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posted by Fnord666 on Sunday February 02 2020, @02:48PM   Printer-friendly
from the gotta-wear-shades dept.

A concept paper published in ACS Photonics, describes a photovoltaic cell that works at night.

According Jeremy Munday, one of the paper's authors and a professor in the Department of Electrical and Computer Engineering at UC Davis

a specially designed photovoltaic cell could generate up to 50 watts of power per square meter under ideal conditions at night, about a quarter of what a conventional solar panel can generate in daytime

The described cell is thermoradiative and emits infrared radiation to space, which has a much lower than Earth temperature of 2.73 Kelvin.

"A regular solar cell generates power by absorbing sunlight, which causes a voltage to appear across the device and for current to flow. In these new devices, light is instead emitted and the current and voltage go in the opposite direction, but you still generate power," Munday said. "You have to use different materials, but the physics is the same."

The device would work during the day as well, if you took steps to either block direct sunlight or pointed it away from the sun. Because this new type of solar cell could potentially operate around the clock, it is an intriguing option to balance the power grid over the day-night cycle.

Journal Reference: [Nighttime Photovoltaic Cells: Electrical Power Generation by Optically Coupling with Deep Space, ACS Photonics (DOI: 10.1021/acsphotonics.9b00679)


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  • (Score: 2) by Arik on Monday February 03 2020, @07:25AM (1 child)

    by Arik (4543) on Monday February 03 2020, @07:25AM (#953055) Journal
    Well, from your first source:

    Professor Aaswath Raman, an applied physicist and engineer at the University of Pennsylvania, explained the process in more detail at TED 2018.

        "That pool of water, like most natural materials, sends out its heat as light. This is a concept known as thermal radiation… The atmosphere and the molecules in it absorb some of that heat and send it back… But here's the critical thing to understand. Our atmosphere doesn't absorb all of that heat… At certain wavelengths, in particular between eight and 13 microns, our atmosphere has what's known as a transmission window. This window allows some of the heat that goes up as infrared light to effectively escape, carrying away that pool's heat… So that pool of water is able to send out more heat to the sky than the sky sends back to it. And because of that, the pool will cool down below its surroundings' temperature."

    This sounds plausible, it might even be right, but someone saying so isn't the same as demonstrating it.

    And the experiment as described wouldn't really seem to demonstrate it. Ice forms in the desert at night a few degrees above the supposed freezing point *because of evaporation.* Even at low temperatures, water will evaporate as long as you have low relative humidity. Desert nights are cool and windy and that wind is bone dry, it soaks up water like a sponge. Every molecule that evaporates robs those remaining of a great deal of heat, so the remaining water becomes cooler than its surroundings quite quickly.

    This effect is easily noticed on a hot day - just soak your shirt in water and let it dry. Don't forget you need low relative humidity, that's much more important than the heat, which is why this works at low temperatures in the desert but will never work in Miami at any temperature. But soak your shirt on a hot day in the desert, and you'll be nice and cool as long as it lasts. Do it *at night* in the desert and you can expect hypothermia to result. So the mere fact that you can leave water out at night in the desert and get ice without the temperature technically going below zero doesn't prove the point.

    Lastly, taking what I quoted as true, that actually changes the picture a bit. It's not postulating that the energy lost is somehow radiating directly to space, but rather that some portion of it makes it through the atmosphere without being absorbed. This is not only plausible, it appears obviously true, as atmospheric gasses are not completely opaque and do affect some wavelengths more than others. But if THAT is what we're talking about, then the key here isn't being in the desert, it's being at high altitude. The wikipedia article on the subject, https://en.wikipedia.org/wiki/Infrared_window, appears to confirm that hunch, as it talks about this particularly working through cloud tops at high altitude.

    At any rate, interesting stuff, thanks for the links.
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  • (Score: 0) by Anonymous Coward on Tuesday February 04 2020, @07:33AM

    by Anonymous Coward on Tuesday February 04 2020, @07:33AM (#953457)
    A good test would be to put a double roof over one puddle of water in the desert and no roof over another puddle of water.

    I'd want to confirm whether the modern stuff still works in places with high humidity. Would be great if it did.