Isaac Azimov’s appearance on David Letterman’s show in 1980 features in a new documentary exploring the promise of space-based solar power. The literary science-fiction pioneer told the Late Night host that he had written about space stations beaming energy back to Earth in 1940. The scientific principles for doing so are sound. The time delay in accomplishing this is attributable to the cost of launching payloads into orbit and the price of energy to make it practical.
As pv magazine reported in 2023, power has indeed been beamed back from a satellite in low-Earth orbit (LEO) in microwave form. This test deployment showed that space structures with photovoltaic generation could indeed be launched economically into space, deploy successfully and transmit solar energy safely back to Earth, albeit just enough to make a few detectors on a rooftop receiver jump and attending Caltech researchers jump for joy.
That moment is also featured in Bright Harvest: Powering Earth from Space, a film directed by Steven Reich that is scheduled for a special screening at Caltech on October 27. As it turns out, the university was a focal point for three professors in different science and engineering disciplines that made the concept of solar energy transfer from space demonstrably possible. Whether it is practical remains to be seen, but the film is persuasively positive.
Harry Atwater, professor of applied physics and material science at Caltech, told pv magazine USA that his early work in thin-film photovoltaic materials produced record-making single-junction cell efficiencies that hold to this day. The combination of high-efficiency cells and flexible substrates make such modules suitable for deployment in space. A serial entrepreneur, Atwater co-founded Alta Devices, which broke records for achieving cell efficiencies of around 30%, and Caelux, which is developing and manufacturing perovskite-on-silicon PV cells.
“We developed very high efficiency gallium arsenide cells by an epitaxial growth process and these cells were able to get to a specific power, which is the figure of merit that is probably most important for space photovoltaics,” Atwater said. “Its power per unit weight – not just absolute power per unit area – is important because you end up having to launch mass and volume into orbit.”
As told in Bright Harvest, Atwater and his Caltech colleagues, Sergio Pellegrino, professor of civil engineering, and Ali Hajimiri, professor of electrical engineering, combined their talents and research on what became the university’s Space-Based Solar Power Project. Pellegrino specializes in developing lightweight space structures with a focus on packaging, deployment and stability in orbit. Hajimiri focuses on developing methods for long-distance wireless energy transfer.
Along with associates and students, the principals of the Space-Based Solar Power Project have designed experimental spacecraft to test various PV module configurations along with a means for transmitting energy in the form of low-intensity microwaves. The key areas of research: lightweight PV cells that can survive in space; space structures that can be deployed from existing rockets; and the underlying electronics for collecting and transmitting power back to Earth combined in the spacecraft that launched from a SpaceX Falcon 9 rocket in January 2023. The satellite successfully beamed energy from LEO the following June.
Rather than launching powerful microwave transmitters, which have both weight and safety constraints, the team integrated numerous low-power microwave beams that can be steered to concentrate on a target equipped with a receiving antenna array for conversion into electricity. The energy density of the transmitted power is described as being akin to sunlight, so it does not produce harmful effects on the ground.
“The power density would be about 100 milliwatts per square centimeter,” Atwater said. “For comparison, if I put my phone next to my ear the power density is about 30 milliwatts per square centimeter, so It’s about two or three times higher than that from the antenna in your smartphone.”
Microwave energy is generated directly from solar cells on a chip. Cell-chip combinations are mountable on flexible substrates that can be rolled and packaged for launch. Beam steering is an established technology for many electronics applications, including radar and communications.
Image: Deer Creek Nature
If all of the individual technologies described in Bright Harvest are known and proven, the key achievement of the Caltech project is integrating them for launch. The 2023 launch included an array of 32 different cells to test their individual characteristics of generating capacity and endurance in the hostile environment of space. A microwave transmitter experiment demonstrated that beams could be steered to a receiver at Caltech. Finally, the effort deployed a space structure that unfolded to a 6’-by-6’ framework that in practice would support flexible solar modules.
