7 ms·
Passive radiative cooling below ambient airtemperature under direct sun (2014) [pdf]
- MauranKilom 5y agoVery cool concept! Has any product come out of this in the 7 years since it was published?
- _Microft 5y agoI know of no products but the area of research seems to be active and the article I linked to is getting cited frequently: https://scholar.google.de/scholar?as_ylo=2021&hl=de&as_sdt=2005&sciodt=0,5&cites=8960583480633878607&scipsc= https://scholar.google.de/scholar?as_ylo=2021&hl=de&as_sdt=2...
- Mengkudulangsat 5y agoAaswath Raman's company [1] https://www.skycoolsystems.com https://www.skycoolsystems.com
- leoedin 5y agoTo compare to an air conditioner. This device has about 40W of cooling per m2. Apparently in Australia you should size between 80 and 120 W/m2 of air conditioning (I think this is cooling watts rather than power usage watts) - https://www.google.com/amp/s/www.crownpower.com.au/blog/choosing-the-right-sized-air-conditioner/ https://www.google.com/amp/s/www.crownpower.com.au/blog/choo... So that means every square meter of living space needs 2 square meters of radiative cooling (assuming no other passive cooling infrastructure). I suspect you'd see further inefficiencies getting the heat to the passive cooler. So it's within the same order of magnitude of an ac, but not powerful enough that it would be straightforward to retrofit.
- varjag 5y agoThere's a lot of potential applications outside of habitat AC in Australia. For example cooling down stand-alone hardware in the field.
- kumarvvr 5y agoA typical AC cycles its power input. Its not constantly On. One benifit of using this system could be to continuously remove heat from the house and then use an AC on top of it. There is always a consumption value to free beer.
- sandworm101 5y ago>> A typical AC cycles its power input. If it has reached the requested temperature. Like basically all consumer thermostats, it is a bang-bang controller. There is no set on-off cycle. If the AC unit is running at capacity, ie it is properly sized for requirements, it will just be on all the time.
- PaulHoule 5y agoBut the temperature goes up and down all the time so there is no "fixed requirement". If the AC is on all the time it is most likely undersized for the requirement at that time and can't maintain the desired temperature.
- silon42 5y agoCheck out interter ACs. Good ones should scale from 20-100% in power or better, so should be able to stay on for most of the time.
- tempestn 5y agoYeah, most central heat pumps installed now are inverter models (also referred to as variable speed). More efficient and more comfortable since you've got a continuous flow of cool air, rather than blasts of cold interspersed with nothing.
- sandworm101 5y agoIn a better-than-consumer setup you will have multiple chillers. Most will just stay on, with one going on-off to handle the variable bit of the load. Starting and stopping electric motors is less efficient tha just keeping them running as much as possible.
- the8472 5y agoThe technology has advanced since then, now white paints with high emissivity in the infrared window are being researched. So if you cover the entire building with that you would get some free, always-on cooling that way.
- PaulHoule 5y agoOpening the windows at night and running fans to equalize the temperature, then putting space blankets over the windows for the day works wonders on hot days in upstate NY.
- dsr_ 5y agoIt's very good as long as two conditions apply: - the humidity has to be comfortably low - the outside temperature has to be low enough Historically, this is usually the case. In the last few weeks, I've had one or both of those fail to apply on the majority of nights. Dropping to 65F doesn't help when the outside air is also at 99% humidity. If the overnight low is 75F, we're not getting much cooling out of it.
- PaulHoule 5y agoAny system has to deal with time-variable conditions. I dream of getting a geothermal heat pump for my 1850s farm house which is normally heated with two wood stoves but has a propane backup. (e.g. the kind of compact heater that you see all the time in people's apartments in anime) At points south the capacity of that kind of system is set by cooling demand but where I live it is set by heating demand. The woodstove could pick up the slack on the coldest days, but that defeats the main selling point of the heat pump which is extreme comfort (e.g. it switches seamlessly from heating to cooling)
- hinkley 5y ago- the outside air quality has to be okay Between pollen count and pollution this is not always the case, and keeping the windows closed and running both the AC and an air filter unfortunately has health benefits for some individuals.
