Different values of "Output:Variable" and "EnergyManagementSystem:OutputVariable" for the same Sensor in EnergyPlus

I am using EMS to compute an intermediate value based on the available sensors. To check that the correct values are used in the EMS calculations, I created customized EMS outputs for those sensors and compared them with their values produced in the ESO file. For example, one of the sensors is the System Node Wetbulb Temperature of the Air Loop Demand Side Outlet as follows:

Actual sensor reading:

Output:Variable,
        Air Loop Demand Side Outlet,  !- Key Value
        System Node Wetbulb Temperature,  !- Variable Name
        timestep;                  !- Reporting Frequency

EMS sensor reading definition and it custom output:

Output:Variable,
        *,                       !- Key Value
        AirLoopWetbulb_Temperature_EMS,                !- Variable Name
        timestep;                  !- Reporting Frequency

    EnergyManagementSystem:Sensor,
        AirLoopWetbulb_Temperature,                !- Name
        Air Loop Demand Side Outlet,  !- Output:Variable or Output:Meter Index Key Name
        System Node Wetbulb Temperature;  !- Output:Variable or Output:Meter Name

    EnergyManagementSystem:OutputVariable,
        AirLoopWetbulb_Temperature_EMS,                !- Name
        AirLoopWetbulb_Temperature,                !- EMS Variable Name
        Summed,                  !- Type of Data in Variable
        !Averaged,                  !- Type of Data in Variable
        ZoneTimeStep ;           !- Update Frequency

image description

Is there a way to eliminate or minimize this error? I tried different EMS Program Calling options as follows:

    EnergyManagementSystem:ProgramCallingManager,
        For EMS Run , !- Name
        BeginTimestepBeforePredictor,  !- EnergyPlus Model Calling Point
        !AfterPredictorBeforeHVACManagers
        !EndOfZoneTimestepAfterZoneReporting
        !EndOfZoneTimestepBeforeZoneReporting
        EMSProgram1;        !- Program Name 1

Thank you.

Mariam

asked 2022-05-18 04:30:10 -0500

14.1k

Aaron Boranian

updated 2022-05-18 09:37:15 -0500

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Comments

Looks like the EMS output is just lagging by one timestep.

JasonGlazer (2022-05-18 07:07:11 -0500) edit

As Jason suggests, EMS output is simply lagging behind every timestep e.g. C1 == B2, and so on. I suspect the lag is due to reporting an HVAC state variable (like node WBT) too soon (i.e. before it gets calculated or initialized at runtime), so it's ends up reporting the previous timestep value. Try EndOfSystemTimestepBeforeHVACReporting (i.e. system variable) to first see if you can at least synchronize reported values, and move on from there. And summed seems odd for a state variable like WBT ... Long story short : correct values (reassuring), yet calling point is off.

Denis Bourgeois (2022-05-18 07:11:25 -0500) edit

Thank you @JasonGlazer and @Denis Bourgeois for your reply. Indeed, I have missed this observation because my question was based on the comparison graphical results I got using Design Builder viewer that I now found out plots the two variables with non-unified Y-axis scales, hence they seems very different. Thanks again for your feedback.

Mariam (2022-05-18 07:21:13 -0500) edit

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Thanks to @JasonGlazer and @Denis Bourgeois, EMS output is simply lagging behind every timestep that can be due to reporting an HVAC state variable (like node WBT) too soon (i.e. before it gets calculated or initialized at runtime), so it's ends up reporting the previous timestep value. I will try EndOfSystemTimestepBeforeHVACReporting (i.e. system variable) as suggested by @Denis Bourgeois to solve the lagging issue. I have missed this observation because my question was based on the comparison graphical results I got using Design Builder viewer that I now found out plots the two variables with non-unified Y-axis scale, hence looking very different.