Hi there - I'm the lead author of this paper. Not a big fan of this particular write-up.. it's not a solar panel at all! But happy to answer questions.
Semi-off topic but you should knowledge-able in the area, but would there be any potential in leveraging the permanent temperature differences which occur 10meters below the ocean? Light does not penetrate below 10meters and old water is denser. Thus the ocean, away from land, has a permanent and substantial potential energy difference between these two layers. Is it viable to harvest this difference?
Yes. It's called an OTEC (Ocean thermal energy converter) and it's a very old idea.
They are quite workable. In the simplest form, you have a 2 vacuum chambers at the surface, and a pipe to the bottom of the ocean. You vacuum out your pressure vessel down to around 3psi and let some warm surface water in one end, while pumping cold bottom water into a radiator in the other. At 3psi, the surface water flashes to steam in vessel A, drives a turbine into vessel B, and then condenses back into liquid water on the radiator. This brings the system pressure back down to where you started.
In practice, you'd probably use the surface water to warm a working fluid that is a gas at surface temperature and pressure, and then the system can stay at atmospheric pressure.
The benefit of this system is that it also creates an artificial upwelling of nutrients from the bottom of the ocean, which can be used to grow all sorts of stuff. Was a big hippy fad idea back in the 70's.
Is there a theoretical limit to the device's performance? Something relatable to power like milliwatts/m^2? How does this theoretical limit relate to the devices you've actually built?
Is it possible for a device to be both a solar panel and a radiative thermoelectric generator? How close to a theoretical limit for radiative thermoelectric generation could a device that was also a solar panel become?
Would capturing heat via mass e.g. warming up a block of cement during the day help improve the efficiency of a radiative thermoelectric generator that sits atop the heat source?
Is there a better term for this other than radiative thermoelectric generation?
There was an analysis done on the theoretical (Carnot/ 2nd Law) limits of using Earth's infrared emissions in this way: https://www.pnas.org/content/111/11/3927.abstract (Roughly 4 W/m2 for a system that purely exploited the radiative mismatch between outgoing and incoming long-wavelength radiation from the sky.
The bigger limit in our case is that we're using a thermoelectric generator - and achieving a relatively small temperature difference. We argued in the paper it might be possible with improved engineering and more favorable weather conditions to push performance to 0.5 W/m2.
In general, solar gets you far more power than this method ever will. The only advantage to combining the two might be to provide incremental power at night that improves the overall energy economics of the footprint associated with the solar panel.
And yes, a heat source would improve the power output. This has been the approach of an entire field of research that one might term 'waste heat recovery'. This encompasses everything from industrial sources to the human body or a campfire. The advantage, such as it is, of what we've done is that you don't need a source of heat besides the air itself.
> Is there a theoretical limit to the device's performance?
Yes.
Let the night-time equilibrium temperature be T_C (temperature_cold). Let the heat reservoir temperature be T_H (temperature_hot). The maximum theoretical efficiency is equal to 1 - T_C / T_H. This is from Carnot’s theorem and the 2nd law of thermodynamics.
The wasted energy is radiated off into space. You can calculate this with the Stefan–Boltzmann law. At 10°C we get 4.6 mW/m^2. (Edit: Whoops, bad arithmetic. Ignore these numbers. Do the math yourself.)
If your heat reservoir is 25°C and your cold temperature is 10°C then you have an efficiency of 5.0%. So you would generate 0.24 mW/m^2 at maximum theoretical efficiency.
You can even solve here for the optimum night-time temperature. Too cold and not enough heat is radiated. Too hot and the efficiency suffers. There is a maximum in the middle (but I am not going to do the math).
There are other interesting calculations I’m sure you can do to figure out maximum and minimum reservoir temperatures, but the challenge here is that you don’t want to harness sunlight to heat up your reservoir—you want to use existing heat that you have lying around.
Apparently, with our atmosphere we can achieve something like 40°C cooling in ideal conditions, and it is claimed that 60°C is possible. Back-of-the-envelope math suggests that you would achieve maximum theoretical power at around ~60°C difference.
With a reservoir temperature of 25°C my estimate is around 40W maximum power (with the correct arithmetic). You can get more power with a hotter reservoir.
