modelChillerStage

Chiller staging controller for plants with two chillers of the same size
Diagram of ChillerStageOpen the diagram
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Information

This model implements the staging control logic as follows:

  • When the plant enabling signal on changes from false to true, one chiller is enabled.
  • When the total cooling load QLoa exceeds 80 percent (adjustable) of one chiller's nominal capacity QChi_nominal, a second chiller is enabled.
  • When the total cooling load QLoa drops below 60 percent (adjustable) of one chiller's nominal capacity QChi_nominal (i.e. 30 percent of both chillers combined), or the plant enabling signal on changes from true to false, the second chiller is disabled.
  • When the plant enabling signal on changes from true to false, the operating chillers will be disabled sequentially.
  • Parameter tWai assures a transitional time is kept between each operation.


It is assumed that both chillers have the same capacity of QChi_nominal.

Note: This model can be used for plants with two chillers with or without waterside econimizer (WSE). For plants with WSE, extra control logic on top of this model needs to be added.

State graph.

Parameters

TypeNameDefaultDescription
Realcp_defaultSpecific heat capacity of the fluid
RealtWaiWaiting time
RealQChi_nominalNominal cooling capacity (negative)
RealstaUpThr-0.8*QChi_nominalStage up load threshold(from one to two chillers)
RealstaDowThr-0.6*QChi_nominalStage down load threshold(from two to one chiller)

Connectors

TypeNameDefaultDescription
Buildings.Controls.OBC.CDL.Interfaces.BooleanInputonEnabling signal of the plant. True: chiller should be enabled
Buildings.Controls.OBC.CDL.Interfaces.RealInputTChiWatRetChilled water return temperature
Buildings.Controls.OBC.CDL.Interfaces.RealInputTChiWatSupChilled water supply temperature
Buildings.Controls.OBC.CDL.Interfaces.RealInputmFloChiWatChilled water mass flow rate
Buildings.Controls.OBC.CDL.Interfaces.BooleanOutput[2]yOn/off signal for the chillers - false: off; true: on

Components

TypeNameDefaultDescription
Modelica.StateGraph.StateGraphRootstateGraphRootState graph root
Modelica.StateGraph.InitialStepoffNo cooling is demanded
Modelica.StateGraph.StepWithSignaloneOnStatus of one chiller on
Modelica.StateGraph.StepWithSignaltwoOnStatus of two chillers on
Modelica.StateGraph.TransitionWithSignaloffToOneCondition of transition from off to one chiller on
Modelica.StateGraph.TransitionWithSignaloneToTwoCondition of transition from one chiller to two chillers
Modelica.StateGraph.TransitionWithSignaltwoToOneCondition of transion from two chillers to one chiller
Modelica.StateGraph.TransitionWithSignaloneToOffCondition of transition from one chiller to off
Buildings.Controls.OBC.CDL.Reals.HysteresisthrOneToTwoThreshold of turning two chillers on
Buildings.Controls.OBC.CDL.Logical.NotthrTwoToOneThreshold of turning off the second chiller
Buildings.Controls.OBC.CDL.Reals.SubtractdTTemperature difference
Buildings.Controls.OBC.CDL.Reals.MultiplyproProduct
Buildings.Controls.OBC.CDL.Reals.MultiplyByParameterplrSpecific heat multiplier to calculate heat flow rate
Buildings.Controls.OBC.CDL.Logical.OrOrOn signal for either chiller
Buildings.Controls.OBC.CDL.Logical.NotnotOnon switches to false
Buildings.Controls.OBC.CDL.Logical.OrTwoToOneConditions that turn off the second chiller

Revisions

  • February 6, 2025, by Jianjun Hu:
    Removed the medium specification. This is for Buildings, #4110.
  • December 10, 2021, by Michael Wetter:
    Corrected parameter value for twoOn.nOut. This correction is required to simulate the model in Dymola 2022 if the model has been updated to MSL 4.0.0. With MSL 3.2.3, the simulation works without this correction.
    This is for Buildings, #1563.
  • August 6, 2020 by Jing Wang:
    First implementation.