Categoria: science

PASADENA — In January, a SpaceX Falcon 9 rocket roared to life, carrying into orbit a demonstration of technologies, developed at Caltech, that could provide a blueprint for an energy revolution, supplying the earth with clean energy from outer space.

To combat climate change, the U.S. government pledged to make its energy sector pollution-free by 2035. Unfortunately, current renewable energy sources lack fossil fuels’ consistency. Solar cells, in particular, suffer atmosphere, weather and nighttime.

One elusive solution, under development for decades, is called space solar power, the process of harnessing solar power in space and beaming energy back to earth using solar power satellites.

Ten years ago, Caltech formed the Space Solar Power Project, which culminated in the Jan. 3 launch milestone. Weeks later, its three lead researchers are still musing on the achievement, enthusiastically talking about how it all works. They’re hopeful about the future of a technology once deemed technically possible but economically infeasible.

Each are experts in a discipline integral to space solar power: Dr. Harry Atwater, Otis Booth Leadership Chair of the Division of Engineering and Applied Science, heads the Project’s photovoltaics research, the solar cells converting sunlight into electricity. Dr. Ali Hajimiri, Bren Professor of Electrical Engineering and Medical Engineering, leads the wireless power transfer technology which would beam harvested energy down to earth. And Dr. Sergio Pellegrino, Joyce and Kent Kresa Professor of Aeronautics and Professor of Civil Engineering, heads the design of lightweight spacecraft that could one day carry the technology in orbit.

Why it matters and how it works

Hajimiri illustrated some of the benefits of space solar power.

“When you are in space, compared to photovoltaics on the ground, there is about 8 to 9 times more power because of the fact that you don’t have day and night, you don’t have seasons, you don’t have clouds,” he explained.

With rocket launches costing multiple dollars per gram of cargo, economics has remained a hurdle to space solar power. Caltech’s team took a new approach to the challenge with their design, cutting down on the mass and footprint of their proposed satellites.

The Project team designed a flat sandwich of components: a thin, photovoltaic cell on one side, a circuitboard in the middle, and an array of transmitters on the other side, all flexible. These sandwiches look like large, flimsy, plastic drink coasters. Combined into panels, they can be suspended in thin, flexible spacecraft, producing lightweight, foldable satellites.

While the orbiting demonstrator is only a series of experiments testing key technologies, the team hopes someday to see thousands of these satellites orbiting the earth.

The power transmitter array in its popped-up configuration. (Photo provided by Caltech) Super thin solar cells

Atwater, who speaks of his work energetically, with proud enthusiasm, designs ultra lightweight photovoltaics, minimizing weight and maximizing efficiency.

“The key guiding principle is to lower the mass to orbit,” he explained. “For a space solar cell, there are two massive elements for a conventional cell. One is that you have the active solar cell which is only a few microns thick and then it’s typically on a wafer that’s a couple hundred microns thick.”

On top of that is a radiation shield, typically a 75- to 100-micron coverglass, similar to a smartphone screen protector. (More on microns below.)

Atwater’s team launched 32 state-of-the-art solar cells, free of wafers and capable of surviving radiation without traditional coverglass, shedding expensive mass. He illustrated the material thicknesses. A human hair is about 50 microns thick. By removing wafer and coverglass, the resulting photovoltaics measure around 5 to 10 microns thick.

Wireless power transfer

Getting energy back to earth requires converting it into microwaves that are transmitted to receivers on the ground.

Hajimiri — whose enthusiasm for the subject is exemplified by his illustrative hand gestures while describing his work — and his team will test wireless power transfer in space; not as simple as pointing antennas at receivers.

Voicing an explainer video on the Project site, Hajimiri illustrated how his team directs microwaves using wave interference.

“If you go sit next to a pond and put both of your hands down into the water and make waves at the same time, what you will probably notice is that there are areas where the waves are much stronger and there are some areas where there are very little waves,” he described.

Engineers carefully lower the DOLCE portion of the Space Solar Power Demonstrator onto the Vigoride spacecraft built by Momentus. (Photo provided by Caltech) The stronger, overlapping, “in-phase” waves are where the waves adds up. Altering the timing of the transmitters (in this example, hands in a pond) allows them to be steered and focused into a beam.

“This ability to control directions by controlling timing,” Dr. Hajimiri explained, “is very critical, because it means that there are no mechanically moving parts and hence, it can be done on the timescale of electronics, at the nanosecond scale.”