- PaulHoule 5y agoThe weakness it has it that it needs a clear sky to work. For overcast muggy days or even a high coverage of cumulus clouds, performance will be absent or degraded. It works at night, however, and for best results you could maximize insulation and "thermal mass" inside the building and minimize radiative transport through the windows. The best thing about air conditioning, however, is de-humidification and that is a matter of cooling the air more than you have to and then re-heating it. I live in an 1850 farmhouse and the reason I want a ground source heat pump is that the humidity destroys books and other printed matter. I have inkjet prints curling off the walls and detailed logs of how 3M's best products only work 90% of the time in my applications.
- eloff 5y agoAdhesives are not built for high-humidity environments. My parents live in the tropical rainforest of Panama. One of the challenges is that anything stuck together with adhesive usually comes apart over time. The glass panel on the door of my mom's oven fell off.
- PaulHoule 5y agoAs an engineer I don't accept that things have to suck. If other people think failure is OK I can't do anything about it, but if I have the problem that "Adhesive X does not work in Environment Y" I am going to change the adhesive, change the environment, or not use an adhesive.
- etskinner 5y agoI'm having trouble understanding why a clear sky is important. Surfaces radiate based on their temperature and emissivity only, right? So why would it matter what the surface is emitting toward? Perhaps what I'm missing is that clouds emit some radiative heat back to the surface, whereas a clear sky emits very little, so the net heat loss from the surface under a cloudy sky would be lower.
- Sanzig 5y ago> Perhaps what I'm missing is that clouds emit some radiative heat back to the surface, whereas a clear sky emits very little, so the net heat loss from the surface under a cloudy sky would be lower. This is exactly it, yes.
- fy20 5y agoMight as well just get solar PV then, that's around 150W per m2.
- hatsunearu 5y ago150W electricity, so you need to run a refrigerator cycle to pump that heat out of your house, which comes at a massive efficiency penalty.
- zplizzi 5y agoRefrigerators actually have greater than 100% efficiency, often like 300% or so. Because you're just moving the heat, not creating it. Sounds like magic but it's not.
- hatsunearu 5y agoah true. though that number is coefficient of performance, not efficiency
- thescriptkiddie 5y agoAnd an efficient heat pump can move four times as much energy as it consumes. But if some special paint/tile/insulation can reduce your needed cooling capacity by 30% you'll save a lot of money on equipment.
- mannykannot 5y agoI am trying to figure out whether one should take into account the cooling power of the surfaces being replaced. The figure of 80-120 W/m2 for air conditioning is presumably based on conventional building materials, which have negative cooling power. In the paper, the figure of 40 W/m2 seems to be the net cooling power, which is defined in equation 1 as being the power radiated away minus various inflows of heat: radiatively, from the atmosphere; radiatively, from the sun; and by conduction and convection. As far as I can see, these corrections are all for this particular surface, not the surface it might be replacing. These will not, in general, be the same, and, given that this new surface is both highly reflective and vacuum-insulated, I would guess that its values for these properties are lower than the conventional building materials on which the a/c rule-of-thumb is based. Nevertheless, I doubt that replacing the entire roof with this material would be sufficient cooling, on its own, in the Australian case, and I agree that this would not likely be a straightforward retrofit, to say the least!
- parineum 5y agoThis was my thought as well. I have an attic fan and monitor the temperature inside to control it. It regularly gets 120+ F in there on an 80 F day. I have to think the majority of the heating of my house is coming from the attic.
- hatsunearu 5y agoKeep in mind there is nothing preventing you from just angling the device (in its extreme, vertically) and just get an arbitrary amount of radiative surface with a given flat footprint. (other than of course, it looking unsightly and construction costs) edit: it would probably help a lot of you angle it such that it is normal to the sun rays, like where i live the sun sweeps from the east to the west, so if you angle the device north or south it would probably work even better.
- jryb 5y agoThat's true, but I don't see why the two couldn't be used in tandem, thus reducing AC power consumption substantially.
- villgax 5y agoThis is as insane as the blackbird land yacht which is a vehicle to go directly downwind faster than the wind!
