Stanford engineers invent a solar panel that generates electricity at night
For researcher Sid Assawaworrarit and his colleagues, this was fantastic news. He told IE that such conditions were “perhaps the best of the year.”
Assaworrarit isn’t grateful that clouds didn’t prevent starlight from passing through the atmosphere and reaching his telescope’s mirror. As an electrical engineer, he was looking forward to the bright nights for a different reason: a clear night allows infrared light from solar panels to easily travel into space.
Because of this flow of energy, the gadget Assaworrarit and his colleagues developed — a regular solar panel with a thermoelectric generator — can generate a modest quantity of power from the temperature difference between surrounding air and the surface of a solar panel directed far 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’s a light coming in from the Sun and hitting the solar cell, but during the night, something of a reverse happens,” Assawaworrarit says.
That’s because solar panels — like everything warmer than absolute zero — emit infrared radiation.
When photons leave the solar panel’s upwards surface, they carry heat with them. That means a solar panel’s surface will be a few degrees cooler than the air around it on a clear night when there are no clouds to bounce infrared light back toward the Earth. Assawaworrarit and his colleagues are taking advantage of the temperature difference. A thermoelectric generator can absorb part of the heat that is being transferred from the warmer air to the cooler solar panel and convert it to energy.
On a clear night, Assawaworrarit’s setup on the Stanford rooftop generates about fifty milliwatts per square meter of the solar panel (50 mW/m2).
“I think that’s probably a record number,” he says. But Assawaworrarit and his team aren’t stopping there. He says that with a couple of improvements (and in a good location) such a device could generate twice that amount of electricity.
“The theoretical limit is probably about one or two watts per square meter,” he says. “That’s not a huge number, but there are a lot of applications” where that kind of energy at night would come in handy.
A major portion of the world’s population, around a billion people, does not have access to an electrical grid, for example. “People in that scenario can rely on solar during the day, but there’s not much they can do at night,” he explains. Unlike batteries, which degrade after a few thousand charge cycles, the thermoelectric generators employed in these solar panels are solid state, he says, “so the lifetime is pretty much forever.”
Another good application for the technology is to power the vast network of environmental sensors that researchers use to track anything from weather to invasive species in far-flung corners of the planet.
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 can get up to a watt per square meter, it would be very attractive from a cost perspective,” Assawaworrarit says.
The invention taps into a source of energy that’s easily overlooked
The Earth receives 173,000 terrawatts of energy from the Sun on a continuous basis. Clouds, atmospheric particles, and reflecting surfaces such as snow-covered mountains immediately reflect 30% of that energy into space. The rest of it warms the earth’s surface, oceans, clouds, atmosphere, and everything else.
That excitement, however, does not linger. Except for the extra heat trapped by greenhouse gases since humans began using large amounts of fossil fuels during the Industrial Revolution, Earth gives forth roughly the same amount of energy that it receives. As a result, the planet emits a mind-boggling amount of energy in the form of infrared radiation.
“It’s a sort of light,” Assawaworrarit says. The infrared radiation that glows from the warm Earth (or anything else) has wavelengths that are too long for eyes to see, but it does carry energy. In fact, more than half of the total amount of solar energy that hits the Earth goes through this process, eventually returning to space.
What Assawaworrarit and his colleagues have done is devise a new way to capture that energy as it departs the planet. They aren’t the first to use a thermoelectric generator to capture this kind of energy (IE covered one of the first big innovations in this space back in 2019). By integrating this new technology with solar panels that generate electricity during the day, the researchers have taken an important step forward in making it possible for ordinary people to capture this energy for themselves.
It all comes down to radiative cooling
Modern scientists are far from the first to realize that a surface facing a cloudless night sky might get cooler than the surrounding air. Radiative cooling is a phenomenon that you’ve probably witnessed first thing in the morning. It’s most noticeable on grass when temperatures drop into the mid-to-low-thirties, but not exactly below freezing.
“The temperature of the [grass] leaf is actually lower, even if the ambient temperature is a few degrees above freezing,” Assawaworrarit says. “If the grass is a few degrees below the ambient temperature and the ambient temperature is just above freezing, the grass may be below the freezing point.”
It’s a strange (if subtle) phenomenon that only happens when skies are clear. That’s because clouds warm the ground by reflecting infrared light back onto 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.
Modern scientists aren’t the first people to put radiative cooling to work either. Southeastern Iran contains the remains of dozens of ice houses, called Yakhchāls, that Ancient Persians used to exploit the phenomenon. When the structures were in operation, people would pour water into shallow pools next to the ice houses. Even if the air temperature was in the high 30s or low 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.
Developing this technology poses several engineering challenges
Understanding the physics behind these solar panels for use at night is only half the battle. Engineers have been working on making them efficient enough to be useful in the real world for years.
During the pandemic, Assawaworrarit and his colleagues began working on the problem.
“At first, we were stuck a lot since the deep number we obtained early on was nowhere near what we expected,” he explains. The team’s initial trial revealed that early prototypes of the device produced about one-tenth the amount of electricity they expected after months of calculations.
It found out that there was a major stumbling block in their way.
“A solar cell is actually not a very good heat conductor,” Assawaworrarit says. “That’s where the problem lies.” The engineers realized that energy escaping the edges of the solar panel wasn’t contributing very much to the system’s energy output because the thermal energy couldn’t easly travel through the solar cell itself.
“Looking back, it sounds straightforward,” he says. “But at that point, it wasn’t obvious.”
The engineers fixed the problem by attaching the solar cell directly to an aluminum plate, which conducts the energy far more efficiently.
“That was sort of an epiphany,” he says.
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