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Effect of Tesla Roof on Home A/C

Discussion in 'Tesla Energy' started by SageBrush, May 13, 2017.

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  1. SageBrush

    SageBrush Active Member

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    Anybody care to estimate the reduced home A/C load in the summer in a home with a Tesla roof compared to other conventional roofs like asphalt shingles or tiles ? Can a Tesla roof modify the need for an attic or insulation ?
     
  2. Ampster

    Ampster Member

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    In California we have a complicated calculation under Title 24 about the building envelope. One would need a lot of data before they could definitively answer your question.

    A simple answer to your question is yes, shade and airflow contribute to keeping that envelope cool in summer.
     
  3. boaterva

    boaterva Supporting Member

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    I'll bite. Why would 'asphalt' tiles be different from an asphalt roof? Do we expect the solar roof to be cooler materially? Hotter? I'm not seeing any difference on first glance here. Solar panels over a standing roof are one thing. Replacing the 'shingles' in contact with the roof base (plywood in the asphalt case, usually) shouldn't make a huge difference.

    What am I missing here?
     
  4. SageBrush

    SageBrush Active Member

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    The solar tiles must have an air space between them and the roof for ventilation, otherwise their performance will be severely compromised.
    There are at least three mechanisms at play that would reduce the temperature of the roof that has overlying PV tiles:

    1. A fraction of the radiation hitting the PV tile is converted to electricity
    2. A fraction of the radiation hitting the PV tile is reflected back, and black body radiation adds some more
    3. The temperature gradient between the PV tile and the roof is modulated by air insulation and ventilation

    I'll guess that the important mechanisms are shade, ventilation, and conversion to electricity but that is about as far as my limited physics takes me. As an empiric guess we could probably track down panel temperatures of old fashioned PV panels on roofs and say that the roof will be no higher. I'm unsure though how much lower.

    Help, @Topher ?
     
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  5. SageBrush

    SageBrush Active Member

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    #5 SageBrush, May 13, 2017
    Last edited: May 13, 2017
    I got around to asking Google
    In San Diego the cooling effect amounted to a 5% discount on the installation price.

    And more good data
     
  6. Topher

    Topher Energy Curmudgeon

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    This is energy calculation and as usual the simple answer is 'it depends.'

    Most roofs are made with ventilation, so I will consider that the same for both types of roof (asphalt shingles vs Tesla shingle). Ordinary solar panels tend to be elevated above the surface of the roof, giving it another avenue for ventilation, that I assume is not available to Tesla Solar Roof.

    The energy from the sun is about 1kW per meter^2 at full sun. Once it hits the roofs, it is either absorbed or reflected. This is dependent almost entirely on color and Infrared reflectivity (the latter is similar for most materials). A well chosen white roof can reflect around 80-95% of the light energy that hits it. A solar cell roof (of any type) will have a reflectivity closer to 5-10% of light energy. Solar cells convert about 15-20% of the light energy into electricity, the rest is converted to heat. That means (very roughly) a solar roof moves at least 60% of the light energy that hits it into the roof materials, as opposed to 10% for a good white roof. Of course, the solar roof has the advantage of 15-20% of light energy over a dark roof.

    My recommendation is to have sufficient insulation between your living areas and outside areas which are unpleasant temperatures. 'Sufficient' means probably around R-60 (US units). Reflective insulation can have a beneficial impact in this application. If one is most concerned about cooling energy, light colored roofs are recommended. Does the increased heat from having a dark colored solar roof outweigh the advantage of having that electricity production? Probably not, but I would have to run the numbers for a specific situation to be sure.

    Thank you kindly.
     
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  7. EV-lutioin

    EV-lutioin Active Member

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    Yes, good point, with solar panels some of the photon energy is converted to electricity instead of heat, and I think we also have to factor in the heat transmission rate through the substrate. How does glass compare to other materials, such as asphalt shingles, in regards to the amount of heat transmitted from the surface of the shingle to the sub-roof materials. Related to that, I think we have to consider that glass reflectivity varies with the season; at the winter solstice angle of 26.5' glass reflects about 20% of the sun energy away from the roof and at summer solstice angle of 73.5' glass reflects about 5%. It would be my hope that the Tesla solar roof has a thin insulating material on the bottom of the single to reduce heat transmission and a reflective material facing upward on the bottom of the glass to increase the albedo of all areas/shingles that are not covered by photovoltaic cells, although added reflectivity could be considered a nuisance (e.g. metal roofs). This image shows about 15-20% of each shingle not covered by photovoltaics.
    0311fxsmoothglass.jpg
     
  8. Topher

    Topher Energy Curmudgeon

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    About the same, and not significant. Asphalt shingles are about R-0.44, Glass is about R-1.0 per inch, and Tesla Solar Roof is probably around 0.5" thick.

    Meh, it also varies over the course of the day. And asphalt shingles probably do the same. If you want to calculate total energy from the sun, include this (which I would do for estimates of specific locations), but for rough estimates we are doing here its insignificant.