The pond example describes a simplified scenario with two transmitters. At full scale, Dr. Hajimiri’s team would need to calculate the real time location and timing for billions of transmitters.

Lightweight spacecraft

The solar power panels will be suspended inside lightweight spacecraft shaped like flattened, trapezoidal picture frames, tens of meters long. Pellegrino, who has the thoughtful, precise demeanor of an engineer, is in charge of designing spacecraft that require no assembly after launch.

“Our concept does not require a robotic, in-space assembly,” Pellegrino explained. “It is a series of free-flying spacecraft and they are unfolded in space individually, and then a formation is created.”

His team’s experiments consist of a scaled-down model observable by cameras and sensors onboard the demonstrator.

“It can be flattened and it can be folded. All of that happens elastically. We do not need any mechanical hinges,” he explained.

Once the spacecraft demonstrator is unfolded in space, the team will observe the structure unfettered by gravity.

The Project team has benefitted from uncommonly long-term collaboration.

“It’s a 10-year project, so that’s pretty unusual in academia that a researcher gets a chance to work on something intensively for a decade,” Atwater pointed out. “So there have been several chapters and in both the wireless power, the ultralight structures and the photovoltaics we’ve had several generations of technology development, which is pretty exciting.”

Not science fiction anymore

If space solar power sounds like the stuff of science fiction, that’s because it was.

The first published mention of the technology was in the short story “Reason”, written by sci-fi author Isaac Asimov, in 1941. In the story, a solar power satellite operator explains: “Our beams feed these worlds energy drawn from one of those huge incandescent globes that happens to be near us. We call that globe the sun…”

The idea was first formalized as a scientifically viable energy source in 1968, by Peter E. Glaser in a paper titled “Power from the Sun: Its Future”, published in the journal Science.

After laying out the basic ideas for space solar power, Glaser proposed that the technology “…may help lead the world into an era in which an abundance of power could free man from his dependence on fire.”

The following decade saw a steady interest globally in harnessing space solar power as a viable future energy source.

Unfortunately, progress on SSP in the U.S. ground to a halt when government-funding for SSP research was cut after it was deemed economically infeasible.

Progress on space solar power in the U.S. ground to a halt when government-funding for its research was cut after being deemed economically infeasible. While other nations continued to pursue space solar power through the 1980s, the topic was out of bounds for researchers in the U.S. until the mid 1990s.

Scientists at Caltech have launched a Space Solar Power Demonstrator prototype into orbit as part of an ambitious effort to harvest solar power in space and beam that energy back to Earth, university officials in Pasadena said. (Courtesy photo) Former NASA engineer and space solar power enthusiast, John C. Mankins, author of The Case For Space Solar Power, was first introduced to the subject in 1995 while working for NASA, in Washington D.C. His supervisor asked him to revisit the concept in light of a decade’s worth of technological advancements. Mankins recalled that the 1980s funding freeze had left a subset of researchers vehemently opposed to the technology.

“When it was canceled, their careers were damaged and they became dedicated opponents of space solar power,” Mankins recalled. “So when you were in these meetings in the 1990s you could not bring up space solar power without running the risk of ridicule.”

In the 1990s, the U.S. government resumed funding for space solar power research.

Atwater made sure to acknowledge that his team relied on decades of publicly funded  research that informed their efforts.

Flashforward to 2013, when after learning about space-based solar energy generation, billionaire philanthropist Donald Bren, chairman of Irvine Company and a lifetime member of the Caltech Board of Trustees, and his wife Brigette, also a Caltech trustee, agreed to a 10-year, $100 million commitment to help start the project.

A unique aspect of Caltech’s project is that the research, development, and execution of the demonstrator all relied solely on private funding.

“It’s kind of remarkable,” Atwater pointed out, “that it was a privately funded project, flying on a private commercial spacecraft launched by a private launch provider. So it’s completely different, you know, 20 years ago it would have been all government.”

For the researchers, the long-term collaboration culminated in the explosive moment when the demonstrator launched into space last month.

“It was sort of ecstatic with all three of us, all three PIs, myself, Sergio and Ali were all there cheering,” Atwater said. “There was a crowd of people cheering when the spacecraft deployed. It all felt very real.”

The Project demonstrator is currently riding a Momentus Vigoride orbital transfer vehicle, a sort of school bus for spacecraft that travels between orbits, dropping off satellites at their individual destinations. The Caltech team eagerly awaits the data that could help shape Earth’s energy future.