New Kind of 'Solar' Cell Shows We Can Generate Electricity Even at Night

By Mike McRae

CONVENTIONAL SOLAR TECHNOLOGY

soaks up rays of incoming sunlight to bump out a voltage. Strange as it seems, some materials are capable of running in reverse, producing power as they radiate heat back into the cold night sky.

A team of engineers in Australia has now demonstrated the theory in action, using the kind of technology commonly found in night-vision goggles to generate power.

So far, the prototype only generates a small amount of power, and is probably unlikely to become a competitive source of renewable power on its own – but coupled with existing photovoltaics technology, it could harness the small amount of energy provided by solar cells cooling after a long, hot day's work. "Photovoltaics, the direct conversion of sunlight into electricity, is an artifi cial process that humans have developed in order to convert the solar energy into power," says Phoebe Pearce, a physicist from the University of New South Wales. "In that sense, the thermoradiative process is similar; we are diverting energy fl owing in the infrared from a warm Earth into the cold Universe."

By setting atoms in any material jiggling with heat, you're forcing their electrons to generate lowenergy ripples of electromagnetic radiation in the form of infrared light.

As lackluster as this electron-shimmy might be, it still has the potential to kick off a slow current of electricity. All that's needed is a one-way electron traffi c signal called a diode.

Made of the right combination of elements, a diode can shuffl e electrons down the street as it slowly loses its heat to a cooler environment.

In this case, the diode is made of mercury cadmium telluride (MCT). Already used in devices that detect infrared light, MCT's ability to absorb mid-and long-range infrared light and turn it into a current is well understood.

What hasn't been entirely clear is how this particular trick might be used effi ciently as an actual power source.

Warmed to around 20 degrees Celsius (nearly 70 degrees Fahrenheit), one of the tested MCT photovoltaic detectors generated a power density

Design News

The UNSW ‘night-time solar’ team captured via infrared camera. They used the same kind of semiconductor technology to produce power for the fi rst ever time

from the emission of light. https://newsroom.unsw.edu.au/

of 2.26 milliwatts per square meter.

Granted, it's not exactly enough to boil a jug of water for your morning coff ee. You'd probably need enough MCT panels to cover a few city blocks for that small task.

But that's not really the point, either, given it's still very early days in the fi eld, and there's potential for the technology to develop signifi cantly further in the future. "Right now, the demonstration we have with the thermoradiative diode is relatively very low power. One of the challenges was actually detecting it," says the study's lead researcher, Ned EkinsDaukes. "But the theory says it is possible for this technology to ultimately produce about 1/10th of the power of a solar cell."

At those kinds of effi ciencies, it might be worth the eff ort weaving MCT diodes into more typical photovoltaic networks so that they continue to top up batteries long after the Sun sets.

To be clear, the idea of using the planet's cooling as a source of low-energy radiation is one engineers have been entertaining for a while now. Diff erent methods have seen diff erent results, all with their own costs and benefi ts.

Yet by testing the limits of each and fi ne-tuning their abilities to soak up more of the infrared bandwidth, we can come up with a suite of technologies capable of wringing every drop of power out of just about any kind of waste heat. "Down the line, this technology could potentially harvest that energy and remove the need for batteries in certain devices – or help to recharge them," says Ekins-Daukes. "That isn't something where conventional solar power would necessarily be a viable option." https://www.sciencealert.com/engineersmeasure-the-potential-of-a-new-kind-of-solar-cellfueled-by-the-night?

This research was published in ACS Photonics.