[Bldg-sim] *****SPAM***** Re: How can Low-e glass have dramatically lower U-Value?

Justin DeBlois Justin.DeBlois at setty.com
Tue Jul 29 12:14:24 PDT 2014


I have been following this discussion and I’m still a little unclear. As I understand it, the U-value for windows from manufacturers includes the long-wave radiation as well – accounting for the effect of the emissivity of the window on the heat transfer and linearizing the radiation component at the winter and summer temperatures to create a combined radiation/conduction U-value.

I am not convinced that using one of these effective U-values in certain softwares will not double count the effect of long wave radiation.  For example, when I look at eQUEST’s inputs for simple window definitions, it has fields for:

-          Shading Coefficient:  to get the short wave solar radiation

-          Glass Conductance (U value)

-          Visible Transmittance: for daylighting calculations only

-          Outside Emissivity: from the DOE2 help: Outside surface IR emissivity of glazing. The default value (0.84) can be used for uncoated glass. Glass with an outside surface metallic coating would have OUTSIDE-EMISS between about 0.03 and 0.4 depending on the type of coating. Used to determine IR radiation leaving the glass and absorbed by the glass.”
So it seems to me that, at least in eQUEST, the IR heat transfer into and out of the room is calculated separately based on the user-input emissivity of the glass. If the U-Value included the effect of the emissivity, wouldn’t that be double counting the effect of short wave radiation?  Someone who knows more about window ratings or modeling software could please help clarify.

Best,

Justin DeBlois |  Energy Analyst
Justin.DeBlois at setty.com<mailto:firstname.lastname at setty.com>
Voice 646-253-9000

From: Bldg-sim [mailto:bldg-sim-bounces at lists.onebuilding.org] On Behalf Of Joe Huang
Sent: Monday, July 28, 2014 8:06 PM
To: bldg-sim at lists.onebuilding.org
Subject: Re: [Bldg-sim] *****SPAM***** Re: How can Low-e glass have dramatically lower U-Value?

Randy,

I'd like to dissuade you of the idea that a U-factor that includes long-wave radiation is some sort of work-around.  In fact, it's the standard definition of U-factor as applied to windows because of their high conductivity.  Actually, the more I think about it, it would be the conduction-only U-factor that would be misleading. I suppose it is possible that a program might use such a U-factor, then also do a external radiative exchange, and thus be double-counting, but that seems unlikely because to do that you would have to solve for the external surface temperature, which would require a detailed heat balance, etc.

Sure, standard weather files have all the information needed to account for long-wave radiation, because during the day, the long-wave radiation from the sun is included in the reported solar radiation, which is all spectrum, while the long-wave radiation exchange with the ground, air, and sky are all calculated using assumed temperatures for each and generally minimal.  During the night, the main long-wave radiation exchange is with the night sky, for which there are various algorithms to estimate the night sky temperature, depending on the atmospherics, clouds, etc.

Joe



Joe Huang

White Box Technologies, Inc.

346 Rheem Blvd., Suite 108D

Moraga CA 94556

yjhuang at whiteboxtechnologies.com<mailto:yjhuang at whiteboxtechnologies.com>

http://weather.whiteboxtechnologies.com for simulation-ready weather data

(o) (925)388-0265

(c) (510)928-2683

"building energy simulations at your fingertips"

On 7/28/2014 2:50 PM, Randy Wilkinson wrote:
Thanks Joe,

So you are saying that use of an improved U-value is a valid work-around in energy modeling software that can't or doesn't model long wave radiation in and out.  What if I put that improved U-value in there and the program then calculates the effect of long wave gains and losses...would we then have double accounted for the radiation effects?

Since I recognize you as also an expert at simulation weather data, do our typical hourly simulation weather data files contain sufficient information to model energy losses and gains from long-wave radiation?  Especially gains from the Sun?

