🌙 What if we could harness solar energy... at night?

Every night, the heat the Sun delivered to Earth during the day radiates back into the cold void of space as infrared light. That "escaping energy" went unused for a long time. Then, using materials from night-vision goggles, scientists built a device that generates electricity from the invisible infrared radiation itself.

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 a conventional solar panel.

Traditional solar panels generate electricity by absorbing photons from the Sun, a very hot object. A thermoradiative diode does the reverse: it generates electricity by emitting infrared photons into a much colder environment: outer space.

"If you look at the Earth at night with an infrared camera, it glows," explains Professor Ned Ekins-Daukes, who leads the UNSW team. "That's because the Earth is radiating heat into the cold universe."

Both exploit the same thing: a temperature difference. Solar panels use the gap between the hot Sun and the cooler panel on the ground; thermoradiative diodes use the gap between the warm surface of the Earth and the extreme cold of space.

From Night-Vision Goggles to Power Generation

The diode uses mercury cadmium telluride (HgCdTe), the same semiconductor found in night-vision goggles. It excels at detecting infrared radiation and has been used for decades in military and civilian night-vision gear.

The concept had been worked out theoretically at Harvard and Stanford, but in 2022 the UNSW team was the first to directly demonstrate electrical power coming out of such a device. With the ground 12.5°C warmer than the air above it, a swing that is common in Australia, they measured 2.26 milliwatts per square meter.

That is roughly 100,000 times less than a conventional solar panel, about enough to run a digital wristwatch off body heat. But the researchers believe optimization could eventually push output to around one-tenth of a solar panel's.

Why Space Is the Perfect Testing Ground

On Earth, atmospheric water vapor and carbon dioxide absorb infrared radiation, shrinking the temperature gap between the ground and the night sky and capping efficiency. In the vacuum of space, there's no atmosphere to get in the way.

Low-Earth orbit satellites are the most promising near-term case. They complete an orbit every 90 minutes, spending about 45 of those in darkness. Today, batteries charged during the sunlit stretch carry the satellite through the eclipse. A thermoradiative diode laminated onto the spacecraft's skin could supply auxiliary power even in the dark.

The payoff is bigger still for deep space. Rovers in the permanently shadowed regions of the Moon and probes far from the Sun rely on radioisotope thermoelectric generators (RTGs), which turn heat from radioactive decay, usually plutonium, into electricity. These units weigh around 45 kilograms and take up roughly 200 liters. A thermoradiative diode offers a lighter, simpler alternative that could reshape how spacecraft are designed.

The Road to Commercial Reality

The UNSW team is currently working on a U.S. Air Force–funded project to optimize the diode for low-Earth orbit satellites, and it is preparing a space flight demonstration, a milestone that would be a big step toward practical use.

Back on Earth, potential uses include power generation in deserts with wide day-night temperature swings, and wearables that run on body heat. Powering something as small as a wristwatch is already within reach.

Commercialization still faces hurdles. HgCdTe is expensive, so manufacturing costs have to come down. The team is exploring materials closer to those in ordinary solar panels, which would let the two share production lines and cut costs.

Ekins-Daukes predicts commercialization could arrive within five years: "If industry can see this is a valuable technology for them, then progress can be extremely fast." For deep space, Dr. Geoffrey Landis of NASA's Glenn Research Center cautions that more work is needed to ensure the semiconductors can survive the high temperatures thrown off by radioisotope heat sources, though he also says that if results keep looking good, a radioisotope-heated thermoradiative system is certainly possible within five to ten years.

Beyond "Solar Doesn't Work at Night"

Thermoradiative diodes turn something we rarely think about, the heat Earth sheds into space every night, into electricity. The output is tiny for now, but space could be the proving ground that eventually carries the idea back down to Earth.

Japan has been pushing its own research into Space Solar Power Systems (SSPS), yet the idea of harvesting "escaping" energy is a fresh one. What about where you live? Are there any conversations or projects around generating power after dark?

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