition among aerospace contractors for a space vehicle with the potential for airline like operation. The winner was Lockheed Martin Skunk Works, legendary innovators in aircraft design from the U-2 to the Stealth fighter. Lockheed Martin plans to to build and test the $1 billion wedge-shaped reusable X-33 - a one-half size, one-eighth mass version of a launch vehicle called Venture Star that would replace the space shuttle for ferrying cargo into low orbit. The target launch cost is $2,200 per kilogram - one-tenth that of a shuttle launch. At that price, space power could become cost-effective if satellites pull double-duty as communications relays and solar-power sources.
A solar power satellite should quickly pay back the energy needed to put it into orbit. Start with the conservative assumption that solar power satellite technology would produce 0.1 kw of electricity on the ground per kilogram of mass in orbit. In that case, the energy expenditure of 10 kw-hours per kilogram to lift the satellite into orbit would repaid in electricity after 100 hours - less than five days.
One way to keep launch costs down is to use an inflatable structure as the solar collector. Doing so would maximize the collector's surface area - important to gathering the greatest amount of solar energy - without imposing a major weight burden on the launch vehicle. Deflated solar collectors could be folded into a compact space on board the spacecraft; once in orbit, gas from a pressurized container would inflate the structure.
Balloon's in space are an old story. In fact, the 1960 vintage satellite known as Echo I was a balloon used to bounce radio waves back to Earth. NASA is now studying the feasibility of inflatable structures in space for antennae, sunshades and solar arrays, although not explicitly for solar power satellite systems. An important experimental milestone was the successful deployment of Space Shuttle Endeavor astronauts in May 1996 of the Spartan Inflatable Antenna Experiment - a 14-meter antenna inflated by a nitrogen gas canister in orbit.
It is not such a very large step from such an experiment to a solar-collecting satellite that could be assembled in orbit from inflated segments. Were NASA to make research on inflatable space structures a high priority, the knowledge base to make cost-effective low-mass power satellites could evolve rapidly.
One Step at a Time
Scaling up to higher power levels would be straightforward, entailing simply the deployment of a larger amount of solar-collecting area in space. Power would be transmitted through the infrastructure of transmitters and receivers that will then be in place for the satellite communications systems. In this regard, microwave transmission has a decided advantage over conventional cable methods of transmitting power. A microwave system that is 80% efficient at sending 1 kW will still be 80% efficient at sending 1MW. This is fundamentally different from an electric utility transmission line, where you need thicker, and costlier, wires to carry more power. If too much power id put through a cable, it will melt the insulation.
Some fear that a network of solar power satellites could turn the atmosphere into one big microwave oven, cooking what ever wanders into the beam's path. In reality, the microwave intensities that we propose would be orders of magnitude below the threshold at which objects begin to heat up. People would be exposed to microwave levels comparable to those from microwave ovens and cellular phones. While some critics speculate that microwaves pose danger non thermal threats to human health, there is no reliable epidemiological evidence for adverse effects from microwaves at these low levels. Higher levels of microwave radiation would be found at the microwave rectennas on which the beams are focussed, but fences and warning signs could demarcate these areas of possible danger. But according to our calculations, microwave intensities even at the perimeter of the rectenna would fall within the the range now deemed safe by the U.S. Occupational Safety and Health Administration.
A bigger potential problem is that of sharing the limited frequencies in the microwave spectrum. Motorola has come under fire, for example, because its planned system will employ frequencies in the 1.616 to 1.626 gigahertz range, which almost overlaps the 1.612 gigahertz frequency that astrophysicists tune to when gathering data about the cosmos. Radio astronomers worry that interference from a solar power satellite will overwhelm the comparatively weak signals they are seeking to detect. Motorola promises to limit spillover of its communications beams into the radio astronomers' frequency niche, but the issue underscores the fact that the microwave spectrum is a limited resource jealously guarded by commercial and nonprofit users alike. Allocation of the spectrum must be addressed promptly and effectively to avoid preemption of space power technology before it's been born.
Whether SPSs become a reality will ultimately depend on the willingness of telecommunications and electric power utility companies to enter the space power business. So far, neither industry has shown much interest. But then, they are for the most part unaware of the commercial possibilities. One has to know that an option exists to choose it. Thirty years ago, communications satellites were a novelty. Ten years ago, no one had heard of the Internet.
What is certain is that the present push for deregulation has led to scramble on the part of telecommunications, computer, cable TV and utilities industries to enter each other's markets. Some electric power companies want to enter the telecommunications business as a way of capitalizing on the huge investment in wire and cable that reaches virtually every building in the country. It makes equal sense to propose that communications companies enter the power business. In practice, consortiums of power and communications companies might develop the proposed technology together.
No single piece of this technology poses a fundamental stumbling block. The physics of photovoltaic cells and microwave generation are well understood. To move to the next stage, though, will require a demonstration that all pieces of this system can work together: the solar panels, the phased-array microwave antennas; the receiving stations that separate the data signals from the power beams, an the computers that tell the satellites where on the ground to aim the beams. NASA could accelerate this development tremendously by placing into orbit a prototype of a solar power satellite.
The benefits are too large to walk away from. a network of solar power satellites such as what we propose could supply the Earth with 10 to 30 trillion watts of electrical power - enough to satisfy the needs of the human race through the next century. Such power satellites thus offer a vision in which energy production moves off the Earth's surface, allowing everyone to live on a "greener" planet. Consider the philosophical implications: no longer need humankind see itself trapped on spaceship Earth with limited resources. We could tap the limitless resources of space, with the planet preserved as a priceless resource of biodiversity.
About the Authors:
Martin I Hoffert is a professor of physics at New York University.
Seth D Potter is a research scientist in physics at New York University.
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