How hot does a glass roof get?

[Dr Barnacle Luffy]

Speaking of hydrogen... I bought a kiln to experiment with making carbon nonotubes out of chicken feathers to store hydrogen. But that project has been delayed by other honey does.

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[ajdelange]

I am talking about compounds not individual atoms.

That was quite clear. I was trying to point out that a molecule is made of atoms, that atoms particles have wave functions and that the wave function determine the Borne hypothesis says that the magnitude of the wave function at a particular point is the probability density at that point. I cited the hydrogen atom's wave function was simple enough that I could describe it as concentric spheres. The next step is to realize that the orbitals of a molecule like methane have wave functions that are, in the LCAO theory of bonding, at least, linear combinatons of the wave functions of hydrogen and carbon (in this particular case) i.e. that the wave function of hydrogen is one of a couple of eigenfunctions that, weighted appropriately, comprise the wave function for the molecule. This is all way beyond my pay grade. The point is that the whole thing is probabilistic. A molecule does not have a fixed structure. The electrons aren't in the same place every time you look at a molecule and neither are the nuclei of the atoms nor the particles within the nuclei. Thus you can't offer a classical explanation of how photons interact with matter (or at least I can't). Futhermore note that quantum mechanics is no more than a set of hypotheses that seem to agree with what experimental evidence shows.

 

[ajdelange]

Thought I'd do another post on this subject based on some additional insight gained by some more thought and some experiments. The question was basically "Does the inside of a car with a glass roof get hot it its parked in the sun? and the answer is "Yes, very hot. Hot enough to kill your kids or pets if you don't do something about it." In order to so something about it it is necessary to know what causes it and what causes it is classic green house effect. In greenhouse effect solar energy is admitted into an enclosure through glass which transmits the wavelengths in which most of solar electromagnetic energy is found which are the visible (380 to 780 nm) range and the near infrared (NIR) which run from 780 up to 1440 or 2000 nm depending on whose definition you use. The red curve below shows approximately how this energy is distributed assuming the sun to be a black body radiator at 6500 K which is the model, called D65, used by illumination people, photographers... to represent sunlight at noon.

The numbers on the left axis represent the percentage of the total energy emitted up to the wavelength indicated on the horizontal axis. Thus less than 0.1% of D65 power is found at wavelength shorter than 160 nm (0.160 micron). The fraction of the power found between any two wavelengths is the difference in the vertical axis readings for those wavelengths. Thus the fraction of the power in the visible band (380 - 780 nm) is 48.7% of the total, the amount between 380 and 1440 which is one definition of where the NIR band ends is 73,8% and the amount found between 380 and 2000 nm which would include the visible and NIR by another definition of the NIR band is 80%, Glass transmits in the visible and NIR bands but not outside them so that most of the solar energy impinging on the glass roof of a car passes through and into its interior. If the interior is white a lot of that gets reflected back out. If you look through the glass you can see the white seats very plainly. If you have the black interior the seats aren't so plain when viewed from outside. A lot of the visible light gets absorbed by the darker material (quite a bit gets absorbed by the white materials too but but not so much which is why we always go for white interior). The visible photons excite the molecules in the seats, dash etc. which sets them vibrating and the materials get hot. Up to 40 or more °C. They then also become black body radiators at temperatures like 293 + 40 = 393K and re-radiate power at longer wave lengths as shown by the blue curve in the picture. Most of that reradiated energy is, as the graph shows, in the band between 4 and 20 microns (4,000 to 20,000 nm). Glass does not pass radiation in that band. If you try to take a picture of the interior of the car with an IR camera you will not be able to see anything. Thus while the visible light energy was able to escape back out through the glass once that has been converted to long wave radiation by warming the seats the energy is trapped. This is how a green house is able to stay warm in January and why your car turns into an oven if parked in the bright sun.

