New Device Harvests Energy in Darkness
nytimes.com
nytimes.com
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...
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.
A dual-mode textile for human body radiative heating and cooling[3]
[1] https://web.stanford.edu/group/fan/ [2] https://web.stanford.edu/group/fan/publication/Goldstein_Nat... [3] https://web.stanford.edu/group/fan/publication/Hsu_ScienceAd...
(I don't have background in physics so apologize if this is a silly question)
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.
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.)
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.
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?
Thanks!
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.
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.
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?
The distances in question are greater than 10m, but yes.
I think you mean more like 200 meters?
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.
paper link: https://www.cell.com/joule/fulltext/S2542-4351(19)30412-X https://doi.org/10.1016/j.joule.2019.08.009
"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.
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.
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.
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.
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.
[1] https://en.wikipedia.org/wiki/Radiative_cooling#Nocturnal_ic...
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.
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).
Screenshot: https://imgur.com/a/Oz4XEhy
This is the URL: http://solarcooking.org/radiant-fridge.htm
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.
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.
And are, therefore, radiating heat back into the small body via parabolic reflector.
As mentioned above, there is a better article that covers this: https://www.nytimes.com/2019/09/12/science/solar-energy-powe...
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 also a proof of concept. Nobody is claiming the device as is should be commercially available.
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.
The former can just be a chunk of repurposed garbage.
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?
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.