Buildings.Fluid.Chillers.BaseClasses

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Buildings.Fluid.Chillers.BaseClasses

Package with base classes for Buildings.Fluid.Chillers

Information

This package contains base classes that are used to construct the models in Buildings.Fluid.Chillers.

Extends from Modelica.Icons.BasesPackage (Icon for packages containing base classes).

Package Content

Name Description

PartialElectric Partial model for electric chiller based on the model in DOE-2, CoolTools and EnergyPlus

warnIfPerformanceOutOfBounds Function that checks the performance and writes a warning if it is outside of 0.9 to 1.1

Name	Description
PartialElectric	Partial model for electric chiller based on the model in DOE-2, CoolTools and EnergyPlus
warnIfPerformanceOutOfBounds	Function that checks the performance and writes a warning if it is outside of 0.9 to 1.1

Buildings.Fluid.Chillers.BaseClasses.PartialElectric

Partial model for electric chiller based on the model in DOE-2, CoolTools and EnergyPlus

Buildings.Fluid.Chillers.BaseClasses.PartialElectric

Information

Base class for model of an electric chiller, based on the DOE-2.1 chiller model and the CoolTools chiller model that are implemented in EnergyPlus as the models Chiller:Electric:EIR and Chiller:Electric:ReformulatedEIR.

The model takes as an input the set point for the leaving chilled water temperature, which is met if the chiller has sufficient capacity. Thus, the model has a built-in, ideal temperature control. The model has three tests on the part load ratio and the cycling ratio:

The test
```
  PLR1 =min(QEva_flow_set/QEva_flow_ava, PLRMax);
```
ensures that the chiller capacity does not exceed the chiller capacity specified by the parameter PLRMax.
The test
```
  CR = min(PLR1/per.PRLMin, 1.0);
```
computes a cycling ratio. This ratio expresses the fraction of time that a chiller would run if it were to cycle because its load is smaller than the minimal load at which it can operature. Notice that this model does continuously operature even if the part load ratio is below the minimum part load ratio. Its leaving evaporator and condenser temperature can therefore be considered as an average temperature between the modes where the compressor is off and on.
The test
```
  PLR2 = max(PLRMinUnl, PLR1);
```
computes the part load ratio of the compressor. The assumption is that for a part load ratio below PLRMinUnl, the chiller uses hot gas bypass to reduce the capacity, while the compressor power draw does not change.

The electric power only contains the power for the compressor, but not any power for pumps or fans.

Implementation

Models that extend from this base class need to provide three functions to predict capacity and power consumption:

A function to predict cooling capacity. The function value needs to be assigned to capFunT.
A function to predict the power input as a function of temperature. The function value needs to be assigned to EIRFunT.
A function to predict the power input as a function of the part load ratio. The function value needs to be assigned to EIRFunPLR.

Extends from Buildings.Fluid.Interfaces.FourPortHeatMassExchanger (Model transporting two fluid streams between four ports with storing mass or energy).

Parameters

Type Name Default Description

replaceable package Medium1 PartialMedium Medium 1 in the component

replaceable package Medium2 PartialMedium Medium 2 in the component

Nominal condition

MassFlowRate m1_flow_nominal mCon_flow_nominal Nominal mass flow rate [kg/s]

MassFlowRate m2_flow_nominal mEva_flow_nominal Nominal mass flow rate [kg/s]

Pressure dp1_nominal Pressure [Pa]

Pressure dp2_nominal Pressure [Pa]

Initialization

MassFlowRate m1_flow.start 0 Mass flow rate from port_a1 to port_b1 (m1_flow > 0 is design flow direction) [kg/s]

Pressure dp1.start 0 Pressure difference between port_a1 and port_b1 [Pa]

MassFlowRate m2_flow.start 0 Mass flow rate from port_a2 to port_b2 (m2_flow > 0 is design flow direction) [kg/s]

Pressure dp2.start 0 Pressure difference between port_a2 and port_b2 [Pa]

Assumptions

Boolean allowFlowReversal1 system.allowFlowReversal = true to allow flow reversal in medium 1, false restricts to design direction (port_a -> port_b)

Boolean allowFlowReversal2 system.allowFlowReversal = true to allow flow reversal in medium 2, false restricts to design direction (port_a -> port_b)

Advanced

MassFlowRate m1_flow_small 1E-4*abs(m1_flow_nominal) Small mass flow rate for regularization of zero flow [kg/s]

