It's quite common that attic temperatures can fall below the outdoor air temperature on clear cold nights due to night sky radiation from the roof. I've seen measured data for residential attics where the temperature depression is around 3-5 F in California and 1-2 F in Florida. 5C or 9F seems pretty large, but not outside the realm of possibility. I don't know what night sky radiation model is currently in EnergyPlus, but in DOE-2 it was based on work done by Marlo Martin at LBNL back in the early 1980's and produced similarly large attic temperature depressions (10F below ambient in some runs I did for California houses) that I have found doubtful.
Joe
Joe Huang White Box Technologies, Inc. 346 Rheem Blvd., Suite 108D Moraga CA 94556 yjhuang@xxxxxxxxxxxxxxxxxxxxxxxx www.whiteboxtechnologies.com (o) (925)388-0265 (c) (510)928-2683 "building energy simulations at your fingertips"
On 2/5/2013 3:26 PM, Griffith, Brent wrote:
Consider thermal radiation to the cold night sky.
From: EnergyPlus_Support@xxxxxxxxxxxxxxx [mailto:EnergyPlus_Support@xxxxxxxxxxxxxxx] On Behalf Of adamsrackes
Sent: Tuesday, February 05, 2013 4:24 PM
To: EnergyPlus_Support@xxxxxxxxxxxxxxx
Subject: [EnergyPlus_Support] Zone colder than outdoors - without mechanical cooling!
I've noticed something strange in the model I've been working with. It's one of the Commercial Reference Buildings (CRB), essentially unmodified: the post-1980 vintage small office in Baltimore. This building has an attic-type roof, which is modeled as zone, but it has no heating or cooling. It gets extremely hot in the summer and extremely cold in the winter.
What doesn't make sense to me is that this attic zone temperature often falls _below_ the ambient temperature at nighttime. I can't see any reason why this should ever happen given that: there is no cooling system, the zone and outdoor temperature at these times are both falling and not rising, and all the other zones in the building are warmer than the ambient. It really doesn't make sense at any time of year, but it's especially hard to understand in the winter, when it's below 0 C outside. The difference is not insignificant--frequently up to about 5 C, or 9 F.
As I said, the model is an unmodified CRB. Those models use the AnalyticalSolution for ZoneAirHeatBalanceAlgorithm. I've also tried ThirdOrderBackwardDifference (the default) and a few other things, a t lots of different timesteps. The results do not change.
One of the following is in error, in decreasing order of likelihood: me, this CRB model, EnergyPlus, or the second law of thermodynamics. Any ideas how/why?
https://dl.dropbox.com/u/47230322/jan.png
https://dl.dropbox.com/u/47230322/jul.png
Thanks,
a
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