Beaming Solar Energy From Space Gets a Step Closer

More than half a century ago, an article titled “Power from the Sun" in the journal Science spelled out the rationale for this high-wireless act. Above Earth’s atmosphere, sunshine is never interrupted by cloudy skies, and there is no day or night. Satellites collecting solar power could theoretically operate around the clock, dispatching emission-free electricity wherever it’s needed, anywhere on Earth. But the concept was long dismissed as too complicated and expensive.

Now it is finally being put to the test.

On Jan. 3, a team at Caltech launched the Space Solar Power Demonstrator, an orbiting suite of experiments to test key components for space-based solar power. It switched on in May and has begun sending back encouraging early results. “People are realizing this isn’t just science fiction," says Ali Hajimiri, an electrical engineer at Caltech and one of the project leaders on the demonstrator. “There may be a pathway to make this reality."

Other related efforts are also gaining momentum. The European Space Agency is drawing up a blueprint for a possible European space-solar network. The China Academy of Space Technology has announced plans for a power-beaming satellite prototype by 2028. And military labs in the U.S. are experimenting with tech that could someday transmit space-based power to remote bases or combat zones.

One of the central challenges for all of these projects is finding a safe, efficient and reliable way to transmit gigawatts of power to the ground and then convert it into electricity that people can use. Microwave beams are the favored technique, in large part because they can travel freely through the air regardless of weather. While similar to those used in microwave ovens, these beams would be nowhere near as concentrated. A recent study by the European Commission found that the incoming microwave beams would be too feeble and diffuse to harm human health. Some involved in these projects say further thorough research will be needed for public acceptance, though.

“It’s basically the same technology as wireless charging for your cellphone," says Chris Rodenbeck, head of the Advanced Projects Group at the U.S. Naval Research Laboratory in Washington, D.C. In 2021, Rodenbeck and his collaborators sent a 1.6-kilowatt beam of microwaves (also similar to those used for Wi-Fi signals, but at a higher frequency) from a transmitter to a receiver two-thirds of a mile away at the U.S. Army Research Field in Blossom Point, Md. The researchers said they used microwaves because they travel freely through the air, unaffected by weather. Nobody has yet pulled off an equivalent feat from orbit, however.

The NRL has been experimenting with beaming technology in space using a device the size of a bread loaf called the photovoltaic radiofrequency antenna module, or PRAM. It flew aboard the U.S. Air Force’s X-37B space plane and effectively converted sunshine into microwaves—but didn’t actually direct the waves anywhere—before returning to Earth last year. Rodenbeck is working on a follow-on project, Arachne, run by the Air Force Research Laboratory in Dayton, Ohio. Arachne is designed to tackle the more challenging task of transmitting power from orbit to a station on the ground. It is scheduled for a 2025 launch.

The Caltech team is attempting to accelerate this process by testing multiple, potentially lower-cost technologies at once, with funding from billionaire real-estate developer and philanthropist Donald Bren, chairman and owner of Irvine Company. Many years ago, he was captivated by an article in Popular Science magazine about harvesting solar energy in space. “I’ve dreamed about how space-based solar power could solve some of humanity’s most urgent challenges," he says. His donation of more than $100 million to Caltech over the past decade supported the creation of the Space Solar Power Demonstrator, the suite of technology tests that launched in January attached to a commercial satellite.

One of the key components on the Caltech demonstrator is a prototype power-beamer called Maple, short for Microwave Array for Power-transfer Low-orbit Experiment. It has generated microwaves and steered them from one part of the satellite to another, lighting up two test LEDs, Hajimiri says. The distance traveled is small, about a foot, but it is the first documented demonstration of power-beaming in space. The device also directed microwaves toward Earth, which were picked up by Caltech’s detectors on the ground.

Maple has a novel modular design that would combine a solar-energy collector and a transmitter into a single, self-contained unit. That approach could help address one of the most daunting obstacles to building solar-power satellites—their shocking size requirements. To match the output of a midsize power plant on Earth, a solar satellite would need at least 1 square mile of light-collecting area.

Rather than attempting to build such an enormous structure all at once, the Caltech team envisions stringing together many small collector-transmitters into expandable configurations. They would operate collectively, eliminating the need for complex wiring and a heavy central antenna. “This is a paradigm shift," Hajimiri says. “The analogy I use is going from one big elephant to an army of ants."

Another Caltech experiment suggests a light, simple way to hold the whole power satellite together. Sergio Pellegrino, a Caltech aeronautical engineer who leads another of the demonstration experiments, has developed expandable space structures that weigh just one-third of an ounce per square foot. A prototype was packed in a tight cylinder aboard the satellite. It is configured to pop out and form a stable set of square frames, 6 feet by 6 feet. “That’s the smallest scale at which we could test the structure and mechanism," Pellegrino says. Like Maple, it’s designed to scale up to vastly larger sizes.

“It is a system that ages gracefully," Pellegrino adds. “If there are failures—for example, micrometeorites or things like that—it’s a small, localized damage rather than global damage."

To beam the energy to earth, the Caltech team would follow the same approach as the experiment at Blossom Point. The solar power satellite would convert electricity into a microwave signal and transmit it toward a receiver—only, in this case, the receiver would be hundreds or thousands of miles away on Earth. That receiver, in turn, would collect the microwaves and use electronics to reverse the process, converting the waves back into electric power. Most of the other concepts for solar-power satellites use this approach as well.

There are many competing visions of where the power would go once it reaches the ground, however.

Rodenbeck of the Naval Research Laboratory sees military goals such as beaming energy to combat locations so that they don’t have to rely on vulnerable convoys of fuel trucks. Hajimiri envisions rolling out flexible antennas as big as a city block to bring in emergency beamed power after natural disasters, or to electrify unwired locations like remote parts of sub-Saharan Africa. Sanjay Vijendran, leader of the European Space Agency’s Solaris space-solar project, is mapping ambitious plans for a fleet of solar-power satellites that would feed directly into the European grid. “We’re looking to make a significant contribution to mitigating climate change," he says.

Bringing space-based solar power to the masses will require not just a lot of satellites but also a lot of antenna farms on the ground. Two gigawatts of beamed power would require about 25 square miles of receiver, according to a Solaris-funded report by the research firm Roland Berger.

Vijendran recognizes the need for a thorough investigation of all possible dangers, from impacts on health to sabotage. There have been many studies of microwave safety, but space-based beaming hasn’t been a focus so far. “People need to see that everyone has done their due diligence and shown conclusively that these things aren’t harmful or have the potential to do harm," he says.

And then there is the question of how much customers will have to pay for their space-based solar electricity. Roland Berger concluded that it could be “a cost-competitive renewable technology," but that depends heavily on the falling cost of space launches and electronics.

Still, Caltech’s Pellegrino sees no choice but to go all-in on testing the technology. “There is an existential need for abundant clean power," he says, “and this could help get us there."