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SBSP test
An image from Caltech’s SSPD-1 mission, showing arrays for transmitting (right) and receiving wireless power mounted on the Vigoride-5 bus. (credit: Caltech SSPP)

A small step forward for space-based solar power technology

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Space-based solar power (SBSP) is one of those concepts endlessly debated without little obvious progress or resolution of those debates. For more than half a century, advocates have described SBSP as a solution to growing energy demands while also serving as a source of green energy in an era of growing alarm about climate change. Critics describe the severe technological challenges and costs that make large-scale SBSP unrealistic to them.

The debates have taken place with very little work to advance SBSP technology itself. Most of the recent work on SBSP has been with studies examining broad questions of feasibility. Actual technical work has been very limited.

The experiments fit to a specific architecture for a space solar power system called the “Caltech concept,” explained Pellegrino. “It is actually not a single giant piece of infrastructure but a constellation of spacecraft.”

An exception to this is a project at the California Institute of Technology called the Space Solar Power Project. That effort, funded primarily through a $100 million donation by real estate investor Donald Bren a decade ago, has been working on several new technologies that, if successful, would bolster the case for space-based solar power. And that project is wrapping up its first mission to test those technologies in space.

The Space Solar Power Demonstrator (SSPD-1) mission launched in January as a hosted payload on the Vigoride-5 orbital transfer vehicle by Momentus. The payload included three separate experiments designed to test key technologies in photovoltaics, large structures, and wireless power transmission needed for SBSP.

All three fit into a specific architecture for a space solar power system called the “Caltech concept,” explained Sergio Pellegrino, one of the Caltech professors involved with the project, during a presentation last month at a Caltech alumni event. “It is actually not a single giant piece of infrastructure but a constellation of spacecraft.”

That concept calls not for a large, monolithic solar array but instead a cluster of mass-produced smaller spacecraft, deploying solar arrays 60 by 60 meters across. “It’s a scalable system. You make it larger simply by adding to it.”

Pellegrino led one of the three experiments on SSPD-1, the Deployable on-Orbit ultraLight Composite Experiment (DOLCE). It was designed to test one way to deploy the arrays by curling them up for launch and then using a set of booms and cables to move them into place.

DOLCE tested the technique for an array 1.7 meters on a side. The initial phases went well in May, he said, including the deployment of one boom with a camera to monitor later phases of the deployment. However, one of the four diagonal booms used to deploy the array itself stopped short of full deployment, which he said was likely caused by a frayed cable.

The project team spent the next few months studying the problem, trying to move the boom only a few millimeters at a time. “We were worried that, if we did the wrong thing, we would break that connection,” he said.

Those efforts took on new urgency by September given concerns the mission might soon be ending. Engineers commanded larger motions of the boom and, though some trial and error, they were able to get the array nearly fully deployed by late September. “We are satisfied with our accomplishment. We learned a huge amount from all this,” Pellegrino said.

Another experiment on SSPD-1 looked at solar cells. While that technology may seem mature, Harry Atwater, a Caltech professor of applied physics and materials science, noted that solar cells used in space have a much higher performance than terrestrial solar cells, producing ten times the power per unit mass as terrestrial cells—just at 100 times the cost per square meter. “If you’re now building spacecraft at the hundreds of meters to kilometer scale, this is not going to be a scalable proposition.”

That meant rethinking how solar cells are produced. The traditional approach, he said, is to invent a new cell, think about how to make it, and only then do an economic analysis. Here, “we’re going to start with an economic analysis, what kind of cell manufacturing process would be compatible with that, and then, how do we make the cell?”

“If you’re now building spacecraft at the hundreds of meters to kilometer scale,” Atwater said of conventional solar cells, “this is not going to be a scalable proposition.”

That work led them to new lightweight solar cells that would produce 33 times as much power per unit mass as traditional space-rated solar cells but cost the same to manufacture as terrestrial cells. “What that means is that we’re going to be able to take advantage of the kinds of production tools for solar power that are already scaled on Earth,” he said.

The ALBA experiment on SSPD-1 tested 32 candidate cells. Those came together quickly: Atwater said the cells were produced in November and integrated on the spacecraft in December for its launch in early January. Some of the cells used ultralight perovskite that is sensitive to moisture: not an issue in space, but exposure to moisture in pre-launch processing in Florida did damage the cells. (He added later the moisture was not linked to the Vigoride tug’s own water-based thrusters.)

“We learned a great deal about a variety of solar technologies that had never been tested before,” he said.

The third experiment on SSPD-1 was Microwave Array for Power-transfer Low-orbit Experiment (MAPLE). It was intended to test beamforming technology placed in the arrays for transmitting the power they create down to Earth using microwaves.

That requires dealing with a “dynamically active” structure that is deployed after launch, said Ali Hajimiri, a Caltech electrical engineering professor. That means that the structure needs to know its own shape to be able to adjust timing to make the beamforming work properly.

MAPLE planned to test that technology in space, using a transmitter and two receivers about 30 centimeters away on the spacecraft. “We can send energy dynamically to different places using our flexible structure and integrated circuits to do that,” he said.

The experiment then tried something more challenging that was “a little bit of an afterthought,” he said: could the transmitter on the spacecraft direct a beam down to Earth and be detected? The project tried it, setting up a receiver on the roof of a Caltech lab that tracked the spacecraft as it passed overhead. To their surprise, they were able to detect the microwaves, even on a “dry run” just to test the equipment.

SSPD-1 is wrapping up: Hajimiri said that the mission was ending in “a couple weeks,” although there has not been any formal announcement yet by Caltech that the project was ending.

All three researchers involved with the mission said they’re not planning an immediate followup, but instead taking the time to digest the results from the mission and plan next steps. “I described the work we did as lab to orbit. Now it’s time to go back to the lab,” said Atwater.

“We’ve learned a lot, so there’s more research coming,” said Pellegrino. A long-term vision is an integrated experiment that combines the deployable array technology, new photovoltaic cells, and power beaming into a single package. That could fly in space, he said, but “this is thinking several years out.”

“To use a blood type analogy, it’s the O negative of all powers,” Hajimiri said of SBSP. “sYou can administer it at any time.”

Notably, while achieving considerable success on this mission, the researchers are still cautious about the long-term potential of SBSP. Atwater said the project has done a “preliminary techno-economic analysis” that concluded a scaled-up space solar power system might produce power at rates of tens of cents to one dollar per kilowatt-hour. That’s higher than most terrestrial power, he acknowledged, although there are cases when it might be competitive.

Hajimiri suggested it might be useful in remote areas or in disaster or emergency zones where there are not terrestrial alternatives (but, presumably, the ability to get a large receiving rectenna in place.) “To use a blood type analogy, it’s the O negative of all powers. You can administer it at any time.”

Pellegrino added that the concept uses large numbers of identical spacecraft, which will drive down costs in ways that conventional models used for space missions don’t capture well. “That’s exactly the scaling of the PV from Earth,” he said. “That revolution has already happened. We just need to make it happen for space solar power.”

Whether that revolution happens remains to be seen, but at least the project is moving the debate about SBSP from positions little changed in the last few decades.

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