Unless I'm missing something here, this test tells me very little that removing the walls and floors from a zone model has no effect on the simulation results. What you've done is compare two thermally identical zone models, one with only a roof and the other with adiabatic walls and floors, i.e., a perfect refrigerator with a roof on top.
My main concern about this approach has always been how different would it be compared to a more realistic attic model where there is radiative exchange between the underside of the roof and the other surfaces, as well as conductive heat flows through those other surfaces? One of the main drivers for dynamic simulations in the early 1980's was the mantra that "heat gain does not equal cooling load". For a relatively "light" structure such as a roof, the differences are huge.
With EnergyPlus, this concern is compounded by the fact that it does a full radiative heat transfer between the attic surfaces, so my first question was whether the View Factors are still computed correctly, because if not, some of the radiative exchange will be lost. Even if the View Factors are correct, I would still be concerned how realistic are the results because the adiabatic walls and floor are simply reflecting all the radiative
exchange back to the zone.
So, my suggestion is to compare either of your adiabatic models to a more realistic model with reasonable amounts and characteristics of walls and floor and see what you get. If they're still the same, or very close, then I'll shut up.
In answer to Adil, I don't understand what you meant that my concern (expanded perhaps in more detail above) would apply only if EnergyPlus used "finite volume heat and mass transfer code" ? As far as I know, dating back to being involved in EnergyPlus development 1997-2005 (but only on the air flow modeling :-)), the main characteristic of the EnergyPlus zone model was its Heat Balance solution that's very much dependent on having the correct view factors.
Joe
Joe Huang White Box Technologies, Inc. 346 Rheem Blvd., Suite 108D Moraga CA 94556 yjhuang@xxxxxxxxxxxxxxxxxxxxxxxx http://weather.whiteboxtechnologies.com for simulation-ready weather data (o) (925)388-0265 (c) (510)928-2683 "building energy simulations at your fingertips"
On 2/25/2014 10:20 AM, Jeremiah Crossett wrote:
I did a simple test on a classroom zone, 10,000 square feet x 10 ft height = 100,000 cubic feet. One model has no walls or floors and uses only zone area, zone height and roof surface size, the other has adiabatic walls and floor for the same zone size.
I am typically looking at total energy in my studies, so less then 1% difference is good.
Here is a table, and attached are the input & output files..
Total Energy [kWh] BOX 239419 ROOF 240733 DIFF -1314 DIFF % -0.55%
Jeremiah D. Crossett | Senior Analyst | Phase Change Energy Solutions120 E. Pritchard St. | Asheboro, NC 27203| Mobile 503-688-8951
On Sun, Feb 23, 2014 at 11:56 PM, Adil <smhadil78@xxxxxxxxx> wrote:
Joe,
Your concern would have absolute meaning if energy plus would have used finite volume heat and mass transfer code for doing heat balance. Unfortunately it doesnt. Jean email clarifies how it works. Yes there will be warning related to soler distribution as pointed out by Linda, similar to window mutipliar warnings. There are too many other greater approximations that make view factor concern negligibleand therefore for initial studies it can be workable according to my opinion.
Regards,
Adil
www.geedindia.org
Just a note of caution. Not meaning to be confrontational in any way, but I'd like to know why Adil feels not having closure "doesn't matter"?
I haven't looked at this aspect of EnergyPlus, but from what I know of the heat balance method, the view factors should add up to 1.0. If you tell me that the Simple Model simply (:-)) prorates view factors by the surface areas in that zone, adding up to 1.0, then that's fine, but if not, you might be having some "heat leak".
Joe
On 2/22/2014 11:33 AM, Jeremiah Crossett wrote:
Adil,Thank you much for taking a look and shearing your insight. Great to know that my simplified modeling approach is reasonable and that the warnings can be safely ignored.The concept of modeling only roof or wall is really useful for initial studies, and I hope will be useful for exemplary calculations for LEED models that have been generated in other software that can not model PCM's. The idea is to model only the building elements of interest with an idealized HVAC system so that analysis and post processing are as simple as possible..
