SCIENCE

Space-based solar power is getting serious—can it solve Earth’s energy woes?

Late last month in Munich, engineers at the European aerospace firm Airbus showed off what might be the future of clean energy. They collected sunlight with solar panels, transformed it into microwaves, and beamed the energy across an aircraft hangar, where it was turned back to electricity that, among other things, lit up a model of a city. The demo delivered just 2 kilowatts over 36 meters, but it raised a serious question: Is it time to resurrect a scheme long derided as science fiction and launch giant satellites to collect solar energy in space? In a high orbit, liberated from clouds and nighttime, they could generate power 24 hours a day and beam it down to Earth.

“It’s not new science, it’s an engineering problem,” says Airbus engineer Jean-Dominique Coste. “But it’s never been done at [large] scale.”

The urgent need for green energy, cheaper access to space, and improvements in technology could finally change that, proponents of space solar power believe. “Once someone makes the commercial investment, it will bloom. It could be a trillion-dollar industry,” says former NASA researcher John Mankins, who evaluated space solar power for the agency a decade ago.

Major investments are likely far in the future, and myriad questions remain including whether beaming gigawatts of power down to the planet can be done efficiently—and without frying birds, if not people. But the idea is moving from concept papers to an increasing number of tests on the ground and in space. The European Space Agency (ESA)—which sponsored the Munich demo—will next month propose to its member states a program of ground experiments to assess the viability of the scheme. The U.K. government this year offered up to £6 million in grants to test technologies. Chinese, Japanese, South Korean, and U.S. agencies all have small efforts underway. “The tone and tenor of the whole conversation has changed,” says NASA policy analyst Nikolai Joseph, author of an assessment NASA plans to release in the coming weeks. What once seemed impossible, space policy analyst Karen Jones of Aerospace Corporation says, may now be a matter of “pulling it all together and making it work.”

NASA first investigated the concept of space solar power during the mid-1970s fuel crisis. But a proposed space demonstration mission—with ’70s technology lofted in the Space Shuttle and assembled by astronauts—would have cost about $1 trillion. The idea was shelved and, according to Mankins, remains a taboo subject for many at the agency.

Today, both space and solar power technology have changed beyond recognition. The efficiency of photovoltaic (PV) solar cells has increased 25% over the past decade, Jones says, while costs have plummeted. Microwave transmitters and receivers are a well-developed technology in the telecoms industry. Robots being developed to repair and refuel satellites in orbit could be turned to building giant solar arrays.

But the biggest boost for the idea has come from falling launch costs. A solar power satellite big enough to replace a typical nuclear or coal-powered station will need to be kilometers across, demanding hundreds of launches. “It would require a large-scale construction site in orbit,” says ESA space scientist Sanjay Vijendran.

Private space company SpaceX has made the notion seem less outlandish. A SpaceX Falcon 9 rocket lofts cargo at about $2600 per kilogram—less than 5% of what it cost on the Space Shuttle—and the company promises rates of just $10 per kilogram on its gigantic Starship, due for its first launch this year. “It’s changing the equation,” Jones says. “Economics is everything.”

Similarly, mass production is reducing the cost of space hardware. Satellites are typically one-offs built with expensive space-rated components. NASA’s Perseverance rover on Mars, for example, cost $2 million per kilogram. In contrast, SpaceX can churn out its Starlink communication satellites for less than $1000 per kilogram. That approach could work for giant space structures made of huge numbers of identical low-cost components, Mankins, now with the consultancy Artemis Innovation Management Solutions, has long argued. Combine low-cost launches and this “hypermodularity,” he says, and “suddenly the economics of space solar power become obvious.”

(GRAPHIC) C. BICKEL/SCIENCE; (IMAGE) SATELLITE APPLICATIONS CATAPULT

Better engineering could make those economics more favorable. Coste says Airbus’s demo in Munich was 5% efficient overall, comparing the input of solar energy with the output of electricity. Ground-based solar arrays do better, but only when the Sun shines. If space solar can achieve 20% efficiency, recent studies say it could compete with existing energy sources on price.

Lower weight components will also improve the cost calculus. “Sandwich panels,” pizza box–size devices with PV cells on one side, electronics in the middle, and a microwave transmitter on the other, could help. Put thousands of these together like a tiled floor and they form the basis of a space solar satellite without a lot of heavy cabling to shift power around. Researchers have been testing prototypes on the ground for years, but in 2020 a team at the U.S. Naval Research Laboratory (NRL) got its aboard the Air Force’s X-37B experimental space plane.

“It’s still in orbit, producing data the whole time,” says project leader Paul Jaffe of NRL. The panel is 8% efficient at converting solar power into microwaves but does not send them to Earth. Next year, however, the Air Force plans to test a sandwich panel that will beam its energy down. And a team at the California Institute of Technology will launch its prototype panel in December with SpaceX.

The drawback of sandwich panels is that the microwave side must always face toward Earth so, as the satellite orbits, the PV side sometimes turns away from the Sun. To maintain 24-hour power, a satellite will need mirrors to keep that side illuminated, with the added benefit that the mirrors can also concentrate light onto the PV. A 2012 NASA study by Mankins put forward a design in which a bowl-shaped structure with thousands of individually steerable thin-film mirrors directs light onto the PV array.

Ian Cash of the United Kingdom’s International Electric Company has developed a different approach. His proposed satellite uses large, fixed mirrors angled to deflect light onto a PV and microwave array while the whole structure rotates to keep the mirrors pointing sunward (see graphic, above). Power from the PV cells is converted to microwaves and fed to 1 billion small perpendicular antennas, which together act as a “phased array,” electronically steering the beam toward Earth whatever the satellite’s orientation. This design, Cash says, delivers the most power for its mass, making it “the most competitive economically.”

If a space-based power station ever does fly, the power it generates will need to get to the ground efficiently and safely. In a recent ground-based test, Jaffe’s team at NRL beamed 1.6 kilowatts over 1 kilometer, and teams in Japan, China, and South Korea have similar efforts. But current transmitters and receivers lose half their input power. For space solar, power beaming needs 75% efficiency, Vijendran says, “ideally 90%.”

The safety of beaming gigawatts through the atmosphere also needs testing. Most designs aim to produce a beam kilometers wide so that any spacecraft, plane, person, or bird that strays into it only receives a tiny—hopefully harmless—portion of the 2-gigawatt transmission. Receiving antennas are cheap to build but they “need a lot of real estate,” Jones says, although she says you could grow crops under them or site them offshore.

For now, Europe is where public agencies are taking space solar power most seriously. “There’s a commitment there that you don’t see in the U.S.,” Jones says. Last year, ESA commissioned two cost/benefit studies of space solar. Vijendran says they concluded it could conceivably match ground-based renewables on cost. But even at a higher price, comparable to nuclear power, its around-the-clock availability—unlike conventional solar or wind—would make it competitive.

In November, ESA will ask member states to fund an assessment of whether the technical hurdles can be overcome. If the news is good, the agency will lay out plans for a full effort in 2025. Armed with €15 billion to €20 billion, ESA could put a megawatt-scale demonstration facility in orbit by 2030 and scale up to gigawatts—the equivalent of a conventional power station—by 2040, Vijendran says. “It’s like a moonshot.”

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