On the night of May 22, a group of researchers and students gathered around a computer monitor on the roof of Caltech’s electrical engineering department. The monitors were connected to equipment designed to detect microwave radiation received from a satellite in space. And about 300 miles above them, far over the night’s thick cover of clouds, that satellite was about to pass overhead, equipped as a test bed for technologies they had developed to gather solar energy in space and project it down to Earth.
The researchers weren’t expecting much. They had already accomplished their primary objective back in March: using microwave radiation to project electricity across a gap of a few inches to light up a pair of LEDs onboard the spacecraft to test whether their power transfer system, essential for one day getting solar power down to Earth, would hold up in the harsh environment of space. There was a lot of uncertainty over whether they could get a tiny quantity of measurable power down to Earth on their first try. Still, they grew quiet as the time of the satellite pass overhead grew nearer. At 9:57 p.m., the monitors began showing the background radiation the receivers were picking up coalescing into something else: an electrical signal that matched what was being projected by their satellite. They had successfully detected the microwave energy that their novel power transfer system was directing toward Earth. “It took it a few moments to sink in,” says Ali Hajimiri, a professor of electrical engineering at Caltech. “Then everyone got really excited.”
Hajimiri leads a component of a larger endeavor by Caltech researchers to develop technology that could gather the sun’s energy in massive satellites orbiting Earth and beam it down to power the grid. It’s an audacious concept, with world-changing benefits should such orbiting solar power plants ever be built. Solar panels on Earth only work during the day, and they don’t produce much power on cloudy days or when the sun is low in the evening or early morning. In orbit, however, such panels would produce a constant stream of zero-emission power. “In space, it’s always noon on a sunny day,” says Hajimiri.
It’s an idea that has captured the imagination of writers and futurists for decades—the first published mention of the concept likely came in a 1941 short story by I, Robot writer Issac Asimov. But even as communication satellites, moon landings, and probes to Mars became reality, solar power stations remained in the realm of science fiction. Many components necessary for such a system were developed through the years, but the physical problems of getting that theoretical power station off the ground were more difficult—any system capable of generating a useful quantity of electricity would be far too heavy to feisably heft into orbit.
But researchers on Caltech’s Space Solar Power Project say that new technological developments—particularly the potential for extremely light, flexible solar panels and lightweight energy transfer systems to replace bulky antennas—have brought the idea into the realm of reality. Caltech’s Space Solar Power Demonstrator, launched in January, includes an array of different types of advanced solar panels to test which will work best for a space solar power station, as well as a test system designed to unfold into a 6-by-6-ft. structure that could be used to hold solar panels, alongside Hajimiri’s energy transfer system.
Caltech isn’t the only organization that has become interested in solar power stations. The Chinese government is planning a 2028 mission to demonstrate the technology in low Earth orbit. And last November, science ministers in the E.U. greenlit Solaris, a joint project between the European Space Agency (ESA) and aerospace company Airbus to look into the possibility of building gigantic solar power stations in geostationary orbit over Europe. (Whether intentional or not, the linkage to the world of mid-century sci-fi remains, with the project sharing the title of Stanislaw Lem’s classic 1961 novel.)
Caltech’s Space Solar Power Demonstrator isn’t actually a prototype power station. Rather, it’s a collection of three separate experiments to test components that would eventually be integrated into one system. Two of the experiments—the self-assembly system and the solar tests—haven’t produced results yet. The energy transfer component worked in sending electricity a few inches. When directed towards Earth, it spread most of its power output over a very wide area, as expected, only managing to get an extremely tiny fraction of its energy onto the receiver (a larger system could focus the energy onto a much smaller area). In a fundamental sense, the way that system works is no different from the typical way that satellites communicate with Earth by projecting microwave radiation that gets converted into electrical energy at a receiver. But Hajimiri’s system is designed differently, based on a concept that would allow it to scale up to focus large amounts of power onto small Earth-based receivers. With more funding and research, the engineers working on the project are optimistic that the technology could reach commercialization in the decades ahead. In about five years, they think they might be able to build a system that could transfer enough solar power to charge a laptop from space. From there, it’s a matter of scaling up further to build a full-fledged commercial power station.
“We are currently building things in our university labs, and so we are necessarily small in scale,” says Sergio Pellegrino, a Caltech professor of aerospace and civil engineering working on the space solar project. “This infrastructure is going to be very large, so a transition to an industrial production facility is key to the next step.
“It takes a few years, with the right amount of money,” he adds. “It doesn’t take very many years.”
The researchers at Caltech came to be in the same league as the Chinese government and the ESA thanks to the interest of one man. In 2011, Donald Bren, 91, a billionaire California real estate magnate and lifetime member of Caltech’s board of trustees, read a Popular Science article about space-based solar power. Intrigued by the potential of the technology, he began funding a program at Caltech to pursue the idea, eventually contributing more than $100 million dollars.
Three Caltech professors came on board. Pellegrino researched lightweight, self-assembling structures—something that could fit in a small space in a rocket, but then unfold in orbit to absorb the sun’s rays. Harry Atwater, a professor of applied physics and materials science, looked into finding the right solar panels for the power station. Traditional solar arrays on satellites use glass to protect underlying systems, but such a solution would be too heavy for the solar power station. “We’ve been investigating how we can make things intrinsically radiation hard, and therefore we could get rid of that piece of glass,” Atwater says.
Hajimiri, meanwhile, headed up the portion of the project investigating energy transfer. Over video chat, he shows his solution, a flexible sheet covered with a grid pattern. Rather than pointing a single huge antenna at a target, each point on Hajimiri’s grid system emits a small amount of microwave radiation. Computers minutely adjust the frequencies of the overlapping radiation, creating a kind of lensing effect using constructive and destructive interference, like overlapping ripples in a pond, to focus the energy on a particular point. “You’ve gone from a giant elephant to an army of ants of these individual transmitters,” says Hajimiri.
A power station using such technology could send electricity to any point below it on Earth, could switch transfer locations almost instantly, or even direct energy to multiple locations at once. It opens up the possibility of easily sending power to places around the world that need it most, or directing power to specific locations after natural disasters. All first responders would have to do is unroll a receiver on the ground to receive a steady supply of electricity, even on cloudy days or at night.
The new results from last Monday’s experiment have proven that the technology could actually work from space. The next step is to sort out small irregularities to improve the next generation of the power transmission system, a process that can take months. Then it’s on to the task of integrating all the experimental components into a bigger system. “While there are still hurdles to overcome for large scale wireless energy transfer of space solar,” says Hajimiri, “this gets us closer.”
Correction, June 1
The original version of this story misstated the name of a Caltech professor. He is Harry Atwater, not Henry Atwater.
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