High-Efficiency Solar Cells Power Satellites—Can They Come Down to Earth?

NREL's push for a 50% efficient solar cell finds more use outside Earth's atmosphere than on the ground—so far

3 min read
6-junction solar cell
Scientists prepare their new 6-junction solar cell for testing on the T-HIPSS tool in the Solar Energy Research Facility (SERF).
Photo: Dennis Schroeder/NREL

Following the arrival of a fresh fleet of robot explorers, the eyes of the space-watching world may be fixed on Mars right now, but later this year, NASA’s ambitious Lucy mission is set to steal some of the limelight. Launching on October 16 on a decade-plus-long mission, Lucy is bound for the trojans, distant asteroids that follow in Jupiter’s orbit.

Most previous forays to the outer solar system relied on expensive radioisotope thermoelectric generators, such as with the Voyager and New Horizons probes. Instead, Lucy carries a pair of solar panel arrays. They unfurl like giant hand fans, each array 6 meters (20 feet) in diameter. Going beyond Jupiter’s orbit will take Lucy farther from the Sun than any prior solar panel—the previous record was for Juno, which is currently in Jovian orbit.

The “extreme power requirements” out there are “pretty much the crux of the reason why the solar array is so huge and amazing,” says Katie Oakman, Lucy’s Spacecraft Structures and Mechanisms Lead.

This has meant pushing photovolatic research in different directions to research that has focused on developing solar power for use on Earth. While cheap silicon photovoltaic cells fuel the clean energy transition on Earth, space solar must rely on other types of solar panels.

Conditions vary, but photovoltaics in space face a number of challenges. Even in Earth’s humble orbit, satellites operate outside the protection of an atmosphere. That means space-based solar panels face drastic temperature swings between cold shade and searing sunlight. And that sunlight is different: it has more ultraviolet than we see on the ground, for instance.

There’s another danger in space: ionizing radiation. Solar panels in Earth’s orbit may face the maelstrom of the Van Allen belts while solar panels elsewhere might need to weather the Sun’s solar wind. Over time, such radiation eats away at solar cells’ effectiveness. A solar cell that spends 15 years in Earth’s geosynchronous orbit, about 36,000 kilometres (22,000 miles) high, will drop to around 80 percent effectiveness.

The very early satellites of the 1950s and 1960s used silicon photovoltaic cells. But designers soon began to look at so-called multi-junction (MJ) III-V solar cells, making them from layers of multiple materials that each absorbed a different wavelength of light. MJ cells are more efficient, more resilient in space’s unearthly conditions, and lighter.

”Space industries have used III-V solar cells for quite some time now, due to those things,” says Ryan France, a scientist at the National Renewable Energy Laboratory (NREL) in Golden, Colorado, outside Denver.

NREL pioneered MJ III-V cells in past decades, and more recently, researchers there have pushed MJ cells towards the vaunted 50 percent efficiency milestone. Most MJ cells consist of three layers, but in 2020, NREL researchers presented a six-junction cell that reached 47 percent efficiency under concentrated sunlight, superior to silicon photovoltaic cells.

Bringing it Back to Earth

On Earth, however, MJ cells face an economic obstacle: Silicon photovoltaic cells’ low cost and overwhelming market dominance, combined, make it a struggle to commercialize MJ cells. Silicon cell prices have fallen sixtyfold since 1980, in a decline that shows little sign of stopping.

So far, MJ cells are largely used in concentrated solar power, which uses mirrors or lenses to focus and intensify sunlight upon the solar panel. This generates impressive efficiency figures, but outside the lab, it hasn’t been able to compete with cheap silicon.

But NREL researchers have now turned their focus away from the 50 percent performance mark in favor of lowering the costs of MJ cells. “The more we lower the costs,” France says, “the more markets we potentially have.” They want to pave the road for MJ cells to reach new applications, although it’s not yet clear what all those applications might be.

But cheaper solar cells will no doubt benefit space applications in turn. And Oakman thinks that unfurling solar panels like Lucy’s also have a place in humanity’s off-world future—especially near Jupiter, where temperatures and light levels are much, much lower than near Earth.

“If we want to continue to send spacecraft to the outer solar system,” she says, “then I think there are all kinds of opportunities to use this type of design in the future.”

The Conversation (0)

Smokey the AI

Smart image analysis algorithms, fed by cameras carried by drones and ground vehicles, can help power companies prevent forest fires

7 min read
Smokey the AI

The 2021 Dixie Fire in northern California is suspected of being caused by Pacific Gas & Electric's equipment. The fire is the second-largest in California history.

Robyn Beck/AFP/Getty Images

The 2020 fire season in the United States was the worst in at least 70 years, with some 4 million hectares burned on the west coast alone. These West Coast fires killed at least 37 people, destroyed hundreds of structures, caused nearly US $20 billion in damage, and filled the air with smoke that threatened the health of millions of people. And this was on top of a 2018 fire season that burned more than 700,000 hectares of land in California, and a 2019-to-2020 wildfire season in Australia that torched nearly 18 million hectares.

While some of these fires started from human carelessness—or arson—far too many were sparked and spread by the electrical power infrastructure and power lines. The California Department of Forestry and Fire Protection (Cal Fire) calculates that nearly 100,000 burned hectares of those 2018 California fires were the fault of the electric power infrastructure, including the devastating Camp Fire, which wiped out most of the town of Paradise. And in July of this year, Pacific Gas & Electric indicated that blown fuses on one of its utility poles may have sparked the Dixie Fire, which burned nearly 400,000 hectares.

Until these recent disasters, most people, even those living in vulnerable areas, didn't give much thought to the fire risk from the electrical infrastructure. Power companies trim trees and inspect lines on a regular—if not particularly frequent—basis.

However, the frequency of these inspections has changed little over the years, even though climate change is causing drier and hotter weather conditions that lead up to more intense wildfires. In addition, many key electrical components are beyond their shelf lives, including insulators, transformers, arrestors, and splices that are more than 40 years old. Many transmission towers, most built for a 40-year lifespan, are entering their final decade.

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