Did a NASA study pull the plug on space solar power?
by Jeff Foust
|“We found that these space-based solar power designs are expensive. They are 12 to 80 times more expensive than if you were going to have renewable energy on the ground,” said Rodgers.
Well, as the old saying goes, be careful what you wish for: you may get it. The report, originally expected to be released in the fall of 2022, was finally published by NASA’s Office of Technology, Policy and Strategy (OTPS) last month. The result was, for those advocates, a stunningly negative assessment of the economics of solar power from space as an alternative to terrestrial renewable energy sources.
“We found that these space-based solar power designs are expensive. They are 12 to 80 times more expensive than if you were going to have renewable energy on the ground,” said Erica Rodgers, science and technology partnership forum lead in NASA’s Office of the Chief Technologist, in a talk at AIAA’s SciTech Forum conference in Orlando, Florida, January 11, where the agency unveiled the report.
The NASA study examined two architectures previously proposed for SBSP systems, one “innovative” approach that uses heliostats to collect and focus sunlight and the other, more “mature,” concept that uses planar arrays. Each system would generate two gigawatts of power. The study examined the costs to build, operate, and eventually decommission each system, based on various assumptions.
The study concluded that the innovative design, called representative design 1 or RD1 in the report, would produce power at a lifecycle cost of $0.61 per kilowatt-hour. The mature alternative, RD2, came in at $1.59 per kilowatt-hour. By contrast, the average energy cost to American consumers is $0.167 per kilowatt-hour, and terrestrial renewable systems—hydropower, nuclear, wind, and terrestrial solar—offer costs as low as $0.02 to 0.05 per kilowatt-hour.
“Therefore, our baseline analysis of SBSP designs does not return cost competitive results relative to terrestrial alternatives,” the report concluded.
Moreover, the study found that space solar power was not significantly greener that terrestrial alternatives. The study measured the greenhouse gas “emissions intensity” of the two SBSP systems, measured as the grams of carbon dioxide equivalent produced per kilowatt-hour. That figure was 26 for RD1 and 40 for RD2, about an order of magnitude lower than the US electrical grid average of 385. But, the report added, terrestrial renewable systems have similar emissions intensities as SBSP systems.
The two SBSP systems in the NASA study, Representative Design One and Two, fared poorly against terrestrial renewable energy sources. (credit: NASA)
The study’s results both disappointed and frustrated supporters of space solar power. “It’s just weird,” said an exasperated John Mankins in an interview. He is a longtime proponent of SBSP who led NASA’s “fresh look” study of the topic in the late 1990s. The RD1 architecture included in the study is based on a design he developed through a study funded by the NASA Innovative Advanced Concepts (NIAC) program more than a decade ago, the most recent NASA-funded study of SBSP before this new report.
The problem with the report, he said, was not with its methodology. “The things that I thought were most admirable were the general methodology, the modeling, and the economic emphasis,” he said. “They looked at lots of different cases and tried to model a wide variety of different parameters.”
What was a problem, though, was what values the study used for those parameters. “It seems to be driven entirely by a wide variety of assumptions that are, in combination, the worst possible of the worst possible cases from years ago,” he argued.
|“It seems to be driven entirely by a wide variety of assumptions that are, in combination, the worst possible of the worst possible cases from years ago,” Mankins argued.
Perhaps the biggest issue was with how the report accounted for launching the systems. Launch was the biggest cost factor, accounting for more than 70% of the costs of the two architectures. The report assumed SpaceX’s Starship would be used for launching the systems because its capacity is the largest among those vehicles with data available to NASA.
However, the report assumed a Starship launch cost of $1,000 per kilogram, with a 15% block buy discount. That is not much less than current Falcon 9 and Falcon Heavy prices, while SpaceX has been promoting much steeper cost reductions for Starship. At a panel discussion last week during the Space Mobility conference, also in Orlando, officials from both SpaceX and the Air Force Research Lab projected a future where Starship costs would approach as little as $20 to 30 per kilogram. There is plenty of time to try to reach that cost: the NASA study projected that the SBSP systems would be launched in the 2040s.
Mankins noted that SpaceX is not alone in seeking to sharply reduce launch costs. “If it was really true that everybody believed that there was never going to be any improvement in launch beyond the Falcon 9 reusable, I don’t think Blue Origin would be wasting their time working on New Glenn,” he said.
He also took issue with how the system was launched as well. The NASA study followed an approach similar to what SpaceX is using for the Human Landing System (HLS), launching tanker Starships to a propellant depot in low Earth orbit, which would then fuel another Starship carrying the SBSP components so it could deliver them to geostationary orbit. (The NASA study assumed a dozen tankers per payload Starship, citing figures used in unsuccessful protests by Blue Origin and Dynetics of SpaceX’s HLS award in 2021.) That payload Starship would then be discarded. Mankins argued that using tugs with electric propulsion would be a far more efficient approach for getting SBSP hardware to GEO, sharply reducing the number of launches.
