It doesn’t generate much power, but it works during the one time of day that solar cells can’t: night.
Aaswath Raman was driving through a village in Sierra Leone in 2013 when an idea came to him as suddenly as, perhaps, a light bulb switching on.
The village was not equipped with electricity, and Dr. Raman, an electrical engineer at the University of California, Los Angeles, was unaware he was in a village until he heard the voices of shadowed human figures.
“It took us about five minutes to realize we were passing through a town, because it was completely dark,” Dr. Raman said. “There wasn’t a single light on.”
Dr. Raman wondered whether he could use all that darkness to make something to light it up, not unlike the way that solar panels generate electricity from the sun’s heat and light.
He did. In new research published on Thursday in the journal Joule, Dr. Raman demonstrated a way to harness a dark night sky to power a light bulb.
His prototype device employs radiative cooling, the phenomenon that makes buildings and parks feel cooler than the surrounding air after sunset. As Dr. Raman’s device releases heat, it does so unevenly, the top side cooling more than the bottom. It then converts the difference in heat into electricity. In the paper, Dr. Raman described how the device, when connected to a voltage converter, was able to power a white LED.
“The core enabling feature of this device is that it can cool down,” Dr. Raman said.
Jeffrey C. Grossman, a materials scientist at the Massachusetts Institute of Technology who studies passive cooling and solar technology, said the work was “quite exciting” and showed promise for the development of low-power applications at night.
“They have suggested reasonable paths for increasing the performance of their device,” Dr. Grossman said. “But there is definitely a long way to go if they want to use it as an alternative to adding battery storage for solar cells.”
Everything emits heat, according to the laws of thermodynamics. At night, when one side of Earth turns away from the sun, its buildings, streets and jacket-less people cool off. If no clouds are present to trap warmth, objects on the Earth can lose so much heat that they reach a lower temperature than the air surrounding them. This is why blades of grass may be glazed in frost on clear fall mornings, even when the air temperature is above freezing. The cloudless atmosphere becomes a porthole to the void, through which warmth flows like air through a porch screen.
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