New Optical Antennas Harvest 100 Times More Electricity from Heat

Quantum tunneling helps electrons pass through barriers in new "rectenna" device

2 min read
This scanning electron microscope image shows the distinct bow tie shape of an optical rectenna.
This scanning electron microscope image shows the distinct bow tie shape of an optical rectenna.
Image: Moddel Lab

The world's most efficient optical rectennas yet can harvest over 100 times more energy from waste heat compared to previous devices, although a new study finds that much work is needed before they can achieve practical value.

Rectennas—short for "rectifying antennas"—pick up electromagnetic waves much like car antennas. When a rectenna’s antenna receives a signal, it generates oscillating charges that move through attached rectifier diodes. These rectifiers then convert these fluctuations to a direct electric current. 

In theory, rectennas could harvest energy from heat that would ordinarily go to waste. "It would be great if these could help out with climate change," says study lead author Amina Belkadi, an electrical engineer at the University of Colorado at Boulder. "You could imagine adding them to solar cells so you can get even more energy from them."

However, the conversion efficiency of optical rectennas has proven far too low to make them useful for such applications. The problem is that in order to capture thermal radiation, rectennas have to be extraordinarily tiny, but the smaller they are, the more their resistance grows, which can shrink their power output.

Now Belkadi and her colleagues have found a way to dramatically boost optical rectenna efficiency using a quantum effect roughly equivalent to electrons walking through walls. They detailed their findings online May 18 in the journal Nature Communications.

In conventional rectennas, electrons must pass through an insulator to generate power. These insulators add a lot of resistance, reducing the amount of electricity these devices can produce.

Using a counterintuitive strategy, the researchers added two insulators to their rectennas instead of just one. If the right thickness and materials are chosen for this narrow trench—a so-called "quantum well"—electrons hitting it with just the right amount of energy can tunnel past it, experiencing no resistance in the process. This is the first time scientists have proven such "resonant tunneling" is possible with optical rectennas.

The scientists tested an array of more than 250,000 bowtie-shaped rectennas made of nickel, nickel oxide, aluminum oxide, chromium and gold that were each only rough 11 nm long and 6 nm wide. They found their devices displayed conversion efficiencies 100 to 1000 times greater than previous optical rectennas.

Still, the conversion efficiencies of these new rectennas remains small—just 0.001%. "We still have a long way to go," Belkadi says.

One potential way to further boost optical rectenna efficiency is to experiment with different materials that more electrons can pass through. "Perhaps we can push the conversion efficiency another 1,000 times," Belkadi says. "At conversion efficiencies of 1 to 2%, given the huge amount of energy lost as waste heat, people are going to start slapping such rectennas on walls."

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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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