Randy

On 07/28/2014 02:34 PM, Joe Huang wrote:
Randy,

I think you're being misled in a way. ALL building simulation programs model long-wave radiation between the building surfaces and the environment, because otherwise you would get erroneous results, a case in point being  night-sky radiation that causes roofs to be significantly colder than the outdoor air at sunrise.  How different programs handle long-wave radiation varies, but that's more an issue of modeling methodology, whether to combine the radiative with the convective or calculating them separately, what temperature to assume for the
environment (ground, sky, air, etc.), etc.

As for the LBNL Suite of window simulation software (Window/Therm/Optics), I don't know of anyone except the NFRC Simulation Laboratories that use all three, and only for the purpose of getting an NFRC rating  of a specific product.  Outside of that context, the most I've seen people
do in building energy simulations is to obtain or create a "Window-4" file using Window (but not Therm or Optics) and then import that into their building energy software.  Even there, the main advantage is to get better representation of the angular-dependent properties of the window.
As far as capturing the long-wave radiation, inputting the U-value from an NFRC Rating or a Window-4 file should work fine.

Joe

Joe Huang

White Box Technologies, Inc.

346 Rheem Blvd., Suite 108D

Moraga CA 94556

yjhuang at whiteboxtechnologies.com<mailto:yjhuang at whiteboxtechnologies.com>

http://weather.whiteboxtechnologies.com for simulation-ready weather data

(o) (925)388-0265

(c) (510)928-2683

"building energy simulations at your fingertips"

On 7/28/2014 1:45 PM, Randy Wilkinson wrote:
This is exactly what I mean by asking if our energy modeling software is inadequate.  Maybe a Senior Analyst or Building Scientist can do this, I don't think I can, or should.  If it takes specialty software to model long wave radiation coming in AND going out, then it seems like the functionality of Window/Therm/Optics should be built into our energy modeling software.

Thanks,

Randy

On 07/28/2014 12:03 PM, Jeremiah Crossett wrote:
Dear Randy,
What software are you using?

To properly model window coatings you could first use a 2D FEA package such as Window, then for framing Therm, and for optical you could use Optics.
Then you can use the 2D model results as inputs to 1D software such as Energy Plus.
http://windows.lbl.gov/software/default.htm

Also a nice, quick way to do analysis is to use COMFIN, (in same link) a graphical UI to E+ that is setup to model windows that have been calculated with Window/Therm/Optics.




​ ​
Jeremiah D. Crossett    | Senior  Analyst    |  LEED Green Associate
​ ​
120 E. Pritchard St.  | Asheboro, NC 27203
​ ​
 | Mobile 503-688-8951
www.phasechange.com<http://www.phasechange.com/>





On Mon, Jul 28, 2014 at 11:55 AM, Randy Wilkinson <randallcwilkinson at gmail.com<mailto:randallcwilkinson at gmail.com>> wrote:
Bldg-Simers,

I wanted to see if Low-e glass saves energy in the far North (60 deg. N latitude or more).  My thought was to use the same U-value for the glass, but change the SHGC to account for the difference in solar heat gain due to the Low-e coatings.  To my surprise, manufacturers data for Low-e glass lists much lower U-values for the same double glazed units except with a Low-e coating on surface #3.

I'm having a hard time understanding how a coating a few molecules thick, improves the U-value so much.  The Architects in my firm say that the manufacturers are calculating an improved U-value to account for energy saved by blocking radiant heat lost (going from inside, out) in Winter.  They surmize this is done because our energy loads and modeling software cannot calculate radiant heat loses in Winter.  I'm not sure the weather data we use has hourly long wave radiation data that can be used to determine the available IR heat that can be blocked by the Low-e coating.  I don't think our energy modeling software can account for radiant heat leaving the building in Winter.

For example,

Pilkington 1" double pane clear glass using air, has a Winter U-value of 0.47 Btu/hr.sq ft F and an SHGC of 0.71

The same Pilkington unit with their Energy Advantage Low-e coating has a Winter U-value of 0.33  and an SHGC of 0.67

PPG lists similar improvement for their Low-e coating


Is our energy modeling software inadequate to accurately model the effects of Low-e coating on glass? Both Summer and Winter?

Can we trust that the glass manufactures are giving us improved U-Values due to Low-e coatings that are valid?

Randy Wilkinson
Spokane, WA







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