Scientists at the University of New South Wales (UNSW) in Sydney, Australia, are developing devices that generate electricity by emitting light, described as "reverse solar panels." This research focuses on harnessing energy released as infrared radiation, particularly at night, after the Earth has absorbed solar energy during the day.
Thermoradiative Diode Technology
The UNSW team, led by Professor Ned Ekins-Daukes, is working on a semiconductor called a thermoradiative diode. This device converts infrared radiation, which is felt as heat, into electricity. While other institutions had previously worked on this technology, the UNSW team was the first to directly demonstrate electrical power from such a device in 2022.
Currently, the device generates a small amount of electricity, approximately 100,000 times less than a conventional solar panel. This output is determined by the temperature difference between the heat source and its environment. On Earth, factors like water vapor and gases in the atmosphere reduce this temperature difference, limiting the diode's power density to about one watt per square meter.
Space Applications
Researchers believe the technology's primary potential lies in space, where the absence of an atmosphere provides a much colder environment for the diode to operate efficiently.
- Low-Earth Orbit Satellites: Ekins-Daukes suggests the diodes could provide auxiliary power to satellites, especially during periods of darkness when solar panels are inactive. Satellites in low orbit experience cycles of sunlight and darkness, typically relying on batteries during dark phases. Thermoradiative diodes could generate additional power from heat absorbed by the satellite radiating into cold space, potentially enabling smaller, more efficient satellites. The team plans a balloon test flight this year.
- Deep Space Missions: Dr. Geoffrey Landis and Dr. Stephen Polly at NASA's John Glenn Research Center are exploring the use of thermoradiative diodes for deep space missions or lunar rovers in permanently shadowed regions. These missions currently use heavy, expensive thermoelectric generators powered by radioactive isotopes like plutonium. While plutonium would still be needed as a heat source, diodes could offer a simpler, more efficient, and lighter alternative, maximizing the use of the rare resource.
Challenges and Future Outlook
A significant challenge for deep space applications is the durability of current thermoradiative diodes at the high temperatures (up to 1,000° Celsius) produced by decaying radioactive isotopes. Current diodes use semiconductive materials similar to those in night-vision goggles. NASA researchers are investigating new materials to enable operation at temperatures up to 375° Celsius (707° Fahrenheit) and ensure longevity for missions lasting decades.
The UNSW team has received funding from the United States Air Force to improve the diode's efficiency for low-Earth satellite use, with the sun as the sole heat source. They are also exploring materials compatible with conventional solar cell manufacturing processes to accelerate commercial availability, which they hope could occur within five years.