Is the radiative cooler really just an aluminum disc painted black? Why this material versus some of the other designs out there (some groups have made these out of wood, others with glass microspheres in polypropylene, etc)
Yes! Most natural materials have a relatively high emissivity at the infrared wavelengths associated with "typical" room / terrestrial temperatures. So in that sense, pretty much any material you might have (except for a highly polished metal that might have low emissivity) is suitable to get some cooling using the radiative cooling effect at night.
The fancier materials work is for two things: 1) selective emission which can allow the radiative cooler to get to a colder temperature than a natural material (many/most of which have relatively uniform emissivity), and 2) high solar reflectance at the same time, which can allow radiative cooling during the day as well.
The OP prompted me to check on Shanhui Fan's group[1], which had done some sub-ambient radiative cooling in sunlight[2], and I stumbled on this recent work:
A dual-mode textile for human body radiative heating and cooling[3]
Unless you go out in the sun. But yes, if you're hot indoors, wearing black clothing can help by radiating more heat. (And white clothing can help in the sun by reflecting more incoming radiation.)
On behalf of all us, thanks for your work. Does the distance between the two surfaces at different temperatures matter? I'm sure this has occurred to you and your team, but the ambient temperature inside of a bedroom, beneath the roof, could provide a greater temperature differential.
Yes that distance matters in so far as getting the heat to the thermoelectric can be a bit more challenging. However I believe this has been investigated before and there are likely ways of doing it at least somewhat well.
Heat doesn't "really" get trapped by greenhouse gases.
The earth is constantly heated by the sun's light (light hits the air, heats the air; it hits the ground, it heats the ground) and it's constantly radiating heat away (the ground emits infrared and cools down). The hotter something is, the faster it emits heat, so based on the amount of incoming heat, the irradiated body hits a temperature where income=outgoing. More greenhouse gases just change the radiation/heat profile of the planet so that the point where income=outgoing ends up at a higher temperature.
You could actively cool the planet in principle, of course; but to do anything noticeable, you'd have to operate on geographical scales. You'd probably be better off building towers to the edge of the atmosphere and putting infrared radiators like this on top of those; otherwise, the your best bet would be to replace a few million square kilometres of a hot region with black paint and make sure there are never any clouds overhead.
> otherwise, the your best bet would be to replace a few million square kilometres of a hot region with black paint and make sure there are never any clouds overhead.
That would be counterproductive since black would absorb more of the sun's energy during the day than it would radiate at night. What you'd need is a way to have a black surface during the night and white during the day. (But barring that, white all the time is better than nothing, because it reflects more of the incoming sunlight. This is why melting of polar ice caps can accelerate climate change.)
One way to understand this cooling effect is that it occurs because at wavelengths where greenhouse gases are not substantially absorptive, heat can effectively escape out (or at least get absorbed and sent back to you at a higher altitude). The actual mechanisms are more complex than I'm describing as the atmosphere's temperature and composition varies with altitude, but the net effect from the perspective of a surface facing the sky is that, if you're at the same temperature as the air around you, you will radiate more heat out than the sky sends back to you.
All that being said, this is not in and of itself a climate change solution in the way you might be imagining. Most surfaces on Earth are effective at radiating heat already, and do so (it's in climate models). The difference here is we're thinking about actively making use of the cooling effect from a device, or building-scale to offset energy uses.
Huh. So this is why cars parked on one side of a street (with no tree cover) will be covered in condensation, while those parked on the other side (which has tree cover) will not?
I don't think it will reduce the need for energy storage. If it does, it might be in some marginal cases and by a small amount. This is because the power generated using this approach is quite a bit less than what you can get from solar. So for any conventional uses, PV+storage will always be the winner. I think the real advantage for this might be for low-power, long-duration applications where battery cycles can be a challenge. The other big scenario is polar climates, where there's low solar insolation for several months.
> I think the real advantage for this might be for low-power, long-duration applications where battery cycles can be a challenge
Correct me if I'm wrong but I guess it wouldn't make much sense to use something like that on a LEO small satellite right? But sounds pretty cool and handy to have such a setup on something larger like a lunar base right?