How to improve the situation? Obviously by keeping the shortwave (Vis, NIR) from getting in and/or by letting the longwave (IR) get out. There are lots of ways to do the former - the simplest being parking in the shade. Sun screens in various forms are available and using special glasses that block Vis and NIR will help. Such glasses are made for just this purpose. The shape of the curve, steeper at lower wavelength, says that I get more bang for my buck if I tint at the blue end of the visible spectrum than I do if I tint at the red end but of course that colors the glass. Of course "tinting" in the NIR band doesn't add a color cast at all because we can't see at those wavelengths. So there are games for the engineers to play in deciding where (in frequency) to tint and how much.

There really isn't much you can do in terms of making it easier for the trapped heat to escape. Even if the interior gets as hot as 40 the curve shows little energy (around 1%) at wavelength less than 4 microns (4,000 nm). Most glasses just won't transmit in that range. Interestingly enough there is a manufacturer who makes a special glass for automotive use with enhanced IR transmission out to 1650 nm. That's not far enough to help with this problem so what's it good for? It's intended to be used if you want to put LIDAR (or other IR sensors) inside the windscreen. There is a lot of talk that the "glass" in the CT will be ALON (aluminum oxy nitride) which does transmit up to 4,000 nm. That still won't help here and I doubt very much that the windows will be made of that stuff anyway.

Thus the only way to keep the interior of the car from becoming a greenhouse is to keep the sun from shining on it. Tesla can mitigate the problem by tinting the roof glass appropriately which it does in its current vehicles.

One final thing I'd like to clear up here is that in previous posts I frequently referred to the IR being "reflected" back into the car and this was based on seeing a reflected IR image in glass. Yes, some is reflected in the traditional sense which would allow for an image to be formed but more is absorbed at the surface of the glass which warms it thus causing it to become a black body radiator just like the seats and steering wheel etc. Thus it reradiates rather than reflects in the usual meaning of the term. I guess you could call that reflection but it would be a stretch!

 

[azjohn]

I think DR barnacle and AJ have just become BFFs

 

[ajdelange]

I am certainly grateful to him. In trying to convince him that glass blocks IR I've gained deeper insight into how green house effect works than I ever had before. Everyone knows that IR blockage is what causes greenhouse effect but I'd never thought about examining the relative distributions of radiation with wavelength for sunlight and of the seats etc. in a hot interior.

 

[TexasTesla]

I live in Texas and have never had a glass roof vehicle. The sun is intense here and I wonder how hot an all glass roof will make the vehicle feel inside. It looks like the roof is heavily tinted based on the few images from above I've seen. I'm hoping this keeps the open air and cavernous feel inside the truck without causing major solar heating inside the cabin. In my current truck that has an amazing AC, the side with the sun on them is much warmer than the other while driving. I'm wondering how hot the entire truck will feel in August when it is 100F outside with the sun blazing down. Anyone with a glass roof car in the deep south have some experience they can share?

My girl and I own a Model Y... I live in the Texas Hill Country, and I can assure you, it won't be an issue.

 

[VFRMike]

I have not felt heat coming in from the glass roof of my Model 3. I do have a ceramic tint on the glass roof (55%), it was not hot before having the tint installed. We get pretty hot here, hitting 100+ in the summer.

 

[rr6013]

SO there’s two truths.
1) IF you have clear glass or tinted, sunroof is hot. Ceramic will help but not to the extent you might expect.
2) Factory Solar tint sunroof is not hot. Ceramic will make it transfer even less heat.
I ceramic coated a tinted w/s on my 4Runner. It helped. I could feel the difference. But it was still warm in direct sunlight. Ceramic cut the UV.
I had extra Ceramic film leftover. So the installer added Ceramic to the factory solar sunroof. It was still too warm in direct sunlight. I had to close its cover.
OTOH, my Tahoe has dark brown solar Glass. That you can’t see thru. But you can from the inside to the outside. That is completely cool in direct sunlight - in the desert.
SO it depends. I believe members when they attest that Tesla sunroofs are cool, as in not hot. Factory solar glass can be all you ever need -as my surround greenhouse on the Tahoe proves.