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?