"We have highlighted the remarkable possibility and potential of generating small amounts of power by radiative cooling at night using low-cost, off-the-shelf, commodity components (less than $30 USD for our initial proof of concept demonstration). In off-grid locations throughout the world, this approach of generating light from darkness highlights an entirely new way of maintaining lighting, entirely passively, at night. The power generated could also be used to power small sensors in remote locations, with their lifetimes not being limited by batteries but the lifetime of the thermoelectric module, which can be an order of magnitude longer"
The invention seems more fitting for certain remote sensor applications rather than say, lighting for an off-grid home that's already served by the various solar-battery-lighting companies that've been around for over a decade.
Sure, it'll never be a significant source of energy [if we plastered literally the entire earth's surface with them at maximum theoretical efficiency, we'd provide 1% of our electricity consumption].
But the sheer concept of "you know what, lets generate energy from THE VOID OF SPACE, the ABSENCE OF HEAT" is deeply entertaining.
I had this weird, uncanny moment when I first realized that atmosphere being transparent at some infrared bands means a device exploiting this is literally cooled as if it was exposed to interplanetary space. It looks into the abyss.
You can have a fun and cool similar effect if you get a $10 infrared thermometer (I would say one with a laser for helping to point it). Useful for cooking and food safety.
But then...go outside a night and point it first at the ground, then at any clouds, and then right up into darkness. You'll see a big drop in temperature, which is the absence of heat energy in the atmosphere itself...all the way out into space.
I believe that the sensor has some type of angle to what it can detect, so you're getting the average reading across a large cone of air/space at that point. Also, the radiated heat along the depth of the cone would be "stacked", so that you are viewing a 'cumulative' reading from the entire depths of that cone, like an integral going into space.
Spacecraft already need large radiators to keep from overheating, so you could theoretically use this to generate a bit of power in the process, especially if it means reducing the amount of battery you need for when you go into the shade of a planet/moon/etc...
In practical terms it probably isn't efficient enough to be worth the weight, but the idea is neat nonetheless.
The new(ish) bit is the fact that with a heatsink it can radiate heat from the earth into the atmosphere.
however its 25mw per m2, to put that in comparison, this photdiode here: https://www.sparkfun.com/products/9541 looks like it can produce 11mw in full sun.
The newish bit is that it's bypassing the atmosphere and radiating directly into space. They actually try to keep the atmosphere off of it with insulation, since the atmosphere warms it up to ambient temperature. The night sky is cold enough to freeze water (and has been used for thousands of years to make ice even in countries where the nighttime temperature is above freezing).
There is a Peltier device at the core of it. That's old news. The new news is increasing the temperature differential that drives the device. None of that is new physics. It's just a first stab at trying to make one practical.
Take a heavily insulated box with a window on one side and put a parabolic reflector in it. Point the reflector at the clear night sky, and you can create ice at the focus.
All objects are in a balance between incoming and outgoing heat, so when all the heat going out it sent into a sink - such as the clear night sky - the object will drop in temperature.
The effect isn't huge, but it's real. The system described in the submitted article here has nothing like the energy generating potential required to become a sensible source of renewable energy, but it's a nice effect, and a nice system.
Use this as the low-T side of a heat engine, and you've got a nice little generating technology.
Maybe filtering the outgoing radiation to enhance an available low-absorbance band (or bands) and you might be able to help yourself to the T of the background sky.
I've actually thought about this a fair bit for things like low-T geothermal resources (increasing the Carnot Delta T by working on the low-T side).
Any optical physicists want to prove to me that I'm full of crap?
I’m trying to wrap my head around this. While shielding the focus from receiving any thermal radiation via surrounding objects ( ground, etc) you also provide a 360 degree thermal radiation heat sink, making it as easy as possible for whatever is at the focus to shed heat?
Your description does not appear to be correct. Ice is not being formed at the "focal point," ice is being formed under the reflector.
You can't focus coldness. "Cold" radiation is simply less radiation. When you focus it, you get more radiation, not less of it. More radiation means more energy.
Instead you're simply seeing the effect of radiative cooling. See "Nocturnal ice making in Early India and Iran" [1]. By insulating the sides of a pan of water, and leaving the only window for radiation pointed to the night sky, you're ensuring that more radiation is leaving the water than entering it. Thus the water cools down.
So it would work equally with an insulated box with high walls. It's not about focusing the coldness.
