Stanford engineers invent a solar panel that generates electricity at night

Radiative cooling could reduce the need for expensive batteries in some applications.

Sid Assawaworrarit has developed a device, an ordinary solar panel fitted with a thermoelectric generator, which generates a small amount of electricity from the slight temperature difference between the surrounding air and the surface of a solar panel pointing towards space.

The new technology takes advantage of a surprising fact about solar panels. During the day, there is light that comes from the Sun and hits the solar cell, but during the night, the reverse happens.

This is because solar panels, like anything hotter than absolute zero, emit infrared radiation.

In fact, there is light coming 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.

Sid Assawaworrarit

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 surrounding air.

Assawaworrarit and his colleagues take advantage of this temperature difference. 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 Assaworrarit device tested on the roof of Stanford generates about fifty milliwatts for each square meter of solar panel (50 mW/m2).

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 one or two watts per square meter. That’s not a huge number, but there are plenty of applications where that kind of power at night would come in handy.

Sid Assawaworrarit

The type of thermoelectric generators used in these solar panels are solid state, so their useful life is practically eternal.

One of the uses of this technology is to power the vast network of environmental sensors that researchers use to monitor everything from weather conditions to invasive species in remote corners of the planet. Again, solar panels that generate a small amount of electricity at night could reduce the need for batteries and associated maintenance and replacement costs.

If you could achieve one watt per square meter, that would be very attractive from a cost perspective.

Sid Assawaworrarit

The invention exploits a power source that is easily overlooked.

The Earth permanently receives a huge amount of energy from the Sun, in the order of 173,000 TW. Clouds, particles in the atmosphere, and reflective surfaces such as snow-capped mountains immediately reflect 30% of this energy back into space. The rest ends up heating the earth, the oceans, the clouds, the atmosphere and everything else on the planet.

But that energy does not stay here. With the exception of the extra heat that greenhouse gases have trapped since humans started burning large amounts of fossil fuels since the Industrial Revolution, Earth sends out as much energy as it receives. This is why the planet emits a truly mind-boggling amount of energy in the form of infrared radiation.

It is a kind of light. Infrared radiation shining on hot Earth (or anything else) has wavelengths too long for our eyes to see, but it carries energy. In fact, more than half of the total amount of solar energy that reaches Earth goes through this process, eventually returning to space.

Sid Assawaworrarit

What Assawaworrarit and his colleagues have done is find 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. By integrating this new technology with solar panels that generate electricity during the day, the researchers have taken an important step towards enabling ordinary people to harness this energy for themselves.

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