MassFlowRate m2_flow_small 1E-4*abs(m2_flow_nominal) Small mass flow rate for regularization of zero flow [kg/s]

Boolean homotopyInitialization true = true, use homotopy method

Diagnostics

Boolean show_V_flow false = true, if volume flow rate at inflowing port is computed

Boolean show_T false = true, if actual temperature at port is computed (may lead to events)

Flow resistance

Medium 1

Boolean from_dp1 false = true, use m_flow = f(dp) else dp = f(m_flow)

Boolean linearizeFlowResistance1 false = true, use linear relation between m_flow and dp for any flow rate

Real deltaM1 0.1 Fraction of nominal flow rate where flow transitions to laminar

Medium 2

Boolean from_dp2 false = true, use m_flow = f(dp) else dp = f(m_flow)

Boolean linearizeFlowResistance2 false = true, use linear relation between m_flow and dp for any flow rate

Real deltaM2 0.1 Fraction of nominal flow rate where flow transitions to laminar

Dynamics

Nominal condition

Time tau1 30 Time constant at nominal flow [s]

Time tau2 30 Time constant at nominal flow [s]

Equations

Dynamics energyDynamics Modelica.Fluid.Types.Dynamic... Formulation of energy balance

Dynamics massDynamics energyDynamics Formulation of mass balance

Initialization

Medium 1

AbsolutePressure p1_start Medium1.p_default Start value of pressure [Pa]

Temperature T1_start 273.15 + 25 Start value of temperature [K]

MassFraction X1_start[Medium1.nX] Medium1.X_default Start value of mass fractions m_i/m [kg/kg]

ExtraProperty C1_start[Medium1.nC] fill(0, Medium1.nC) Start value of trace substances

ExtraProperty C1_nominal[Medium1.nC] fill(1E-2, Medium1.nC) Nominal value of trace substances. (Set to typical order of magnitude.)

Medium 2

AbsolutePressure p2_start Medium2.p_default Start value of pressure [Pa]

Temperature T2_start 273.15 + 5 Start value of temperature [K]

MassFraction X2_start[Medium2.nX] Medium2.X_default Start value of mass fractions m_i/m [kg/kg]

ExtraProperty C2_start[Medium2.nC] fill(0, Medium2.nC) Start value of trace substances

ExtraProperty C2_nominal[Medium2.nC] fill(1E-2, Medium2.nC) Nominal value of trace substances. (Set to typical order of magnitude.)

