The sun’s rays glistening on the water and seeping into the skin and sand are part of Australia’s identity. And in Sydney, solar scientists are harnessing the sun’s power to produce energy, but not in the way you might expect.
“We’re working on a device that generates electricity by emitting light rather than absorbing it,” said Jamie Hanson, a graduate student at the University of New South Wales (UNSW). “It’s like a solar panel upside down,” he added.
Hanson is part of a team of researchers in the university’s School of Photovoltaic and Renewable Energy Engineering who have been exploring new ways to produce electricity from solar power. rear The sun has set.
Energy absorbed by the Earth from the Sun during the day is emitted as infrared radiation at night. Infrared light is invisible to the human eye, but is a type of light that can be felt as heat. UNSW researchers are working on a semiconductor called a thermally emitting diode that can convert infrared light into electricity.
“When you look at the Earth at night, what you see with an infrared camera is that it’s glowing,” says Professor Ned Ekins-Dawkes, who leads the team at UNSW. “What’s happening is the Earth is radiating heat into cold space,” he added.
UNSW scientists were not the first to develop thermally emitting diodes. However, based on research from Harvard University and Stanford University in the US, the team conducted the first experiment to directly demonstrate power from one of these devices in 2022.
So far, this device can only generate very small amounts of electricity. This is about 100,000 times cheaper than traditional solar panels.
“It’s enough to power a Casio digital watch from your body heat,” Ekins-Doakes says, explaining that it’s the temperature difference between the heat source and the surrounding environment that determines how much power the diode can generate.
Even when operating at optimal efficiency, diodes would be able to generate power at a power density of just 1 watt per square meter on Earth, Ekins-Doakes said.
This is because gases such as water vapor and carbon dioxide in the atmosphere also absorb heat from the sun, reducing the temperature difference between the earth’s surface and the night sky.
But in Ekins-Doakes’ view, the technology’s real potential lies in space, where the absence of an atmosphere provides a cooler ambient environment for the diodes to operate.
He hopes the technology will be used to power satellites. These are typically powered via solar panels, but Ekins-Dawkes stresses that this has limitations, particularly during periods when the satellite is not in direct sunlight.
“Especially in low Earth orbit… you have 45 minutes of sunlight and then 45 minutes of darkness,” he says. “Obviously, solar panels only work when the sun is shining, so the opportunity here is… to use other surfaces on the spacecraft and provide supplemental power rather than fully powering it,” he explains.
The diode will generate electricity from the heat absorbed by the satellite while the sun is visible, which is then radiated into space, which is “incredibly cold” during hours of darkness, Ekins-Dawkes said.
Currently, during darkness, the satellite is powered by a battery that is charged during sunlight hours. But Ekins-Dawkes said the diodes offer “an opportunity to squeeze a little more power out of the surface of the satellite.”
“There is a trend in space technology to create smaller satellites that retain the same functionality as larger satellites while flying in lower orbits,” he says. “This is why thermal radiating diodes are useful: they are lightweight and generate power from unused surfaces.”
The team is planning a balloon test flight this year that could test the technology in space for the first time.
Dr. Jeffrey Landis, a scientist working on thermal radiation technology at NASA’s John Glenn Research Center, said the technology could work on low-orbit satellites, but only if it could be done at “very low cost.”
“Batteries are cheap,” he says. “You could use a thermal emitting diode, but it would probably be more expensive than using a battery for 45 minutes,” he added.
Instead, Landis’ research focuses on using thermally emitting diodes on satellites for deep-space missions to the outer planets of the solar system and for land probes on permanently shadowed regions of the moon.
Such missions are currently realized by special systems. thermoelectric generator It converts the heat produced by the decay of radioactive isotopes such as plutonium into electricity.
“These things are heavy. They weigh 45 kilograms or so and have a volume of about 200 liters… They’re saved for large flagship missions because they’re very expensive and they have to make plutonium. Plutonium is a hard to make, expensive, scarce resource,” says Dr. Stephen Polly, who works with Landis at NASA.
He said plutonium is still needed to provide the heat source for thermally emitting diodes in deep space, but diodes are much simpler and have fewer moving parts than traditional thermoelectric generators.
Many small diodes will be connected together to create a panel similar to the solar arrays currently used to power satellites, Polly said.
“Because the panels themselves are emitting waste heat as light, they can be made much smaller and more efficient, making better use of the plutonium resource,” he says.
Thermal-emitting diodes are currently made from the same semiconductor material used in night-vision goggles, but more research is needed to assess their survivability when exposed to high temperatures that produce decaying radioactive isotopes, Landis said.
Current thermoelectric systems in space that use these isotopes as heat sources are temperature Approximately 540 degrees Celsius or 1,000 degrees Celsius (1,004 degrees Fahrenheit and 1,832 degrees Fahrenheit).
“We don’t really know how long this semiconductor will last because no one has thought of operating this kind of semiconductor at high temperatures. And for space missions, we hope these semiconductors will last 10, 20 years, maybe even longer,” he added.
Landis and Polly are investigating. new material It will be used to manufacture and test thermal radiation cells, and Polly says the system should be able to operate at temperatures up to 375 degrees Celsius (707 degrees Fahrenheit).
He said that “if research results remain promising,” the use of thermal radiation systems heated by radioactive isotopes “will certainly become possible within the next five to 10 years.”
At UNSW, Ekins-Dawkes’ team received funding from the US Air Force to perfect the diode, allowing it to operate more efficiently and generate more power when used in low-Earth satellites using radiation from the sun as their sole heat source.
Ekins-Doakes said his team is also considering using different materials similar to those used to make conventional solar cells, which would allow them to “piggyback” on the solar cell manufacturing process and scale up production more quickly when the diode becomes commercially available, which he hopes will happen within the next five years.