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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: 3, Interesting) by ledow on Sunday February 02 2020, @04:01PM (16 children)

    by ledow (5567) on Sunday February 02 2020, @04:01PM (#952726) Homepage

    1) Sounds like pie in the sky before it even starts. It "could make up to" but they haven't yet made one that does anywhere near that.
    2) I'm not at all sure about the physics - because the NIGHT SKY is almost zero Kelvin... you're going to get energy by emitting heat to it? But... you're not in the night sky. Unless you're inside space. In which case this is an expensive and inefficient solution, requiring you to emit heat into the void.

    It sounds like, at absolute best, it'll be one of those "we generate 0.1mW more than we spend" things that might be useful on a life-long satellite mission to keep things ticking over. They're painting it like you'll stick this on the back of a solar panel and at night it'll run the grid.

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  • (Score: 5, Informative) by maxwell demon on Sunday February 02 2020, @04:16PM (12 children)

    by maxwell demon (1608) on Sunday February 02 2020, @04:16PM (#952734) Journal

    you're going to get energy by emitting heat to it?

    That's how thermodynamic machines work: You have a temperature difference and use that to extract energy from the warmer reservoir (in this case, the heat surrounding it), but it only can work if some energy is given to the colder reservoir (in this case, the night sky). Basically, you're taking part of the energy that would be radiated to the night sky anyway, and turn it into electricity (that is, the amount of energy this device radiates to the sky is less than what a perfectly black surface of the same temperature would radiate, and the difference — or at least part of it, because real devices are never perfect — is turned into electric energy).

    So at least from the view of thermodynamics, the concept is sound.

    --
    The Tao of math: The numbers you can count are not the real numbers.
    • (Score: 1) by Arik on Sunday February 02 2020, @05:02PM (11 children)

      by Arik (4543) on Sunday February 02 2020, @05:02PM (#952751) Journal
      "That's how thermodynamic machines work: You have a temperature difference and use that to extract energy from the warmer reservoir (in this case, the heat surrounding it), but it only can work if some energy is given to the colder reservoir (in this case, the night sky)."

      It's easy for me to see how that works in the case of a Stirling engine. In this case, not so much.

      The first thing that seems suspicious is they're talking about radiating to outerspace, but that's a long ways off. It seems like what they'd actually be radiating to is the atmosphere instead, for a much smaller temperature difference, at best.

      "Basically, you're taking part of the energy that would be radiated to the night sky anyway, and turn it into electricity (that is, the amount of energy this device radiates to the sky is less than what a perfectly black surface of the same temperature would radiate, and the difference — or at least part of it, because real devices are never perfect — is turned into electric energy)."

      So, the black surface soaks up solar radiation during the day, it gets quite warm. Then it cools down at night, and as it does so, we convert a little of that heat to electricity instead? If that's basically what we're talking about, it seems like you'd get a lot more power just by converting that same solar radiation to electricity immediately, probably even enough more to make up for losses if you need to store it for use at night. No?
      --
      If laughter is the best medicine, who are the best doctors?
      • (Score: 2) by JoeMerchant on Sunday February 02 2020, @05:10PM (7 children)

        by JoeMerchant (3937) on Sunday February 02 2020, @05:10PM (#952752)

        In this case, not so much.

        You don't so much see this case as feel it.

        Go to the desert, on a clear and preferably windless night, stand outside and feel the heat leave your body and the landscape into the sky. That is the principle this operates on, and it is a powerful one.

        --
        🌻🌻 [google.com]
        • (Score: 2) by Arik on Sunday February 02 2020, @06:40PM (5 children)

          by Arik (4543) on Sunday February 02 2020, @06:40PM (#952779) Journal
          Been there, done that, and yes, it's a powerful force, but it seems to be what I would expect from the air temperature, not what I would expect from "outer space." The difference is not insignificant.
          --
          If laughter is the best medicine, who are the best doctors?
          • (Score: 0) by Anonymous Coward on Sunday February 02 2020, @07:16PM (1 child)

            by Anonymous Coward on Sunday February 02 2020, @07:16PM (#952797)

            Compare going outside on two windless nights, both with air temperatures of 10C. One night with clear skies, one with clouds. The difference is significant.

            • (Score: 2) by Arik on Sunday February 02 2020, @08:38PM

              by Arik (4543) on Sunday February 02 2020, @08:38PM (#952837) Journal
              But it's never cloudy in the desert.
              --
              If laughter is the best medicine, who are the best doctors?
          • (Score: 2) by JoeMerchant on Sunday February 02 2020, @09:08PM (2 children)

            by JoeMerchant (3937) on Sunday February 02 2020, @09:08PM (#952853)

            The air temperature falls because of space... it's not as if the ground under your feet sucks up all the heat...

            As AC says: the effect is much MUCH reduced with heavy cloud cover - one demonstration of a greenhouse type effect.

            The other thing to remember, space is cold, really really cold - 3K cold. So, when your ground temp falls from 40C to -10C, that's only dropping from a 313/3K differential to a 263/3K differential - even at -10C you've got a hell of a long way further you could fall if the Earth didn't rotate you back to face the sun.

            --
            🌻🌻 [google.com]
            • (Score: 2) by Arik on Sunday February 02 2020, @10:15PM (1 child)

              by Arik (4543) on Sunday February 02 2020, @10:15PM (#952890) Journal
              "So, when your ground temp falls from 40C to -10C, that's only dropping from a 313/3K differential to a 263/3K differential"

              Exactly my point.

