[Equest-users] (no subject)
Nick Caton
ncaton at smithboucher.com
Thu Aug 12 15:50:26 PDT 2010
Namrata,
I have patched Bill Bishop's latest response in below in case you missed it. He's a proven smart cookie, and if he's telling me I've got my head wrapped around the problem the wrong way then I do believe him...
...but with that caveat, the manufacturer implying zero heat transfer through the entire window assembly (because two of at least 4 surfaces have equal temperature), sounds like flat-out misinformation to me. If two surfaces of a material have equal temperature (say, 100 °F), then yeah, the conduction between them would be zero, but that won't prevent convection/radiation from sweeping a majority of heat away when one side is subject to sub-freezing temperatures! My car for example has a heated rear window which helps me dislodge nightly glacier-formations in the wintertime. On a cold day, the heat generated in that glass may help heat the interior air somewhat, but surely a majority is going straight to the literally frozen exterior, especially once the interior air has risen to a temperature in which I can breathe regularly.
I remain skeptical of the concept of using heated windows as a good idea for a space heating source (as opposed to a draft or condensation mitigation strategy - perhaps to melt snow in a skylight as well?). Still, if these units do put a significant fraction of heat into the space, I think modeling electric baseboard heating will at the least be a good start to emulate the space temperature behavior the actual system may help achieve - though I'm still stuck wondering how you might account for the amount of energy that is lost to the exterior as the units operate... hear me out as I think my ducks are in a row here =)!
I'd be cautious that your manufacturer may be, intentionally or not, dumbing down the discussion by framing their "effective U-value" as a material property like "conductivity." Conductivity is an arbitrary material property that does not change with temperature - (i.e. steel has a higher conductivity than air). U-values (and R-values) on the other hand have that tricky little °F (or °C) in their units, because they are tied directly to a heat transfer rate, which requires having a delta-T - in other words, for a given set of material properties, the rate of heat transfer is dependent on the difference in temperature the assembly is subject to. That's one reason we come across "summer" and "winter" U-values for window assemblies.
It's my intuition that eQuest (among other variables) uses the hourly exterior temperature from the weather file alongside the hourly calculated interior temperature for the parent zone to determine what temperature difference to use when figuring the hourly heat loss through a window assembly. A heated window however (when on) should be using a fundamentally different figure for the interior surface temperature than, say, 70°F when determining a temperature difference to find the hourly heat loss for the zone.
I think I'm understanding now what we're all getting at when we speak of "effective U-values:" If a baseboard heating element in eQuest (or other software) uniformly distributes 100% of its heat to the zone, then we might account for the reality of a significant (?) fraction being lost straight out the window by increasing the window's effective U-value, in turn modeling a realistically higher heat loss through the assembly on account of the higher-than-normal heat temperature difference between the innermost and outermost surfaces of the window, right?
I could certainly be wrong, but an approach defining an electric internal equipment load with an custom equipment power curve (see Bill, below) *might* be easier than an approach defining dynamic window properties (which I do not think is feasible in eQuest).
Again, I am not writing with full confidence that I completely understand exactly how heated windows truly function, but critically thinking this through is helping me get there! I look forward to anyone stepping up to the plate to affirm or refute my current understanding =).
~Nick
NICK CATON, E.I.T.
PROJECT ENGINEER
25501 west valley parkway
olathe ks 66061
direct 913 344.0036
fax 913 345.0617
Check out our new web-site @ www.smithboucher.com
From: Namrata Vora [mailto:namrata.vora at soladigm.com]
Sent: Thursday, August 12, 2010 2:08 PM
To: Nick Caton; Sami, Vikram; Sam Mason; Bishop, Bill; equest-users at lists.onebuilding.org
Subject: active heating window modeling
All,
Thanks for your prompt responses
The mfg of this technology claim that since the temp difference b/n surface 3 & 4 is zero or minimal, there is no heat transfer out through the windows. In addition the radiant nature of the heat allows the room temperature to be set lower.
