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Aaswath Raman was crossing a village in Sierra Leone in 2013 when an idea came to her as suddenly as maybe a light lighted bulb.
The village was not equipped with electricity, and Dr. Raman,an electrical engineer at The University of California Los Angeles did not know that he was in a village until he heard the voice of dark human figures.
"It took us about five minutes to realize we were going through a city because it was completely dark," said Dr. Raman. "There was not a single light on."
Dr. Raman wondered if he could use all this darkness to light his light, as solar panels do to produce electricity from heat and sunlight.
He did. In a new published research On Thursday, in the Joule newspaper, Dr. Raman presented a way to exploit a dark night sky to power a light bulb.
Its prototype uses radiative cooling, the phenomenon that makes buildings and parks cooler than the ambient air after sunset. Since Dr. Raman's apparatus gives off heat, it does so unevenly, the top is cooler than the bottom. So converts the difference into heat in electricity. In his article, Mr. Raman explained how the device, when connected to a voltage converter, could power a white LED.
"The main 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 "pretty exciting" and was promising for application development. low power at night.
"They suggested reasonable solutions to increase the performance of their device," said Dr. Grossman. "But there is still 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 the Earth turns away from the sun, its buildings, streets and inhabitants without a jacket cool off. If no clouds are present to capture the heat, objects on the Earth can lose as much heat as they reach a lower temperature than the air around them. This is why grass strands can be iced in freezing weather, in the autumn mornings even when the air temperature is above freezing. The cloudless atmosphere becomes a porthole towards the void, through which heat flows like air through a porch screen.
Humans have benefited from this effect for millennia. Six thousand years ago, the inhabitants of present-day Iran and Afghanistan built huge structures in the form of hives called yakhchal, who used this passive cooling effect to create and store ice in the desert.
Modern scientists have studied how to harness the energy of Earth's day / night temperature variations, but most of this work has remained theoretical. In 2014, researchers led by Federico Capasso, professor of electrical engineering at Harvard, calculated that at best, about 4 watts of energy alone could be extracted from a square meter of space. 39, cold space. In contrast, a photovoltaic panel, the most common type of solar panel, generates about 200 watts per square meter of direct sunlight.
Nevertheless, a device capable of producing any amount of electricity at night would be valuable; after sunset, solar cells no longer work and winds often go out, even when the demand for lighting peaks.
Shanhui Fan, an electrical engineer at Stanford and author of Dr. Raman's study, has been at the forefront of this research. Last fall, Dr. Fan's team describes a device that can generate electricity through solar panels during the day, then use the passive cooling effect to cool a building at night. Earlier this year, they also tested an infrared photodiode, similar to the technology used in most solar cells, but that uses heat, not sunlight, to generate electricity streams in the dark. .
The prototype built by Mr. Raman looks like a hockey puck in a heated dish. The puck is a disc of polystyrene covered with black paint and covered with a windbreaker. At its heart, a commercially available gadget, called a thermoelectric generator, uses the temperature difference between opposite sides of the device to generate a current. A similar device powers the NASA Curiosity robot on Mars; its thermoelectric generator draws its heat from the plutonium radiation.
In general, the temperature difference between these generators is striking and they are carefully designed to separate the hot from the cold. Dr. Raman's device instead uses the ambient temperature of the atmosphere as a source of heat. The passage from hot to cold is very light, which means that the device can not produce a lot of energy.
His puck-in-a-dish is raised on aluminum feet, allowing air to circulate around him. As the dark puck heats up in the night sky, the side facing the stars becomes colder than the one facing the warmed-up deck. This slight difference in temperature generates a flow of electricity.
When it is associated with a voltage converter, the prototype product 25 milliwatts of power per square meter. This is about three orders of magnitude less than a typical solar panel produces, and well below the maximum efficiency of 4 watts for such devices. However, several experts said the prototype was a significant contribution to a new, relatively unusual space in the renewable energy sector.
"It's a perfect combination of radiative cooling – a technique in which Raman was the first to use devices that actually work – with thermoelectric materials that generate electricity if one side is hotter than the other." , said Ellen D. Williams, Professor of Physics at the University of Maryland and former Director of the Advanced Research Projects Agency of the Department of Energy – Energy. "Both technologies are proven and practical, but I have not seen them combined in this way. They did this with inexpensive materials, suggesting that it could be turned into useful products for the developing world. "
One of the challenges will be to improve efficiency without increasing costs, said Lance Wheeler, a materials scientist at the National Renewable Energy Laboratory in Golden, Colorado. Although thermoelectric devices are less efficient and more expensive than photovoltaic cells, they can be more durable.
"You could call it a long piece," he said. "It's just a piece of metal with spray paint. This could take a very long time, but not its competitors, photovoltaic cells and batteries. It can enhance any thermoelectric device provided it is on the outside, facing the stars. "
Raman said thermoelectric devices could complement solar-powered lights in areas where battery replacement is a challenge, such as street lights or remote areas far from power grids.
"I thought the amount of electricity we could get would be pretty small, and that was the case," he said. "But while walking in Sierra Leone, I realized that lighting was a big problem, so it's also an opportunity."
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