An artist's impression of what JAXA's Space Solar Power System might look like if it used laser transmission to beam solar power down to Earth.
Space solar power advocates may soon get their day in the sun, as different projects aimed at beaming energy to Earth from orbit begin to take shape. But at least one space power scientist worries that a U.S.-based project may be promising too much, too soon.
Last week, California regulators proposed a plan to approve a 15-year contract with the American company Solaren Corp. to supply space-based solar power to utility giant Pacific Gas & Electric (PG&E) by 2016. The Japan Aerospace Exploration Agency (JAXA) has also teamed up with a private Japanese coalition to design a solar space station for launch by the 2030s.
Such projects encourage scientists who dream of harnessing the sun's power directly, without the interruption of cloudy skies and Earth's day-night cycle. Marty Hoffert, a physicist at New York University and one of the staunchest supporters of space solar power, suggests that today's technologies allow space solar power to provide energy as cheaply as the usual solar panel arrays on Earth.
"The problem is that we're treating space solar power as something that has to compete with coal right now," said Hoffert, who gave a recent talk on beamed power at the New Jersey Institute of Technology. "Nothing can compete with coal."
Despite his enthusiasm, Hoffert remains skeptical of Solaren's plan. And he warns that failure to deliver could deal a life-threatening blow to the dream of space solar power.
A decision by the California Power Utility Commission on Solaren?s plan for PG&E could come as early as Thursday, according to a Dow Jones wire report.
Harnessing the sun
Solar panels in space can receive seven times more solar energy per unit than ones on Earth and don't have to deal with weather or darkness. The challenge in harnessing that energy comes from the expensive costs of launching material into space, as well as figuring out how to beam energy back down to Earth.
Microwave beaming has long been the favored delivery option for space solar power advocates. Space power stations using this method would convert the electricity generated by solar panels into radio frequency (RF) waves for beaming down to an Earth receiver several kilometers wide.
A former NASA scientist demonstrated the RF concept last year by beaming 20 watts between two Hawaiian islands ? barely enough energy to power a dim light bulb. That experiment cost just $1 million. A full-scale space solar power setup would require much bigger and more costly receivers.
Another more recent choice has arisen in the form of solid-state lasers. Such lasers now have enough power to deliver energy as a tightly focused optical beam that requires much less costly equipment in space and on the ground. But unlike RF, lasers can run into bigger problems with atmospheric interference and weather.
"Microwaves can beam through clouds, which lasers can't," Hoffert explained. "With lasers you're going to have to have receivers in desert sites that are cloud free, and maybe backup receivers in several sites."
Hoffert still favors lasers because of the lower costs required up front for a tech demonstration. By contrast, Solaren weighed its choices and decided to go with RF technology.
"Basically we chose RF because it is more efficient and has all-weather capability for the reliable delivery of electricity to our customers," said Cal Boerman, Solaren's director of energy services.
The cost of space power
Hoffert is wary of Solaren's latest step forward and the company's promise of delivering 200 megawatts to PG&E utility customers in California by 2016.
Hoffert estimates that Solaren could manage to get about 50 percent transmission efficiency in a best-case scenario, meaning that half of the energy collected by space solar panels would be lost in the transfer down to Earth.
Solaren would then need to launch a solar panel array capable of generating 400 megawatts. The total launch weight of all the equipment would be the equivalent of about 400 metric tons, or 20 shuttle-sized launches, according to Hoffert.
But Solaren says that it would just require four or five heavy-lift rocket launches capable of carrying 25 metric tons, or about one fourth of Hoffert's weight estimate. The company is relying on developing more efficient photovoltaic technology for the solar panels, as well as mirrors that help focus sunlight.
"Solaren?s patented SSP [space solar power] system dramatically reduces the SSP space segment mass compared to previous concepts," Boerman told SPACE.com.
Solaren has not provided details on just how its technology works, citing intellectual property concerns. But it expects that its space solar power can convert to RF energy with greater than 80 percent efficiency, and expects similar conversion efficiency for converting the RF energy back to DC electricity on the ground in California. The company also anticipates minimal transmission losses from the space to the ground.
Hoffert remains unconvinced without knowing the details of Solaren's technology. He frets that "premature optimism" over unproven and perhaps scientifically implausible concepts could end up ruining the reputation of space solar power, even as advocates desperately want to see their vision come true.
"Too many space power guys have been silent, perhaps to not give comfort to opponents," Hoffert noted in a recent e-mail to colleagues. "But scientists should not do this."
Beaming into the future
Hoffert still believes strongly in the promise of space solar power, and has calculated that it can even prove as cost-effective as ground-based solar panels. That's because solar farms on Earth must build expensive storage systems to hold energy reserves during cloudy days or nighttime ? although Hoffert still sees solar farms as an ideal complement to space solar power.
Space solar power has to deal mainly with expensive launch costs of about $15,000 per kilogram, as well as the huge capital costs of building ground arrays if RF technology is involved. Hoffert has pushed for the laser beaming approach as newly effective cost-cutting measure, and even submitted a proposal with his son to ARPA-E, the U.S. Department of Energy's new agency.
"The cost to first power doesn't have to be in the hundreds of billions," Hoffert said. His proposal includes laser transmission tests on the ground in an NYU lab, and then a space experiment launched to the International Space Station.
Such beaming tests could even provide temporary power to isolated places on Earth along the space station's ground track, although a true solar space power station would sit in geostationary orbit.
Hoffert approved of Japan's own space solar power effort, led by JAXA, which would test both RF technology and lasers as means of energy transmission. He envisions the possibility of space solar power becoming commercially viable within a decade ? but only if all the science bears out the technology behind private efforts.
"Some of it is physics and engineering, and some of it is business and promotion," Hoffert said. "But in the long run, you can't fool Mother Nature."
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