How is Glazing Conduction defined?
I’m using the Honeybee_EnergyPlus Window Material, and when I halve the U Value, the Glazing Conduction in the peak month only changes by 12%. I would expect these to be more closely related.
When you get down to U-values that low, the glazing conduction from purely inside/outside temperature difference is going to be insignificant in relation to the other parts of the energy balance. You’ll find things like solar energy that’s absorbed by the glass and then conducting to the interior/exterior will dominate the conduction term rather than this temperature-driven flow that you’re expecting to see here.
So you should see a more significant difference if you were to compare a U-value of 1 to 0.5 (IP). But it’s never going to be a perfectly linear relationship. Especially on a monthly basis like that where you’re averaging hours when your loosing heat through the glass with the hours when you’re gaining heat through the glass.
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Thanks @chris! I just want to be clear on the line between “Solar” and “Glazing Conduction.” Say we have double pane windows with a low-e coating on the inside of the outer pane, is the following correct?
- Solar radiation hitting the outside pane warms the outside pane. This energy is transferred through the window (affected by U value, not SHGC) and goes to Glazing Conduction.
- Solar radiation hitting the inside pane warms the inside pane. This energy (affected by SHGC, not U value) goes to Glazing Conduction.
- Solar radiation hitting interior surfaces other than windows (floors and walls) goes to Solar.
- Heat transfer through glazing from air temperature differences are affected by glazing U value and go to Glazing Conduction.
- Heat transfer through exterior walls from air temperature differences are affected by wall U value and go to Opaque Conduction.
- Solar radiation hitting exterior wall surfaces warms the walls. This energy is transferred through the walls (affected by U value) and goes to Opaque Conduction (not Solar).
Hi @mikeheiss ,
Yes, the window conduction term in Honeybee accounts for both the conduction that happens as a result of indoor/outdoor temperature difference as well as solar heat that gets absorbed by the glass and conducts to the interior. The solar term that you see in Honeybee accounts for only solar that is directly transmitted through the glass (and then absorbed by interior surfaces). And, yes, solar heat hitting opaque walls that then conducts to the interior will be included in the Opaque Conduction just like Window Conduction.
Still, I sense there’s a slight misunderstanding in your descriptions:
Note that the SHGC metric includes BOTH directly transmitted solar gain as well as solar heat that gets absorbed by the glass and then conducts to the interior under standard “summertime” thermal conditions. I think the term you are looking for is “Solar Transmittance” and not “SHGC” since Solar Transmittance refers exclusively to the fraction of incident solar heat that does NOT get reflected or absorbed as it passes through the glass.
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This is an interesting thread. I just realize that Honeybee has glazing conduction output. This is certainly not an EnergyPlus output.
@chris Could you elaborate how Honeybee calculates the glazing conduction? A pointer to the source code would suffice.
@mikeheiss There are a number of factors that lead to the result you showed.
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You are using the simplified window definition (i.e. specifying only U-value and SHGC). When this is used, EnergyPlus will create a full definition of the window, i.e. with spectral average. The resulted spectral model is reported in the EIO file. However, as the manual warns, if you plug in the spectral model into the window definition, you might get a different result, compared to the one using simplified window definition. A better approach is to find a real glass in LBNL Window program that closely represents the U-value and SHGC you want.
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The combination of U-value and SHGC might not be realistic. You changed the U-value and not the SHGC. This may or may not be realistic. Remember that EnergyPlus will create a spectral average from the simplified window definition.
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EnergyPlus does not report conduction, simply because the zone cannot “see” the conduction. All the zone can see is the convection from the wall to the air. That’s it. More conduction will be felt as more convection by the zone. That is how EnergyPlus is designed. I found it interesting that Honeybee does report the conduction. I am wondering how is the conduction value isolated.
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If you want to see the effect of glass conduction, you should test when there is no solar radiation. Disable the heating and cooling during the day, and set it on during the night. The difference in the total energy for heating and cooling is caused by window conduction. The combined effect of radiation, convection, conduction and absorption is a lot more complex than a simple comparison like you did.
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Finally, you should check the EnergyPlus manual. Here is an example, note there is no conduction.
Cheers.
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Thanks for elaborating the explanation, @erydjunaedy .
We literally just do the following:
Window_Conduction =
"Surface Window Heat Gain Energy"
- "Surface Window Heat Loss Energy"
- "Zone Windows Total Transmitted Solar Radiation Energy"
Granted, we only offer this on a Zone-by-Zone basis, so we sum all of the “Surface” window conduction terms over the Zone. Here’s the part of the source code where we do the subtraction:
FYI, we’ve verified that this produces fully-balanced load graphics from the EnergyPlus results like this:
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