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Chiller loop getting to 322C in first timestep of design day and causing Severe error

asked 2019-04-18 10:38:22 -0500

mldichter's avatar

updated 2019-04-18 11:12:41 -0500

I'm having some difficulty running a simulation. The error is below. There's no reason why the chiller loop should be this hot. I think it's just a numerical error because it's during the simulation of the first time step of a design day, the previous design days simulate just fine, and I bet the rest of the simulation would run just fine if I could get EnergyPlus to ignore the high temperature for a couple time steps. Oddly, I can get the error to go away by changing the storage capacity of the thermal energy storage ice tank that the chiller is feeding into. Any ideas on how I could get energyplus to ignore the high temp for just a little bit?

Workaround below, but still no solution.

** Severe  **
Plant temperatures are getting far too hot, check controls and relative loads and capacities
During Warmup, Environment=TORRANCE-MUNICIPAL ANN CLG 1% CONDNS DB=>MCWB, at Simulation time=08/21 00:04 - 00:06
PlantLoop Setpoint Temperature=-5.0 {C}

PlantLoop Outlet Node (SupplySide) "CHILLED WATER LOOP CHW PRI SUPPLY OUTLET" has temperature=322.3 {C}
PlantLoop  Inlet Node (SupplySide) "CHILLED WATER LOOP CHW PRI SUPPLY INLET" has temperature=-4.5 {C}

Post Script
I just tried the obvious idea of changing the time step. A timestep of 10 minutes gave the 322C error, a timestep of 1 minute gave a 491C error, and a timestep of 30 minutes gave a 359C error.
Post Post Script
I came up with a workaround. I increased the Maximum Loop Temperature field of the PlantLoop object to 400C and got these errors for a simulation running from 5/1 to 10/1.

Warning GetSpecificHeatGlycol: Temperature out of range (too high) for fluid [ETHYLENEGLYCOL40PERCENT] specific heat 
   *************  **   ~~~   **   This error occurred 718 total times;
   *************  **   ~~~   **   during Warmup 0 times;
   *************  **   ~~~   **   during Sizing 0 times.
   *************  **   ~~~   **   Max=595.421006 {C}  Min=127.188615 {C}
   *************  ** Warning ** GetDensityGlycol: Temperature out of range (too high) for fluid [ETHYLENEGLYCOL40PERCENT] density **
   *************  **   ~~~   **   This error occurred 144 total times;
   *************  **   ~~~   **   during Warmup 0 times;
   *************  **   ~~~   **   during Sizing 0 times.
   *************  **   ~~~   **   Max=595.421006 {C}  Min=127.191025 {C}
   *************  ** Warning ** Plant loop exceeding upper temperature limit, PlantLoop="CHILLED WATER LOOP PRIMARY WATER LOOP ICE TANK CHILLER HEAT EXCHANGER HALF"
   *************  **   ~~~   **   This error occurred 2 total times;
   *************  **   ~~~   **   during Warmup 0 times;
   *************  **   ~~~   **   during Sizing 0 times.
   *************  **   ~~~   **   Max=521.753501

I also looked the System Node Temperature for the node that was getting to 322C before and it does not report a termperature outside -10C to 10C. So looks like something's wrong with energyplus, possibly as simple as an overly aggressive error condition.

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answered 2019-04-18 11:35:06 -0500

The only equipment in a chilled water loop that should add heat to the loop is the pump. I suggest adding:

Output:Diagnostics, ReportDuringWarmup;

to your input file and then examining how the plant loop temperature changes as the simulation warms up. Include the pump inlet and outlet node temperatures to see the pump heat.

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@rraustad I did that and the maximum temp of both the inlet and outlet of the pump was 24.60275C in CSV columns with the extra Output:Diagnostics you recommended.

Looking back at the original error, the simulation time of the 322C error was Simulation time=08/21 00:04 - 00:06, but the timestep is 10 minutes long in that simulation. The high temp error is occurring in the middle of the timestep. Can the plantloop timestep be smaller than the simulation timestep?

mldichter's avatar mldichter  ( 2019-04-18 11:58:30 -0500 )edit

There is a zone (building) timestep and a system (HVAC and plant) timestep. These can be adjusted in this object:

 15,      !- Minimum System Timestep {minutes}
 20,      !- Maximum HVAC Iterations
 2,        !- Minimum Plant Iterations
 8;        !- Maximum Plant Iterations

N1 , \field Minimum System Timestep
 \note 0 sets the minimum to the zone timestep (ref: Timestep)
 \note 1 is normal (ratchet down to 1 min)
 \note setting greater than zone timestep (in minutes) will effectively set to zone timestep
 \minimum 0
 \maximum 60
rraustad's avatar rraustad  ( 2019-04-18 12:36:03 -0500 )edit

The zone timestep is set here (and 4 timesteps per hour is the same as 15 minutes per timestep):

  4;                                      !- Number of Timesteps per Hour
rraustad's avatar rraustad  ( 2019-04-18 12:38:09 -0500 )edit

@rraustad I tried fiddling around a bit, but increasing the PlantLoop upper temperature limit works pretty well. I don't get any huge temperatures in the nodes reported in output variables, and the number of times the error occurs is about the number of days in the simulation, which probably means the high temp occurs when the loop turns on. This is strange program behavior, but pretty low on the priority list for energyplus.

mldichter's avatar mldichter  ( 2019-04-19 10:24:00 -0500 )edit

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Asked: 2019-04-18 10:38:22 -0500

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Last updated: Apr 18 '19