🌙 What if we could harness solar energy... at night?
Every night, the heat that the Sun delivered to Earth during the day radiates back into the cold void of space as infrared light. This "escaping energy" has long been ignored—until now. Using materials from night-vision goggles, scientists have created a device that generates electricity from this invisible infrared radiation. Welcome to the age of "night-time solar power."
Solar Panels in Reverse
A research team at the University of New South Wales (UNSW) in Sydney, Australia has developed a "thermoradiative diode"—a device that works on the exact opposite principle of conventional solar panels.
Traditional solar panels generate electricity by absorbing photons from the Sun, a very hot object. In contrast, thermoradiative diodes generate electricity by emitting infrared photons into a much colder environment—outer space.
Professor Ned Ekins-Daukes, who leads the UNSW team, explains: "If you look at the Earth at night with an infrared camera, it glows. That's because the Earth is radiating heat into the cold universe."
The key insight is that both technologies exploit temperature differences. Solar panels use the difference between the hot Sun and the relatively cool panel on Earth's surface. Thermoradiative diodes use the difference between the warm Earth's surface and the extreme cold of space.
From Night-Vision Goggles to Power Generation
The thermoradiative diode uses mercury cadmium telluride (HgCdTe), the same semiconductor material found in night-vision goggles. This material excels at detecting infrared radiation and has been used for decades in military and civilian night-vision applications.
In 2022, the UNSW team achieved a world first: directly demonstrating that power can be generated from infrared emission. In their experiments, they generated 2.26 milliwatts per square meter from a temperature difference of just 12.5°C.
This output is approximately 100,000 times less than a conventional solar panel—roughly enough to power a digital wristwatch from body heat. However, researchers believe that with optimization, the technology could eventually reach about one-tenth of solar panel output.
Why Space Is the Perfect Testing Ground
On Earth, atmospheric gases like water vapor and carbon dioxide absorb infrared radiation, reducing the temperature difference between the ground and the night sky and limiting efficiency. In the vacuum of space, however, there's no atmosphere to interfere.
Low-Earth orbit satellites are particularly promising. These satellites complete an orbit every 90 minutes, spending about 45 minutes in darkness. Currently, batteries charged during sunlit periods power the satellite during these eclipse periods. Thermoradiative diodes laminated onto a spacecraft's surface could generate auxiliary power even in the dark.
The potential is even greater for deep space missions. Currently, rovers exploring permanently shadowed regions of the Moon and deep space probes rely on radioisotope thermoelectric generators (RTGs), which convert heat from radioactive decay (typically plutonium) into electricity. These devices weigh around 45 kilograms and occupy about 200 liters of space. Thermoradiative diodes offer a lighter, simpler alternative that could transform spacecraft design.
The Road to Commercial Reality
The UNSW team is currently working on a project funded by the U.S. Air Force to optimize thermoradiative diodes for low-Earth orbit satellites. They aim to conduct a space flight demonstration between 2025 and 2026—a milestone that could dramatically advance the technology's practical applications.
On Earth, potential applications include power generation in deserts where day-night temperature differences are large, and providing power to wearable devices using body heat. The technology is already capable of powering small devices like wristwatches.
However, several challenges remain before commercialization. The HgCdTe material currently used is expensive, and manufacturing costs need to be reduced. The research team is exploring the use of materials similar to conventional solar panels, which would allow manufacturing processes to be combined and costs to be lowered.
Professor Ekins-Daukes predicts commercialization could happen within five years: "If industry can see this is a valuable technology for them, then progress can be extremely fast."
NASA researchers are also exploring this technology for deep space applications. Dr. Geoffrey Landis at NASA's Glenn Research Center notes that while thermoradiative diodes show promise for missions beyond low-Earth orbit, more research is needed to ensure the semiconductors can withstand the high temperatures generated by radioisotope heat sources.
Conclusion: A New Frontier for Solar Energy
Thermoradiative diodes represent a paradigm shift—harvesting energy from heat that Earth naturally releases into space every night. While current power output is minimal, space applications could serve as a proving ground that leads to broader terrestrial use.
This technology challenges the fundamental limitation of solar power: that it doesn't work at night. As the world seeks diverse renewable energy solutions, "night-time solar power" offers an intriguing new frontier.
Japan has been advancing its own research into Space Solar Power Systems (SSPS), but the concept of harvesting "escaping" energy is a fresh perspective. What about in your country? Are there discussions or initiatives around night-time power generation technologies? We'd love to hear your thoughts!
