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Old 05-18-2024, 02:49 AM   #1 (permalink)
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Lightbulb Passive passenger compartment cooling

I’m very interested in maximizing passive cabin cooling, both at rest and in motion. Here is a list I’ve come up with, along with some thoughts on each item. This list assumes a near-worst case scenario. Imagine the vehicle sitting in the hot Arizona sun.
  1. Paint car/roof white
    Self explanatory. White paint reflects a lot of sunlight. I’ve personally had extremely positive experiences with elastomeric white roof paint.
  2. Tint/film on windows
    Virtually eliminates heat transfer through windows in motion, and significantly delays it while stationary.
  3. Reflective sunshades in windows (when parked)
    Self explanatory.
  4. Light colored interior
    Reflect more sunlight/heat. I’m a big fan of beige.
  5. Dark colored headliner
    Improve reflected sunlight/heat capture from the light interior, and improve transfer thru to the cool white roof. Basically traps heat up high, where it’s easy to get rid of it.
  6. Evaporative cooler
    Especially in dry areas, and even more especially if it can be plumbed into the vent system on the firewall, downstream of the blower motor. Fill with water, turn the blower motor on, and voila.
  7. Solar vent system (while parked)
    Install a vent fan in the ceiling, pulling from the shaded area underneath the car, creating a draft through the interior and (potentially) evacuating the hottest air.

I’d like to add that stationary passive cooling is very important, it extends the lifespan of your car interior, and also improves interior conditions when you get in the car. For something with no A/C, any passive improvement is very valuable, and even with A/C, a passively cooled vehicle reduces cooling requirements, maximizing MPG and/or power on hot days.

So—any other ideas?

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Old 05-18-2024, 03:59 AM   #2 (permalink)
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Safari windows front and rear, pop-outs and an oversized swamp cooler.
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Old 05-18-2024, 05:36 PM   #3 (permalink)
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Probably not easy to impliment yet, but scientists have discovered that just as colored paints reflect certain wavelengths of light and reflect others, one that reflects visible and ultraviolet light, yet is dark and absortive in the infrared spectrum, can maintain a temperature below ambient air temperatures by several degrees. Add a dark roof, perhaps no liner at all, just this material on the roof, and you have an interior passive cooling system.

The way it works is that the hotter something is, the higher the wavelenghts it gives off (get something hot and it starts to glow in the visible spectrum, first red, then yellow, then eventually blue or even violet until it starts producing ultraviolet). At around ambient temperatures, however things usually give off infrared radiation, which cools it. The problem is that this mainly applies to non-reflective surfaces (infrared guns have a hard or impossible time telling the temperature of a reflective surface). But the dilema is that by making something non-reflective in infrared, it usually gives off infrared, but if it's also non-reflective in visible and ultraviolet light, it then absorbs those. And if it's reflective in visible and ultraviolet, like your "cool white roof" it also will not shed heat in infrared. So the key is to make it non-reflective and dark in infrared, but reflective or white in visible and ultraviolet.
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Old 05-18-2024, 06:12 PM   #4 (permalink)
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Quote:
Probably not easy to impl[e]ment yet, but scientists have discovered that just as colored paints reflect certain wavelengths of light and reflect others....
Not sure I follow, let's ask Wikipedia:

en.wikipedia.org/wiki/Atmospheric_window

Quote:
An atmospheric window is a region of the electromagnetic spectrum that can pass through the atmosphere of Earth. The optical, infrared and radio windows comprise the three main atmospheric windows.[2] The windows provide direct channels for Earth's surface to receive electromagnetic energy from the Sun, and for thermal radiation from the surface to leave to space.[3] Atmospheric windows are useful for astronomy, remote sensing, telecommunications and other science and technology applications.

Out of an average 340 watts per square meter (W/m2) of solar irradiance at the top of the atmosphere, about 200 W/m2 reaches the surface via windows, mostly the optical and infrared. Also, out of about 340 W/m2 of reflected shortwave (105 W/m2) plus outgoing longwave radiation (235 W/m2), 80-100 W/m2 exits to space through the infrared window depending on cloudiness. About 40 W/m2 of this transmitted amount is emitted by the surface, while most of the remainder comes from lower regions of the atmosphere. In a complimentary manner, the infrared window also transmits to the surface a portion of down-welling thermal radiation that is emitted within colder upper regions of the atmosphere.[3]

Now let's ask Ecomodder: ecomodder.com/forum/showthread.php/energy-free-cooling-through-physics-40537.html#post693050

Given the fabrication process it might be difficult to embody as a paint, but hypershift pearls suggest otherwise.
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Old 05-18-2024, 09:09 PM   #5 (permalink)
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Here's a design that reflects solar light yet also radiates heat into space in the same time.

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Old 05-18-2024, 10:20 PM   #6 (permalink)
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Nor sure why they call it passive, when there're pumps and circulating fluids. At 5:36 he asserts it's not a heat pump. Because the condenser is a plate radiator?

Compare the claim of 95% efficiency with the 99.6% of nano-scale aluminum foam. Not sure how the diffusion reaction process compares to roll-to-roll thin films for manufacturability.

Cars might have white hexagonal tile roofs? Maybe dragon scale vortex generators, like shark skin.
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Old 05-19-2024, 12:17 AM   #7 (permalink)
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Originally Posted by freebeard View Post
Nor sure why they call it passive, when there're pumps and circulating fluids.
You know about the difference between an air-cooled engine and a water-cooled engine. Both have cooling fins somewhere, but which needs a water pump and why?

The same thing with the panels. The idea is to move the heat towards the panels from around the inside of a building and also to have a way to control it (you probably don't want a chilling effect during the middle of winter).

But if temperature control weren't a requirement, then you could just put this material on the roof as the roofing material, so to speak. The bottom line is that it radiates more heat than it assorbs, making it cooler than ambient temperature by as much as 15 degrees. Most materials don't do that, most either assorb a lot of heat from the light from the Sun or they reflect most of that heat without being able to radiate heat.

It's like the valve covers on a VW air-cooled engine. Black valve covers are usually preffered because black tends to radiate more heat (infrared). But reportedly VW would sell air-cooled engines with chrome valve covers in artic areas to help hold heat in.

But this material both reflects and radiates at the same time. It would be nice if they could design a material like this for engines so we could have nice chrome looking engines that also radiate heat as if they were painted black.
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Old 05-19-2024, 12:35 AM   #8 (permalink)
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One thing I've thought about is dash cooling. Run waterlines right under the dash and then up to a radiator with a fan, but could be powered by a small solar panel. It just seems to me that the dash is one of the main places that heat gets into the car (roof can be white or covered in aluminum foil or chrome or something like that). You could have a reflective dash, but that would be hard to see as it would reflect onto the windshield. Hence, why adding cooling to the dash makes sense to me, that way less heat will get into the car.
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Old 05-19-2024, 01:19 AM   #9 (permalink)
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Old 05-19-2024, 01:43 AM   #10 (permalink)
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Nice dash cooler!

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