First look at Rivian Electrochromic Roof in action

[skyote]

While a non-glass roof will be offered, it is possible if not probable that the electrochromic option will get prioritized for early deliveries.

I'd be in for the electrochromic roof if it rejects heat as well as the tinted moon roof on my Volvo with the perforated sun shade closed. That still allows a little light to pass without turning the interior into a furnace in the Texas sun and heat. If the AC can keep things nice and chill inside without massively draining the battery on a 105-110 degree afternoon then all will be good. But if the AC is going to struggle to keep up, then I'd much prefer the metal roof.

Exactly. I'd like to understand quantitative measures of the differences between roof options for heat. With Rivian being an engineering-focused organization, I hope this is something where they will provide us the data necessary to make an informed decision.

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[ajdelange]

Rivian is unlikely to release the engineering data because most people wouldn't know how to interpret it which wouldn't stop many of them from interpreting it anyway. You can, of course, do WAG calculations. If, as offered in the previous post, a roof has 0.5% transmission, then it will let in 5 watts of heat per square meter more than a solid metal roof which will admit virtualy none. If a glass roof is tinted to optical density 1.0 then it will pass 10% of 1 kW/m^2 i.e. 100 W/m^2.

But keep in mind it is not the amount of heat that comes in, it is that minus the amount that goes out. When the car gets hot enough inside then heat starts to escape via conduction (it can escape via radiation because glass blocks the longer wavelengths - greenhouse effect), My experience always has been that it doesn't much matter whether you remember to close your sunroof (I had one car with a "moonroof" - never understood what the difference was and it didn't matter) or not. The interior is still bloody hot if you leave the car in the sun.

 

[Jehorton]

Still think tinting could be an option to make the roof slightly darker

 

[electruck]

Still think tinting could be an option to make the roof slightly darker

It's an option but at this point we have no idea whether it would even be necessary.

 

[skyote]

Rivian is unlikely to release the engineering data because most people wouldn't know how to interpret it which wouldn't stop many of them from interpreting it anyway. You can, of course, do WAG calculations. If, as offered in the previous post, a roof has 0.5% transmission, then it will let in 5 watts of heat per square meter more than a solid metal roof which will admit virtualy none. If a glass roof is tinted to optical density 1.0 then it will pass 10% of 1 kW/m^2 i.e. 100 W/m^2.

But keep in mind it is not the amount of heat that comes in, it is that minus the amount that goes out. When the car gets hot enough inside then heat starts to escape via conduction (it can escape via radiation because glass blocks the longer wavelengths - greenhouse effect), My experience always has been that it doesn't much matter whether you remember to close your sunroof (I had one car with a "moonroof" - never understood what the difference was and it didn't matter) or not. The interior is still bloody hot if you leave the car in the sun.

Not looking for anything quite so scientific. Maybe just interior temp readings from two of the same vehicles (including paint color), but with different roof options. Would also be cool to show the effects of different opacity settings on the electrochromic glass.

Simple, effective "real world" tests.

 

[ja_kub_sz]

Not looking for anything quite so scientific. Maybe just interior temp readings from two of the same vehicles (including paint color), but with different roof options. Would also be cool to show the effects of different opacity settings on the electrochromic glass.

Simple, effective "real world" tests.

I was always thinking about this with my Model S, interior temps and all, but temp controls with the all took care of all that for me.

I do however hope that the glass roof will have the same darkness as standard privacy glass included in the majority of cars on the market today.

 

[Surferdude]

I believe a white metal roof with standard insulation would far outperform any variant of electrochromic regardless of its ability to darken to 100% opacity. A black roof whether it's made of glass or metal, will generate more heat in the cabin by radiation (glass cannot block 100% of radiated heat from the sun) and conduction (with the absence of any insulation, the glass will heat up and the inside surface of the glass will radiate heat into the cabin). A white roof will reflect much of the solar radiation simply because of the color of the roof and any insulation below the metal roof will reduce the metal's ability to radiate and dissipate heat into the cabin. An electrochromic roof, if effective at blocking out light, will effectively become a black body which will absorb much of the sun's radiation and transmit that into the cabin. I could imagine there would be some reduction in the heat entering the cabin if the glass can get to near 100% opacity but a good old fashioned metal (sorry, not steel) roof with standard insulation will far outperform any glass product.