For the original users question, I agree with Joe, Jean and Niraj that unless you are intending to model a part of a building such as I have that you should enclose your zones, but just mentioned my model to give an example how E+ could be used in this way..
Kind regards
Jeremiah D. Crossett | Senior Analyst | Phase Change Energy Solutions120 E. Pritchard St. | Asheboro, NC 27203 | Mobile 503-688-8951
On Sat, Feb 22, 2014 at 11:23 AM, Adil <smhadil78@xxxxxxxxx> wrote:
Jermiah,
These warnings might have been put in EP for geometry integrity testing. Any cube or energy plus zone enclosure must have atleast 6 surface provided that you have define the zone by using buildingsurface:detailed object (this is default surface with many geometry export). In my opinion it dosent really matter for your model. You can safely ignore it. May be ep development team can note it and instigate this warning only incase of Buildingsurface detailed objects and not in surface objects that you have used.
Adil
www.geedindia.org
From: Jeremiah Crossett
Sent: 20-02-2014 03:01
To: EnergyPlus_Support@xxxxxxxxxxxxxxx
Subject: Re: [EnergyPlus_Support] Help [1 Attachment]
Yea, it is quite useful to be able to do a simple model with the roof simple object-- so now I am a bit concerned-- if anyone has further opinions on the attached model that uses only a roof, zone area and ceiling height to calculate volume I would like to hear.. The model uses loads and schedules from the commercial reference primary school, but with a roof of an actual school. The goal of the model is to evaluate roof PCM capacity, SRI and U value and nothing more- so a surface model is used. The file will open in open studio, but has some "Material Property Phase Change Dual Curve" objects from a custom version of E+ I use.
The specific error I get from the attached file is:
Program Version,EnergyPlus, Version 8.1, YMD=2014.02.18 22:51,IDD_Version 8.1.0.008************* Beginning Zone Sizing Calculations** Warning ** GetSurfaceData: The total number of floors, walls, roofs and internal mass surfaces in Zone GYMNASIUM** ~~~ ** is < 6. This may cause an inaccurate zone heat balance calculation.** Warning ** GetSurfaceData: The total number of floors, walls, roofs and internal mass surfaces in Zone CAFETERIA** ~~~ ** is < 6. This may cause an inaccurate zone heat balance calculation.** Warning ** GetSurfaceData: The total number of floors, walls, roofs and internal mass surfaces in Zone CLASSROOM** ~~~ ** is < 6. This may cause an inaccurate zone heat balance calculation.** Warning ** GetSurfaceData: The total number of floors, walls, roofs and internal mass surfaces in Zone CORRIDOR & OTHER** ~~~ ** is < 6. This may cause an inaccurate zone heat balance calculation.** Warning ** Surfaces in Zone="GYMNASIUM" do not define an enclosure.** ~~~ ** Number of surfaces <= 3, view factors are set to force reciprocity.** Warning ** Surfaces in Zone="CAFETERIA" do not define an enclosure.** ~~~ ** Number of surfaces <= 3, view factors are set to force reciprocity.************* Testing Individual Branch Integrity************* All Branches passed integrity testing************* Testing Individual Supply Air Path Integrity************* All Supply Air Paths passed integrity testing************* Testing Individual Return Air Path Integrity************* All Return Air Paths passed integrity testing************* No node connection errors were found.************* Beginning Simulation************* Simulation Error Summary ************************** EnergyPlus Warmup Error Summary. During Warmup: 0 Warning; 0 Severe Errors.************* EnergyPlus Sizing Error Summary. During Sizing: 6 Warning; 0 Severe Errors.************* EnergyPlus Completed Successfully-- 6 Warning; 0 Severe Errors; Elapsed Time=00hr 01min 46.73sec
Jeremiah D. Crossett | Senior Analyst | Phase Change Energy Solutions120 E. Pritchard St. | Asheboro, NC 27203 | Mobile 503-688-8951
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