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Passive radiative cooling below ambient airtemperature under direct sun (2014) [pdf]

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Re: Passive radiative cooling below ambient airtemperature under direct sun (2014) [pdf]

#31

Post by oezi »

PaulHoule wrote: post #15
the8472 wrote: post #8

Earlier 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.

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.

Re: Passive radiative cooling below ambient airtemperature under direct sun (2014) [pdf]

#32

Post by pengaru »

villgax wrote: post #6

This is as insane as the blackbird land yacht which is a vehicle to go directly downwind faster than the wind!

There's nothing insane about the blackbird land yacht, except that so many people seem to think it's insane.

Re: Passive radiative cooling below ambient airtemperature under direct sun (2014) [pdf]

#33

Post by hinkley »

dsr_ wrote: post #19
PaulHoule wrote: post #15

Earlier quoted context omitted.

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.

- 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.

Re: Passive radiative cooling below ambient airtemperature under direct sun (2014) [pdf]

#34

Post by tempestn »

silon42 wrote: post #17
PaulHoule wrote: post #14

Earlier quoted context omitted.

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.

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.

Re: Passive radiative cooling below ambient airtemperature under direct sun (2014) [pdf]

#35

Post by occamrazor »

oezi wrote: post #31
PaulHoule wrote: post #15

Earlier quoted context omitted.

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.

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.

Open two windows. Only if needed, put the fan somewhere in between the two windows to facilitate airflow.

Re: Passive radiative cooling below ambient airtemperature under direct sun (2014) [pdf]

#36

Post by cowvin »

oezi wrote: post #31
PaulHoule wrote: post #15

Earlier quoted context omitted.

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.

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.

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.

Re: Passive radiative cooling below ambient airtemperature under direct sun (2014) [pdf]

#37

Post by tempestn »

oezi wrote: post #31
PaulHoule wrote: post #15

Earlier quoted context omitted.

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.

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.

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.

Re: Passive radiative cooling below ambient airtemperature under direct sun (2014) [pdf]

#38

Post by eloff »

PaulHoule wrote: post #16
leoedin wrote: post #4

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 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…

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.

Re: Passive radiative cooling below ambient airtemperature under direct sun (2014) [pdf]

#39

Post by etskinner »

PaulHoule wrote: post #16
leoedin wrote: post #4

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 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…

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.

Re: Passive radiative cooling below ambient airtemperature under direct sun (2014) [pdf]

#40

Post by Sanzig »

etskinner wrote: post #39
PaulHoule wrote: post #16

Earlier quoted context omitted.

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…

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.

> 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.

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