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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)
(Score: 4, Interesting) by JoeMerchant on Sunday February 02 2020, @02:53PM (7 children)
Does this mean that space based solar arrays can become two sided for a 25% output boost?
🌻🌻 [google.com]
(Score: 1) by RandomFactor on Sunday February 02 2020, @03:34PM (1 child)
It sounds like a good fit, spacecraft already have to radiate heat anyway so getting power from doing so could make sense.
I thought about the feasibility of making two sided arrays and flipping them for daytime vs nighttime use on Earth. Same general idea, although I wouldn't want a bunch of flipping/shuffling panels on my roof, however it might work for industrial sized solar farms.
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(Score: 3, Interesting) by Coward, Anonymous on Sunday February 02 2020, @04:11PM
Yay, let's add thermal resistance to the dissipation path! Engineers are going to love that.
(Score: 4, Informative) by Immerman on Sunday February 02 2020, @03:38PM (4 children)
Nope. Shipping to orbit is so expensive that you'll generally go with the high-end efficient solar panels of at 30+% efficiency. You also get almost 40% more sunlight reaching the panel without the atmosphere in the the way. Combined, you're generating at least 1400Winsolation * 30%(?) efficiency = 420W
Assuming these panels worked just as well in orbit, they'd only produce an extra 50W, or around 12%
That said, they might be more efficient as well - the solar panels likely get lot hotter than ambient earth conditions, and in space the radiating side of the panel isn't also recieiving radiation from the atmosphere, which probably cuts efficiency on Earth.
Of course, the big question is weight - if these could be laminated to the back of solar panels with no additional structure and minimal additional weight, then they could well be worth it. Otherwise it's probably far more weigh-efficient to just add more solar panels.
(Score: 2) by JoeMerchant on Sunday February 02 2020, @04:37PM
That was my thought, you're making these big "wings" with an essentially unused surface, so why not use it?
🌻🌻 [google.com]
(Score: 0) by Anonymous Coward on Sunday February 02 2020, @07:11PM (2 children)
They wouldn't work in space. They work on the heat difference between the ambient environment and the radiant temperature of the sky. Without the ambient heat flowing into the back side, power output would drop to zero at the temperature equalised.
They might be useful in tropical cities, as they are basically ejecting heat to space and would cool the area around them. Not as much as just painting stuff black, but if you do that it gets hot during the day. I do like the idea of putting them on all the industrial roofs, with solar cells on the other side and flipping them sunrise/sunset.
(Score: 3, Informative) by Immerman on Sunday February 02 2020, @07:39PM (1 child)
The "ambient environment" on the warm side would be the solar panels themselves - which might well be considerably warmer than they would be on Earth since they're big dark panels that can only shed energy through radiant cooling.
Even if the solar panels are 30% efficient, that still leaves 70% of the incoming energy being either reflected or absorbed and converted to heat - and 70% of orbital solar energy is roughly the same as 100% of sea-level solar energy.
The radiant temperature of the sky would also be FAR lower in orbit. On Earth it averages around around 0C, though it cloudless desert nights can commonly get as cold as -50C, so you're talking maybe a 20-60C difference. The tropics are actually the worst, as warm humid air can bring the sky temperature up to 20C or more, not much cooler than ambient. Space in comparison is around -270C, so about 5x to 15x the thermal difference. If power output from the panels scaled similarly it you might end up actually rivaling the power from the solar panels.
(Score: 0) by Anonymous Coward on Monday February 03 2020, @02:32AM
Solar panel have to eject a significant amount of heat. Putting these on the back to extract energy would block that cooling and probably reduce their efficiency and cost you more than these produce.
If the stuff can be made cheap and efficient enough it might be economical to face the 'hot' side towards the sun, and use a mirror so that the 'cold' side sees a different part of the sky. Flipping it so that at night the 'hot' side faces the ground and the cold side faces space could give you panels than generate continuously,
(Score: 3, Interesting) by ledow on Sunday February 02 2020, @04:01PM (16 children)
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.
