The article is confusing, at least to me. At first glance it appears to be a highly efficient mirror that is very good at reflecting those wavelengths that can most easily penetrate the atmosphere.
Fair enough, that could protect a structure from sunlight (by reflecting it) and therefore reduce the heating that would otherwise be caused by that sunlight. But that’s not really cooling, it’s preventing some amount of heating.
Yet the article uses phrases like “net cooling power” and says
It seems to be saying things that violate the laws of thermodynamics. You can’t move heat in this manner without expending energy, yet it says that it requires no energy.
I suspect that it’s just poor reporting and that it does require energy, the energy in the sunlight, to somehow move heat without using any kind of compressor.
Am I understanding this right? Am I missing something here? Is the whole thing bullshit?
Yeah, I see what you’re saying although they do seem to try to emphasize the “radiative” aspect of it. IDK. Regardless, as best as I can tell, I would peg this as the key paragraph.
I think what they’re getting at is that, in addition to essentially providing shade for the structure, the panels are good at absorbing heat from the structure they’re attached to and radiating it away.
For example: say you install some of these panels on a typical asphalt-shingle roof, covering half the surface. (I don’t know if they would replace the shingles or just be mounted flush to the surface, but they would need to have a large contact area, I think.) The first effect is that that part of the roof is now highly reflective, and neither the attic directly under it nor the panels themselves heat up as much as the rest of the roof. That differential means that heat will transfer from the hotter parts of the roof and attic to the parts sheltered by the panels. The panels will absorb some of the excess heat from inside the attic and radiate it away in a specific infrared band that can escape efficiently (which mostly means that it won’t heat the air directly above the panels as much).
That’s my best guess, anyway. Does it make more sense that way?
That doesn’t sound quite right to me. Unless the panels are cooler than the inside of the house, heat isn’t going to move passively from the house to the panels and then unless the air is cooler than the panels, heat isn’t going to move from the panels to the air. So you wouldn’t have a net cooling effect. The house could never become cooler than the outside air.
The panels would presumably be cooler than the air inside the hypothetical attic (or similar space), because they are absorbing less heat from the sun than the unprotected parts of the roof, which are heating the inside air. If that’s the case, they can absorb heat from the inside air. Believe me, attics are ovens in the summer, and cooling the attic does a lot to keep the temperature down in the rest of the house.
The outside air temperature doesn’t matter when we’re talking about radiating heat away. If the air is hotter than the panels, they couldn’t conduct heat away, but objects emit infrared regardless of the temperature of their surroundings. However, if the outside air is hotter than the inside air and warms the panels to that temperature via conduction, the panels would start heating the interior instead.
It seems obvious to me that they’ve envisioned more ways of deploying it than that, but I don’t know what they are. I’m just offering an example in which it could plausibly do at least some of what they claim. It’s not agin the Laws of Thermodynamics, anyway. I don’t think it could actually replace active cooling in a truly hot climate, but I could see it reducing the load on the active cooling system.
I didn’t read the article but there are plenty of strategies that take advantage of the various energy flows that are always available around buildings. You could design a building to stay cooler (or warmer) than the average air temperature without using any purchased energy.
For example, you could give the building very little mass, and insulate it very well, and during the coolest 5% of each 24 hour cycle, you suddenly pop out huge high performance conductive fins that conduct heat from the building out into the environment. You could make the building’s average temperature be in something like the 5th or 10th percentile of the environmental temperature distribution that way.
As far as radiation goes, most of the sky is often a very cold radiative sink.
Not sure what “this manner” is, but there’s no general thermodynamic reason why moving heat from hotter to colder requires external energy. Indeed, most machines do the opposite, use heat flows to put energy into some external mechanism.
Is it moving from hotter to colder? That wasn’t my impression. Obviously heat can move from hotter to colder, but the article seemed to be saying the opposite.
