Radiating Clean Energy From Space – Caltech’s “Extraordinary And Unprecedented Project”

A technology capable of harvesting solar energy in space and beaming it down to Earth to provide a global supply of clean and affordable energy was once considered science fiction. Now it’s getting closer to reality. As part of the Space-based Solar Power Project (SSPP), a team of researchers from the California Institute of Technology (Caltech) is working to deploy a constellation of modular spacecraft that collect sunlight, convert it into electricity, and then wirelessly transmit that electricity to where he is needed . They could even send it to places that don’t currently have access to reliable electricity.

“This is an extraordinary and unprecedented project,” says Harry Atwater, an SSPP researcher and Otis Booth Leadership Chair in Caltech’s Division of Engineering and Applied Sciences. “It exemplifies the courage and ambition it takes to tackle one of the most significant challenges of our time and bring clean and affordable energy to the world.”

Atwater, who is also the Howard Hughes Professor of Applied Physics and Materials Science, is co-leading the project with two other researchers: Ali Hajimiri, Bren Professor of Electrical Engineering and co-director of the SSPP; and Sergio Pellegrino, Joyce and Kent Kresa Professor of Aerospace and Civil Engineering, Co-Director of the SSPP and Senior Research Scientist at the Jet Propulsion Laboratory (

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Harnessing solar energy in space is based on breakthrough advances in three main areas:

• Atwater’s research group is developing ultra-lightweight, high-efficiency photovoltaics (materials that convert light into electricity) optimized for space conditions and compatible with an integrated modular energy conversion and transmission system.
• Hajimiri’s research team is developing the inexpensive and lightweight technology needed to convert direct current into radio frequency energy (used, for example, to transmit cell phone signals) and send it to Earth as microwaves. The process is safe, explains Hajimiri. Non-ionizing radiation at the surface is significantly less harmful than standing in the sun. In addition, the system could be switched off quickly in the event of damage or a fault.
• Pellegrino’s group invents foldable, ultra-thin, and ultra-lightweight space structures to support photovoltaics, as well as the components needed to convert, transmit, and direct radio frequency energy where it’s needed.

The basic unit of the system the researchers envision is a 4-by-4-inch tile that weighs less than a tenth of an ounce. Hundreds of thousands of these tiles would combine into a system of flying carpet-like satellites that, once unfolded, would create an area of ​​3.5 square miles for collecting sunlight.

Work on the SSPP has received more than $100 million in support from Donald Bren, Chairman of the Irvine Company and a lifelong member of the Caltech community, and his wife, Brigitte Bren, a trustee of Caltech. Northrup Grumman Corporation provided funding for initial feasibility studies.

Atwater, Hajimiri and Pellegrino discussed their progress – and the transformational potential of space-based solar power – as the project nears a significant milestone: a test launch of prototypes into space in December 2022.

A series of small solar panels, part of the Space Solar Power Project, integrate photovoltaics, power transmission circuitry and beam steering. Photo credit: Caltech

Describe the vision behind the space-based solar energy project. How did the project come about?

Sergio Pellegrino: More than 10 years ago, in 2011, conversations started with Donald Bren when he asked if Caltech had any ideas when it came to sustainable energy and space research. We began discussing in a group of faculty members how we could build on our interests and what’s happening in each of our areas, which could lead to a very impactful research initiative. Over a period of a few months, we had a vision – I called it a dream – of three or four technological breakthroughs that combined would change the way solar power was approached in space.

Ali Hajimiri: This concept was real science fiction in the past. What allowed us to think about bringing it from the realm of science fiction to the realm of reality was the combination of developments in photovoltaics in Harry’s lab, in structures in Sergio’s lab, and in wireless power transmission, which was done in mine happens lab. We realized that we can now power solar power for space in a way that is becoming both practical and economical.

One of the first questions everyone asks is, “Why do you want to put photovoltaics in space?” Well, in space, where there’s no day and night and clouds and the like, you get about eight times more energy. The vision of this program is to give you as much energy as you need, where you need it, when you need it.

(Left to right) Professor Sergio Pellegrino, Harry Atwater and Ali Hajimiri. Photo credit: Steve Babuljak for Caltech

What progress have you made in realizing this ambitious vision?

Pellegrino: Over a period of two years, we built and demonstrated a prototype tile. This is the key modular element that captures the sunlight and transmits the energy. Through this process we have learned many things about how to design highly integrated and ultra-lightweight systems of this type. We then developed a second prototype that is 33 percent lighter than the first.

Hajimiri: This tile, as Sergio mentioned, is the building block of the larger system. It must be fully functional, compatible and scalable. While it may sound simple, it’s actually quite sophisticated. These tiles are mounted on a very flexible structure that can be folded to fit into a launch vehicle. Once erected, the structure expands and the tiles work together and in sync to generate, transform and transmit energy exactly where you need it and nowhere else.

What can you tell us about the next phase of this project?

Atwater: It doesn’t become real until you actually fly into space. As described by Sergio and Ali, we demonstrated this key element of the unit called a tile in our labs. One of the lessons learned from this series of demonstrations was that the path we had to take for photovoltaics needed to be fundamentally changed. We were working with what I call conventional photovoltaic materials that had to be designed in a form that would make it difficult to meet the mass-per-area and specific performance targets, so we had to fundamentally rethink the photovoltaic strategy completely. As a result, the classes of photovoltaic devices we are testing in space have never flown in space before.

Pellegrino: Most spacecraft today have solar cells – photovoltaic cells connected to a support structure – but not with this type of material and not folded to the dimensions that we have achieved. By using novel folding techniques inspired by origami, we are able to significantly reduce the dimensions of a huge spacecraft for launch. The packaging is so tight that it is essentially free of voids.

Hajimiri: Wireless power transmission of this type has not been demonstrated in space. We also demonstrate it with our flexible, lightweight material, which is not necessarily a rigid structure. That adds complexity.

What impact could it have on society if solar energy becomes a reality in space?

Hajimiri: It will revolutionize the nature of energy and access to it so that it becomes ubiquitous, it becomes distributable energy. You can send it to where you need it. This energy deflection takes place purely electrically via a focusing array without mechanical movements and is therefore extremely fast.

Atwater: I think it’s fair to say that the Brens’ vision really was to do something that, as Ali mentioned, originally came almost from science fiction, to do something that would become a major power source for the world.

Pellegrino: We have added JPL staff to our team and this collaboration has become powerful and useful for us as we start thinking about these space demonstrations. The discussion of energy, which was implicitly limited to supplying energy to the earth, actually extends to space exploration. We’re opening new chapters in the way JPL thinks about future missions.

Speaking of collaboration, cross-disciplinary work has been a key part of SSPP’s success. What was it like working together so intensively over the course of a long-term project?

Hajimiri: The students, the postdocs, we all worked very closely together and learned a lot about each other’s areas. The result is something greater than the sum of its parts, both in terms of the end result of the project and in terms of the education the students receive. This training is incredibly important for the future of space technology, be it wireless power transmission, communications, space structures, or any other possible applications that we haven’t even thought of.

Atwater: I have worked in photovoltaics for a past life but never in my wildest dreams thought I would get involved in space until this opportunity presented itself. And for me it was a window into a whole new world of science. That was tremendously exciting.

Pellegrino: Sometimes it feels like we’re pushing our colleagues to do something that they clearly think is impossible, but later turns out to be not impossible. It’s just a wonderful feeling. It’s a different kind of research, where you do the best you can in your own field, but you also use the interface with other fields, a collective system that really benefits society. Benefiting society is a much more complex matter than doing a good job in one’s own field. It’s so much more challenging.