SageBrush
REJECT Fascism
No kidding!
Rather like this question: if a pot of water is set to heat using more or less flame, which will end up with the hottest water ?
;-)
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Rather like this question: if a pot of water is set to heat using more or less flame, which will end up with the hottest water ?
;-)
SageBrush
REJECT Fascism
This makes some sense. Since heat transfer depends on the temperature difference, temperature in a car, with its limited insulation, and large amounts of windows, is going to reach an equilibrium temperature fairly quickly, after which increase in temperature is matched by increase in heat loss (due to the higher temperature). Equilibrium temperature will be somewhat higher in the dark car, but probably not hugely.
As always, it depends. But if the equilibrium temperature is higher than the boiling point, than both will be the same temperature. Phase change occurs at a specific temperature, with a lot of energy being required before the temp can change again. This (hopefully) isn't the case with cars.
Very roughly, If the car has a cross section normal to the sun direction of 3 meters^2, that means about 3 kW of power is being input to the car. To cool that with a heat pump, that could be something like 1kW. So range loss would be approximately 3-4 miles per hour in the sun. [Note: that if you only turn on the cooling just before you use the car, this number goes way down, since you only need to cool from the equilibrium temperature. How much you save depends on many things like how powerful the heat pump is.]
Thank you kindly.
SageBrush
REJECT Fascism
Thank you -- I came up with similar numbers. I was really only trying to encourage the person who wrote "minimal" to avoid rather useless designations.
However, I am not sure you are right that starting the heat pump only a few minutes before you get in the car to drive off is going to be a big savings. The pump still has to remove the retained heat in the materials.
Yes, but the heat will NOT be the sum total of all energy input (which it would be if the cooling was done continuously). Once the car exceeds the outside temperature, it will be giving off heat naturally to the environment. All that heat energy is lost for free. If the car is never allowed to exceed outside temperature, the heat energy added by the sun, needs to all be removed at cost.
Yes, most of the heat in a hot car will be in the materials, not the air. But I never claimed it would be a 'few minutes', it will be whatever it takes. I would be happy to undertake the real calculation, if Tesla sends me all the specs for the car, and my usual fee for such services.
Thank you kindly.
JeffK
Well-Known Member
Jason Bourne
Member
In my area, white Teslas are much more rare than dark Teslas. I saw a white S on the road the other day and I wet my pants. It was so gorgeous.
JeffK
Well-Known Member
Jason Bourne
Member
Damn, I might steal that idea to darken the factory wheels. I'm also going to tint all the windows.
Will find a good place to have a proper powder coat on the wheels - the last thing I want is bits chipping away and exposing original color.
Would like to tint all windows but too many rules and regulations here affecting front windows so that is a no-go.
So I will do a very light tint on back door windows but the large rear glass will get a gradual tint if possible so that the section just above rear passengers will be very dark to the point it is almost opaque and towards the bottom it will be a very light tint. Just don't like the idea of a visible glass section above passenger seat. I reckon the next best thing to a full tint is a light tint on back windows so even though the fronts are not tinted, it will not be noticeable.
alseTrick
Active Member
D.E.
Uncorked
The heat comes in as visible light. That is absorbed by the interior and reradiated as infrared. Glass is opaque to infrared so the heat is trapped. The white or light colored interior will reflect more of the incoming visible light so will absorb less and should stay somewhat cooler. One would think the actual heat from the sun would heat the inside but the long wave incoming IR should be stopped by the IR opaque glass. The surface of the glass will be heated, though, then there will be conductive heating of the glass inside surface, then that heat will be radiated inside. There will be convective heat transfer from the inner glass surface to the air inside the car as well. Then there's the effect of the surface temperature of the outside of the car. Dark colors will get hotter than light colors because they absorb more light instead of reflecting it. Depending on the insulation values of the materials used between the outer metal surface of the car and the interior, that exterior dark color may increase the inside temperature. When the car is moving, there is more convective cooling of the outside surface of the car so the dark exteriors should heat a moving car less than a parked car.
Red Sage
The Cybernetic Samurai
No. The heat energy enters the car as a combination of Ultraviolet and Infrared in addition to visible light. Tesla Motors treats their panoramic roofs so that they greatly reduce the Ultraviolet portion, which is responsible for sunburn.
It is the heated surfaces within the car that in turn heat the air within the car, that causes a convection effect, as the hot air rises, cooler air drops, comes in contact with the hot surfaces, and rises again. As long as the same heated air is circulating through the car over and over again it gets hotter and hotter inside the car. This is exacerbated by the curvature of the glass on the car which may act as a lens at certain angles with the incoming light concentrating the heat on the interior.
