Stanford engineers have invented a solar panel that generates electricity at night

The skies over Stanford, California, a few nights last October were unusually clear.

That was good news for researcher Sid Asavorvorrit and colleagues. Those conditions were “probably the best of the year,” he says IE.

Assavorrarite is not an astronomer, grateful that the clouds did not prevent the light from the stars to pass through the atmosphere and reach the mirror of his telescope. An electrical engineer, he welcomed cloudless nights for a completely different reason: a clear night means that infrared light from the surface of solar panels can radiate freely into space.

This flow of energy allows the device created by Assavorite and his colleagues – a conventional solar panel equipped with a thermoelectric generator – to generate a small amount of electricity from a small temperature difference between the ambient air and the surface of the solar panel directed inward. space.

At night, solar panels turn the table and emit photons

New technology takes advantage of the amazing fact about solar panels.

“During the day, the light coming from the sun hits the solar panel, but at night something like the opposite happens,” says Osavavorrarrit.

This is because solar panels – like anything warmer than absolute zero – emit infrared radiation.

“Actually the light goes out [from the solar panel]and we use this to generate electricity at night. The photons that go out into the night sky actually cool the solar element, ”he says.

When these photons leave the surface of the solar panel, which is controlled in 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 the solar panel will be a few degrees colder than the air around it. This temperature difference is what Asavorvorrit and his colleagues use. A device called a thermoelectric generator can capture some of the heat coming from warmer air to colder solar panels, and convert it into electricity.

On a clear night, the Assawaworrarit device tested on Stanford’s roof produces about fifty milliwatts per square meter of solar panel (50 mW / m).2).

“I think that’s probably a record number,” he says. But Assawaworrarit and his team do not stop there. He says that with a couple of improvements (and in a good location) such a device could generate twice as much electricity.

“The theoretical limit is probably about one or two watts per square meter,” he says. “It’s not a big number, but there are a lot of apps” where such energy at night may be needed.

For example, much of the world’s population – about a billion people – do not have access to electricity. “People who live in this situation can count on solar power during the day, but they can do little at night,” he said. Unlike batteries, which degrade significantly after several thousand charge cycles, the type of thermoelectric generators used in these solar panels are solid-state, “so the lifespan is almost eternal,” he says.

Another good application of this technology is the power supply of a huge network of environmental sensors that researchers use to monitor everything from weather conditions to invasive species in remote corners of the globe. Again, solar panels that generate a small amount of electricity at night can reduce the need for batteries as well as reduce the maintenance and replacement costs they carry.

“If you can get up to watts per square meter, it would be very attractive in terms of cost,” says Asavorvorrit.

The invention uses a source of energy that is easy to miss

The earth constantly receives from the Sun a huge amount of energy in the amount of 173,000 terawatts. Clouds, atmospheric particles, and reflective surfaces, such as snow-capped mountains, immediately reflect 30 percent of that energy into space. The rest ends with the heating of the earth, oceans, clouds, atmosphere and everything else on the planet.

But this energy does not stay here. With the exception of the extra heat trapped by greenhouse gases once people have started burning large amounts of fossil fuels since the Industrial Revolution, the Earth sends out about as much energy as it receives. That’s why the planet emits a truly staggering amount of energy in the form of infrared radiation.

“It’s a kind of light,” says Asavavorrarit. Infrared radiation that glows from a warm Earth (or anything else) has too long wavelengths for the eyes to see, but it carries energy. In fact, more than half of the total amount of solar energy that enters the Earth goes through this process, eventually returning to space.

Osavavorrarit and his colleagues have developed a new way to capture this energy when it leaves the planet. They are not the first to use a thermoelectric generator to capture this kind of energy (IE covered one of the first major innovations in this space back in 2019). By combining this new technology with solar panels, which generate electricity during the day, the researchers took an important step forward so that ordinary people could capture this energy for themselves.

It all comes down to radiation cooling

Modern scientists are hardly the first to notice that the surface, directed towards the cloudless night sky, can become colder than the air around it. This phenomenon is called radiation cooling, and you’ve probably seen it yourself in the morning. This is most evident on the grass after the temperature has dropped to mid or low 30, but not quite below zero.

“Even if the ambient temperature is a few degrees above zero, the temperature [grass] the letter is actually below, “Osavovorrarit. “If the grass is a few degrees below ambient temperature and the environment is just above zero, then the grass may be below freezing.”

This is a strange (though inconspicuous) phenomenon that occurs only when the sky is clear. This is because clouds heat the earth, reflecting infrared light back to the Earth’s surface. “You won’t be able to see it because it’s happening at a wavelength that people don’t see,” but radiation cooling happens all the time, Assawaworrarit says.

Modern scientists are also not the first to include radiation cooling. In southeastern Iran, the remains of dozens of ice houses called Yakhala have survived, which the ancient Persians used to exploit the phenomenon. When the structures worked, people poured water into shallow pools near glaciers. Even if the air temperature was in the high 30 or low 40, the water would freeze. In the morning, people collected ice and transferred it to a nearby beehive-like structure that used a different set of passive cooling techniques to keep the ice below zero throughout the summer.

The development of this technology poses several engineering challenges

Understanding the physics of these nocturnal solar panels is only part of the battle. Engineers have worked for years to make them efficient enough to be used in the real world.

Osavovorrarit and his colleagues began working on the problem during the pandemic.

“At first we were very stuck because the figure we got in the beginning was not what we expected,” he said. After months of analyzing the numbers, the team’s first experiment showed that the first iterations of the device produced about one-tenth of the amount of electricity they expected.

It turned out that one big problem arose on their way.

“The solar cell isn’t really a very good heat conductor,” Assawaworrarit says. – That’s the problem. Engineers have realized that the energy coming out from under the edges of the solar cell does not make a big contribution to the energy production of the system because thermal energy cannot easily pass through the solar cell itself.

“Looking back, it sounds simple,” he says. “But at the time it wasn’t obvious.”

Engineers fixed the problem by attaching a solar cell directly to an aluminum plate that conducts energy much more efficiently.

“It was kind of a revelation,” he says.

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