“When I was first presented with the idea of solar power from space, I was skeptical,” says Space Solar Power Project Director Dr. Harry Atwater, Howard Hughes Professor of Applied Physics and Materials Science at Caltech. “The concept was simply too heavy and too expensive to be realistic. But when we realized that the makings of an ultralight alternative were within reach, I became convinced. It was clear that we were looking at a component-led revolution.”
Weight matters
The Caltech initiative has led to new component designs using integrated tile arrays that harvest sunlight and convert it to electric power. Ultralight gallium-arsenide photovoltaic cells are attached to tiles about the size of a frisbee. Each tile performs the role of a miniature solar installation, complete with photovoltaics, electronic components, and a microwave transmitter.
The tiles act in concert, but without physical connection, eliminating cables and support structures, thus bringing down weight. Groups of tiles form modules of up to 60 square meters, with all the modules together forming a hexagonal power station approximately three kilometers in length. Reduced weight is the game-changer: compared to specific power of 100-200 watts per kilogram using standard technology, the Caltech concept can promise 1-20 kilowatts per kilogram.
“By bringing the overall weight down from one kilogram per square meter to 10-20 grams, we are reducing the mass needed to achieve the necessary area, meaning we can bring the payload for a megawatt system down to the hundreds of kilos,” Atwater says. “We are not reimagining how a system for collecting sunlight would work, but rather reimagining the array of components and taking mass out of the system.”
A lifetime of sun seeking
Atwater has been deeply involved in photovoltaic (PV) research for much of his career, first developing PV technologies commercially for terrestrial use, then moving on to unmanned electrical airplanes. After that came investigations into using PV in space based on direct current (DC) to radio frequency (RF) power, and ultimately the Space Solar Power project, working together with Caltech colleagues Ali Hajimiri, Professor of Electrical Engineering and Project Co-Director, and Sergio Pelegrino, Head of the Space Structures Lab.
Referring to NASA’s James Webb space telescope launched in early January 2022, with its deployable sunshield the size of a tennis court, Atwater relates that the impetus for innovation in the project is designing deployable ultralight PV to RF power systems.
“We have developed integrated prototypes and demonstrated the concept successfully. Now we are working on further reduction of weight and mass,” Atwater says. A first space flight demo is planned for the latter part of 2022. “The goal is not to demonstrate the full concept, but to prove that the disruptive elements work in space. For example, we will conduct a power transmission demo from one end of the spacecraft to other, with the aim of testing the spaceworthiness of components.”
Beam me down
Capturing solar power in space, where clouds do not form and the sun never sets, may seem feasible enough, but beaming it back to earth sounds almost fantastical. So what makes this scenario straight out of science fiction a potential reality?
“The solar power conversion rate with current PV technology is already high, and terrestrial solar power has gotten cheaper, but it is not continuous 24 hours a day. An orbiting spacecraft sees more sunlight as it is more continuously exposed to the sun. This allows the system to harvest nine times more power than on earth, in addition to ensuring continuous, baseload solar power production,” Atwater explains.
The system converts sunlight to DC power, then to phased array for transmission. Radar penetrates cloud cover, so transmission is not impeded by atmospheric conditions. Some power losses are incurred in the process, but end-to-end the system will recover about ten percent of the incident solar power, which Atwater assures is acceptable.
The laws of diffraction guarantee that the transmission system will not be able to focus power beyond a certain density, thus ensuring safety and security, Atwater confirms. “It comes down to an issue of power density, and the energy density of the microwave beam will be equal to the power density in sunlight.”
Banking on the sun
As to when we might first harness the power of the sun from space, Atwater is clear that a specific timeline is hard to pin down. “To exist in the marketplace, the concept has to be bankable. These would be the largest deployable space structures ever built, and they have to be manufacturable to be realized at scale. I believe we will see ultralight PV and RF components used in space systems on a commercial scale in roughly ten years. In the meantime, commercial launch players will continue to drive launch costs down.”
Initially sponsored by aerospace giant Northrup Grumman, philanthropic support for further development has been provided by Donald Bren, a California-based real estate developer. “As a serial entrepreneur myself, I know the risks of commercialization,” Atwater relates. “The compelling element here is the chip-based components. These have intrinsic commercial value for other uses, for example to revolutionize the existing space power system market.” Other uses for chip-based RF systems include the continuing global Internet buildout, he says. “We need an early beachhead market to spur manufacturing. Then we can piggyback on other needs and industries to bring the concept further along.”
Atwater sees the Space Solar Power project as both a national endeavor, and one that will eventually benefit from international cooperation, similar to collaboration on the International Space Station. “There would be multiple beneficiaries of this system globally. This is one reason I think we will eventually see broad international buy-in.”
The project has already captured attention of other entities around the world, including the space agencies of Britain, China, and Japan, in addition to numerous private enterprises. “Ground stations can be co-located with existing utility-scale power installations. There is also a great need to supply more power to remote areas and developing countries, and dispatchable power stations would make this possible without having to invest in costly physical infrastructure,” Atwater concludes.