Type	Name	Default	Description
replaceable package Medium1	PartialMedium	Medium 1 in the component
replaceable package Medium2	PartialMedium	Medium 2 in the component
Nominal condition
MassFlowRate	m1_flow_nominal	mCon_flow_nominal	Nominal mass flow rate [kg/s]
MassFlowRate	m2_flow_nominal	mEva_flow_nominal	Nominal mass flow rate [kg/s]
Pressure	dp1_nominal		Pressure [Pa]
Pressure	dp2_nominal		Pressure [Pa]
Initialization
MassFlowRate	m1_flow.start	0	Mass flow rate from port_a1 to port_b1 (m1_flow > 0 is design flow direction) [kg/s]
Pressure	dp1.start	0	Pressure difference between port_a1 and port_b1 [Pa]
MassFlowRate	m2_flow.start	0	Mass flow rate from port_a2 to port_b2 (m2_flow > 0 is design flow direction) [kg/s]
Pressure	dp2.start	0	Pressure difference between port_a2 and port_b2 [Pa]
Assumptions
Boolean	allowFlowReversal1	system.allowFlowReversal	= true to allow flow reversal in medium 1, false restricts to design direction (port_a -> port_b)
Boolean	allowFlowReversal2	system.allowFlowReversal	= true to allow flow reversal in medium 2, false restricts to design direction (port_a -> port_b)
Advanced
MassFlowRate	m1_flow_small	1E-4*abs(m1_flow_nominal)	Small mass flow rate for regularization of zero flow [kg/s]
MassFlowRate	m2_flow_small	1E-4*abs(m2_flow_nominal)	Small mass flow rate for regularization of zero flow [kg/s]
Boolean	homotopyInitialization	true	= true, use homotopy method
Diagnostics
Boolean	show_V_flow	false	= true, if volume flow rate at inflowing port is computed
Boolean	show_T	false	= true, if actual temperature at port is computed (may lead to events)
Flow resistance
Medium 1
Boolean	from_dp1	false	= true, use m_flow = f(dp) else dp = f(m_flow)
Boolean	linearizeFlowResistance1	false	= true, use linear relation between m_flow and dp for any flow rate
Real	deltaM1	0.1	Fraction of nominal flow rate where flow transitions to laminar
Medium 2
Boolean	from_dp2	false	= true, use m_flow = f(dp) else dp = f(m_flow)
Boolean	linearizeFlowResistance2	false	= true, use linear relation between m_flow and dp for any flow rate
Real	deltaM2	0.1	Fraction of nominal flow rate where flow transitions to laminar
Dynamics
Nominal condition
Time	tau1	30	Time constant at nominal flow [s]
Time	tau2	30	Time constant at nominal flow [s]
Equations
Dynamics	energyDynamics	Modelica.Fluid.Types.Dynamic...	Formulation of energy balance
Dynamics	massDynamics	energyDynamics	Formulation of mass balance
Initialization
Medium 1
AbsolutePressure	p1_start	Medium1.p_default	Start value of pressure [Pa]
Temperature	T1_start	273.15 + 25	Start value of temperature [K]
MassFraction	X1_start[Medium1.nX]	Medium1.X_default	Start value of mass fractions m_i/m [kg/kg]
ExtraProperty	C1_start[Medium1.nC]	fill(0, Medium1.nC)	Start value of trace substances
ExtraProperty	C1_nominal[Medium1.nC]	fill(1E-2, Medium1.nC)	Nominal value of trace substances. (Set to typical order of magnitude.)
Medium 2
AbsolutePressure	p2_start	Medium2.p_default	Start value of pressure [Pa]
Temperature	T2_start	273.15 + 5	Start value of temperature [K]
MassFraction	X2_start[Medium2.nX]	Medium2.X_default	Start value of mass fractions m_i/m [kg/kg]
ExtraProperty	C2_start[Medium2.nC]	fill(0, Medium2.nC)	Start value of trace substances
ExtraProperty	C2_nominal[Medium2.nC]	fill(1E-2, Medium2.nC)	Nominal value of trace substances. (Set to typical order of magnitude.)

Connectors

Type Name Description

FluidPort_a port_a1 Fluid connector a1 (positive design flow direction is from port_a1 to port_b1)

FluidPort_b port_b1 Fluid connector b1 (positive design flow direction is from port_a1 to port_b1)

FluidPort_a port_a2 Fluid connector a2 (positive design flow direction is from port_a2 to port_b2)

FluidPort_b port_b2 Fluid connector b2 (positive design flow direction is from port_a2 to port_b2)

input BooleanInput on Set to true to enable compressor, or false to disable compressor

input RealInput TSet Set point for leaving chilled water temperature [K]

output RealOutput P Electric power consumed by compressor [W]

Type	Name	Description
FluidPort_a	port_a1	Fluid connector a1 (positive design flow direction is from port_a1 to port_b1)
FluidPort_b	port_b1	Fluid connector b1 (positive design flow direction is from port_a1 to port_b1)
FluidPort_a	port_a2	Fluid connector a2 (positive design flow direction is from port_a2 to port_b2)
FluidPort_b	port_b2	Fluid connector b2 (positive design flow direction is from port_a2 to port_b2)
input BooleanInput	on	Set to true to enable compressor, or false to disable compressor
input RealInput	TSet	Set point for leaving chilled water temperature [K]
output RealOutput	P	Electric power consumed by compressor [W]

Modelica definition

partial model PartialElectric 
  "Partial model for electric chiller based on the model in DOE-2, CoolTools and EnergyPlus"
  extends Buildings.Fluid.Interfaces.FourPortHeatMassExchanger(
   m1_flow_nominal = mCon_flow_nominal,
   m2_flow_nominal = mEva_flow_nominal,
   T1_start = 273.15+25,
   T2_start = 273.15+5,
   redeclare final Buildings.Fluid.MixingVolumes.MixingVolume vol2(
      V=m2_flow_nominal*tau2/rho2_nominal,
      nPorts=2,
      final prescribedHeatFlowRate=true),
    vol1(
      final prescribedHeatFlowRate=true));

  Modelica.Blocks.Interfaces.BooleanInput on 
    "Set to true to enable compressor, or false to disable compressor";
  Modelica.Blocks.Interfaces.RealInput TSet(unit="K", displayUnit="degC") 
    "Set point for leaving chilled water temperature";