(Score: 5, Informative) by maxwell demon on Sunday February 02 2020, @04:16PM (12 children)
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)
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)
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: 5, Interesting) by pTamok on Sunday February 02 2020, @05:44PM
This is how ancient Egyptians and ancient Persians made ice [fieldstudyoftheworld.com].
p 41, Icehouses, Tim Buxbaum [google.co.uk]
New insight on passive ice making and seasonal storage of the Iranian Yakhchal and their potential for contemporary applications (Hareth Pochee, John Gunstone, Oliver Wilton) [maxfordham.com]
(Score: 2) by Arik on Sunday February 02 2020, @06:40PM (5 children)
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)
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
If laughter is the best medicine, who are the best doctors?
(Score: 2) by JoeMerchant on Sunday February 02 2020, @09:08PM (2 children)
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)
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
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)
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)
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
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)
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.
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(Score: 0) by Anonymous Coward on Monday February 03 2020, @04:06AM
Great link.
(Score: 2) by Immerman on Sunday February 02 2020, @07:49PM
> 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.
(Score: 2, Interesting) by Anonymous Coward on Sunday February 02 2020, @06:04PM (4 children)
How does this concept not violate the 2nd law of thermodynamics? I understand the part where you create a voltage differential by thermal radiation, but how does that generate power? Since the device itself is radiating, it is a net power drain, not a power source.
Sounds to me they're describing a thermal battery: store energy on the radiative surface, extract that energy when cooling down. That is not a "generator" is any scientific sense of the word, it is a transducer.
(Score: 1, Interesting) by Anonymous Coward on Sunday February 02 2020, @07:07PM
Yeah, I kinda/sorta get how this works but I don't know if I would call it a photovoltaic cell either? IIRC, they had an article similar to this in the past on Soylentnews (or was it the other site?) but I'm too lazy to search for it.
(Score: 3, Informative) by Immerman on Sunday February 02 2020, @08:09PM (2 children)
Think thermocouples - lots of documentation of those you can browse.
Heat comes in one side, somewhat less goes out the other side, and the difference is converted to electricity.
The key component is that you have an energy flow. Where energy is flowing, some of it can be intercepted to do work (like generating electricity)
(Score: 0) by Anonymous Coward on Sunday February 02 2020, @08:22PM (1 child)
The problem is they are wildly terrible at it.
Not sure if he has found something novel here or just another iteration on the a thermometric generator. https://en.wikipedia.org/wiki/Thermoelectric_generator [wikipedia.org] You can buy them by the gross on alibaba and mouser.
The issue with those usually is the heat dissipation and eventual equilibrium between the two sides. Usually the heat is removed from the system with one big giant heat sink on one side (a radiating body).
So wait and see if it is something novel or just more of the same before getting too excited.
They are neat though.
(Score: 0) by Anonymous Coward on Sunday February 02 2020, @11:19PM
I'm pretty sure it's more of the same, just about all of these articles end up being more of the same.
(Score: 2, Funny) by Anonymous Coward on Sunday February 02 2020, @06:15PM
Another successful project from University of Transylvania's nighttime physics department.
(Score: 1, Interesting) by Anonymous Coward on Sunday February 02 2020, @06:56PM
This cannot generate enough power to justify constructing anything to use it... unless a framework was already built. It could be combined with existing solar farms to generate power at night. Perhaps put it on shutters that close over the photovoltaic at night then open in morning
(Score: 0) by Anonymous Coward on Sunday February 02 2020, @07:14PM (1 child)
oh no you diiieeennn!
(Score: 0) by Anonymous Coward on Monday February 03 2020, @06:16PM
It is weird when some shit poster is so bad at it that you feel sorry for them.
(Score: 2) by Username on Monday February 03 2020, @01:58AM (1 child)
So where are they getting the "current and voltage" used to create the light to emit to the sky in order to harvest electricity from? Is this just an LED with a thermalcouple for a heatsink? How do they get around clouds? I can see clouds reflecting the infrared light and scattering it back at town. Which might be cool, we can create new filters for windshields so we dont need headlights. This also would blind most unfiltered cameras. The MIB will need new spy satellites.
(Score: 2) by ChrisMaple on Monday February 03 2020, @02:58AM
On Earth, this acts as a thermocouple using the atmosphere (by conduction and convection) as the warm reference and space (by radiation) and the cold reference.
In space, the backside of sun-facing photocells is the warm reference by conduction, and space by radiation is again the cold reference.