NEW YORK: 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 radioactive 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 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 contrast, 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.”
Source: Dharmakshethra