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EnergyPlus fenestration solar cooling load very low

asked 2015-07-21 09:59:48 -0600

updated 2016-03-31 09:51:21 -0600

I did a model in openstudio in order to calculate HVAC cooling loads. I compared e+ results with trace700. I found that e+ has very low cooling loads. I checked the componens zising summary in e+ and found that the Fenestration solar load is very low (1/3 of the one in Trace700). I have the same U value and SHGC in both softwars. I expected to have some discrepancies in my calculations but not 3 times.

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Could there possibly be an ip/si mixup between the two models?

David Goldwasser gravatar image David Goldwasser  ( 2015-07-22 22:29:15 -0600 )edit

Thanks David. I am sure they are the same units. I have seen different post about trace e+ diferences in cooling loads. But any answer so far.

obuchely gravatar image obuchely  ( 2015-07-23 08:11:51 -0600 )edit

I have worked in the comparative results between Trace and EnergyPlus some time. The Trace results are always bigger. When the spaces have significant amount of fenestration, the differences are around 300%. I have already put a few questions in this forum and other discussion centers. So far no effective conclusion.

Geraldo Pithon gravatar image Geraldo Pithon  ( 2015-07-23 11:56:52 -0600 )edit

I am starting simulations in other software, particularly HAP Carrier, to attempt a comparison and point out a more fide result. I have always used the same envelope in the analysis, with the same climate file and indoor conditions. Shades are always drawn from the templates to simplify the analysis. If you come to some solution, please share with us, it will be of great importance.

Geraldo Pithon gravatar image Geraldo Pithon  ( 2015-07-23 11:57:55 -0600 )edit

A question for both of you. Did you make the model in Trace from scratch or did you export the OpenStudio model to gbXML and import that into Trace?

Also, just to point out one inherent difference in Trace vs. EnergyPlus geometry, Trace tracks geometry by area and orientation but it doesn't exist at a specific point in space, as a result there is no self shading from other base surfaces like you would see in EnergyPlus. That may make Trace solar gain slightly higher, but not 300% (possible Trace has some internal self shading coef. that is applied, but maybe not?).

David Goldwasser gravatar image David Goldwasser  ( 2015-07-23 12:39:56 -0600 )edit

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answered 2015-08-11 13:32:35 -0600

SEI_Associates gravatar image

updated 2015-08-11 14:04:04 -0600

The E+ Helpdesk passed your files to me since I am the principal developer for Trane Trace700. Here are some general comments:

  1. Trace700 usually will run all 12 cooling design months and so it may happen that a zone does not peak in same month as the summer design OADB and this may be due to a higher solar component in the off month. So I always recommend, whether running Trace or E+, to run all 12 months for cooling design.

  2. Trace allows the user to choose from several cooling load methodologies, some going back to the 1970's (such as TETD and CLTD/CLF) which are generally quite conservative compared to more recent methods. The most accurate of the Trace load methods is the RTS Heat Balance method which uses the ASHRAE Loads Toolkit algorithms to calculate both the hourly loads and the "room load" components. This method nearly always generates lower cooling loads than the older ASHRAE load methods. For solar calculations, the RTS method is the only Trace load method which automatically accounts for solar that reflects off the floor then back out through the windows. (Though in your case, this loss was quite small.)

  3. You chose the correct E+ summary report to compare against Trace but I'll repeat for other users' benefit: When comparing the load design output from Trace700, the closest report in EnergyPlus is the Zone Component Load Summary report which is automatically generated by the keyword Output:Table:SummaryReports,AllSummaryAndSizingPeriod. These reports are meant to display both the instantaneous and delayed load components so the user has an idea which part of the building envelope or interior loads contributed to the HVAC sizing requirements under Ideal Load conditions. The E+ "Sensible Heat Gain Summary" summary report is also very useful for displaying realtime simulation heat gains but is not meant for to show load sizing components.

  4. The window "90.1 Window Zone 3 Metal All Other" referenced throughout these datasets uses the simplified glass model for both Trace and E+. However, the E+ "simplified" model (WindowMaterial:SimpleGlazingSystem) can produce different results than the Trace700 "Std DS Glass" model. Just because the SC and SHGC can be made to match, each have different methods for calculating the SHGC incidence angle modifier. Perhaps, I'll compare these two methods at a later time.

  5. It is important to note how E+ and Trace report the glass solar and glass conduction. For example, in Trace, the solar absorbed by the window is added to the solar heat gain, not the window conduction (which is simply UATD in Trace) whereas in EnergyPlus, the the opposite occurs, i.e. solar absorbed by the window is part of the conduction load component. So the correct procedure is to compare E+ (Fenestration Solar + Conduction) vs Trace (Glass Solar + Conduction).

With that as background, I looked at the E+ IDF and Trace file you sent in. For comparison purposes I only looked at Office 2 since that was the one zone ... (more)

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Thank you very much!! Your information is very valuable. I am sure I was not the only one trying to find out why the differnces.

obuchely gravatar image obuchely  ( 2015-08-11 14:41:56 -0600 )edit

yes, this was very helpful. I was getting hung up on exactly the same differences in glass solar loads between E+ and Trace 700. Thanks.

SVVitale gravatar image SVVitale  ( 2015-11-24 15:03:54 -0600 )edit

Great explanation. I'm always cautious with sizing so it is good to see a clear comparison.

J gravatar image J  ( 2015-12-15 22:01:49 -0600 )edit

Great info!

There are monthly design days for EnergyPlus based on the 2013 ASHRAE HOF Design Days available through the EnergyPlus helpdesk. These are necessary for proper equipment sizing and I wish they were included with the weather file ddy.

I was just troubleshooting a model and noticed that the angle of incident modifier in the simple glazing system in EnergyPlus is much lower than the default for Trace. This is effecting the solar gains significantly. I would recommend that this is editable in EnergyPlus.

jmcneill gravatar image jmcneill  ( 2016-03-28 14:18:20 -0600 )edit

Awesome info SEI , do you by any chance have the trace engineering reference that explains there calculations...

Arif Hanif gravatar image Arif Hanif  ( 2016-03-28 17:53:03 -0600 )edit

answered 2015-07-22 22:13:28 -0600

hongtz gravatar image

Are you using same weather data? Did you model window frames explicitly? Shading on windows will reduce solar loads.

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Thanks, I am using for e+ epw from the energy pluss weather data. For trace 700 I am using IWC weather form Trane web page. both of them are from the same airport weather station and 8760 hours.

obuchely gravatar image obuchely  ( 2015-07-23 08:14:14 -0600 )edit

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Asked: 2015-07-21 09:59:48 -0600

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Last updated: Mar 31 '16