I have a feeling we're talking past each other, and I'm not going to bother addressing everything you've said point-by-point. Experiments show that have a parabolic reflector configured correctly results in a stronger effect. You're not focusing coldness, but the radiation from the body being cooled emerges in every direction, so putting it at a parabolic focus ensures that all the radiated heat gets directed outwards to the night sky.
Yes, simply having an insulated container and having an opening pointing at the sky does work, but it's not the best you can do.
The focusing effect will help, because you're focusing the infrared radiation from the object away.
If you have an insulated box with tall sides, radiation from the object will reflect from the sides of the box back to it, keeping it warm (or less cold).
Pull the source and look at the last few lines. Alternately, hover your mouse over the area at the bottom of the page and look at the URLs it would go to if you click.
I've tried contacting the author but the email no longer works. I've not yet contacted the site owner.
Consider a small body at the focal point of a parabolic reflector, and that reflector pointed at empty sky at night.
You effectively have the body radiatively thermally connected to space, and space is cold. So heat radiated from the object goes out to space, and nothing comes back, and thus the object gets colder.
If doesn't work if there are clouds, because clouds are quite warm.
Not much is going on in terms of the 2nd law, because this device is not a closed system (heat is coming in and then later going out) so the 2nd law doesn't apply.
Does anyone know of a way to prevent scroll-jacking everywhere? I can't stand it when I run across it and it seems to be infesting more and more sites.
Very interested in this, as someone thinking about how to set up an off grid cabin here in Alaska. Last winter we hit a 112°F difference between inside our cabin at 72° and outside at -40°. This seems like a way to harness some of the heat drain while compensating for the accompanying darkness of the winter months that would drive less photovoltaic generation. I don't know much about solar alternatives so would love to know more from anyone who could suggest things...
It's seems to be a variation on a simple thermocouple [0]. Underlying the technology is geocooling and geoheating which have been around for quite some time and are fairly mature technologies [1].
It is not like a heat pump at all, it is a thermoelectric material. This has been around for ages. Basically, the thermal gradient across a thermoelectric material causes the flow of charged particles. See some of the links below.
They basically built an efficient heat sink (a material good at cooling itself and anything that it is attached to) that radiates heat from one side of their thermoelectric crystal while keeping the other side at room (or other) temperature. This temperature gradient creates the current that they say can run an LED.
Yeah, but it's a gimmick. There's no way to make it useful because even at perfect efficiency, it's not possible to generate a useful amount of power given the required surface area.
Useful for what? The paper mentions off-grid sensors, which seems like a useful application. Looks like the numbers are 0.5W/m² which means a square meter of these could power e.g. an ESP8266.
It's obviously not going to power your home with those numbers, but it's a little unimaginative to say that it has no useful application.
I’m not sure what scenario you’re thinking of where a reservoir, thermoelectric chip, and radiative cooler is easier than a (much smaller) battery and photovoltaic cell. Care to explain?
Could you bury something like this in the ground all the time since the earth is ~50° at 11’[1]. You could have those plates touching the earth which would conduct heat away from the plate. 50° is pretty cool and would be running 24/7.
Would surrounding earth be able to sustain cooling a disk like this or would the area just warm up right away and you’d lose any gains?
That's really neat! If you only need a little power, you might even be able to harness the 100 W of heat our bodies continually give off. You could press the warm end against you to bring it up to human body temperature rather than just ambient, which should increase the thermodynamic efficiency by quite a bit. I'm imagining, perhaps, a handheld flashlight that powers itself using the heat from your hand alone.
At it's core, yes, but I think it's less "cold air" and more thermally equilibrating with the radiative temperature of the night sky which is more like 3 degrees Kelvin. So in essence you are lowering the temperature of the cold sink until the ambient temperature is comparatively hot to extract work out of your heat pump.
Almost, but same concept. It looks to be taking advantage of the temperature of space (or at least as much as you can on the ground) as it does have a cover that blocks heat transfer to/from the wind.
Its basically using a standard thermopile/seebeck module or basically a peltier module that is optimized for generating power, rather than moving heat, to generate power from the temperature difference.
The NYT actually had a detailed write-up that I thought was quite accurate: https://www.nytimes.com/2019/09/12/science/solar-energy-powe...