No. The Infrared entered originally through the same glass, which isn't opaque to infrared at all. Pretty much anything that visible light can pass through Infrared can get through as well.
This is true. While the ambient temperature in the car was a reasonable 74ºF, the seats were a bit warmer but not anything near the earlier ambient temperature of 98ºF. Materials inside the car will retain heat for quite awhile after the interior cabin air is cooled. Better than entering a car that's 105-115ºF and sitting on seats that are scorching.
D.E.
Uncorked
Infrared comprises a comparitively broad part of the spectrum. The wavelengths associated with the temperatures under discussion are far infrared so are indeed blocked by glass. Have a look at thermal imaging documentation. You may have in mind other frequencies in the infrared spectrum, near infrared will be passed, far infrared will be blocked by most glasses.
I'm not sure about your lensing statements. I doubt the focal length of a curved windshield would be short enough to make much of a difference. If you have data or references we can discuss it. The hot seats will heat the air and cause convection currents as you describe. They'll also radiate heat energy. When the butt contacts the seat, there will be conductive transfer. If the butt sweats, there will be evaporative heat losses. All mechanisms of heat transfer come into play. If you really want to get technical, convection will include radiation as the mechanism for heat transfer to the air, so the argument of convection vs radiation isn't wrong, it's just incomplete.
If you look at the wavelength vs transparency curve of the atmosphere, you'll see that the frequencies transmitted fairly closely approximate the sensitivity range of the human eye. This no doubt is why our eyes are sensitive to 400 to 800 nm frequency range. The amount of transmitted light falls off fairly sharply as you go above or below the visible spectrum. Practically this means most of the delivered energy is going to be in the visible range. If you want to quibble about infrared and ultraviolet that is slightly outside the visible spectrum, well that's OK but I don't think those ranges are going to be meaningful. Since energy increases with shorter wavelengths, the energy content should be more significant for the near UV than for the near IR. As you point out, the Tesla glass blocks the UV.
I suspect we probably agree more than we disagree.
Off the topic but interesting, did you know that the human eye has far fewer blue cone cells than green and red?
I'm not sure about your lensing statements. I doubt the focal length of a curved windshield would be short enough to make much of a difference. If you have data or references we can discuss it. The hot seats will heat the air and cause convection currents as you describe. They'll also radiate heat energy. When the butt contacts the seat, there will be conductive transfer. If the butt sweats, there will be evaporative heat losses. All mechanisms of heat transfer come into play. If you really want to get technical, convection will include radiation as the mechanism for heat transfer to the air, so the argument of convection vs radiation isn't wrong, it's just incomplete.
If you look at the wavelength vs transparency curve of the atmosphere, you'll see that the frequencies transmitted fairly closely approximate the sensitivity range of the human eye. This no doubt is why our eyes are sensitive to 400 to 800 nm frequency range. The amount of transmitted light falls off fairly sharply as you go above or below the visible spectrum. Practically this means most of the delivered energy is going to be in the visible range. If you want to quibble about infrared and ultraviolet that is slightly outside the visible spectrum, well that's OK but I don't think those ranges are going to be meaningful. Since energy increases with shorter wavelengths, the energy content should be more significant for the near UV than for the near IR. As you point out, the Tesla glass blocks the UV.
I suspect we probably agree more than we disagree.
Off the topic but interesting, did you know that the human eye has far fewer blue cone cells than green and red?
Red Sage
The Cybernetic Samurai
Possibly. I might have known something similar once. It's been a long time since 3rd Grade Science class.
True, but most of it is in the visible range.
Not really. The vast majority of the heat in a car resides in materials, not air. If the car is reasonably well sealed, convection will have only small effects on the temperature in the car. The rising temperature problem is simply an imbalance between incoming energy and outgoing energy. Since the car is sealed (for our purposes), the energy can leave as heat only through conduction and radiation. Conduction is ineffective because since the outside of the car only touches air, which is a terrible conductor of heat. So, the job falls mostly to radiation. Some of which could exit through a window, if it is transparent to that frequency.
False. The exterior aperture of the window glass is nearly exactly the same as the interior. No concentration of total energy occurs (to first approximation). Some parts may get hotter than others, but that will even out (everywhere but where you put your skin
).False. The infrared ENTERING is not the same as the infrared LEAVING. Solar infrared is (mostly) in the near Infrared (since the sun is very hot) Objects in a hot car are radiating in the long infrared band, 8-15 µm where normal glass only passes about 20% of that radiation.
Thank you kindly.
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