The NASA study did examine how lower launch costs and the use of electric propulsion would affect launch costs. It also took in other factors such as increased lifetime of the SBSP hardware and improved solar cell efficiency. When all combined, it produced systems with lifecycle costs of $0.04 to 0.08 per kilowatt-hour, comparable to terrestrial renewable alternatives. It also reduced the emissions intensity to levels below terrestrial systems.
That approach bothered Mankins. “Normally, when you’re doing a systems analysis study, you pick a middle-of-the-road estimate” as the baseline, he said. In this study, though, every change to the parameters they tested resulted in lowering the cost of the system, making the baseline more of a worst-case estimate. “They took the very worst possible combination of parameters.”
His frustration was shared by other advocates of space solar power. In a January 17 statement, the National Space Society “congratulated” NASA for publishing the report while also suggesting there is room for improvement. “NSS feels that the NASA OTPS report, while a welcome revisiting of a technology that was first conceived in the US, offers an incomplete assessment of the viability of space-based solar power as a solution to the world’s rapidly expanding energy needs,” it stated.
The Space Frontier Foundation took a slightly different approach. “This report retires the concerns that space based solar power is science fiction, and shows that NASA and the US government are recognizing the climate-friendly economic benefits of global leadership of this new energy system,” said the organization’s executive director, Sean Mahoney.
However, it added that the organization wants to work with other government agencies, like the Department of Energy (DOE) and Department of Defense (DOD) as well as companies in the utility industry, to advance SBSP. “Timely progress on SBSP will require a whole-of-government approach involving NASA, DOE, DOD, and the commercial space and energy sectors,” said Srikanth Raviprasad, the Space Frontier Foundation’s SBSP project manager.
|“Solar power beamed from space at commercial rates, lighting the globe, is still a future prospect,” said Caltech’s Rosenbaum, but added it should be “an achievable future.”
Mankins thinks the issue is a lack of interest at NASA about space solar power. “NASA really doesn’t want to work on this in general. NASA has its major missions. They’re Earth and space science, human spaceflight, and aeronautics, period. NASA has never really been looking for an additional mission,” he said. “I think the assumptions that drove the systems modeling in this report are a reflection of that.”
NASA, in the report, said that some of the technologies it is working on now for other applications, like in-space assembly and manufacturing, could also benefit space solar power. “NASA could maintain its focus on core Agency missions and technologies, while documenting their relevance to SBSP,” the report stated, recommending “regular reviews of global SBSP developments and focused analyses of SBSP designs.”
It's unclear that other agencies are willing to take on SBSP, though. The Space Frontier Foundation noted in its statement that the technology is among those included in a bill that passed the House late last year regarding interagency cooperation between NASA and DOE. But DOE itself has shown little public interest—or, more importantly, funding—for the technology yet. Similarly, an amendment to the House’s Commercial Space Act approved by the House Science Committee in November would direct NASA and the Office of Space Commerce to collaborate on a new SBSP report, but again without funding.
There is international interest in SBSP, something that NASA’s Rodgers noted at the AIAA conference. “We were motivated because space-based solar power research is picking up globally. It’s been accelerating over the past five years,” she said. “We wanted to better understand why there’s this acceleration.”
Mankins, though, worried the NASA report might dampen that enthusiasm. “That will have a significant chilling influence, particularly in Europe and the UK, and in the US and among US companies,” he cautioned. “In the US, any document that comes out with the [NASA] meatball on it is regarded almost as gospel.”
All those efforts, though, are still in their early stages with modest amounts of funding: while ESA, for example, backed a SBSP study project called Solaris at its 2022 ministerial meeting, it is spending only a few tens of millions of euros on it.
One of the best funded SBSP efforts in the West has been a private one: the Space Solar Power Project at the California Institute of Technology, established with a $100 million donation from real estate investor Donald Bren a decade ago and a smaller contribution from Northrop Grumman. That effort resulted in a technology demonstration flown last year as a hosted payload to test deployable strucutres, wireless power transmission and advanced solar cells (see “A small step forward for space-based solar power technology”, The Space Review, November 13, 2023.)
Last month, days after the release of the NASA report, Caltech confirmed that that flight experiment had ended, as expected. The project is not planning any immediate followup flight experiments, as the project leaders said last fall, instead working in the lab “to identify the next set of fundamental research challenges for the project to tackle,” according to a Caltech statement.
“Solar power beamed from space at commercial rates, lighting the globe, is still a future prospect,” Thomas Rosenbaum, president of Caltech, said in a statement. “But this critical mission demonstrated that it should be an achievable future.” Exactly when—and if—that future will be achieved, and how, remains uncertain.
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