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Huge difference in cooling loads between Openstudio and EnergyPlus

asked 2015-04-17 09:29:22 -0600

oat gravatar image

updated 2015-07-10 21:26:19 -0600

I have a simple one zone model (W=5, L=5, H=3) with some typical materials and construction, no window, no internal gains.

I simulated the annul cooling loads for this model in two methods, however, the results are hugely different.

Method 1: the model is created using Openstudio plugin for sketchup, and then exported as IDF file. The exported IDF file was added with an ideal loads air system, equipment list and equipment connection for the only zone, and a single cooling thermostat with a setpoint temperature of 24C specified for the ideal loads air system which is scheduled to be active 24/7 throughout the whole year.

Method 2: the model is created using Openstudio plugin for sketchup and further edited in Openstudio stand alone application, through which a dual setpoint thermostat is added with the cooling setpoint of 24c and heating setpoint 20c. This is because only dual setpoint thermostat is available in Openstudio, although only cooling setpoint will be used because I'm using Singapore's weather file which is in tropical region. The ideal air system is turned on for the only zone.

The annual cooling loads for method 1 is 9579.21kWh, whereas the result for method 2 is 21.29GJ which is about 5912kWh.

I'd like why there is such a huge difference between the two methods, both aim at modeling the annual cooling loads for a simple zone with an ideal loads air system to maintain 24°C throughout the whole year.

The IDF file and the OSM model are shared in the following link:

Hope I explained my problem clearly, and hope you can kindly advise! Thank you very much!

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answered 2015-04-17 10:15:09 -0600

At first glance it looks like method 1 shown on the left of screenshot below has infiltration, while the OSM model doesn't

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Yes, David, you're right. Method 1 does have a infiltration definition. However, there's no window or door for this simple model, the infiltration definition shouldn't affect the cooling loads, right?

Please kindly advise what the possible factors are that causes the difference in cooling loads!

Thank you very much!

oat gravatar image oat  ( 2015-04-18 10:14:22 -0600 )edit

The IDF has infiltration specified as always_on at 0.8 AirChanges/Hour. This object will provide infiltration whether or not there are windows or doors in the geometry. See the documentation for more info about how this object works. This will definitely impact your cooling energy over the year.

aparker gravatar image aparker  ( 2015-04-18 13:59:55 -0600 )edit

Dear aparker, thank you very much! you're right that the ACH of 0.8/h in method one affect the cooling loads a lot.

However, when I change it to zero, the result for method one is 5881.48kWh which is about 0.52% lower than the result of Method two.

I wonder why the results are still not close enough .... or Is this amount of difference acceptable and can be ignored?

May I ask everybody here what is the fundamental difference between the two methods?

Thank you all very much in advance!

oat gravatar image oat  ( 2015-04-24 12:14:07 -0600 )edit

@oat a difference of 0.52% is, IMO, totally ignorable. The fundamental difference in your two methods is that you are making EnergyPlus IDF files in two different ways (OS creates an IDF file before running the simulation), and the defaults are likely different. To find the difference, you could open both IDF files in IDF editor, and compare them object-by-object, field-by-field.

aparker gravatar image aparker  ( 2015-05-08 09:47:42 -0600 )edit

Thanks, aparker!

Is the "ultimate" IDF file generated by OpenStudio for EnergyPlus simulation in the OpenStuido project folder "...\run\4-EnergyPlusPreProcess-0" with the name "out.idf"?

oat gravatar image oat  ( 2015-05-10 09:10:49 -0600 )edit

answered 2015-04-18 14:24:44 -0600

I am not sure about open studio but I think when you model ideal loads in EnergyPlus it automatically assumes outdoor air goes through the system although I took a look at your idf file and you didn't have a fresh object specified so from that I couldn't figure out whether it did or not. It is worth reading the I/O guide in EnergyPlus for that object to fully understand what it is doing and make sure this is the same in Open Studio.

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Asked: 2015-04-17 09:29:22 -0600

Seen: 438 times

Last updated: Apr 18 '15