Harnessing the Night Sky to Generate Electricity Can Complement Solar, Scientists Say

A team of researchers have discovered a way to generate electricity at night using the same devices that are used for infrared sensing, offering a potential sister device to the solar panel that works in the dark.

In a new paper published in the peer-reviewed journal ACS Photonics this month, scientists at the University of New South Wales (UNSW) detail a new use for what’s called a thermoradiative diode that is made of the same material as night-vision goggles. While commercially available and typically used for imaging purposes, like documenting gas leaks, these devices also have the ability to generate power from heat that’s emitted via infrared light. The result is a nighttime counterpart to the photovoltaic cell that works in the opposite direction.

Just as the sun emits energy to the Earth via solar radiation—some of which is absorbed, warming the planet—and Earth emits energy back into the atmosphere at night in the form of infrared radiation. These are wavelengths that are typically naked to the human eye, but can be felt as heat on the skin, and are possible to capture via infrared camera. 

“Infrared light (or thermal energy) is given off by any hot object depending on how hot it is. The hotter the object is, the closer to visible light the thermal energy it is giving off becomes,” Dr. Michael Nielsen, lecturer in the school of photovoltaic and renewable energy engineering at UNSW and co-author on the paper, explained to Motherboard in an email.

“The Earth needs to re-radiate to space approximately the same amount of energy it receives from the Sun each day so that the planet doesn’t overheat,” Nielsen continued. “This is given off as infrared thermal energy, the radiation of which the thermoradiative diode might harness.”

The diodes rely upon a temperature differential between infrared radiation absorbed on one side  and the temperature of outer space—effectively one giant cold sink—to generate current, in a similar but opposite fashion to how  solar cells absorb energy coming from the hot Sun to the cold Earth, Nielsen explained. 

Scientists have explored this idea before, even coming up with a $30 device that sits on the ground, facing the night sky, and generates a tiny amount of current. But the current study is the first time this effect has been precisely measured, the researchers say. The team tested the device at a temperature differential of 12.5 °C on a photodiode emitting close to 4.7 micrometers of infrared radiation and generated 2.26 milliwatts per square meter, a volume they predict could increase to up to 19.4 milliwatts per square meter under different conditions.

Nielsen told Motherboard it took some self-convincing to embrace the idea that thermodiodes could generate usable electrical current: “Only the theoretical underpinning as well as the efforts by researchers that came before us that made it clear this is possible kept us going,” he said. 

But he hopes to see the technology scaled up; they would be useful for generating power on satellites, for example, he says.

“In principle they could be used wherever there is a temperature differential between what they are attached to and what they view,” he said. “The larger the differential, the more power generated.”