  Modelica.SIunits.Temperature TEvaEnt "Evaporator entering temperature";
  Modelica.SIunits.Temperature TEvaLvg "Evaporator leaving temperature";
  Modelica.SIunits.Temperature TConEnt "Condenser entering temperature";
  Modelica.SIunits.Temperature TConLvg "Condenser leaving temperature";

  Real COP(min=0) "Coefficient of performance";
  Modelica.SIunits.HeatFlowRate QCon_flow "Condenser heat input";
  Modelica.SIunits.HeatFlowRate QEva_flow "Evaporator heat input";
  Modelica.Blocks.Interfaces.RealOutput P(unit="W") 
    "Electric power consumed by compressor";

  Real capFunT(min=0, nominal=1, start=1) 
    "Cooling capacity factor function of temperature curve";
  Real EIRFunT(min=0, nominal=1, start=1) 
    "Power input to cooling capacity ratio function of temperature curve";
  Real EIRFunPLR(min=0, nominal=1, start=1) 
    "Power input to cooling capacity ratio function of part load ratio";
  Real PLR1(min=0, nominal=1, start=1) "Part load ratio";
  Real PLR2(min=0, nominal=1, start=1) "Part load ratio";
  Real CR(min=0, nominal=1,  start=1) "Cycling ratio";

protected 
  Modelica.SIunits.HeatFlowRate QEva_flow_ava(nominal=QEva_flow_nominal,start=QEva_flow_nominal) 
    "Cooling capacity available at evaporator";
  Modelica.SIunits.HeatFlowRate QEva_flow_set(nominal=QEva_flow_nominal,start=QEva_flow_nominal) 
    "Cooling capacity required to cool to set point temperature";
  Modelica.SIunits.SpecificEnthalpy hSet 
    "Enthalpy setpoint for leaving chilled water";
  // Performance data
  parameter Modelica.SIunits.HeatFlowRate QEva_flow_nominal(max=0) 
    "Reference capacity (negative number)";
  parameter Real COP_nominal "Reference coefficient of performance";
  parameter Real PLRMax(min=0) "Maximum part load ratio";
  parameter Real PLRMinUnl(min=0) "Minimum part unload ratio";
  parameter Real PLRMin(min=0) "Minimum part load ratio";
  parameter Real etaMotor(min=0, max=1) 
    "Fraction of compressor motor heat entering refrigerant";
  parameter Modelica.SIunits.MassFlowRate mEva_flow_nominal 
    "Nominal mass flow at evaporator";
  parameter Modelica.SIunits.MassFlowRate mCon_flow_nominal 
    "Nominal mass flow at condenser";
  parameter Modelica.SIunits.Temperature TEvaLvg_nominal 
    "Temperature of fluid leaving evaporator at nominal condition";
  final parameter Modelica.SIunits.Conversions.NonSIunits.Temperature_degC
    TEvaLvg_nominal_degC=
    Modelica.SIunits.Conversions.to_degC(TEvaLvg_nominal) 
    "Temperature of fluid leaving evaporator at nominal condition";
  Modelica.SIunits.Conversions.NonSIunits.Temperature_degC TEvaLvg_degC 
    "Temperature of fluid leaving evaporator";
  parameter Modelica.SIunits.HeatFlowRate Q_flow_small = QEva_flow_nominal*1E-9 
    "Small value for heat flow rate or power, used to avoid division by zero";
  Buildings.HeatTransfer.Sources.PrescribedHeatFlow preHeaFloEva 
    "Prescribed heat flow rate";
  Buildings.HeatTransfer.Sources.PrescribedHeatFlow preHeaFloCon 
    "Prescribed heat flow rate";
  Modelica.Blocks.Sources.RealExpression QEva_flow_in(y=QEva_flow) 
    "Evaporator heat flow rate";
  Modelica.Blocks.Sources.RealExpression QCon_flow_in(y=QCon_flow) 
    "Condenser heat flow rate";

initial equation 
  assert(QEva_flow_nominal < 0, "Parameter QEva_flow_nominal must be smaller than zero.");
  assert(PLRMinUnl >= PLRMin, "Parameter PLRMinUnl must be bigger or equal to PLRMin");
  assert(PLRMax > PLRMinUnl, "Parameter PLRMax must be bigger than PLRMinUnl");
equation 
  // Condenser temperatures
  TConEnt = Medium1.temperature(Medium1.setState_phX(port_a1.p, inStream(port_a1.h_outflow)));
  TConLvg = vol1.heatPort.T;
  // Evaporator temperatures
  TEvaEnt = Medium2.temperature(Medium2.setState_phX(port_a2.p, inStream(port_a2.h_outflow)));
  TEvaLvg = vol2.heatPort.T;
  TEvaLvg_degC=Modelica.SIunits.Conversions.to_degC(TEvaLvg);

