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Severe error calculated density of air is negative

asked 2020-12-04 02:33:16 -0600

maria123098 gravatar image

updated 2020-12-07 09:31:34 -0600

Hello everyone! I am modeling a solar cooling system in EnergyPlus and while running the simulation I've got this severe error:

** Severe  ** PsyRhoAirFnPbTdbW: RhoAir (Density of Air) is calculated <= 0 [-9.04677].
**   ~~~   ** pb =[101100.00], tdb=[-312.09], w=[0.0000000].
**   ~~~   **  Routine=CorrectZoneHumRat, During Warmup, Environment=DSDAYTANGIER COOLING 0.4%, at Simulation time=08/21 00:12 - 00:13
**  Fatal  ** Program terminates due to preceding condition.

I also got a warning that the calculated cooling load is zero, and also the water heater's Recovery Efficiency and Energy Factor could not be calculated during the test for standard ratings.

I tried to hard size the cooling coil but the results are the same.

If anyone could help I'll be grateful. For more details you could find my IDF file in this link:

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tdb=[-312.09]. That is quite cold. Especially when you consider that absolute zero is 0 K = -273.15 °C :)

Julien Marrec gravatar image Julien Marrec  ( 2020-12-08 04:34:19 -0600 )edit

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answered 2020-12-08 04:47:44 -0600

updated 2020-12-08 08:19:53 -0600

I have upgraded the provided defect file (v8.7) to energyplus v9.4.0 and got an actual hard crash. So I went ahead and created an issue at NREL/EnergyPlus#8412. I will report back if a resolution is found.

Workaround: The problem is explained in more details on, but basically you want to NOT autosize the tank (volume and height at least) for now.

For eg, given that your WaterHeater:Sizing objects indicate "Nominal Tank Volume for Autosizing Plant Connections" of 30 m3 for HW ST and 0.1 m3 for HW Aux, do something like this in the WaterHeater:Stratified:

    HW ST,                   !- Name
    General,                 !- End-Use Subcategory
    30.0    ,                !- Tank Volume {m3} <= Enter hard value
    2.0,                     !- Tank Height {m} <= Enter hard value

    HW Aux,                  !- Name
    General,                 !- End-Use Subcategory
    0.1,                     !- Tank Volume {m3} <= Enter hard value
    1,                       !- Tank Height {m} <= Enter hard value

With that change, the file wile run to completion.

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Thank you for your contribution, it was very helpful

maria123098 gravatar image maria123098  ( 2020-12-08 13:19:50 -0600 )edit

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Asked: 2020-12-04 02:33:16 -0600

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Last updated: Dec 08 '20