Passive radiative cooling below ambient airtemperature under direct sun (2014) [pdf]
web.stanford.edu
web.stanford.edu
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
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?
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.
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.
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
so the math works out even better than it seems...?
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.
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.
For example cooling down stand-alone hardware in the field.
There is always a consumption value to free beer.
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.
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.
- 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.
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)
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.
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.
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.
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.
> 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.
- Blowing out is better than blowing in.
- Fan should be some distance to window otherwise it is ineffective.
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.
I got decent cooling results with two separate window fans, opposite sides of house, one blowing in one blowing out. Of course, all other windows are closed.
Depending on your window frame style, these types of dual fans fit inside a window frame and don't leak air. They can be set to: both in, 1 in 1 out, both out. So quite flexible. https://www.amazon.com/Bionaire-BW2300-N-Reversible-Airflow-...
A single window fan of the above style can cool a single room quite well, blowing cool air in and blowing hot air out.
As others have mentioned, depending on breeze and temperature, opening all the windows can sometimes work better than multiple fans.
Fans are just no substitute for a real AC unit, which can lower indoor air temp to below outside air temp and can also extract moisture from the inside air.
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.
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.
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.
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!
(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.
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.
https://en.wikipedia.org/wiki/Passive_cooling
https://scholar.google.de/scholar?as_ylo=2021&hl=de&as_sdt=2...
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?
The surface being pushed by the wind is moving slower than the wind.