This particular company claims a u-value of 0.14 in non-heated state. I tried creating such a low u-value dual pane IGU using WINDOW5 and was unable to get that. So I created it directly in eQuest using simplified glass type method. Now will try adding the radiant heating as electric baseboard as suggested.
http://www.iqglass.com/products_iqglass/products_specs.html
I agree w/ Nick's comment. In addition I wouldn't think the window would be in heating mode all the time. I would need to figure out when to trigger the heating.
Namrata
Cell: 408-893-3729
From: Bishop, Bill [mailto:wbishop at pathfinder-ea.com]
Sent: Tuesday, August 10, 2010 1:36 PM
To: Nick Caton; equest-users at lists.onebuilding.org
Subject: RE: [Equest-users] (no subject)
Nick,
"The "efficiency" of these units ... is going to vary hourly (probably dramatically) based on the outside-to-interior temperature difference." Don't think of it as less heat to the space, but more heat to the exterior. The window U-factor should take care of this.
"apply an ON/OFF/TEMP schedule to an equipment or internal energy load?" You can apply an equipment power curve as a function of space temperature (EQUIP-PWR-FT).
"I'm concerned I might be missing something" For the same glazing properties, the heat loss to the exterior should be the same, EXCEPT that the interior surface is now warmer, so radiative and convective losses should be higher. Hopefully the manufacturers account for that in their rated U-factor. I'm not convinced yet that there is any energy benefit to this system.
Regards,
Bill
________________________________
From: Nick Caton [mailto:ncaton at smithboucher.com]
Sent: Tuesday, August 10, 2010 12:15 PM
To: Sami, Vikram; Sam Mason; Bishop, Bill; Namrata Vora; equest-users at lists.onebuilding.org
Subject: RE: [Equest-users] (no subject)
On further thought, I think the equipment referenced would be over-simplified if you just modeled an electric baseboard (that was my intuitive reaction also - a few beat me to the punch)...
The "efficiency" of these units, meaning the btu/h delivered to the space for every input kW, is going to vary hourly (probably dramatically) based on the outside-to-interior temperature difference. If it's -10F outside and 70F inside, these are going to lose a ton of the input heat to the exterior for the amount desired heat flowing inside - heat will flow proportionately to where it's coldest.
Time allowing, I'd propose achieving a much higher degree of modeled accuracy by defining a series of equipment loads, each with a custom multiplier schedule (even if the units in reality are under some kind of thermostatic control) to account for when the units would be turned on. This would be more work, but then you're able to define a distinct sensible HG ratio to account for the fraction of heat actually entering the space (instead of lost to the exterior). As you can define up to 5 separate equipment load/schedule combinations for one space, you could come up with a weighted efficiency to apply to the SHG ratio for up to 5 different blocks of time. Further explanation: This would involve creating hourly reports to compare the hourly exterior temps to the target heating temp for the interior, for the times the units would be operating, and to use that info to figure out an hourly efficiency for the system based on how much heat would actually find it's way into the interior zone. Those hourly results could be used to come up with weighted SHG ratio values for 5 blocks of time.
On a related note, I've got a question for everyone: Does anyone know of any workaround (aside from extensive excel number-crunching) to apply an ON/OFF/TEMP schedule to an equipment or internal energy load?
On another tangent, for my own education, am I wrong to consider heated glass as an intrinsically inefficient approach to space heating? It seems most appropriately applied as a "band-aid" or "patch" solution to a decision to put too much glazing where drafts will cause discomfort. One of the previous links has a claim: "When installed as part of a zone heating strategy, heated windows can actually reduce the overall energy consumption of a building by decreasing dependence on an inefficient HVAC system." Wouldn't this need to be a terribly inefficient heating system to compare unfavorably to a heating element losing >50% of its energy to the exterior? I'm concerned I might be missing something fundamental for HVAC design here... =/
~Nick
NICK CATON, E.I.T.
