An inexpensive device generates electricity naturally in the dark

• A thermoelectric generator is built whose cold side radiates heat to the sky
• The night current generation of 25 mW / m2 is demonstrated, sufficient for one LED
• Performance trajectories> 0.5 W / m2 using existing basic components exist
• This approach is immediately convenient for lighting and off-grid sensors

When frost forms on the ground during the night, even when the temperature is well above the freezing point, or as water droplets appear on the windshield of a car, even by clear night, the cause is often a phenomenon called radiative cooling of the sky.

In an article published in the journal JouleResearchers led by a UCLA materials scientist report that they have used the principles of radiative cooling of the sky to develop an innovative way to produce renewable energy at night.

The approach could be adapted into a low cost technology that could possibly be a boon to more than a billion people worldwide who, according to the International Energy Agency, only do not have reliable access to electricity. The concept could be used as a stand-alone technology or be associated with solar energy to produce electricity day and night.

Radiative cooling of the sky is a natural phenomenon in which a surface that faces the sky rejects its heat in the air in the form of thermal radiation. Some of this heat eventually reaches the upper atmosphere and then into colder areas of space.

"This effect occurs naturally all the time, especially on a clear day," said Aaswath Raman, assistant professor of materials science and engineering at UCLA's Samueli School of Engineering, who led the study. . "The result is that the object emitting heat, be it a car, the ground or a building, will be slightly colder than the ambient temperature."

The new technology takes advantage of this temperature difference by capturing some of the heat from the ambient air that would otherwise be emitted into the sky and converting it into electricity.

The researchers developed a simple and inexpensive device to demonstrate the technology: all the parts used in the experiment were purchased from computer hardware stores and electronic devices for a total cost of less than $ 30.

Their installation, arranged on the roof of a building, included a black painted aluminum disc on one side, which faced the sky. The researchers used the disc to emit the heat released by the ambient air. A thermoelectric generator – a device that produces an electrical voltage in response to a temperature difference – then converts that heat into electricity.

The device has generated up to 25 milliwatts per square meter, enough to power a single LED bulb. Although the device generates much less energy than a solar cell of similar size, Raman said that it could be used to generate energy at night, in remote areas of the power grid or for users who do not have easy access to the batteries.

Raman said the technology could be improved with better components and that it could potentially generate up to 0.5 watts per square meter – about 20 times more than the device demonstrated by researchers – particularly in hot and dry climates, where the effect of radiative cooling is the best effect. The Strongest.

With this release, he said, a small installation on the roof of a house could provide enough energy during the night to charge a cell phone or to light a room with LED bulbs.

"We believe it 's an intriguing demonstration of how the cold of space can be used as a source of renewable energy and generate quantities of energy. electricity modest but usable, "said Raman. "We believe that this could also be the basis of a technology that complements solar energy. Although the power output is still significantly lower than that of solar devices, this new technology can operate at times when solar cells can not. "

The other authors of the study are Wei Li, a postdoctoral researcher, and Shanhui Fan, professor of electrical engineering, both at Stanford.

Radiative cooling of the sky has become an active topic of research over the past decade. In a study published in Nature in 2014, Raman, Fan and his colleagues showed how this phenomenon can be used to cool objects, such as buildings, in warm, sunny weather. This study was an important step in the use of this phenomenon for energy applications.

The study was funded by the US Department of Energy.

Reference: "Generating Light of Darkness" by Aaswath P. Raman, Wei Li and Shanhui Fan, September 12, 2019, Joule.
DOI: 10.1016 / j.joule.2019.08.009