Seems like similar tech is now available in paint form? https://www.purdue.edu/newsroom/releases/2021/Q2/the-whitest...
Passive radiative cooling below ambient airtemperature under direct sun (2014) [pdf]
11–20 of 68 posts
Re: Passive radiative cooling below ambient airtemperature under direct sun (2014) [pdf]
#12I'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
Re: Passive radiative cooling below ambient airtemperature under direct sun (2014) [pdf]
#13This is as insane as the blackbird land yacht which is a vehicle to go directly downwind faster than the wind!
Re: Passive radiative cooling below ambient airtemperature under direct sun (2014) [pdf]
#14Earlier quoted context omitted.
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.
>> 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.
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.
Re: Passive radiative cooling below ambient airtemperature under direct sun (2014) [pdf]
#15To 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 in…
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.
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.
Re: Passive radiative cooling below ambient airtemperature under direct sun (2014) [pdf]
#16To 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 in…
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.
Re: Passive radiative cooling below ambient airtemperature under direct sun (2014) [pdf]
#17Earlier quoted context omitted.
>> 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.
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.
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.
Re: Passive radiative cooling below ambient airtemperature under direct sun (2014) [pdf]
#18Very cool concept! Has any product come out of this in the 7 years since it was published?
Aaswath Raman's company [1] https://www.skycoolsystems.com
Re: Passive radiative cooling below ambient airtemperature under direct sun (2014) [pdf]
#19Earlier quoted context omitted.
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.
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.
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.
Re: Passive radiative cooling below ambient airtemperature under direct sun (2014) [pdf]
#20To 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 in…
Might as well just get solar PV then, that's around 150W per m2.