# Heat transfer between double window panes

#### zzinfinity

I'm designing a vertical farm prototype and I'm trying to do some preliminary calculations to get an idea of how hot my crops are going to get. The basic design consists of a low-e glass (SHGC=.39) in the front and an aerogel panel in back(see attached diagram). I'm calculating for the hottest scenario, south facing glass at noon in July. The design is in Boulder Colorado (Lat~40N) and from what I can tell from tables, I can expect .1233 kWh/(m^2*h) to be gained through the low-e glass during the hottest part of the day. What I'm not sure how to calculate, is how much heat I can expect to loose through the aerogel panel in the back.

Does any one know of a way to approximate this, or have a link to a site that can point me in the right direction? Can I just treat the aerogel panels like any other insulating material or do I need a SHGC for it? Don't need a high order of accuracy, just trying to approximate max and min temperatures for my cavity.

Thanks!

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#### anorlunda

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Gold Member
Yes, you use the R insulation values of each layer and add them up. You can use manufacturer's info for the specific glass. You can google the R value of aerogel.

#### trurle

I'm designing a vertical farm prototype and I'm trying to do some preliminary calculations to get an idea of how hot my crops are going to get. The basic design consists of a low-e glass (SHGC=.39) in the front and an aerogel panel in back(see attached diagram). I'm calculating for the hottest scenario, south facing glass at noon in July. The design is in Boulder Colorado (Lat~40N) and from what I can tell from tables, I can expect .1233 kWh/(m^2*h) to be gained through the low-e glass during the hottest part of the day. What I'm not sure how to calculate, is how much heat I can expect to loose through the aerogel panel in the back.

Does any one know of a way to approximate this, or have a link to a site that can point me in the right direction? Can I just treat the aerogel panels like any other insulating material or do I need a SHGC for it? Don't need a high order of accuracy, just trying to approximate max and min temperatures for my cavity.

Thanks!
You can get absolute maximum limit of temperature rise by just neglecting inside-to-outside heat transfer through the (aerogel-insulated) panels, and using approximation of 20 W/m2 for non-insulated fraction of greenhouse surface area.
For 75% of area been insulated, and 25% not insulated,
you get
(123.3*0.75)/(20*0.25) degrees temperature rise at maximum.

#### Henryk

Gold Member
Your thermal conductivity value for aerogel seems rather high. I would expect something around 0.03 $\frac W {m^2K}$ but there is more to it.
First, you need to calculate the thermal conductivity to the ground
Next, I suppose you want to use transparent aerogel in order to pass the solar radiation through. That, would mean that there will be certain amount of radiation emitted by your soil in the infrared region. I do not know the aerogel transparency in the infrared. You would have to find that data and incorporate into your calculations.
From what I know, the conductivity aerogels at around ambient temperature is much lower than the energy transfer via radiation. That's why most commonly used aerogel materials have something added to reduce radiative heat transfer. Either white material (to scatter infrared) or black (to absorb it) but for your application you cannot use either.
The SHGC number is no good, it applies only to solar radiation.
I do have a feeling that using aerogel is not a good idea. It is expensive and does not offer your much advantage of having just one pane or double pane glass window (at much lower price). Besides, there is such thing as too hot for the plants.

#### anorlunda

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Gold Member
www.aerogeltechnologies.com

Try contacting those people. Their business is tailoring aerogel properties for special purposes. It was the subject of this recent video

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