Earth continuously receives huge amounts of solar energy from the Sun of which around 30% is instantly reflected from surfaces like snow, particles in the atmosphere, and clouds back into space. The remaining 70% warms the planet.
However, that energy doesn’t remain. Apart from additional heat generated by greenhouse gases, the earth emits as much energy as it receives which contains huge amounts infrared radiation.
Researchers led by electrical engineer Sid Assaworrarit have created a solar panel device, adapted using a thermoelectric generator, to utilise the way infra-red light captured by solar panels can radiate back into space.
The device exploits the difference in temperature between the surface of the solar panel and the background air to generate a slight amount of electricity.
Light particles or photons hit the solar panels during the day, but at night the process is reversed. Just like everything else at a higher temperature than absolute zero, solar panels discharge infrared radiation. The photons radiating from the solar panels at night carry heat with them, cooling the panel. In a phenomenon known as radioactive cooling, which is happening all the time, a surface pointed toward the cloudless night sky can become colder than the air around it. This means that on a clear night, without clouds to reflect the infrared light back, the solar panel’s surface is slightly colder than the surrounding air.
The researchers’ adapted solar panel, re-engineered to incorporate a thermoelectric generator, can capture some of the heat flowing from the warmer air to the cooler solar panel and leverage the temperature differential, converting it into electricity.
In fact, for every metre square of solar panel, the testing device generated approximately fifty milliwatts of electricity (50 mW/m2).
According to Assawaworrarit, there is potential for significantly more,
“The theoretical limit is probably about one or two watts per square meter. That’s not a huge number, but there are a lot of applications where that kind of energy at night would come in handy.”
These could include providing energy at night in the areas worldwide lacking an electrical grid, which hitherto have only been able to rely on solar power during the day. Similarly, environmental sensors which, for example, monitor weather conditions and must be permanently in-action, could be significantly more cost-effective if the need for the maintenance, upkeep, or replacement of batteries was reduced.
Inevitably, batteries degrade after repeated use, but because the kind of thermoelectric generators used in these solar panels are solid, their longevity isn’t limited. Radiative cooling might reduce the need for costly batteries in some applications – the challenge is to make the panels efficient enough to be real world usable.