One of my biggest gripes with our Tesla is the gimmicky glass roof that heats up our cabin and brings unwanted light into the space. I hope Rivian avoids the gimmicks and provides optimal utility and function. This is why I am buying a Rivian.

 

[Samwhitney]

Just love the clear glass roof look and feel until it is not so cool. Hope Rivian can get the range from clear to dark right. The Mercedes S Class has it right.

 

[Hmp10]

I've been reading up on sun-blocking films for sunroofs, and one of the most frequent warnings is to avoid a film that is too dark. While a darker film will be more effective at dealing with sun glare, it will transmit more heat into the cabin than a lighter reflective film that is engineered more toward blocking IR radiation.

If the issues with topical film also apply to the integrated electrochromic films, there seems to be an unavoidable trade-off involved in glass roofs: the more visible light blocking, the more heat transmission.

 

[Samwhitney]

Yes, Diamler and all the others use an IR block in combination with the tinting tech. Daimler seems to have the best range and comfort so far.

 

[ajdelange]

All glass blocks IR transmission. The majority of the heat transmitted to the cabin is in the visible and NIR. Thus a cabin with a 1 m^2 roof of clear glass will receive 1 kW of radiation if the sun be directly overhead and that will all pass into the cabin where it will strike upholstery etc and be converted to IR which the glass will not pass and the cabin will get hot (greenhouse effect). Now if the glass is absorbtive with density .1 then only 100 W pass into the cabin and the rest gets absorbed by the glass which as a consequence gets warm causing it to radiate IR into the cabin and up into the sky. The static situation is a bit tricky to assess but the dynamic one, in which the car is moving, is fairly simple. The black body radiation back into the car will be dependent on the (fourth power of) the cabin temperature as the inner surface of the glass is going to be close to that. The 900 W/m^2 heat from absorbtion of solar energy is going to be mostly carried away by the slipstream and the outside surface of the glass will be, thus, close to the outside air temperature. So lets say you have the A/C set for 22 °C and it is 35°C outside and lets assume that the drop across the glass is such that the inner surface of the glass is at 27 °C. The heat radiated into the cabin would be 30 W/m^2 and, as the glass had 10% visible, there would be an addition 100 W/m^2 from transmitted light. The total load on the A/C would be 130 W/m^2. With clear glass it would be about 1020 W/m^2 (a little less from radiation as the glass wouldn't get as warm).

In the static case (car parked in the sun) the outer surface of the glass is going to get hotter as the airstream is not there to carry that heat away. There is convective cooling, of course, but it is not as effective and glass temperature will rise above air temperature. Let's say the inside of the glass goes to 35 °C. With the cabin set for 22° C the energy radiated into the cabin with 0.1D absorbtive glass would be about 81 W/m^2 making the A/C load 181 W/m^2. This is clearly better than the 1020 W/m^2.

Bottom line is that no, there isn't a trade between darkening and heat transmission. The darker the glass the less heat gets in.

 

[ajdelange]

Yes, Diamler and all the others use an IR block in combination with the tinting tech. Daimler seems to have the best range and comfort so far.

I'm not sure what the point of that would be as not much of insolation is in the IR (100 w/m^2 ?). But I guess 100W eliminated is 100W that doesn't have to be dealt with. Of course this worsens greenhouse effect. Sounds good in the advertising I suppose.

Maybe they mean the NIR. That would help but you wouldn't see the glass as being darker.

 

[Hmp10]

All glass blocks IR transmission. The majority of the heat transmitted to the cabin is in the visible and NIR.

My understanding is that the term "near infrared" does not mean the wavelength is near the infrared spectrum. It means the wavelength is in the infrared spectrum, but at the wavelengths nearer to the visible light spectrum. Thus NIR is still infrared light.

According to the article below, glass does not block NIR light . . . and NIR light feels hot on the skin.

https://climaterx.wordpress.com/2013/04/11/can-infrared-light-pass-through-glass/

"This radiation, called near infrared, does pass through glass. A better way to look at it is to say that it is not absorbed by the glass. It’s energy is too large to excite atoms in molecules to higher vibrational states. If you own an electric stove, you will experience this light just before the coils begin to glow a dull red. If you doubt that it is there, put your hand near a coil. Your skin actually “sees” this light."

This sensation of heat on the skin from NIR radiation would seem to be why sunlight coming in through a windshield can uncomfortably heat your chest (especially when wearing a dark shirt) or sunlight coming in through a sunroof can make the top of your head hot.

