[HN Gopher] Passive radiative cooling below ambient airtemperatu...
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Passive radiative cooling below ambient airtemperature under direct
sun (2014) [pdf]
Author : _Microft
Score : 127 points
Date : 2021-07-09 10:52 UTC (12 hours ago)
(HTM) web link (web.stanford.edu)
(TXT) w3m dump (web.stanford.edu)
| huachimingo wrote:
| Video from QuantumFracture explaining it:
| https://youtu.be/wzPdcqrDKzw (You can use subs)
| infogulch wrote:
| I 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.
| villgax wrote:
| This is as insane as the blackbird land yacht which is a vehicle
| to go directly downwind faster than the wind!
| nayuki wrote:
| See https://www.youtube.com/watch?v=jyQwgBAaBag ;
| https://www.youtube.com/watch?v=yCsgoLc_fzI ;
| https://www.youtube.com/watch?v=VUgajGv4Aok
| pengaru wrote:
| There's nothing insane about the blackbird land yacht, except
| that so many people seem to think it's insane.
| speed_spread wrote:
| Seems like similar tech is now available in paint form?
| https://www.purdue.edu/newsroom/releases/2021/Q2/the-whitest...
| coolradmab wrote:
| I'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/S030626191...
| Shameless plug: https://www.scihouse.space
| hatsunearu wrote:
| wouldn'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 wrote:
| Short answer not really https://what-if.xkcd.com/84/
| Tade0 wrote:
| This makes me think: Earth's energy imbalance is around
| 0.5W/m2, while such a paint sends how much, 40W/m2 through
| the transparency window?
|
| So we'd only have to paint 1/80 of the Earth. That's
| ~6.4mln km2, or 2/3 the area of the USA. Still a lot, but
| not impossible.
|
| I'm sure paint manufacturing scales better than li-ion
| batteries, and those more than doubled in production volume
| over the last decade.
| david-gpu wrote:
| Interesting, but even if we solved the temperature
| problem, we would still have the issue of the
| acidification of the oceans due to excess CO2. In the end
| we must remove CO2 from the atmosphere one way or
| another.
| wolfram74 wrote:
| That's more a "we haven't made enough paint to cover a
| large fraction of the planet" argument than a "what would
| the thermal ramifications of such an act be" argument.
| Which I was excited to read about, but alas.
| jschwartzi wrote:
| You would need a really big roller with a lot of knap
| too. Although I suppose you could paint most of the
| midwest and avoid the mountains with a flatter roller.
| samatman wrote:
| For an interesting toy model related to this, check out
| Lovelock's Daisyworld simulation:
|
| https://en.wikipedia.org/wiki/Daisyworld
|
| The argument (and the related Gaia hypothesis) has some
| important and subtle connections to the facts of climate
| change. Though even if it's correct, and the biosphere
| will tend to naturally reassert homeostasis, there's no
| guarantee we'll enjoy living through it.
| alex_h wrote:
| There's some numbers here for what it would take.
|
| https://www.cell.com/joule/pdf/S2542-4351(19)30354-X.pdf
|
| Basically we need 1W/m^2 of cooling for the earth, so if you
| could get a radiative cooling device with 100W/m^2 you'd need
| to cover about 1% of Earth's area
| 99_00 wrote:
| I'm really ignorant about this whole field.
|
| Your work on passive radiative cooling doesn't sound like
| biotech or related to biotech but your link
| https://www.scihouse.space is a biotech lab.
|
| I was just wondering the kind of education/knowledge someone
| who is working on the cooling technology would have, and was
| surprised to see a biotech background. So am wondering on the
| journey to get from biotech to whatever is needed for the
| cooling tech.
| cdata wrote:
| I'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 wrote:
| Silica better yet a net http://nnf.mit.edu/sites/default/file
| s/publications/files/GH...
| leoedin wrote:
| To 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/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.
| hatsunearu wrote:
| Keep 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 wrote:
| That's true, but I don't see why the two couldn't be used in
| tandem, thus reducing AC power consumption substantially.