- nayuki 5y agoSee https://www.youtube.com/watch?v=jyQwgBAaBag https://www.youtube.com/watch?v=jyQwgBAaBag ; https://www.youtube.com/watch?v=yCsgoLc_fzI https://www.youtube.com/watch?v=yCsgoLc_fzI ; https://www.youtube.com/watch?v=VUgajGv4Aok https://www.youtube.com/watch?v=VUgajGv4Aok
- pengaru 5y agoThere's nothing insane about the blackbird land yacht, except that so many people seem to think it's insane.
- pontifier 5y agoIf you just look at the blades as some structure that is moving slower than ground speed through the transmission it becomes much easier to think about. The surface being pushed by the wind is moving slower than the wind.
- huachimingo 5y agoVideo from QuantumFracture explaining it: https://youtu.be/wzPdcqrDKzw https://youtu.be/wzPdcqrDKzw (You can use subs)
- speed_spread 5y agoSeems like similar tech is now available in paint form? https://www.purdue.edu/newsroom/releases/2021/Q2/the-whitest-paint-is-here-and-its-the-coolest.-literally..html https://www.purdue.edu/newsroom/releases/2021/Q2/the-whitest...
- coolradmab 5y agoI've been actively working on this technology, goal is making it cheaper and simplify installation. Stanford's a highly reflective surface ~95% combined with stacks layers of silica oxide on a wafer under vacume. The trick too achieving bellow ambient temperature is too reflect nearly all solar energy while emitting strongly in the "atmospheric window". Most silica compounds are well suited as emitters, however the hard part is adding a reflector too the silica and minimising heat transfer from the environment. I've managed to make a meta material paint, reflector and emmiter that achieved bellow ambient temperature, with bulky conventional insulation. as for any effective cooling bellow ambient. Radiative cooling is just not that strong of heat transfer, what you want too look our for is the research into reflective coatings needrthese systems too function. Review paper: https://www.sciencedirect.com/science/article/pii/S0306261918318373 https://www.sciencedirect.com/science/article/pii/S030626191... Shameless plug: https://www.scihouse.space https://www.scihouse.space
- cdata 5y agoI'm eager to experiment with a material like this for the application of passive water harvesting in a high humidity environment. Would you be able to recommend some materials that are perhaps sub-optimal for the task but trivial to assemble from commodity sources to produce this effect?
- coolradmab 5y agoSilica better yet a net http://nnf.mit.edu/sites/default/files/publications/files/GHM275.pdf http://nnf.mit.edu/sites/default/files/publications/files/GH...
- hatsunearu 5y agowouldn't large scale usage of a device like this essentially increase the planetary albedo and help fight climate change? especially if you just skip the "environmental heat transfer" part
- coolradmab 5y ago
- infogulch 5y agoI think I remember seeing a paper where they designed a metamaterial with vertical microstructures that passively lased input heat as narrow band IR within the frequency range that is transparent to the atmosphere. But I can't find it now, perhaps I'm mistaken.
- debacle 5y agoAre there any resources that catalog historical methods of passive cooling? Many of these methods are space efficient but not cost efficient, and many areas of the developing world (where these issues have the greatest impact) have all the space in the world and very limited access to funds.
- danans 5y agoHistorical passive cooling methods usually involve using shade and high thermal mass like a stone floor - basically simulating a cave. More sophisticated historical methods include wind catchers used in Persia. But historically stone houses weren't cheap - poor people lived in straw huts, and most people probably just put up with the heat the best they could cope.
- _Microft 5y agoI found the question interesting and found a few Wikipedia articles that might help: https://en.wikipedia.org/wiki/Passive_cooling https://en.wikipedia.org/wiki/Passive_cooling https://en.wikipedia.org/wiki/Bâdgir https://en.wikipedia.org/wiki/Bâdgir https://en.wikipedia.org/wiki/Yakhch%C4%81l https://en.wikipedia.org/wiki/Yakhch%C4%81l
- thedudeabides5 5y agoSo conceptually, this tech could be used for air-conditioning, during the bright summer day, without consuming electricity? If so, that's pretty sweet. Like at what scale does this work? How much is needed to cool, say, a 1000 sq ft apartment in nyc?