It would also seem that in order for a glass car roof to reduce the amount of heat felt on the skin, some additional steps would have to be taken other than just being glass. One of those steps would be to use a film or tint engineered to block some of the NIR radiation, as MB does.

(The article is by a PhD and Professor Emeritus of Chemistry at Southern Connecticut State University.)

 

[ajdelange]

My understanding is that the term "near infrared" does not mean the wavelength is near the infrared spectrum. It means the wavelength is in the infrared spectrum, but at the wavelengths nearer to the visible light spectrum. Thus NIR is still infrared light.

"Near" in this case refers to being near the visible spectrum (380 - 780 nm). Radiation of around 1000 nm to say 7000 nm woould be consiederd NIR. Glass passes visible and NIR radiation. It does not pass radiation at longer wavelengths. Most of the energy in sunlight is in the UV, visible and NIR parts of the spectrum.

According to the article below, glass does not block NIR light . . . and NIR light feels hot on the skin.

Article or no, glass passes NIR. This is why digital (and film) cameras can take IR photographs (unless equipped with an "IR" blocking filter, which is actually a NIR blocking filter, as they all are today.

"This radiation, called near infrared, does pass through glass."

Everybody knows that.

A better way to look at it is to say that it is not absorbed by the glass. It’s energy is too large to excite atoms in molecules to higher vibrational states.

No, its energy is not too high to excite molecules to higher vibrational states. Its energy is not in the correct band to excite molecules in glass to vibrate. UV light which is at higher energy still does excite glass. It is absorbed. Visible and NIR do excite molecules in other substances such as your skin and the upholstery and fittings of the car. This is why sunlight feels warm on your skin.

If you own an electric stove, you will experience this light just before the coils begin to glow a dull red. If you doubt that it is there, put your hand near a coil. Your skin actually “sees” this light."

It really would be better if you left the physics to the physicists but the heat you feell before yoy see visible light is not NIR. It is (deep) IR. The distribution of wavelength depends on the temperature. Your body radiates long IR (you can take a picture of a human in long IR). When you turn on a stove the wavelength of the peak radiation shifts higher and higher as the element warms. The total radiation increases as the 4th power of the temperature so while you feel the heat at lower temperature (IR) the quantity radiated does not get really large until K is high enough that most of the radiation is in the high visible and NIR,

This sensation of heat on the skin from NIR radiation would seem to be why sunlight coming in through a windshield can uncomfortably heat your chest (especially when wearing a dark shirt) or sunlight coming in through a sunroof can make the top of your head hot.

The reason your skin or an object fells warm in direct sunlight is because both NIR, visible and UV excite the molecules in your skin or any opaque object. It was mentioned in a previous post that most of the energy in sunlight is in the visible. It therefore follows that most of the heat delivered to your skin or the interior of the car comes from the visible part of the spectrum (glass also blocks UV).

It would also seem that in order for a glass car roof to reduce the amount of heat felt on the skin, some additional steps would have to be taken other than just being glass. One of those steps would be to use a film or tint engineered to block some of the NIR radiation, as MB does.

As most of the energy entering the vehicle is is the visible, not the NIR, the biggest gain comes from reducing transmission in the visible. Blocking in the NIR may pick up another few % rejection.

But I already said all this.[/QUOTE]

 

[Hmp10]

[Can't get the quote feature to work, so . . . ]

Ajdelange wrote:

"It really would be better if you left the physics to the physicists but the heat you feel before you see visible light is not NIR."

It might be better not to open a post with a categorical statement that "all glass blocks IR transmission" when, in fact, glass only blocks part of the IR spectrum.

In the article I cited, the author discussed skin feeling heat in the paragraph about NIR in order to illustrate his point that NIR can be perceived by the skin if not the eyes. He didn't begin to discuss different wavelengths of IR and their effects until the following paragraphs.

You've said you're an engineer . . . and now a physicist, it seems. The author's curriculum vitae shows he was a PhD in chemistry and a retired university professor.

So, whom to believe? Toss up?

Ajdelange also wrote:

". . . glass also blocks UV".

Once again, that is a categorical statement that goes against what I've read; i. e., untreated glass blocks almost all UV-B rays but only a small portion of UV-A rays. Glass that blocks a wider range of UV rays is somehow treated, often by application of coatings or films or, in the case of laminated glass, with the film between the panes.

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