How to Harness the Power of 70,000 Suns

NC State designs new connection to boost efficiency of stacked solar cells

2 min read
How to Harness the Power of 70,000 Suns

Stacked solar cells are already the most efficient solar cells available, but researchers at North Carolina State University have found a technique to boost the cells' effectiveness even further.

The electrical engineering team from NC State focused on the connection junctions between the layers of the stacked cells, also known as multi-junction cells. The research appeared in a paper published in the online September 5 issue of Applied Physics Letters.

Stacked solar consists of layers of cells that can capture a larger portion of the solar spectrum because photons not absorbed by the first cell are transmitted to the second and sometimes third cell, where some of the remaining solar radiation is then absorbed. 

One of the limitations of how much solar energy can be converted to electricity is the connecting junctions between the cells. The junctions absorb some portion of the solar energy and also siphon off the voltage produced by the cells, according to the researchers.

“We have discovered that by inserting a very thin film of gallium arsenide into the connecting junction of stacked cells we can virtually eliminate voltage loss without blocking any of the solar energy,” Salah Bedair, a professor of electrical engineering at NC State and senior author of the paper, said in a statement.

Currently, stacked solar can convert about 45 percent of solar energy into electricity, which is far higher than other technologies. Polycrystalline silicon solar modules, for example, are about 15 percent efficient. The goal in multi-junction solar cells is to break the 50 percent conversion efficiency barrier. Sharp currently has a world-record 44.4 percent for its triple-junction solar cell.

Multi-junction cells are used in concentrated solar power (CSP) applications, where lenses are used to concentrate the sun’s energy onto the panels. Concentrating the energy can increase the power of one sun to thousands of suns.

One of the limitations of concentrated solar power is that when the solar energy is intensified to about 700 suns or more, the connecting junctions being to lose voltage, according to the researchers.

“Now we have created a connecting junction that loses almost no voltage, even when the stacked solar cell is exposed to 70,000 suns of solar energy,” Bedair said. “And that is more than sufficient for practical purposes, since concentrating lenses are unlikely to create more than 4,000 or 5,000 suns worth of energy.” Although the novel connecting junctions would have less voltage loss, it is not exactly clear at this time how much more conversion efficiency it would ultimately allow for in commercially-produced cells. 

There are various CSP projects being built across the U.S., such as BrightSource Energy’s 370-megawatt Ivanpah project and the Middle East and South Africa are also increasingly active CSP markets.

One of the limitations of CSP is its high cost, but Bedair said that the more effective junctions should help drive down the overall cost because producers could make much smaller cells that produce as much electricity or more compared to larger cells.

Photo: NC State University

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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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