There is free energy raining down on Earth in the form of sunlight, but harnessing all of that energy has proven difficult. The sun only shines during the day, and those solar panels are useless at night, or at least they are right now. Researchers from the University of California Davis suggest you could generate power at night using “anti-solar panels.” Of course, it’s all so obvious when you think about it.

Researchers around the world are working to improve the efficiency of solar panels, which currently only capture a fraction of the total solar energy falling on them. Even high-efficiency solar panels are at a disadvantage compared with non-renewable energy because you can’t generate solar power at night. So, you need to capture enough during the day to store for usage when the sun isn’t shining. However, battery technology has been similarly slow to improve. The anti-solar panels described in the journal ACS Photonicscould fill the gap to supplement power generation at night when solar panels and batteries aren’t good enough.

Solar panels work because they’re cold compared with the sun, so they can absorb sunlight and convert it to energy. Space, and therefore the night sky, is cold. Therefore, you can point a warmer panel on Earth toward the sky to radiate energy outward as infrared light. That’s what the University of California team is proposing—it’s essentially a heat engine.

These devices don’t use the same technology as solar panels, although they’d probably look similar. Solar panels rely on photovoltaic cells that absorb photons to create electron-hole pairs across the semiconductor, generating a working voltage. A nighttime panel would use a thermoradiative cell to emit infrared radiation from the Earth into space to create electron-hole pairs.

The team estimates that thermoradiative cells would only be able to generate about a quarter as much power as a solar panel of the same area. That’s mainly a consequence of the lower energy of infrared light. Silicon is the current material of choice for solar panels as it’s good at capturing light in the visible wavelengths. It may be possible to boost the efficiency of thermoradiative cells by using materials that can better interact with longer wavelengths of light, for example, mercury alloys.

The University of California study is just an initial proposal for nighttime energy generation. The next step is to start building the devices to see how well they perform.

Top image credit: Getty Images

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