A novel solar cell generates electricity despite the total darkness of night

It's a backward process but it works.
Derya Ozdemir
The thermoradiative diode (left), and a solar array at night (right).1, 2

A team of engineers successfully generated electricity from a technology commonly found in night-vision goggles — called "nighttime" solar power, according to a recent study published in the journal ACS Photonics

Scientists at the University of New South Wales (UNSW) demonstrated viable power generation using "the inverse of a conventional solar cell," which could eventually produce up to one-tenth the power of a solar panel, said the scientists in an initial New Atlas report

Even after the sun has set.

Producing electricity from infrared thermal radiation 

Sunlight is converted into electrical energy either by photovoltaic (PV) panels or by concentrating solar radiation. The energy in this source can be used to generate electricity, or stored in batteries or thermal energy storage devices to be used later. Interestingly, some materials can run in reverse, producing power as they radiate heat back out into the night air.

There's some common sense to this: whenever an atom in a material gets heated up, it produces ripples of electromagnetic radiation in the form of infrared light. As the Earth cools by radiating energy into space at night, the researchers were able to generate electricity from just that.

The device created by the researchers is called a thermoradiative diode. And it's now been proven to produce electricity using this process, accepting heat radiated upward from the Earth and converting it into electrical potential by using the temperature differential. Some of the materials widely used in night vision goggles were also used in the creation of this device.

"Photovoltaics, the direct conversion of sunlight into electricity, is an artificial process that humans have developed in order to convert the solar energy into power," explained Phoebe Pearce, a physicist from the University of New South Wales, in a press release. "In that sense, the thermoradiative process is similar; we are diverting energy flowing in the infrared from a warm Earth into the cold Universe."

Early days in night-time power generation

During a test, one of the tested MCT photovoltaic detectors warmed up to 70 degrees Fahrenheit (21.11 degrees Celsius) and generated 2.26 milliwatts per square meter, according to the study.

The amount of energy generated is admittedly very small — 100,000 times less than a solar panel, which is why it's too early to think of it as a competitive source of renewable power on its own, at this point.

However, the researchers think they can improve it in the future, and, paired with existing PV technology, it could harness the small amount of energy provided by solar cells cooling after a day's work. A team of Stanford engineers has already invented a solar panel that generates electricity at night.

In fact, the technology could even help generate power from anything that shines through the eyes of a thermal camera (in other words, emits heat), with the possibility of harvesting heat from industrial waste or making bionic devices like artificial hearts that run off the body's heat.

"Even if the commercialization of these technologies is still a way down the road, being at the very beginning of an evolving idea is such an exciting place to be as a researcher," said Michael Nielsen, co-author of the paper. "By leveraging our knowledge of how to design and optimize solar cells and borrowing materials from the existing mid-infrared photodetector community, we hope for rapid progress towards delivering the dream of solar power at night."

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