Stanford engineers have discovered a solar panel that produces electricity at night

The sky above Stanford, California was unusually clear for several nights last October.

That was good news Researcher Sid Assawaworrarit and his colleagues. Those conditions, he says, are “probably the best this year.” IE.

Azovorrit is not a grateful astronomer for the fact that clouds do not block the starlight that travels through the atmosphere and reaches the mirror of his telescope. As an electrical engineer, he welcomed cloudless nights for a completely different reason: a clear night is when infrared light from the surface of the solar panels is emitted freely into space.

That energy flow activates the device created by Azovorrit and his colleagues – an ordinary solar panel fitted with a thermoelectric generator – that generates small amounts of electricity from the ambient air and the small difference in temperature at the surface of the solar panel. Space.

At night, solar panels turn the desk and emit photons

New technology is taking advantage of the surprising fact about solar panels.

“During the day, a light from the sun comes and strikes the solar cell, but at night, something happens upside down,” Azovorrit says.

This is because solar panels – all hotter than absolute zero – emit infrared radiation.

“The light really goes out [from the solar panel], And we use it to produce electricity at night. The photons going into the night sky really cool the solar cell, ”he says.

When those photons leave the surface of the solar panel towards the sky, they carry heat with them. That is, on a clear night – when there are no clouds to reflect infrared light towards the Earth – the surface of a solar panel will be a few degrees cooler than the air around it. Azavoor and his colleagues take advantage of that temperature difference. The device, called a thermoelectric generator, captures some of the heat flowing from the hot air to the cool solar panel and converts it into electricity.

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

“I think it’s probably a registration number,” he says. But Azovorrit and his team did not stop there. He says that with one or two improvements (and in one good place) such a device could generate twice as much electricity.

“The theoretical limit is one or two watts per square meter,” he says. “It’s not a huge number, but there are a lot of applications” That kind of energy comes to hand at night.

For example, large sections of the world’s population – about one billion people – do not have access to the power grid. People living in that situation “can rely on solar power during the day, but at night they can not do much,” he says. Unlike batteries that decompose significantly after a few thousand charge cycles, the thermoelectric generators used in these solar panels are in solid state, “so the lifespan will be very long,” he says.

Another good use of technology is enabling an enormous network of environmental sensors used by researchers to keep tabs on everything from the weather to invasive species in remote corners of the world. Again, solar panels that generate a small amount of electricity at night can 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 will be very attractive from a cost perspective,” says Azzovorrit.

Discovery easily taps into an unnoticed energy source

The Earth constantly receives the largest amount of energy from the sun, up to 173,000 terawatts. Reflective surfaces such as clouds, particles in the atmosphere and snow-capped mountains immediately reflect 30 percent of that energy into space. The rest heats the land, the oceans, the clouds, the atmosphere and everything on the planet.

But that energy does not stay here. After the Industrial Revolution, as humans began to burn large quantities of fossil fuels, the Earth sent out as much energy as it could, except for the extra heat trapped by greenhouse gases. That is why this planet emits a truly fascinating amount of energy as infrared radiation.

“It’s a kind of light,” Assawaworrarit says. Infrared infrared radiation from warm earth (or anything else) has wavelengths that are invisible to the naked eye, but it carries energy. In fact, more than half of the total solar energy that strikes the Earth goes through this process and eventually returns to space.

What Asavo and his colleagues did was to create a new way to capture that energy as they left the planet. They were not the first to use a thermoelectric generator to capture this kind of energy (IE Includes one of the first major discoveries in this space in 2019). By integrating this new technology with solar panels that generate electricity during the day, researchers have taken an important step in making it possible for ordinary people to capture this energy for themselves.

It all comes down to radiant cooling

Modern scientists have not noticed that a surface facing the cloudless night sky becomes colder than the air around it. This phenomenon is called radiation cooling, and you may have seen it first thing in the morning. This is most evident in the grass after temperatures drop in the mid or lower 30s, but not below freezing.

“Even if the ambient temperature is a few degrees higher than the freezing temperature [grass] The leaf is really low, ”Assawaworrarit. “If the grass is a few degrees below ambient temperature, and the environment is slightly above freezing, the grass may actually be below freezing.”

This only happens when a strange (if subtle) sky is clear. This is because clouds heat the ground by reflecting infrared light back to the earth’s surface. “You can’t see it because it happens at a wavelength that humans can’t see,” but the radiation cold happens all the time, Assawaworrarit says.

Modern scientists are not the first people to work with radiation cooling. In southeastern Iran there are the remains of dozens of ice houses known as yaksals, which the ancient Persians used for this purpose. When the structures are operational, people will pour water into shallow pools next to snow houses. Even if the air temperature is 30s or less than 40 seconds, the water will freeze. In the morning, people collect snow and transfer it to a nearby beehive-like structure, which uses different passive cooling techniques that freeze the snow throughout the summer.

Improving this technology presents many engineering challenges

Understanding the physics behind these nocturnal solar panels is only part of the battle. Engineers have worked for many years and are qualified to use them in the real world.

Azavovar and his colleagues began to work on the issue during epidemics.

“Initially we were a little bit stuck because the deep figure we got in the beginning was not close to what we expected,” he says. Several months after the numbers were crushed, the team’s first experiment showed that initial repetitions of the device produced one tenth of the electricity they expected.

A big problem they knew was coming their way.

“A solar cell is not really a good conductor of heat.” Says Assawaworrarit. “That’s where the problem lies.” The engineers realized that the energy leaving the edges of the solar panel did not contribute much to the system’s energy output because thermal energy could not easily travel through the solar cell.

“Looking back, it seems straightforward,” he says. “But at the time, it was not obvious.”

The engineers fixed the problem by attaching the solar cell directly to an aluminum plate, which transmits energy more efficiently.

“It’s an epiphany,” he says.

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