A/C in a paint can?

nicholas bonham-carter

Something does not compute for me with this:

https://www.usatoday.com/story/news/nation/2021/09/17/whitest-paint-created-global-warming/8378579002/

jumper

How long does white stay white?

Steamhead

When I had my roof replaced, I went to a lighter color and it made a very noticeable reduction in summer heat gain. Not sure how this paint gets to a negative value, but if it rejects the most heat gain of anything out there, it should be worth using.

SlamDunk

That is interesting because I have an eccentric neighbor around the corner who years ago painted his roof shingles with this "NASA created" coating, thick like batter, and swore his house was cooler. He even painted the tops of his cars! Fast forward to today and he has new shingles and different cars but still....

RayWohlfarth

I read that and shook my head. Wondering how well it removes the latent heat

ChrisJ

RayWohlfarth said:

I read that and shook my head. Wondering how well it removes the latent heat

Many areas do not have issues with latent heat, in fact, they even use swamp coolers with good success.

RayWohlfarth

@ChrisJ They would not work up here but I would love to see one in person

jumper

Won't white radiate heat slower at night?

unclejohn

When we bought our first house it happen pretty quick. House went on the market and that day we made and offer and they excepted. It was a divorce and the husband was already gone. At settelment I asked the lady what brand A/C unit she had. She said the house dosen't have A/C, it didn't need it because there is a breeze. Probably why we got the house at 5k under asking price. And she was right about the breeze for the first two weeks in May the house was nice and cool. Then the breeze stopped.

ChrisJ

unclejohn said:

When we bought our first house it happen pretty quick. House went on the market and that day we made and offer and they excepted. It was a divorce and the husband was already gone. At settelment I asked the lady what brand A/C unit she had. She said the house dosen't have A/C, it didn't need it because there is a breeze. Probably why we got the house at 5k under asking price. And she was right about the breeze for the first two weeks in May the house was nice and cool. Then the breeze stopped.

A breeze would be convection.

Ironman

You guys help me because I think that I’m missing something here.
What about the benefit of the heat that’s absorbed from sunlight (passive solar) during heating season? Would that be lost or greatly reduced? In our area, the heating season is 7-8 months of the year and cooling is 3-4. In the overall picture wouldn’t I be better off without this wonderful paint?

ChrisJ

Ironman said:

You guys help me because I think that I’m missing something here.
What about the benefit of the heat that’s absorbed from sunlight (passive solar) during heating season? Would that be lost or greatly reduced? In our area, the heating season is 7-8 months of the year and cooling is 3-4. In the overall picture wouldn’t I be better off without this wonderful paint?

Just like anything in HVAC, it depends on your area and situation.
Some never need AC. Some need AC 365 days a year. Some rarely need sensible cooling but need a lot of latent cooling. Some need zero latent cooling but a ton of sensible.


I'm not sure what you're missing but not everyone lives where you do.

Hot_water_fan

You guys help me because I think that I’m missing something here.
What about the benefit of the heat that’s absorbed from sunlight (passive solar) during heating season? Would that be lost or greatly reduced? In our area, the heating season is 7-8 months of the year and cooling is 3-4. In the overall picture wouldn’t I be better off without this wonderful paint?

There's a lot less sunlight to go around in the heating months and the sun is lower so the net benefit is usually pretty high.

Jamie Hall

There is something to be said for decreasing radiant heat gain during the sunnier and warmer months. Fancy paint isn't the only solution -- in many areas there are such things as trees...

Not, I grant, everywhere!

Swamp coolers are really great in areas where there is low humidity. They can't get the air temperature too low, but they work fine where the ambient relative humidity is in the 20s or 30s. If the dewpoint is below freezing, as it is in some true desert climates, in principle they can even create -- or at least keep -- ice! A principle known several millennia ago in Persia.

All that said, while a reduction in radiant heat gain is always going to help in warmer weather, it's not going to keep you comfortable in, say, Houston or Miami in the summer, with the air temperature hovering around 100 F and the relative humidity around 90...

ChrisJ

Is there a such thing as an absorption type air conditioner that uses heat from the roof?

jumper

ChrisJ said:

Is there a such thing as an absorption type air conditioner that uses heat from the roof?

Some Japanese maybe Yazaki offered an absorption chiller that could be fired by water from solar thermal collectors. That was in the seventies. I think the condenser was expected to be evaporatively cooled. If you're in the middle of nowhere and rich.....

Indirect evaporative cooling can often work somewhat. Especially for agriculture where animals cannot complain.

delcrossv

I have to call BS on the article. it's 98% reflective, so it's 2% absorbent. Absorbing energy and getting cooler violates thermodynamics. And thermodynamics is NEVER violated.
Wonder how it lasts on window sashes.

Zipper13

Most articles about this are very limited in their discussion, but that article specifically offers VERY little information. From what I've read, the groundbreaking part of this paint isn't so much about it having more white pigment in it, but rather the physical structure of the particles that make that pigment. The shapes, sizes, and physical properties of the chemical particles making the pigment allow it to reflect and efficiently scatter more than previous paints.

to the point about it ONLY reflecting 98% so it must be absorbing 2%. Consider that it just says "solar radiation" and doesn't clarify which constituent wavelengths and energies are preferentially reflected/absorbed. Further, the paint is said to also emit IR, so it follows that at least some amount of the absorbed 2% is subsequently re-emitted leading to less heat gain in the surface relative to an untreated surface. The fact that is emits IR must mean that cooling is occurring. Indeed, pretty much everything emits IR, but if you can reduce absorption and increase emission, I'd call that a "cooling effect" even if its nothing like standing in front of an AC fan.

If you don't believe in the "cooling" power of reflection, lay a reflective blanket over a patch of snow in your yard this winter and notice how much more slowly that patch melts than the surrounding unprotected snow.

I suspect that all of these articles, like nearly all science-centric articles have latched onto a soundbite that the authors think is most relatable to readers like us, but present it in the wrong context.

Zipper13

delcrossv said:

I have to call BS on the article. it's 98% reflective, so it's 2% absorbent. Absorbing energy and getting cooler violates thermodynamics. And thermodynamics is NEVER violated.
Wonder how it lasts on window sashes.

I do not know if this paint is such a material, but there are indeed substances that can absorb energy and store it as latent heat without raising the material's temperature. For these substances, the absorbed energy does work to change the substance's structure rather than raising the temperature. Consider how much energy boiling water will absorb without getting any hotter! That energy does work to produce a phase change from liquid to steam. Thermodynamics is still happy here.