| kumarvvr wrote:
| A 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 wrote:
| >> 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 wrote:
| But 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 wrote:
| Check 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 wrote:
| Yeah, 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 wrote:
| In 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.
| PaulHoule wrote:
| The 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 wrote:
| Adhesives 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 wrote:
| As 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 wrote:
| I'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.
| coolradmab wrote:
| Water vapour re absolves the IR akin too blowing on your
| sails however it's affect is reduced when it's cloudy can
| still function just not as well
| Sanzig wrote:
| > 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.
| the8472 wrote:
| The 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 wrote:
| Opening 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.
| oezi wrote:
| One question that I always wonder when hot days strike:
|
| Given that I only have one portable fan, what is the best
| setup at night if it is colder outside than inside:
|
| 1. Open the windows and put fan so that it blows air out of
| one window
|
| 2. Open the windows and have fan mix the air inside the
| room
|
| 3. Open the windows and put fan on balcony to blow air from
| outside in.
| tempestn wrote:
| 4. Buy additional fan for ~$15.
|
| Seriously though, it depends. If you have no other way to
| intake or exhaust air, probably 3, since fans are more
| effective at blowing than sucking. (IE: 1. would spend
| some of its power recirculating inside and outside air
| rather than just pulling inside air out.)
|
| Most likely your bathroom and hopefully stove have
| exhaust fans, so even better would be to turn one or both
| of those on, and have the fan blow in a window on the
| opposite side of the house. It may not even be ideal to
| open _all_ the windows. You want cool air flowing through
| the whole house. In an extreme example, if you have the
| fan blowing in the balcony and an open window right
| beside the balcony, it could just circulate air there,
| rather than reaching the rest. Likewise with exhaust, if
| you have a fan in the bathroom and the bathroom window
| open. So you 'd need to experiment a bit to see what
| flows air best through the house.
|
| Things also change if you have a central blower.
| spearo77 wrote:
| Matthias tries a few options and evaluates them-
| https://www.youtube.com/watch?v=1L2ef1CP-yw
|
| > Experiments and anemometer measurements to figure out
| where to best place a fan to optimally air out the house
| to cool it down at night.
| occamrazor wrote:
| Open two windows. Only if needed, put the fan somewhere
| in between the two windows to facilitate airflow.
| clipradiowallet wrote:
| Another suggestion from your 3 - use an exhaust fan on
| one end of your home, blowing air from your ceiling out a
| window(you want to blow the air near the ceiling, it is
| warmest). Use an intake fan at the lowest elevation
| possible.
|
| The premise is...cooler air falls, warmer air rises. You
| want to blow in the low(cool) elevation air, and exhaust
| the high(warm) air.
| cowvin wrote:
| I wonder the same thing. If you get a chance, try to test
| the various configurations!
|
| I would suspect you will also see different results
| depending if you have multiple windows or just 1. For
| example, if you have more than 1 window and can seal the
| opening except the fan, then the fan will move x amount
| of air in or out and will have similar results.
|
| Simply mixing the air inside the room seems like it's
| probably the least effective because it will result in
| very little heat exchange at the windows themselves.
| However, making the temperature within your house more
| even may make it more comfortable on the average inside
| your house.
| dsr_ wrote:
| It'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 wrote:
| Any 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 wrote:
| - 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.
| mannykannot wrote:
| I 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 wrote:
| This 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.
| fy20 wrote:
| Might as well just get solar PV then, that's around 150W per
| m2.
| hatsunearu wrote:
| 150W electricity, so you need to run a refrigerator cycle to
| pump that heat out of your house, which comes at a massive
| efficiency penalty.
| varjag wrote:
| There's a lot of potential applications outside of habitat AC
| in Australia.
|
| For example cooling down stand-alone hardware in the field.
| debacle wrote:
| Are 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 wrote:
| Historical 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 wrote:
| I 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/Badgir
|
| https://en.wikipedia.org/wiki/Yakhch%C4%81l
| MauranKilom wrote:
| Very cool concept! Has any product come out of this in the 7
| years since it was published?
| _Microft wrote:
| I 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=2...
| Mengkudulangsat wrote:
| Aaswath Raman's company [1] https://www.skycoolsystems.com
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