PROJECT ENGINEER
25501 west valley parkway
olathe ks 66061
direct 913 344.0036
fax 913 345.0617
Check out our new web-site @ www.smithboucher.com
From: equest-users-bounces at lists.onebuilding.org [mailto:equest-users-bounces at lists.onebuilding.org] On Behalf Of Sami, Vikram
Sent: Tuesday, August 10, 2010 11:07 AM
To: Sam Mason; Bishop, Bill; Namrata Vora; equest-users at lists.onebuilding.org
Subject: Re: [Equest-users] (no subject)
I stand corrected - it is a good U-Value J
Vikram Sami, LEED AP
Sustainable Design Analyst
1382 Peachtree St. NE, Atlanta, GA 30309
t: 404-443-7462 f: 404.892.5823 e: vikram.sami at perkinswill.com www.perkinswill.com <http://www.perkinswill.com/>
Perkins+Will. Ideas + buildings that honor the broader goals of society
From: Sam Mason [mailto:sam.mason at atelierten.com]
Sent: Tuesday, August 10, 2010 12:00 PM
To: Sami, Vikram; Bishop, Bill; Namrata Vora; equest-users at lists.onebuilding.org
Subject: RE: [Equest-users] (no subject)
Vikram,
The conversion from W/sq.m.-K to Btu/hr-sq.ft.-F is 5.68. This gives a U-value of 0.162 Btu/hr-sq.ft.-F.
I would like to see a study of the energy consumption of this window compared to a building with normal low-e IGU and actual radiators.
Sam
--
Sam Mason
Atelier Ten
From: equest-users-bounces at lists.onebuilding.org [mailto:equest-users-bounces at lists.onebuilding.org] On Behalf Of Sami, Vikram
Sent: Tuesday, August 10, 2010 11:47 AM
To: Bishop, Bill; Namrata Vora; equest-users at lists.onebuilding.org
Subject: Re: [Equest-users] (no subject)
I'm a little curious - is 0.92 W/m2k a good U-value? Just poking around on online converters - it looks like the IP version of that is around 0.56 or something in that region - doesn't seem spectacular (you get 0.25 with a lot of double pane low-e IGUs). Unless the 0.922 is for the entire glazing assembly - in which case its around the same as other glass types.
I might have got the conversion factor wrong.
Vikram Sami, LEED AP
Sustainable Design Analyst
1382 Peachtree St. NE, Atlanta, GA 30309
t: 404-443-7462 f: 404.892.5823 e: vikram.sami at perkinswill.com www.perkinswill.com <http://www.perkinswill.com/>
Perkins+Will. Ideas + buildings that honor the broader goals of society
From: equest-users-bounces at lists.onebuilding.org [mailto:equest-users-bounces at lists.onebuilding.org] On Behalf Of Bishop, Bill
Sent: Tuesday, August 10, 2010 11:25 AM
To: Namrata Vora; equest-users at lists.onebuilding.org
Subject: Re: [Equest-users] (no subject)
Namrata,
I would model the heating as electric baseboards and select a glazing type based on the U-value and SHGC.
Regards,
Bill
William Bishop, PE, BEMP, LEED® AP | Pathfinder Engineers & Architects LLP
Mechanical Engineer
134 South Fitzhugh Street
Rochester, NY 14608
T: (585) 325-6004 Ext. 114
F: (585) 325-6005
wbishop at pathfinder-ea.com
www.pathfinder-ea.com
P Sustainability - less is more.
________________________________
From: equest-users-bounces at lists.onebuilding.org [mailto:equest-users-bounces at lists.onebuilding.org] On Behalf Of Namrata Vora
Sent: Tuesday, August 10, 2010 10:34 AM
To: equest-users at lists.onebuilding.org
Subject: [Equest-users] (no subject)
How does one model active heating windows like the ones listed below.
http://www.iqglass.co.uk/p/h/Products/U_Values_%26_Benefits/218/
http://www.thermiquetech.com/about_heated_glass.html
Namrata Vora
408-893-3729
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