  // Enthalpy of temperature setpoint
  hSet = Medium2.specificEnthalpy_pTX(port_b2.p, TSet, port_b2.Xi_outflow);

  if on then
    // Available cooling capacity
    QEva_flow_ava = QEva_flow_nominal*capFunT;
    // Cooling capacity required to chill water to setpoint
    QEva_flow_set = min(m2_flow*(hSet-inStream(port_a2.h_outflow)),0);

    // Part load ratio
    PLR1 = min(QEva_flow_set/(QEva_flow_ava+Q_flow_small), PLRMax);
    // PLR2 is the compressor part load ratio. The lower bound PLRMinUnl is
    // since for PLR1<PLRMinUnl, the chiller uses hot gas bypass, under which
    // condition the compressor power is assumed to be the same as if the chiller
    // were to operate at PLRMinUnl
    PLR2 = max(PLRMinUnl, PLR1);
    // Cycling ratio
    CR = min(PLR1/PLRMin,1.0);

    // Compressor power.
    P = -QEva_flow_ava/COP_nominal*EIRFunT*EIRFunPLR*CR;
    // Heat flow rates into evaporator and condenser
    QEva_flow = max(QEva_flow_set, QEva_flow_ava);
    QCon_flow = -QEva_flow + P*etaMotor;
    // Coefficient of performance
    COP = -QEva_flow/(P-Q_flow_small);
  else
    QEva_flow_ava = 0;
    QEva_flow_set = 0;
    PLR1 = 0;
    PLR2 = 0;
    CR   = 0;
    P    = 0;
    QEva_flow = 0;
    QCon_flow = 0;
    COP  = 0;
  end if;

  connect(QCon_flow_in.y, preHeaFloCon.Q_flow);
  connect(preHeaFloCon.port, vol1.heatPort);
  connect(QEva_flow_in.y, preHeaFloEva.Q_flow);
  connect(preHeaFloEva.port, vol2.heatPort);
end PartialElectric;

Buildings.Fluid.Chillers.BaseClasses.warnIfPerformanceOutOfBounds

Function that checks the performance and writes a warning if it is outside of 0.9 to 1.1

Information

This function checks if the numeric argument is outside of the interval 0.9 to 1.1. If this is the case, the function writes a warning.

Inputs

Type Name Default Description

Real x Argument to be checked

String msg String to be added to warning message

String curveName Name of the curve that was tested

Type	Name	Description
Real	x	Argument to be checked
String	msg	String to be added to warning message
String	curveName	Name of the curve that was tested

Outputs

Type Name Description

Integer retVal 0 if x is inside bounds, -1 if it is below bounds, or +1 if it is above bounds

Type	Name	Description
Integer	retVal	0 if x is inside bounds, -1 if it is below bounds, or +1 if it is above bounds

Modelica definition

function warnIfPerformanceOutOfBounds 
  "Function that checks the performance and writes a warning if it is outside of 0.9 to 1.1"
  input Real x "Argument to be checked";
  input String msg "String to be added to warning message";
  input String curveName "Name of the curve that was tested";
  output Integer retVal 
    "0 if x is inside bounds, -1 if it is below bounds, or +1 if it is above bounds";

algorithm 
  if (x > 1.1) then
    retVal :=1;
  elseif ( x < 0.9) then
      retVal :=-1;
  else
    retVal :=0;
  end if;
  if (retVal <> 0) then
      Modelica.Utilities.Streams.print("************************************************************************************");
      Modelica.Utilities.Streams.print("*** Warning: Chiller performance curves at nominal conditions are outside of bounds.");
      Modelica.Utilities.Streams.print(msg + " is outside of bounds 0.9 to 1.1.");
      Modelica.Utilities.Streams.print("The value of the curve fit is " + String(x));
      Modelica.Utilities.Streams.print("Check the coefficients of the function " + curveName);
      Modelica.Utilities.Streams.print("************************************************************************************");

  end if;

end warnIfPerformanceOutOfBounds;

Automatically generated Tue Jan 8 08:29:21 2013.