Conventional solar technology absorbs incoming sunlight to increase voltage. Strange as it may sound, some materials are able to work in reverse, generating power by radiating heat back into the cold night sky.
A team of engineers in Australia have now demonstrated the theory in action, using the kind of technology commonly found in night vision goggles to generate power.
So far, the prototype generates only a small amount of energy and is unlikely to become a competitive source of renewable energy on its own – but coupled with existing photovoltaic technology, it could exploit the small amount of energy provided by solar cells cooling down after a long, hot day’s work.
“Photovoltaics, the direct conversion of sunlight into electricity, is an artificial process that humans have developed to convert solar energy into electricity,” says University of New South Wales physicist Phoebe Pearce.
“In this sense, the thermoradiative process is similar; we divert energy flowing in the infrared from a hot Earth to the cold Universe.”
By placing atoms in any material stirred by heat, you force their electrons to generate ripples of low-energy electromagnetic radiation in the form of infrared light.
As dull as this flicker of electrons may be, it still has the potential to trigger a slow electrical current. All that is needed is a unidirectional electron traffic signal called a diode.
Made up of the right combination of elements, a diode can mix electrons on the street slowly losing 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, the ability of MCT to absorb mid- and long-range infrared light and turn it into current is well understood.
What’s not entirely clear is how this particular trick could be effectively used as an actual power source.
Warmed to around 20 degrees Celsius (almost 70 degrees Fahrenheit), one of the MCT photovoltaic detectors tested generated a power density of 2.26 milliwatts per square meter.
Admittedly, just boiling a jug of water for your morning coffee is not enough. You would probably need enough MCT panels to cover a few city blocks for this small task.
But that’s not really the point either, given that it’s still very early in the game, and there’s potential for the technology to grow significantly 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 Ekins-Daukes.
“But the theory says it’s possible that this technology will ultimately produce about 1/10th the power of a solar cell.”
With these kinds of efficiencies, it might be worth weaving MCT diodes into more typical photovoltaic arrays so that they continue to charge the batteries long after the sun goes down.
To be clear, the idea of using planetary cooling as a low-energy source of radiation is something engineers have been considering for some time now. Different methods yielded different results, each with their own costs and benefits.
Yet by testing the limits of each and refining their abilities to absorb more of the infrared bandwidth, we can deliver a suite of technologies capable of extracting every drop of energy from almost any type of waste heat.
“Ultimately, this technology could potentially harvest that energy and remove the need for batteries in some devices — or help charge them,” Ekins-Daukes says.
“It’s not something where conventional solar power would necessarily be a viable option.”
This research was published in ACS Photonics.