Lasers challenge microwaves for power-relay system

A Congressional hearing in September examined a possible use of lasers that ordinarily is not on the minds of lawmakers: transmitting electrical power to distant corners of Earth. At the hearing, the Space and Aeronautics Subcommittee of the House of Representatives Science Committee examined the technical prospects for using satellites to either collect solar power in orbit and beam it to Earth or to serve as relay stations for power being sent from one point on the planet to another.

In general, engineers have envisioned using microwaves rather than lasers for relaying power in this fashion. But microwave transmission may not be feasible or cost-effective because such a system would require large ground installations and reflecting satellites, John C. Mankins, manager of advanced concepts studies at NASA, told the committee. It's a very severe challenge, but using lasers may be a more practical idea," he said.

That position was endorsed even more strongly by Jerry Grey, director of aeronautics and science policy for the American Institute of Aeronautics and Astronautics (AIAA; Reston, VA). Microwaves may prove impractical because of the possibility that such beams could interfere with satellites such as communications satellites that orbit at the same altitude.

The AIAA recently completed an independent review of NASA's Space Solar Power Exploratory Research and Technology Program—31 projects that have been addressing technical issues in space-power research since 1999, totaling about $22 million. The AIAA's review, to be released this fall, found that lasers have emerged as a serious competitor to microwaves, Grey said.

"Laser technology is advancing rapidly, the technical risk does not appear excessive, and the use of lasers resolves many of the concerns with the more-mature microwave transmission technology," he told the subcommittee. "The now-promising use of laser power transmission opens a new vista on the satellite relay concept."

A power-relay system would include uplink stations that would convert electricity into laser beams and aim them skyward. Mirrors would orbit about 22,000 miles above Earth in a stationary position over the equator; because they would not appear to move, the uplink lasers would not have to change the direction in which they pointed. The mirrors would reflect the lasers back toward a photovoltaic solar-power plant on Earth's surface, where the beam would be converted back into electricity for use in the area.

Grey said that such a system probably would use a laser wavelength of about 1 µm—a wavelength that poses little risk to the human retina and skin. However, that wavelength is unable to penetrate clouds or rain, he said.

System difficulties

But, although the system sounds simple, there are hurdles, Grey told the committee. One is the low efficiency of the technology, which would likely run at 20% or less using current solid-state laser technology, although he feels that efficiencies are likely to improve.

Another hurdle is the fact that the system would have to incorporate multiple downlink sites because of the possibility that any particular site might be blocked from receiving the laser beams due to clouds or rain. (However, the extra downlink sites would not lay wasted during time when the laser downlink was being directed to other locations, Grey said. If the weather were clear, the receiving sites could simply generate electricity from conventional sunlight, as any photovoltaic installation could.)

Rep. Vernon Ehlers (R-MI), the only physicist in Congress, pointed out another possible problem with a space-based power-relay system: the prospect that a terrorist group or rogue nation might attack the orbiting mirrors with a rocket. "They could really incapacitate our nation very easily if we were dependent on a lot of space-based power," Ehlers said.

Nevertheless, Ehlers declared that laser transmission is "clearly a better way" of moving electricity. Told by Grey that photovoltaic installations covering a square, 100 miles on each side, could supply Earth's entire demand for electricity, Rep. Ehlers replied: "The figures are amazing, but the capital investment is huge."

And that may be the biggest hurdle of all for the program: finding a way to foot the bill. No one at the hearing offered a price tag for a laser-based power-relay system. Simply getting the mirrors into space would cost a fortune, whether using expendable rockets or the space shuttle.

"Let's write the check," an enthusiastic Rep. Dana Rohrabacher (R-CA), the subcommittee chairman, joked at the conclusion of the hearing. Of course, getting money for an expensive new project is never so easy. But the congressman suggested that he will continue to push for power-relay systems and said he was impressed by the potential offered by laser technology. "It just seems to me that we should move forward with this," Rep. Rohrabacher said. "We need to keep our eye on this idea."