According to Atwater, a functioning solar power space station would have an area of about a square kilometer when deployed. Such a structure would be launchable as a single unit from a SpaceX Starship, when that vehicle becomes commercially available. Constellations of smaller satellites could be launched from rockets currently in service.
On the ground station side, Atwater said his team is eying existing utility-scale PV arrays as being perfect locations. The microwave receivers are optically transparent and could in effect be overlaid on top of ground-mounted solar modules. The PV wouldn’t play any role in energy conversion, but a hosting project already has suitable cleared land and grid interconnections.
“Our vision is that in the existing solar power station produces power in the daytime, whereas the space solar power station uses the same infrastructure to produce power 24 hours a day,” he said.
Bright Harvest producer Brigitte Bren told pv magazine USA that the film brings together three passions of hers: clean energy, environmental stewardship and bold innovation.
“Caltech’s scientists are turning sunlight in space into power on Earth, which feels both visionary and practical,” Bren said. “I wanted to help share that story of hope, ingenuity and possibility for our planet’s future.”
Bren, who is the co-founder of International Strategic Planning, Inc., a business consulting firm, said the efforts of Caltech’s Space-Based Solar Power Project are receiving a tailwind from the dramatic fall in commercial launch costs in recent years, adding that advances from SpaceX, Blue Origin and others are changing what’s possible.
“As launch costs drop and capacity rises, building solar power stations in orbit moves from dream to reality,” she said. “Their progress makes space solar not just inspiring, but increasingly within reach.” Caltech’s Space-Based Solar Power Project is one of several efforts aiming to beam energy to Earth from orbit. California-based Aetherflux is developing an approach that uses many satellites in lower orbits that transmit power by infrared laser to small ground stations. The company has a U.S. Department of Defense contract to deploy a prototype and expects to launch its first spacecraft next year with SpaceX. UK-based Space Solar is working with the British Antarctic Survey and others on a concept for solar satellites in LEO to delivering power to remote locations where electricity is expensive or non-existent.
For his part, Atwater says the Caltech team is looking for partners to commercialize its concept. First and foremost, the Space-Based Solar Power Project is a research and development effort. Commercialization will require manufacturing and systems integration that only industry can provide. Nevertheless, the R&D continues: Atwater says the team is delivering new experimental set of test cells to SpaceX in November for a planned launch to the International Space Station, likely next year.
All of the technology and effort will remain experimental so long as the levelized cost of energy (LCOE) for space-based solar exceeds that for terrestrial sources. Solar in general has become competitive with other sources, even with the drawbacks of intermittency. While solar power stations in appropriate orbits would essentially operate without obstructions, cost is still very much a factor.
Atwater is convinced the Caltech team can turn the experimental into reality: “I’m going to be somewhat audacious and claim that scaling up the technology we currently have could produce a LCOE of $0.09 per kWh. Going forward we will be able to deliver $0.03 per kWh. Space-based solar will be competitive with utility-scale PV.”
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JPL did a study (1) of space based power in 1975 with aggressive assumptions on PV performance, low weight structures, heavy loft launch vehicle performance, microwave transmission efficiency, etc. and compared it to projected performance of nuclear, coal, ground solar thermal and PV. They concluded that it would be too expensive as a commercial venture with a high initial R&D investment. The recommendation was to take a look again in 25 years.
Well, it’s been 50 years and we may be there with the advances that have taken place and the clever design modifications taken by the Caltech team.
It is especially noteworthy that their approach avoids the achilles heel of prior designs that had a single microwave beam of high intensity. If diverted by a rogue actor by sending an “I’m here” signal from in the middle of a city, it could cause enormous damage. The multiple low intensity microwave beams avoids this defect.
I wish them well!
1. Caputo, R., “An Initial Comparative Assessment of Orbital and Terrestrial Central Power Systems, Final Report,” JPL 900-780, March 1977