[Bldg-sim] How can Low-e glass have dramatically lower U-Value?
Randy Wilkinson
randallcwilkinson at gmail.com
Mon Jul 28 18:31:25 PDT 2014
There is my problem. For decades I've been trained to understand that
energy exchange thru glass contains 2 components: 1) Conduction -
Calculated using a U-Value with units of Btu per hour per SF per degree
F., and 2) solar - calculated using the sun's energy output, adjusted
for latitude, shading factor for glass, tinting, drapes, etc.
The U-Value is only for conduction...by definition, and is confirmed by
looking at it's units (per degree F). Now a few molecules of metallic
coatings added to one surface adds to the U-value? That's the part I
find hard to believe. Or at least I did until today.
I can accept that an improved U-value can be determined that would make
it easy to calculate the overall equivalent heat transfer, averaged over
a season, including the effect of long wave infrared radiation from the
Low-e coatings. That would make less sophisticated simulation programs
be able to get better results. I guess I just need to verify that the
improved U-value given is an NFRC value that has been determined using
LBNL's Window software.
Thanks to all for helping me understand. As others pointed out, E+ can
accept a Window glass definition file and can also allow you to define
glass parameters using layers. I better figure that out next.
Randy
On 07/28/2014 05:05 PM, Joe Huang wrote:
> 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
> 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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