(Left to Right) Professors Sergio Pellegrino, Harry Atwater, and Ali Hajimiri. (Image: Steve Babuljak for Caltech)

Once considered science fiction, technology capable of collecting solar power in space and beaming it to Earth to provide a global supply of clean and affordable energy is moving closer to reality. Through the Space-based Solar Power Project (SSPP), a team of Caltech researchers is working to deploy a constellation of modular spacecraft that collect sunlight, transform it into electricity, then wirelessly transmit that electricity wherever it is needed — including to places that currently have no access to reliable power.

"This is an extraordinary and unprecedented project," said SSPP researcher Harry Atwater, "It exemplifies the boldness and ambition needed to address one of the most significant challenges of our time, providing clean and affordable energy to the world."

The project is led jointly by Atwater, Ali Hajimiri, and Sergio Pellegrino.

Harnessing solar power in space relies on breakthrough advances in three main areas:

  • Atwater's research group is designing ultralight high-efficiency photovoltaics that are optimized for space conditions and compatible with an integrated modular power conversion and transmission system.
  • Hajimiri's research team is developing the low-cost and lightweight technology needed to convert direct current power to radio frequency power that can be sent to Earth as microwaves. The process is safe, Hajimiri explains. Non-ionizing radiation at the surface is significantly less harmful than standing in the sun. In addition, the system could be quickly shut down in the event of damage or malfunction.
  • Pellegrino's group is inventing foldable, ultrathin, and ultralight space structures to support the photovoltaics as well as the components needed to convert, transmit, and steer radio frequency power to where it is needed.

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.

The vision:

Sergio Pellegrino: “It was more than 10 years ago, in 2011, that conversations began with Donald Bren asking whether Caltech had any ideas when it came to research in the field of sustainable energy and space. We started discussing, in a group of faculty members, ways of building on our interests and what was happening in each of our areas that might lead to a very impactful research initiative. Over a period of a few months, we came up with a vision — I called it a dream — of three or four technology breakthroughs that, in combination, would transform the way space solar power had been previously approached.”

Ali Hajimiri: “This concept was, in the past, truly science fiction. What made it possible for us to consider taking it from the realm of science fiction to the realm of reality was the combination of developments happening in photovoltaics in Harry's lab, in structures in Sergio's lab, and in wireless power transfer, which is happening in my lab. We realized that we can now pursue space solar power in a way that is becoming both practical and economical.”

One of the first questions that anyone asks is, "Why do you want to put photovoltaics in space? Well, in space, where you don't have day and night and clouds and things of that sort, you get about eight times more energy. The vision of this program is to be able to provide as much power as you need, where you need it, and when you need it.”

Progress report:

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 power. Through that process, we learned many things about how to design highly integrated and ultralight systems of this sort. We then developed a second prototype, 33 percent lighter than the first.”

Hajimiri: “This tile is the building block, as Sergio mentioned, of the larger system. It has to be fully functional, compatible, and scalable. Although it may sound simple, it's actually quite sophisticated. These tiles are mounted on a very flexible structure that can be folded to fit in a launch vehicle. Once deployed, the structure expands, and the tiles work in concert and in synchronization to generate energy, convert it, and transfer it exactly where you need it and nowhere else.”

The next phase:

Atwater: “It doesn't get real until you actually go to space. As Sergio and Ali described, we demonstrated this key unit element, called a tile, in our labs. One of the lessons from that series of demonstrations was that the pathway we needed to follow for photovoltaics fundamentally had to change. We were working with what I'll call conventional photovoltaic materials, which had to be designed in a form that was going to make it difficult to reach the mass-per-unit area and specific power goals, so we had to rethink the photovoltaic strategy completely. As a result, the classes of photovoltaic devices that we are testing in space have actually never flown in space before.”

Pellegrino: “Most spacecraft today have solar arrays — photovoltaic cells bonded to a carrier structure — but not with this type of material and not folded to the dimensions we've achieved. By using novel folding techniques, inspired by origami, we are able to significantly reduce the dimensions of a giant spacecraft for launch. The packaging is so tight as to be essentially free of any voids.”

Hajimiri: “Wireless power transfer of this nature has not been demonstrated in space. We are also demonstrating it with our flexible, lightweight material, not necessarily a rigid structure. That adds complexity.”

Impact on Society:

Hajimiri: “It is going to revolutionize the nature of energy and access to it so that it becomes ubiquitous, it becomes dispatchable energy. You can send it where you need it. This redirection of energy is done without any mechanical movements, purely through electrical means using a focusing array, which makes it extremely fast.”

Atwater: “I think one can say that the vision really was to do something that, as Ali mentioned, originally emerged almost from science fiction, to do something that would become a large-scale energy source for the world.”

Pellegrino: “We have had JPL collaborators join our team, and that collaboration has become powerful and useful to us as we start thinking about these space demonstrations. The discussion about energy that was implicitly limited to powering the earth actually extends to space exploration also. We're opening new chapters in the way JPL is thinking about future missions.”