The sky over Stanford, California was unusually clear for several nights last October. That was good news for scientist Sid Assawaworrarit and his colleagues. According to him, these conditions were “Probably the best this year.”
Assaworrarit is not grateful that the clouds did not block the starlight from traveling through the atmosphere and reaching the mirror of its telescope. As 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.
Solariums that work even at night
This flow of energy allows the equipment that Assaworrarit and his colleagues have created – which is an ordinary solar panel equipped thermoelectric generator – to produce a small amount of electricity from the slight temperature difference between the ambient air and the surface of the solar panel directed deep into space.
The new technology takes advantage of the surprising fact regarding solar panels. “During the day, light comes from the sun and hits the solar cell, but at night the story is reversed.” explains Assawaworrarit. The reason is that solar panels – like anything warmer than absolute zero – emit infrared radiation.
“The light that we use to generate electricity at night actually comes out of the solar panel. Photons heading into the night sky cool the solar cell, “ he says. As the photons leave the surface of the panel, they carry heat away. This means that on clear nights – when there are no clouds reflecting infrared light back to Earth – the surface of the solar panel will be several degrees colder than the surrounding air.
It is this difference in temperature that Assawaworrarit and his colleagues take advantage of. A device called a thermoelectric generator it 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 panel, which was tested on the roof of Stanford, generates about fifty milliwatts per square meter (50 mW / m2).
It could be even better
Researchers believe this is probably a record number, and with a few improvements, such a device could produce even more electricity. “The theoretical limit is probably about one or two watts per square meter. It’s not a big number, but there are a lot of applications where this kind of energy would come in handy at night. “
For example, a large part of the world’s population – less than a billion people – does not have access to the electricity grid. People living in such a situation cannot rely on energy from the sun at night. Unlike batteries, which degrade considerably after several thousand charging cycles, the thermoelectric generators used in these solar panels remain in good condition, so their longevity is essentially eternal.
Another possible use of this technology is to power a huge network of environmental sensors that scientists use to monitor everything from weather conditions to invasive species in remote corners of the world. Solar panels, which produce a small amount of electricity at night, could again reduce the need for batteries and the cost of maintaining and replacing them.