Glistening on the water, soaking into skin and sand, sunshine is part of Australia’s identity. And in Sydney, solar scientists are trying to harness the power of the sun to produce energy — but not in the way you might expect.
“We’re working on developing devices that generate electricity by emitting light instead of absorbing light,” says Jamie Hanson, a postgrad student at the University of New South Wales (UNSW). “It’s like a reverse solar panel,” he adds.
Hanson is one of a team of researchers at the university’s School of Photovoltaic and Renewable Energy Engineering, who have been looking for new ways to produce power from solar — including after the sun has set.
Energy that’s been absorbed by the Earth from the sun during the day is released at night as infrared radiation — a type of light invisible to the human eye but felt as heat. The UNSW researchers have been working on a semiconductor called a thermoradiative diode, which can convert that infrared radiation into electricity.
“If you were to look at the Earth at night, what you’d see with an infrared camera is the Earth glowing,” says Professor Ned Ekins-Daukes, who leads the team at UNSW. “What’s happening is the Earth is radiating heat out into the cold universe,” he adds.
UNSW scientists were not the first to develop a thermoradiative diode. But, building on work from Harvard and Stanford universities in the USA, the team was the first to directly demonstrate electrical power from one of these devices, in 2022.
So far, the device can generate only a very small amount of electricity — around 100,000 times less than that of a conventional solar panel.
“It’s enough to power a digital Casio wristwatch from your body heat,” says Ekins-Daukes, explaining that what determines the amount of power the diode can generate is the temperature difference between the heat source and the surrounding environment.
Even operating at optimal efficiency, Ekins-Daukes says that on Earth, the diode could generate electricity with a power density of only a single watt per square meter.
That’s because water vapor and gases like 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, as Ekins-Daukes sees it, the real potential for this technology is in space, where the absence of an atmosphere provides a much cooler surrounding environment for the diode to operate in.
He hopes the technology will be used to provide electricity to satellites. These are typically powered via solar panels, but Ekins-Daukes highlights that this has limitations, most notably during periods when the satellite is not in direct sunlight.
“Particularly in lower orbit … you have 45 minutes of sunlight and then 45 minutes of darkness,” he says. “Obviously, your solar panel only works when the sun’s shining. So, the opportunity here is … (to) use other surfaces on the spacecraft, not to totally power it, but provide some auxiliary power,” he explains.
The diode would generate electricity from the heat absorbed by the satellite while in view of the sun, as it radiates out into “incredibly cold” space during periods of darkness, Ekins-Daukes says.
Currently, during darkness satellites are powered by a battery that’s charged during periods of sunlight, but Ekins-Daukes says the diodes present an “opportunity … to squeeze a bit more power off the surface of the satellite.”
“There is a trend in space technology to make smaller satellites that fly in lower orbits, yet retain the same function as larger ones,” he says. “It is for that reason that the thermoradiative diode could be useful — it is lightweight and generates power from unused surfaces.”
The team is planning a ballon test flight this year that would allow them to trial the technology in space for the first time.
Dr Geoffrey Landis, a scientist working on thermoradiative technologies at the NASA John Glenn Research Center, says the technology could work for low orbit satellites, but would only be useful if it could be done at “a very, very low cost.”
“Batteries are cheap,” he says. “You could think about using a thermoradiative diode, but it would probably be more expensive than just using batteries for the 45 minutes,” he adds.
Instead, Landis’ research focusses on using thermoradiative diodes for satellites on deep space missions to the solar system’s outer planets, or land rovers in permanently shadowed regions of the moon.
Such missions are currently powered by special thermoelectric generators that convert heat — produced by the decay of a radioactive isotope, such as plutonium — into electricity.
“These things are heavy. They’re 45 kilograms or so, they’re about 200 liters in volume … They’re very expensive, and they’re saved for big, flagship missions because we have to make plutonium – it’s difficult to make, it’s expensive to make, and it’s a rare resource,” says Dr Stephen Polly, who works with Landis at NASA.
He says that while plutonium would still be required to provide a heat source for thermoradiative diodes in deep space, compared with conventional thermoelectric generators the diodes are much simpler and have fewer moving parts.
Many smaller diodes would be connected to each other to create a panel that looks similar to the solar cell arrays currently used to power satellites, says Polly.
“The panel itself is what’s giving off waste heat as light, so they can be much smaller, much more efficient, and be a better use of that plutonium resource,” he says.
Thermoradiative diodes are currently made of the same semiconductive materials used in night-vision goggles, but Landis says more work is needed to assess their viability when exposed to the high temperatures that decaying radioactive isotopes would produce.
Current thermoelectric systems in space which use these isotopes as heat sources operate at temperatures of around 540° or 1,000° Celsius (1,004° and 1,832° Fahrenheit).
“Nobody has ever thought to operate these types of semiconductors at higher temperatures, so we don’t know a whole lot about the longevity of this. And, for a space mission, we’d want these semiconductors to last for 10 years, 20 years, maybe even longer,” he adds.
Landis and Polly are investigating new materials for the fabrication and testing of a thermoradiative cell, which Polly says should enable the system to operate at temperatures of up to 375° Celsius (707° Fahrenheit).
He says that “if research results continue to look promising,” then the use of a thermoradiative system heated by radioactive isotopes “is certainly possible in the next five to 10 years.”
At UNSW, Ekins-Daukes’ team has received funding from the United States Air Force to perfect the diode so that it can operate more efficiently and generate greater amounts of power when used on low-Earth satellites, with radiation from the sun as the sole heat source.
His team is also looking at using different materials, similar to those used to make conventional solar cells, which Ekins-Daukes says would allow them to “piggyback” on solar cell manufacturing processes, enabling production to be upscaled more quickly when the diode becomes commercially available — which he hopes could be within the next five years.