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NASA eyes solar energy collectors in space by 2015

Monday, July 12, 1999

By Byron Spice, Science Editor, Post-Gazette

Solar panels have been used to help power spacecraft since the earliest days of the space age, and the dream of someday using spacecraft to harness the sun's rays for use on Earth has existed at least as long.

 
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The National Aeronautics and Space Administration studied the concept extensively in the 1970s before rejecting space-based solar power stations as too costly and impractical. But NASA recently dusted off those old plans and is taking a fresh look at the concept.

As now envisioned, each "Sun Tower" might stretch 22 miles in length, with pairs of 100- to 200-meter diameter solar collectors arranged along a backbone. From its perch in geosynchronous orbit, a Sun Tower would beam 1.2 billion watts of power to ground stations via microwave -- enough electricity for 1.2 million homes and 40 percent more than generated by either of the Beaver Valley Nuclear Power Stations.

A pollution-free source of electricity has always been enticing, but the renewed interest in building such gargantuan space facilities can be traced to new technologies that have developed over the past 20 years, said Joe Howell, space solar power project manager at Marshall Space Flight Center in Huntsville, Ala. It now appears possible to slash the weight of the facility by replacing metal structure with plastics and composite materials, boost the efficiency of solar cells and both assemble and operate the Sun Towers autonomously.

When space solar power was studied in the 1970s, NASA assumed the power plants would be constructed by astronauts, Howell said. But, like communications satellites, they would need to be located about 22,300 miles above Earth, in a geosynchronous orbit -- staying above a fixed spot on the globe -- which would be well beyond the reach of the space shuttle.

Moreover, the facilities would be as delicate as they are gigantic, said William "Red" Whittaker, director of Carnegie Mellon University's Field Robotics Center. For instance, the solar collectors, each bigger than a football field, would be made of thin polymer film stretched across a frame. The collectors would serve as lenses, supporting concentric prisms that would concentrate sunlight on a small but highly efficient solar cell.

Both the high orbit and the fragility make it important that the facility be built and maintained by robots, Whittaker and Howell said. A Carnegie Mellon team headed by Whittaker and James Osborn is designing three-limbed, stick-like walking robots that would become part of the structure, drawing their power from it even as they build, inspect and maintain it.

"In many ways, robots of the kind we're talking about are absolutely enabling for space solar power," Whittaker said.

Carnegie Mellon is one of 23 organizations that have received NASA contracts totalling $6.4 million to develop the solar power system. With the nine-month program now getting under way, the contractors will meet for the first time this week in Huntsville.

Other contractors include such aerospace giants as Boeing and Lockheed Martin and academic institutions such as Texas A&M University and the University of Illinois at Chicago. "There are some rock-solid players," Whittaker said. "If we are to proceed with space exploration, we've got to master this remote construction of all kinds of facilities, not just solar power facilities."

Howell said NASA hopes to demonstrate whether solar power stations are feasible by 2015. Future contracts will depend on the findings and progress achieved over the next nine months.

The Carnegie Mellon robots would be similar to an earlier Carnegie Mellon design called the Self-Mobile Space Manipulator, which was designed for space-based construction and was sponsored by a Japanese construction firm. But the new Skywalkers, each about twice the size of a human, would have to be designed to move without shaking or damaging the delicate Sun Towers. They also would be designed to work together in teams; using two of their three limbs to attach themselves to the structure, they could each contribute their remaining manipulator arm to a common task, if necessary.

Carnegie Mellon's $450,000 award is one of four demonstration projects included in the solar power program and will result in construction of a small prototype.

Ground-based photoelectric cells are used in a number of niche applications, often providing power to remote locations beyond existing electrical power grids. Outer space would offer two obvious advantages: unlimited space and sunlight unobstructed by weather, clouds or the day/night cycle. It also has some obvious disadvantages, including the need to transmit power over long distances.

Plans now call for the orbiting stations to transmit power to Earth, to the moon or other destinations by microwave beams. Energy losses would be minimal, Howell said, but the beams would have to be very large so that the energy densities are limited to safe levels. Otherwise, the microwaves might cook birds, planes or anything else they struck in the atmosphere.

As a result, the receiving antennas, or rectennas, on Earth would have to be 10 to 15 miles in diameter. An alternative is to transmit the energy in the form of laser light, but NASA has opted against lasers because of the dangers of the beam and perceptions that they might serve as weapons, Howell said.

Japan's National Aerospace Laboratory and Tohoku University, which also have begun studying space solar power plants, have proposed a combined system that would use lasers to transmit power to the atmosphere, where the laser beams would be intercepted by airships floating 12 miles high, and transformed into microwaves that could be beamed to the ground.

Japanese researchers, who have suggested that solar power stations might become a reality between 2015 and 2050, say this type of solar power eventually might fulfill all of the island nation's energy needs.

Howell said one big question is how much such facilities will cost. He's guessing $1 billion or $2 billion is a possible price tag for the 1.2-gigawatt plants.

"We're trying to get the cost to 5 cents per kilowatt hour," he said, close to the 3 or 4 cents per kilowatt commonly paid for conventional electrical power. "We haven't quite got there yet."



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