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Time of Peak is not in Timestep increments

asked 2024-01-31 03:50:52 -0500

Keigo's avatar

updated 2024-02-01 21:22:51 -0500

A basic question. In the output files (html files), sometimes I see that Time of Peak is not in Timestep increments. Timestep of the cases below is 6, so I expected that the Time of Peak is 10-minutes increments, but it was not.

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Why is Time of Peak not in Timestep increments?

I guess Time of Peak is in Detailed Calculation Step increments and I can get Output:Variable at the Time of Peak by selecting Detailed. However, I'm not sure what's the difference between Timestep and Detailed Calculation Step and why Detailed Calculation Step is not based on Timestep. I would appreciate it if someone could explain in more detail.


@shorowit and @Aaron Boranian,

As a test, I tired to get Output:Variable of this idf file at the Time of Peak by selecting Detailed Reporting Frequency. Timestep is 6.

The Time of Peak is 04-SEP-08:09 (09/04 08:09:00), but I couldn't find the time in the csv file.

How can I get Output:Variable at the Time of Peak? Maybe I should make a new post, but it's closely related to my original question.

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@Keigo it looks like the Time of Peak in the summary report only goes down to minutes, while the Detailed timestep frequency seen in the CSV can go down to seconds. Your highlighted example shows 04-SEP-08:09, and there is row 133220 for 09/04 08:08:34, so perhaps the summary report is rounding up to the nearest minute.

Aaron Boranian's avatar Aaron Boranian  ( 2024-02-12 07:56:10 -0500 )edit

@Aaron Boranian I compared the end use at 04-SEP-08:09 shown in the html file and the end use at 04-SEP-08:08:34 shown in the csv file. They are different.

Cooling: 225,005.09W vs 220,283.61W

Fans: 262,920.25W vs 250,062.67W

Pumps: 96,391.93W vs 95,746.98W

So, there is no way to get Output:Variable at the Time of Peak...

Keigo's avatar Keigo  ( 2024-02-12 19:35:57 -0500 )edit

@Keigo from your final screenshot of output variables, it looks like some related to non-HVAC results (e.g., electric equipment) are only calculated at the model timestep and not the detailed or HVAC timestep. Perhaps there is some kind of interpolation between the model timestep output values that are before and after the peak timestep seen in the summary report?

Unfortunately, I believe it will take digging through EnergyPlus source code to get to the bottom of this.

Aaron Boranian's avatar Aaron Boranian  ( 2024-02-13 09:08:27 -0500 )edit

@Aaron BoranianInterpolate to Timestep in Schedule:Day:Interval is set to "No" for the electric equipment schedule. So, the electric equipment at the Time of Peak (04-SEP-08:09) = the electric equipment after 04-SEP-08:00 till 04-SEP-09:00.

Keigo's avatar Keigo  ( 2024-02-14 01:35:17 -0500 )edit

2 Answers

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answered 2024-01-31 09:26:13 -0500

There are two timesteps in EnergyPlus, the Zone timestep (which is the user input) and a System timestep (which is not). See the description here that states:

There is also second type of timestep inside EnergyPlus that is known as the System Timestep. This is a variable-length timestep that governs the driving timestep for HVAC and Plant system modeling. The user cannot directly control the system timestep (except by use of the ConvergenceLimits object). When the HVAC portion of the simulation begins its solution for the current zone timestep, it uses the zone timestep as its maximum length but then can reduce the timestep, as necessary, to improve the solution. The technical details of the approach are explained in the Engineering Documentation under “Integrated Solution Manager”. Users can see the system timestep used if they select the “detailed” frequency option on an HVAC output variable (e.g. Zone Air Temperature). To contrast, the “Zone” variables will only be reported on the zone timestep (e.g. Zone Mean Air Temperature).

And there is engineering documentation for the Variable (System) Timestep here.

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answered 2024-01-31 09:28:42 -0500

In summary, if a zone air temperature changes by > 0.3 $^0C$ or 0.5 $^0F$, EnergyPlus uses a smaller timestep than what's defined by the Timestep object type to iterate over how the HVAC system serving that zone should respond and avoid instability in the resulting zone air temperature. How small that timestep becomes is dynamic, and you can read more information in the Variable Timestep section of the Engineering Reference.

The "Detailed" reporting frequency option of Output:Variable objects would normally report every model timestep, but also add rows to show those portions of the simulation where this smaller timestep is used. Note that this "Detailed" reporting frequency can ONLY be applied to output variables that have an HVAC timestep instead of a Zone timestep. From the Output Details and Examples section for the eplusout.rdd file:

“Zone” variables are calculated and can be reported after each Zone/Heat Balance timestep (ref: TimeSteps in Hour command). “HVAC” variables are calculated and can be reported with each variable HVAC timestep. “Average” variables will be averaged over the time interval being reported whereas “sum” variables are summed over that time interval.

"Zone" vs. "HVAC" meaning the first word you see in the comment for a given output variable in the RDD output file. Example excerpts from the RDD output file, with ** around the key comment words I'm referring to:

Output:Variable,*,Zone Mean Air Humidity Ratio,hourly; !- **Zone** Average [kgWater/kgDryAir]

"Detailed" reporting frequency would always match the model timestep (Zone).

Output:Variable,*,Zone Air Heat Balance Internal Convective Heat Gain Rate,hourly; !- **HVAC** Average [W]

"Detailed" reporting frequency would show the smaller timesteps, if they are used (HVAC).

You can read this post for another reference, as well as see a diagram for this regarding EMS program calling points.

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Asked: 2024-01-31 03:50:52 -0500

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Last updated: Feb 01