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
John,
Nay. I believe you should be leaving the emissivity alone (or changing
it only slightly). By decreasing the emissivity, you make it more
difficult for the roof to get rid of heat that is absorbed. White/cool
roofs have higher reflectance. You want to change the absorptance (= 1 -
reflectance) of the roof, which is entered in the roof construction in
eQUEST. See my previous email, attached, for more info.
Regards,
William Bishop, PE, BEMP, LEED(r) AP
Hello -
I forwarded this email to colleagues working in LBNL's Cool Roof group
(Jonathan Slack (JLSlack at lbl.gov) and Haley Gilbert (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
Cool roof would have high thermal emittance and low solar absorptance (high solar reflectance). So you need to modify the parameters accordingly.
Som Shrestha, PhD, BEMP
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