BP Desk
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 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, report agencies.
“It took us about five minutes to realize we were passing through a town, because it was completely dark,” Raman said. “There wasn’t a single light on.” 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, 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 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, 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,” 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,” 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 jacketless 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.
Humans have taken advantage of this effect for millenniums. Six thousand years ago, people in what are nowIran and Afghanistan constructed enormous beehive-shaped structures called yakhchal, which used this passive cooling effect to create and store ice in the desert.
Modern scientists have studied how to harness energy from Earth’s day-night swings in temperature, but that work has mostly remained theoretical. In 2014, researchers led by Federico Capasso, an electrical engineering professor at Harvard, calculated that at best only about 4 watts of energy can be extracted from a square meter of cold space. By contrast, a photovoltaic panel, the most common type of solar panel, generates about 200 watts per square meter in direct sunlight.
Nonetheless, a device that could produce any amount of electricity at night would be valuable; after the sun sets, solar cells don’t work and winds often die down, even as demand for lighting peaks.
Shanhui Fan, an electrical engineer at Stanford and an author on Raman’s study, has been at the vanguard of this research. Last fall, Fan’s team described a device that can generate electricity with solar panels during the day, then use the passive cooling effect to chill a building at night. Earlier this year, they also tested an infrared photodiode, similar to the technology used in most solar cells but which uses warmth, not sunlight, to generate wisps of electricity in the darkness.
The prototype built by Raman resembles a hockey puck set inside a chafing dish. The puck is a polystyrene disk coated in black paint and covered with a wind shield. At its heart is an off-the shelf gadget called a thermoelectric generator, which uses the difference in temperature between opposite sides of the device to generate a current. A similar device powers NASA’s Curiosity rover on Mars; its thermoelectric generator derives heat from plutonium radiation.
Usually, the temperature difference in these generators is stark, and they are carefully engineered to separate hot and cold. Raman’s device instead uses the atmosphere’s ambient temperature as the heat source. The shift from warm to cool is very slight, meaning the device can’t produce much power.
His puck-in-a-dish is elevated on aluminum legs, enabling air to flow around it. As the dark puck loses warmth to the night sky, the side facing the stars grows colder than the side facing the air-warmed tabletop. This slight difference in temperature generates a flow of electricity.
When paired with a voltage converter, the prototype produced 25 milliwatts of power per square meter. That is about three orders of magnitude lower than what a typical solar panel produces, and well short of even the roughly 4-watt maximum efficiency for such devices. Still, several experts said the prototype was an important contribution to a new and relatively unusual space in the renewable energy sector.
“This is a neat combination of radiative cooling — a technique where Raman has pioneered real working devices — with thermoelectric materials that generate electricity if one side is hotter than the other side,” said Ellen D. Williams, a physics professor at the University of Maryland and a former director of the Department of Energy’s Advanced Research Projects Agency-Energy. “Both technologies are proven and practical, but I haven’t seen them combined like this. They did this with inexpensive materials, suggesting it could be made into useful products for the developing world.”
One challenge will be improving the device’s efficiency without raising its 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 this a long play,” he said. “It is just a piece of metal with spray paint on it. It could last for a super long time, and its rivals, photovoltaic cells and batteries, don’t. It can enhance any thermoelectric device as long as it’s outside facing the stars.”
Conceivably, Raman said, thermoelectric devices could complement solar-powered lights in areas where changing batteries is a challenge, like on street lamps or in remote areas far from electrical grids.
“I figured the amount of electricity we could get would be pretty small, and it was,” he said. “But walking around in Sierra Leone, I realized lighting remains a big problem, so it’s an opportunity as well.”