I think that this is just a way of cooling an object down to the ambient temperature, in a way that’s more effective than simply putting the object in the shade. Let’s say you’ve got a hot object sitting in broad daylight. We can divide the heat flowing in and out into a few categories: radiative heat absorption mostly from the sun, radiative heat emission to the environment, and then heat conduction to and from the environment that would bring the object’s temperature to ambient. If you put the object in the shade, it eliminates (most) all of the radiative heat transfers – it won’t get hotter sitting in the sun, but it won’t be able to radiate much heat away either. This new material seems to remove radiative heat absorption while still allowing emission. It won’t cool an object below ambient, but it will cool that object down to ambient faster than conduction alone would allow.
Practically speaking, I doubt that this would be something that we would actually put on a roof. Once it gets slightly dirty, its reflective and emissive properties will be reduced. And if we’re talking about buildings, there are much simpler passive cooling technologies that rely on clever uses of shade, convection, and thermal mass.
My WAG is that this material might have practical applications on something like a spacecraft, where you could combine solar heat shields and radiators into a (perhaps) lighter combined system.
This. Radiative cooling is why you can develop frost on outdoor surfaces even when the ambient temperature is well above freezing (up to about 37-38F). This only happens on cloudless nights; if it’s cloudy, the clouds block the earth’s “view” of icy-cold outer space, and so outdoor surfaces end up at a temperature much closer to that of the air.
And so that’s what these miraculous panels are doing: keeping sunlight from being absorbed by the building (by being a very efficient reflector), and also radiating away thermal energy from the building to outer space, providing a very real cooling effect.
Guys, you are all missing the key piece of information from the article:
*The trick, from an engineering standpoint, is two-fold. First, the reflector has to reflect as much of the sunlight as possible. Poor reflectors absorb too much sunlight, heating up in the process and defeating the purpose of cooling.
The second challenge is that the structure must efficiently radiate heat back into space. Thus, the structure must emit thermal radiation very efficiently within a specific wavelength range in which the atmosphere is nearly transparent. Outside this range, Earth’s atmosphere simply reflects the light back down. Most people are familiar with this phenomenon. It’s better known as the greenhouse effect—the cause of global climate change.*So, it accepts incident energy, shifts it to a frequency band that is more-or-less transparent to the atmosphere, and radiates it more efficiently than from a normal blackbody spectrum. There is nothing inherently wrong or in violation of thermodynamics, though I would like to see more detail on how they force energy to flow through that specific band. To get energy concentrated into a narrow band usually requires some kind of resonator (like a magnetron or optical cavity) which is not thermodynamically efficient, i.e. you generate more waste heat than what is pumped into the specific band. Since the process is presented as being “passive” it must be spontaneous and not require an outside source of energy for pumping energy into those frequency bands.
Not quite. Incident sunlight is simply reflected (i.e. same wavelength). And in the infrared, the material radiates energy fairly efficiently - not as good as a true blackbody, but still pretty good (between 50% and 100% depending on wavelength). In technical terms, it’s a surface with very low absorptivity and very high emissivity. The linked article has a link to the abstract which includes an emissivity curve. It’s pretty impressive and exciting.
The idea is not new though. Silver-coated Teflon also has a fairly low absorptivity and high emissivity; this material is used on radiators on some spacecraft (e.g. DS1). Since it absorbs very little sunlight, it can function as a radiator even when sunlight is shining on it. (It’s pretty *$&@ expensive though, I used a couple of square feet of it in one project and it cost us thousands of dollars.)
Actually, I mentioned that in my first post in the thread, “…radiate it away in a specific infrared band that can escape efficiently”. Without that, this would be little better than painting your roof white (which, granted, can be effective in hot climes).
I don’t have access to the full paper, but article says they’re using a nanostructured material to make it work. I presume the trick is in the photonic properties of the nanostructures. I’ve no idea how that works, though. Maybe they’ve managed to come up with some effect similar to pumped lasing with heat as a driver?
I don’t think it would be anything special in space. As Stranger reminds us, the advantage of this system is that it radiates IR in a band that the atmosphere is pretty much transparent to, so it’s got a clean shot at cold space. A radiator in space can get the same effect by radiating at any frequency.