The sky over Stanford, Calif., was unusually clear for several nights last October.
It was good news for researcher Sid Assawaworrarit and his colleagues. These conditions were “probably the best of the year”, he says THAT IS TO SAY.
Assaworrarit is not one grateful astronomer that clouds didn’t block starlight from passing through the atmosphere and reaching the mirror of his telescope. An electrical engineer, he welcomed cloudless nights for an entirely different reason: a clear night means infrared light from the surface of solar panels can radiate freely into space.
This flow of energy allows the device created by Assaworrarit and his colleagues – an ordinary solar panel fitted with a thermoelectric generator – to generate a small amount of electricity from the slight difference in temperature between the surrounding air and the surface of a solar panel pointed deep into space.
At night, solar panels turn the table and emit photons
The new technology takes advantage of a surprising fact about solar panels.
“During the day, there is light coming from the Sun and hitting the solar cell, but during the night, something the opposite happens,” says Assawaworrarit.
This is because solar panels – like anything hotter than absolute zero – emit infrared radiation.
“There’s actually light going out [from the solar panel], and we use it to generate electricity at night. The photons that come out into the night sky actually cool the solar cell,” he says.
When these photons leave the surface of the solar panel towards the sky, they carry heat with them. This means that on a clear night – when there are no clouds to reflect infrared light back to Earth – the surface of a solar panel will be a few degrees cooler than the air above it. ‘surrounded. This temperature differential is what Assawaworrarit and his colleagues take advantage of. A device called a thermoelectric generator can capture some of the heat flowing from the warmer air to the cooler solar panel and convert it into electricity.
On a clear night, the Assawaworrarit device being tested on the Stanford rooftop generates about fifty milliwatts for every square meter of solar array (50 mW/m2).
“I think that’s probably a record number,” he said. But Assawaworrarit and his team don’t stop there. He says that with a few improvements (and in the right location), such a device could generate twice as much electricity.
“The theoretical limit is probably around one or two watts per square meter,” he says. “It’s not a huge number, but there are a lot of applications” where this type of energy at night would be useful.
For example, a large part of the world’s population – around one billion people – does not have access to an electricity grid. People living in this situation “can rely on solar power during the day, but at night they can’t do much,” he says. Unlike batteries that degrade significantly after a few thousand charge cycles, the type of thermoelectric generators used in these solar panels are solid state, “so the lifespan is pretty much forever,” he says.
Another good use of the technology is to power the huge network of environmental sensors that researchers use to keep tabs on everything from weather conditions to invasive species in the farthest corners of the globe. Again, solar panels that generate a small amount of electricity at night could reduce the need for batteries – and the maintenance and replacement costs they incur.
“If you could achieve one watt per square meter, that would be very attractive from a cost perspective,” says Assawaworrarit.
Invention taps into a power source that is easily overlooked
The Earth constantly receives an enormous amount of energy from the Sun, to the tune of 173,000 terrawatts. Clouds, particles in the atmosphere, and reflective surfaces like snow-capped mountains immediately reflect 30% of this energy back into space. The rest ends up heating up the land, the oceans, the clouds, the atmosphere and everything else on the planet.
But that energy does not stay here. Except for the extra heat that greenhouse gases trapped once humans started burning large amounts of fossil fuels since the Industrial Revolution, Earth sends out about as much energy as it does. receives it. This is why the planet emits a truly mind-boggling amount of energy in the form of infrared radiation.
“It’s a kind of light,” says Assawaworrarit. The infrared radiation that shines from the hot Earth (or anything else) has wavelengths too long for the eyes to see, but it carries energy. In fact, more than half of the total amount of solar energy that hits Earth goes through this process, eventually returning to space.
What Assawaworrarit and his colleagues have done is devise a new way to capture this energy as it leaves the planet. They are not the first to use a thermoelectric generator to capture this type of energy (THAT IS TO SAY covered one of the first big innovations in this space in 2019). By integrating this new technology with solar panels that generate electricity during the day, the researchers have taken an important step in allowing ordinary people to harness this energy for themselves.
It all comes down to radiative cooling
Modern scientists aren’t the first to notice that a surface facing the cloudless night sky can become colder than the air around it. The phenomenon is called radiative cooling, and you’ve probably seen it yourself first thing in the morning. It’s most apparent in the grass after temperatures have dropped into the mid to low 30s, but not quite below zero.
“Even if the ambient temperature is a few degrees above freezing point, the temperature of the [grass] the leaf is actually lower,” Assawaworrarit. “If the grass is a few degrees below ambient temperature and the ambient temperature is slightly above freezing, then the grass might actually be below freezing.”
It’s a strange (though subtle) phenomenon that only occurs when the sky is clear. This is because clouds warm the ground by reflecting infrared light off the Earth’s surface. “You won’t be able to see it because it’s happening in a wavelength that humans can’t see,” but radiative cooling happens all the time, Assawaworrarit says.
Nor are modern scientists the first to use radiative cooling. Southeast Iran contains the remains of dozens of glaciers, called Yakhchāls, which the ancient Persians used to exploit the phenomenon. When the structures worked, people poured water into shallow pools next to the coolers. Even if the air temperature was in the 30s or 40s, the water would freeze. In the morning, people would collect the ice and transfer it to a nearby beehive-like structure that used a different set of passive cooling techniques to keep the ice below freezing throughout the summer.
The development of this technology poses several engineering challenges
Understanding the physics behind these nocturnal solar panels is only part of the battle. Engineers have been working for years to make them efficient enough to be usable in the real world.
Assawaworrarit and his colleagues started working on the issue during the pandemic.
“We were stuck a bit at the start because the number we got at the start was far from what we expected,” he says. After months of crunching the numbers, the team’s first experiment showed that early iterations of the device produced about a tenth of the expected amount of electricity.
It turned out that a big problem was bothering them.
“A solar cell is actually not a very good conductor of heat”, said Assawaworrarit. “Therein lies the problem.” The engineers realized that the energy escaping from the edges of the solar panel did not contribute much to the system’s power output because the thermal energy could not easily pass through the solar cell itself.
“In hindsight, it seems simple,” he says. “But at that time it was not obvious.”
Engineers solved the problem by attaching the solar cell directly to an aluminum plate, which conducts energy much more efficiently.
“It was kind of an epiphany,” he says.