[Bldg-sim] "Horses mouth" question.

[Paul Riemer]

John,
Appendix G may or may not be a meaningful reference for  your boss but here goes.
In 90.1-2007, Table G3.1 allows roofs with reflectance greater 0.70 or more and remittance greater than 0.75 (per testing standards) to be modeled as having a reflectance of 0.45.  I presume the difference there is initial vs. maintained.  Baseline and other roofs are to be modeled with a reflectance of 0.30.

In eQUEST/DOE-2, absorptance is a Construction property while emissivity is a Surface property.
As Bill noted absorptance = 1 - reflectance, assuming no transmission (opaque surface).
The help section also has values for different properties.

External shading by buildings, PV panels, etc should also be considered.

Paul Riemer
Dunham


From: bldg-sim-bounces at lists.onebuilding.org [mailto:bldg-sim-bounces at lists.onebuilding.org] On Behalf Of Robin Mitchell
Sent: Monday, October 11, 2010 4:11 PM
To: 'John Aulbach'; bldg-sim at lists.onebuilding.org
Subject: Re: [Bldg-sim] "Horses mouth" question.

Hello -

I forwarded this email to colleagues working in LBNL's Cool Roof group (Jonathan Slack (JLSlack at lbl.gov<mailto:JLSlack at lbl.gov>) and Haley Gilbert (hegilbert at lbl.gov<mailto:hegilbert at lbl.gov> )), and got this response back from them:

***

"By reducing the emissivity, he's reducing the ability of the roofing material to emit thermal IR and thereby drop it's temperature. Hence, the roofing material is increasing in temperature, thus increasing heat flow into the structure, thus increasing demand on the cooling system. The most effective cool roofs are both high in reflectivity and high in thermal emittance. Haley and I have all sort of nice graphics on this if your collaborator would be interested in seeing them.

Bottom line for his model - increase reflectance, increase emissivity. Luckily, all the real world cool roof materials actually do have high emissivities. An example of a poor cool roof material would be polished copper - even though it's reflectance is high, it's low emissivity dooms it to poor performance in a hot climate."
***

Robin Mitchell
Windows & Daylighting Group
Lawrence Berkeley National Laboratory

From: bldg-sim-bounces at lists.onebuilding.org [mailto:bldg-sim-bounces at lists.onebuilding.org] On Behalf Of John Aulbach
Sent: Monday, October 11, 2010 12:01 PM
To: bldg-sim at lists.onebuilding.org
Subject: [Bldg-sim] "Horses mouth" question.

All:

Question on Cool Roof and the way to simulate them in eQuest.

My boss came to me with a cool roof and asked me to do a quick eQuest on it. I did a 25,000 sf single story office in Los Angeles, saw the roof outside emissivity was 0.9.  I changed it to 0.5 and the electric usage went UP. I change it again to 0.12 and the electric usage went up AGAIN. The HVAC is a standard single zone pacakge unit with furnace heat.

Am I seeing reality or what am I missing?

Thanks.


John R. Aulbach, PE, CEM
Senior Energy Engineer
________________________________
Partner Energy
1990 E. Grand Avenue, El Segundo, CA 90245
W: 888-826-1216, X254| D: 310-765-7295 | F: 310-817-2745
www.ptrenergy.com<http://www.ptrenergy.com/> | jaulbach at ptrenergy.com<mailto:%7C%20jaulbach at ptrenergy.com>



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