    A better place for insulation would be below the ventilation. I don't think the Tesla shingles would benefit from reflective coating on back. To the extent that it would reduce light transmission (small I guess), it would ruin the look that you are paying so much more for.

    Thank you kindly.
     
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  9. brucet999

    brucet999 Active Member

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    Unfortunately, neither of those examples would apply to Tesla Solar Roof.

    Solar panels are mounted a few inches above the roof, allowing air circulation between the two and providing no direct path for heat transmission from the solar panels through the roof deck.

    Tesla Solar Roof tiles are the roof, so all the solar energy that is not either reflected or converted to electricity will pass through the roof deck the same as any other tiles.

    Being glass, their specific heat is nearly the same as concrete or clay, but being thinner and lighter, the amount of heat they can store and re-radiate after sundown will be about half as much as concrete or clay; probably about the same as asphalt shingles, which have about the same thickness when installed as the Tesla tiles.
     
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  10. EV-lutioin

    EV-lutioin Active Member

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    I am not sure how Tesla tiles are installed, but it is hard to imagine installing them without a rack or attachment rail of some sort, which may create a small air gap. I am not sure how much difference a small air gap would make, but it has to be better than direct contact.
     
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  11. SageBrush

    SageBrush Active Member

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    Either some ventilation, somehow, or the Tesla PV cell has a different efficiency scale to temperature than I am used to seeing.
     
  12. boaterva

    boaterva Supporting Member

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    This is why I'm confused. How does a 'roof' have an air gap? And conversely how do solar tiles work well without one. A lot we don't know about how the tiles are installed and how they work. The new Tesla Solar Panels look like normal panels installed off the roof with some cosmetic skirt to make it more attractive. All the photos of the Solar Tiles look like everything is as flush (in contact) as a normal shingle/tile. Makes no sense in how we understand solar 'panels' (meaning the tiles) work 'well'.
     
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  13. brucet999

    brucet999 Active Member

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    I think the flat tiles will be installed exactly as shingles are; nailed directly to the roof sheathing and overlapping top inch of the course before. There would be just enough room for a wire to go from tile top edge to the next tile.
     
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  14. brucet999

    brucet999 Active Member

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    I thought a little more about your mounting rack hypothesis and, looking at the pictures on the Tesla Energy website, I see the tiles butted up against the sides of adjacent tiles and overlapped perhaps an inch. But the individual tile pictures show no water channel on one side of the tile, as is customary with concrete and clay tiles, so water will certainly pass between tiles. This means that the underlayment would have to be able to keep water from the plywood sheathing and there would have to be some sort of drain holes at the bottom edge of the roof to let water out. Naturally, such a condition makes the use of horizontal wood battens impossible as they would catch and retain water, quickly rotting away. If the battens were of plastic, I suppose they could be made with transverse slots to allow water flow past them.

    Maybe there are water channels and the pictures online just don't show them. I notice that the nail holes don't show on that picture either.
     
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  15. timpierc

    timpierc Member

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    If I remember correctly, in wk057's thread of his tesla battery/solar system he has a thermal video showing how much cooler his panels are versus the surrounding roof when they're generating current.
     
  16. EV-lutioin

    EV-lutioin Active Member

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    Yes, very good thoughts. This whole roofing thing makes me a bit queasy because a roof is a pretty darn important part of a house and if they don't get this right and there is widespread water damage or storm damage down the road it could drain Tesla's coffers very quickly.
     
  17. Topher

    Topher Energy Curmudgeon

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    Most traditional roofing methods did. For example, cedar shingle would be nailed to 'skip sheathing', that is boards only where nails go, the shingles being (partially) visible from beneath. Only with the modern advent of sheet goods have roofs been (vaguely) air tight.

    Thank you kindly.
     
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  18. boaterva

    boaterva Supporting Member

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    Well, right, but I was discussing asphalt shingles, where both traditional asphalt (used all over the east and northeast USA) and in Elon's demo does not show any gap betweeen the 'shingles' and the roof. So, where is the 'cooling' space coming from, or the room for wiring, inverters, controllers, etc? As I said above somewhere, still a lot we don't know. There may be room in the 'higher two' forms of tiles, but the 'plain' and 'textured' seem to have no room for the 'solar' accessories needed for a functioning solar generation installation.
     
  19. SabrToothSqrl

    SabrToothSqrl Active Member

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    Can't wait to see all the installers who drive a roofing nail right through a solar tile ;)
     
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  20. dhanson865

    dhanson865 Active Member

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    They'd have to be obstinate. The nail would slide off, bounce, not penetrate on the first dozen tries. I bet they couldn't accidentally do it, they'd have to purposefully predrill or tap a starter hole somehow.

    Now I'm sure they could scratch the heck out of it, maybe even crack it with the hammer as it slides off the nail and continues to the glass. I just don't think they'll get a roofing nail to penetrate the outer glass and actually puncture through to the next layer.
     

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