Renewable Energy

Space-Based Solar Power Beamed to Earth

What if you could capture solar power in space, then send it down to Earth? What if you could launch the hundreds of modules for such a satellite, then use robots to assemble the entire array in space? You could power a military installation, a city—even on a cloudy day, even at night. At the U.S. Naval Research Laboratory (NRL), some of the brightest and most daring minds in satellites, space robotics, and radio frequency are building the technologies that could lead to such an achievement. Dr. Paul Jaffe, a spacecraft engineer at NRL, has built and tested a module to capture and transmit solar power. Even Jaffe admits the idea of an orbiting solar array that would beam energy to our planet seems kind of crazy. But, like most novel ideas, he says, “Hard to tell if it’s nuts until you’ve actually tried.”

As the Department of Defense (DoD) presses forward with energy security investments, solar power has already been proven in places like Hawaii and California. And ideally, a solar power satellite would provide power that was cost-competitive to what was locally available: about 10 cents per kilowatt hour in many places. But the military sometimes has energy requirements in very remote areas. The U.S. Marine Corps has successfully used solar panels at Experimental Forward Operating Bases in the Middle East and for humanitarian assistance. Current practices-like running diesel generators, driving fuel over roads in hostile areas, or even dropping in fuel canisters with parachutes-make power extremely expensive and impact mission and safety.

With multiple, potentially hidden receivers, space solar power could ease logistics for DoD’s deployed troops and remote bases. Jaffe has built two different prototypes of a “sandwich” module. In both designs, one side receives solar energy with a photovoltaic panel, electronics in the middle convert that direct current to a radio frequency, and the other side has an antenna to beam power away. Jaffe sometimes gets asked about the efficiency of such a system, but the most important metric is the power cost per pound. “Launching mass into space is very expensive,” says Jaffe, so finding a way to keep the components light is an essential part of his design. He can just cradle one module in his forearms.

His sandwich module is four times more efficient than anything done previously. He also has a “novel approach to solving the thermal problem, using the ‘step’ module.” The step module design, now in the patent process, opens up the sandwich to look more like a zig-zag. This allows heat to radiate more efficiently, so the module can receive greater concentrations of sunlight without overheating.

Additionally, “One of our key, unprecedented contributions has been testing under space-like conditions.” Using a specialized vacuum chamber at another facility would have been too expensive, so in typical NRL spirit, Jaffe built one himself. “It’s cobbled together from borrowed pieces,” he says. The vacuum chamber is just big enough for one module. In it, Jaffe can expose the module to the simulated extreme cold of space and concentrated solar intensities (mimicked by turning on two powerful xenon lamps in the same spectrum as the sun). By hooking the module up to a tangle of red and blue wires, he measures how well it radiates heat.

Jaffe says most solar panels orbiting with today’s satellites are never tested in space-like conditions because the technology is already mature. “But if you wanted to test anything under concentrated sunlight,” he says, “you would need something like the simulator we’ve put together here.”

Through trial and error, Jaffe has learned a lot. “The capability we’ve built up with the testing and vacuum under sun concentration is something that’s pretty unusual. And we’ve actually gotten a couple inquiries from people who may want to use this.” As an example, he’s had to modify how he uses the lamps because they don’t have uniform intensity, which creates hotspots on the modules.

For the antenna, Jaffe partnered with Dr. Michael Nurnberger, an antenna expert at NRL. “Antennas look simple,” Jaffe says, “you would never believe all of the calculations and analysis.” A chess-piece like copper object is encircled by a thin wall of metal. It’s mounted on a circular metal board, about the size of a pie pan. Jaffe and Nurnberger identify the antenna’s radiation pattern in one very unique room at NRL, an anechoic chamber. The anechoic chamber allows researchers to measure how an antenna radiates energy into free space. That signature enables communications between the satellite and Earth.

Except for one part of the floor, the chamber is completely covered in stalactite-like blue foam. The cones prevent waves from bouncing around, including muffling footsteps and voices. One wall, instead of being flat, is pushed out into a sideways pyramid that extends the length of the room to 125 feet. The overall effect is surreal, something out of a funhouse, and almost makes the uninitiated lose balance.

One of the primary objections to space solar power is the idea of an antenna shooting a concentrated beam of energy through our atmosphere. But we already use radiofrequency and microwaves to send smaller amounts of energy all the time. “People might not associate radio waves with carrying energy,” says Jaffe, “because they think of them for communications, like radio, TV, or cell phones. They don’t think about them as carrying usable amounts of power.”

There are a few ways to mitigate this concern. First, the antenna sends energy only to a specific receiver that asks for it. Second, using microwaves to send energy may be less objectionable than the higher power density required for lasers. Third, sending the energy on a lower frequency increases the size of the antenna and receiver, but decreases the concentration of power. As a side benefit, it also lessens the potential disturbances in the ionosphere to interrupt power: “At 2.45 gigahertz,” says Jaffe, “you’ll get power in a monsoon.”

From Jaffe’s perspective as an engineer, however, “The most sobering thing about all of this is scale.” He imagines a one kilometer array of modules-not to mention the auxiliary sun reflectors. The International Space Station is the only satellite that, to date, has come close. It stretches a little longer than an American football field; the array Jaffe is talking about would span nine. The modules would have to be launched separately, and then assembled in space by robots. That research is already being advanced by NRL’s Space Robotics Group.

“Another area ripe for research,” adds Jaffe, “is the system that would reflect and concentrate sunlight onto the modules.” As for where his research could be funded to go next, Jaffe has several proposals. But one challenge to finding a sponsor is that the project cuts across many different federal agencies. (Jaffe’s work to date has been funded from NRL’s base research budget.)

One proposal is to make the module even lighter, by using thinner solar panels, a flatter and lighter antenna, and, “instead of using these chunky prototyping radio frequency boards, you could make what’s called a monolithic microwave integrated circuit and put all that functionality into a little chip.” Another is “a demonstration mission, where you actually manufacture a whole bunch of these things and assemble them as an array in space to investigate some of the other challenges.”

NRL and others have also proposed using similar technology, but instead of deploying it to space, setting it up at a very high altitude in the stratosphere. “You wouldn’t get the same 24-hour energy, but you’d be above the clouds and you’d also have a longer daytime because you’re farther from the horizon.” The International Academy of Astronautics recently predicted space solar power could be viable within the next 30 years. (The idea has been in circulation since the 1970s, promoted in part by a demonstration in 1964 of a beam of microwave power keeping a helicopter aloft.)

But we may be approaching a turning point. In 2009, the California utility company, PG&E, committed to buying such power from Solaren by 2016. Jaffe’s aware of two other projects seeking to turn theory into reality: “Prior to this prototyping effort, there had been two groups that made meaningful sandwich modules, both of them were in Japan. Neither of them were tested in space, and we were more than four times as efficient as the most efficient of those.”

Society has yet to decide if we’ll invest in space solar, but Jaffe’s research informs the economic analysis and has relevance for many other types of projects. As mentioned, he’s improved testing for space hardware, particularly for that designed to perform under concentrated sunlight.

“In terms of the other applications,” he adds, “the space radar is an obvious one.” Jaffe’s concept of building large structures in space from modules applies to large phased-array radars. “The image quality of a radar is related to how big the antennas are and how much power the radar puts out,” he explains. With many antennas, each powered by the sun, “you don’t have to have a huge, heavy bundle of wires that spreads out to each one of them.”

Other applications include the conversion of direct current to radio frequency, microwave power beaming (including as first demonstrated in 1964 to power an aircraft), satellite propulsion, and thermal management architecture (the step module).

U.S. Naval Research Laboratory
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