References
- https://www.cnn.co.jp/fringe/35243048.html
- https://www.unsw.edu.au/news/2024/07/solar-power-at-night-on-earth-and-in-orbit-a-renewable-reality
- https://www.nature.com/articles/d42473-024-00483-8
- https://www.sciencedaily.com/releases/2022/05/220517112246.htm
- https://interestingengineering.com/energy/solar-power-generated-at-night-device
Reactions in Japan
A solar panel that generates power at night—the concept is completely reversed and fascinating. Using radiated heat to space means recovering energy we've been throwing away, which makes perfect sense.
Power output at 1/100,000th of solar panels... seems far from practical use. But it's interesting research, so I hope they keep at it.
Didn't know you could generate power with night-vision goggle materials. Technology transfer is truly fascinating. If it can power a watch from body heat, it could work for wearables.
Japan is researching Space Solar Power Systems (SSPS), but this approach uses heat radiation from Earth to space. Combining both could create a 24-hour power generation system?
If this can solve satellite power issues, it's huge for space development. Perfect technology for low-orbit satellites that enter shadow every 45 minutes.
Isn't mercury cadmium telluride toxic? If mass-produced, I'm concerned about environmental impact.
They say output could theoretically reach 1/10 of solar, but I wonder if that's achievable. Still, efficiency improves in space, so there's hope.
The U.S. Air Force funding this suggests military applications are in mind. Could be useful for reconnaissance satellites.
So it's more efficient where day-night temperature differences are large, like deserts. Might be tough in Japan, but suitable for the Middle East or Australia.
Solar panels that work at night—it's moving that sci-fi concepts are becoming reality. Commercialization in 5 years is surprisingly fast.
If it can replace plutonium thermoelectric generators, space exploration could become safer and cheaper. Weight reduction from 45kg would be significant too.
The challenge with renewables was 'no power at night,' but this could be a solution? Revolutionary if we can supply power without relying on batteries.
Didn't know Australian universities were advancing this research. Japan can't fall behind. Hope JAXA collaborates on this.
'Generating power by emitting light' seems counterintuitive, but thermodynamically possible with temperature differences. Amazing that they're pioneering territory not even in physics textbooks.
Says they'll do space experiments in 2025-26. If successful, this could be a real game-changer. Looking forward to updates.
From Australia here. Proud of UNSW's research. Our country leads in solar power, and it's great to lead in night-time generation too. With our vast deserts, this technology would be perfect for us once commercialized.
Germany is transitioning to renewables, but nighttime generation is a big challenge. Interesting if this works in European climates. Though cloudy regions may have smaller temperature differences, reducing efficiency.
From Saudi Arabia's perspective, this technology is very promising. Our deserts can have day-night temperature differences of over 30°C. We should invest in technologies like this to reduce oil dependence.
Working in the US space industry, I knew the Air Force was funding this project. Satellite weight reduction and power efficiency are constant challenges, so this technology could have major impact if commercialized.
Living in Mexico's northern desert region. Solar power only benefits us during the day, so nighttime generation would be perfect. But I'm curious about the costs.
India has areas with severe power shortages. Technology that generates power 24/7 could transform rural life. But the current low power output is definitely a challenge.
From Ireland, and honestly this might not be efficient in our climate. Lots of clouds and small day-night temperature differences. But space applications sound great.
China invests heavily in renewable energy research. Chinese institutions will likely enter this field too. More competition should accelerate innovation.
Engineering student in Poland. Learned about thermoradiative diode principles in class but didn't know they were being applied to power generation. Impressed by the technological progress.
Offshore wind is dominant in Denmark, but having diverse energy sources is important. This technology could become part of our energy mix once it matures.
Chile's Atacama Desert has some of the most sunny days in the world. We already have large solar plants, but adding night generation could enable 24-hour operation.
Working at a Korean space startup. Power issues for small satellites are always a bottleneck. Reducing batteries would greatly expand design flexibility. Closely watching this technology's progress.
Living in northern Canada where we get almost no sunlight in winter. Temperature difference-based generation is interesting. But I wonder if it works in extreme cold.
From Egypt. We have vast deserts and strong sunlight. If commercialized, this technology could change the energy landscape of the MENA region.
Nuclear engineer from France. Nuclear provides stable power, but diversifying renewables is important too. Night generation is an interesting complementary approach.