              "even at -10C you've got a hell of a long way further you could fall if the Earth didn't rotate you back to face the sun."

              But look, even if we could somehow change the rotation of the Earth to eliminate day and night - to keep the same side facing the sun all year - that would not cause the backside of the Earth to reach 0k. Not ever - not until the Sun itself cools down considerably, at least. The heat absorbed on the side facing the Sun would be passed on around the globe. I suppose the oceans would be eliminated quickly, the atmosphere slowly, and the far side would be quite cold and icy; but not to 0k.

              --
              If laughter is the best medicine, who are the best doctors?
              • (Score: 2) by JoeMerchant on Monday February 03 2020, @02:58AM

                by JoeMerchant (3937) on Monday February 03 2020, @02:58AM (#952980)

                that would not cause the backside of the Earth to reach 0k. Not ever

                Clearly not.

                First, space itself has ~3K background temperature, at least in our neighborhood.

                Second, the atmosphere would conduct a great deal of heat over from the day side.

                Third, something like a grape skin thickness beneath our feet (in relative terms) is magma at roughly 975K.

                However, even with all those in mind, also bear in mind that the night side of Mercury is -180C (155K) while the day side cooks at 430C (700K), precisely due to passive radiation of heat to space and the tidal lock that keeps it from rotating. For reference, at our distance from the sun, the moon swings from -173C (100K) to 127C (400K) as it passes through night and day on it's 29 day solar cycle.

                So, while we've got stuff on the ground "glowing hot" at 260+K, it's not hard at all to imagine extracting energy from that glow before it heads on to the rather large heat-sink of outer space, particularly in the "color bands" (infra red, I believe) where the atmosphere is most transparent to the glowing energy.

                --
                🌻🌻 [google.com]
      • (Score: 0) by Anonymous Coward on Monday February 03 2020, @06:10AM (2 children)

        by Anonymous Coward on Monday February 03 2020, @06:10AM (#953038)

        The first thing that seems suspicious is they're talking about radiating to outerspace, but that's a long ways off. It seems like what they'd actually be radiating to is the atmosphere instead, for a much smaller temperature difference, at best.

        You need to keep up with the times:
        https://www.realclearscience.com/blog/2018/07/09/how_people_created_ice_in_the_desert_2000_years_ago.html [realclearscience.com]

        More recently:
        https://www.nature.com/articles/d41586-019-03911-8 [nature.com]
        https://web.stanford.edu/group/fan/publication/Raman_Nature_515_540_2014.pdf. [stanford.edu]
        https://www.sciencealert.com/this-new-roof-material-stays-colder-than-the-air-around-it-even-in-summer [sciencealert.com]
        https://www.racplus.com/news/researchers-scale-up-cooling-film-technology-to-rooftop-size/10036707.article [racplus.com]

        • (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.
          --
          If laughter is the best medicine, who are the best doctors?
          • (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.
  • (Score: 2, Informative) by RandomFactor on Sunday February 02 2020, @04:33PM (1 child)

    by RandomFactor (3682) Subscriber Badge on Sunday February 02 2020, @04:33PM (#952740) Journal

    2) I'm not at all sure about the physics - because the NIGHT SKY is almost zero Kelvin... you're going to get energy by emitting heat to it? But... you're not in the night sky. Unless you're inside space.

    It's been ages since I took heat transfer, but in a nutshell, there are three basic types of heat transfer mechanism.
     
    Conduction, Convection, & Radiation. [keydifferences.com]
     
    In this case radiation is the mechanism in use and does not require you to be in contact with or surrounded by the recipient of the transfer. It just requires a temperature differential. The greater that differential the greater the efficiency of these panels would be, so space being 2.73K isn't a bad thing.
     
    Radiation is the only one of the three mechanisms available to objects in space, lacking two of the mechanisms is why heat management is a significant consideration on spacecraft.

    --
    В «Правде» нет известий, в «Известиях» нет правды
    • (Score: 0) by Anonymous Coward on Monday February 03 2020, @04:06AM

      by Anonymous Coward on Monday February 03 2020, @04:06AM (#953009)

      Great link.

  • (Score: 2) by Immerman on Sunday February 02 2020, @07:49PM

    by Immerman (3985) on Sunday February 02 2020, @07:49PM (#952810)

    > 2) I'm not at all sure about the physics - because the NIGHT SKY is almost zero Kelvin... you're going to get energy by emitting heat to it? But... you're not in the night sky. Unless you're inside space. In which case this is an expensive and inefficient solution, requiring you to emit heat into the void.

    Any time you have an energy flow, such as radiating thermal infrared photons into the night sky, you can generally intercept some of that energy to do work like generating electricity.

    The night sky is NOT zero Kelvin though. In space it's very close, around 3K or -270C, on Earth though you're right - you've got the atmosphere radiating back, and that's dramatically warmer. Around 0C is common, while a humid tropical sky can be as warm as 20C. Cloudless desert nights are best, and can commonly get as low as -50C

    As for wehther it would be expensive and inefficient in space? Even high efficiency solar panels are typically only around 30% efficient, which means 70% of the energy (equivalent to what reaches Earth's surface) is either reflected or absorbed as heat. If you could convert half that to energy as it radiates away, you could rival the power being generated by the solar panels. Even far more modest gains might be worth it if the system added minimal mass to the solar wings.