How an Indian-origin scientist is harnessing darkness to create light
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.
“It took us about five minutes to realise we were passing through a town, because it was completely dark,” Raman said.
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.
Jeffrey C Grossman, a materials scientist at the Massachusetts Institute of Technology, said the work was “quite exciting” and showed promise for development of lowpower applications at night.
“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,” Grossman adds.
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
device employs radiative cooling, a phenomenon that makes buildings and parks feel cooler than the surrounding air after sunset. His prototype uses a polystyrene disk coated in black paint and covered with a wind shield. At its heart is
a thermoelectric generator
How the device works
temperature than the air surrounding them. The cloudless atmosphere becomes a porthole to the void, through which warmth flows.
Humans have taken advantage of this effect for millenniums. Six thousand years ago, people in what are now Iran 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 metre of cold space. By conHumans have taken advantage of this effect for millennia. Six thousand years ago, people constructed enormous beehive-shaped structures called to create and store ice in the desert
trast, a solar panel generates about 200 watts per square metre 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.
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.
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 device is elevated on aluminium 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 metre. 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 with thermoelectric materials,” said Ellen D Williams, a physics professor at the University of Maryland. “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 US National Renewable Energy Laboratory. Although thermoelectric devices are less efficient and more expensive than photovoltaic cells, they can be more durable. Conceivably, Raman said, thermoelectric devices could complement solarpowered lights in areas where changing batteries is a challenge, like on street lamps or in remote areas.
“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 realised lighting remains a big problem, so it’s an opportunity as well.”