Buildings.Fluid.HeatExchangers.CoolingTowers

Package with cooling tower models

Information

This package contains components models for cooling towers.

The model Buildings.Fluid.HeatExchangers.CoolingTowers.FixedApproach computes a fixed approach temperature.

The model Buildings.Fluid.HeatExchangers.CoolingTowers.YorkCalc computes the cooling tower performance based the York formula.

Extends from Modelica.Icons.VariantsPackage (Icon for package containing variants).

Package Content

Name	Description
FixedApproach	Cooling tower with constant approach temperature
YorkCalc	Cooling tower with variable speed using the York calculation for the approach temperature
Correlations	Package with correlations for cooling tower performance
Examples	Collection of models that illustrate model use and test models
BaseClasses	Package with base classes for Buildings.Fluid.HeatExchangers.CoolingTowers

Buildings.Fluid.HeatExchangers.CoolingTowers.FixedApproach

Cooling tower with constant approach temperature

Buildings.Fluid.HeatExchangers.CoolingTowers.FixedApproach

Information

Model for a steady-state or dynamic cooling tower with constant approach temperature. The approach temperature is the difference between the leaving water temperature and the entering air temperature. The entering air temperature is used from the signal TAir. If connected to the a dry-bulb temperature, then a dry cooling tower is modeled. If connected to a wet-bulb temperature, then a wet cooling tower is modeled.

By connecting a signal that contains either the dry-bulb or the wet-bulb temperature, this model can be used to estimate the water return temperature from a cooling tower. For a more detailed model, use for example the YorkCalc model.

Extends from Buildings.Fluid.HeatExchangers.CoolingTowers.BaseClasses.CoolingTower (Base class for cooling towers).

Parameters

Type	Name	Default	Description
replaceable package Medium		PartialMedium	Medium in the component
TemperatureDifference	TApp	2	Approach temperature difference [K]
Nominal condition
MassFlowRate	m_flow_nominal		Nominal mass flow rate [kg/s]
PressureDifference	dp_nominal		Pressure difference [Pa]
Assumptions
Boolean	allowFlowReversal	true	= false to simplify equations, assuming, but not enforcing, no flow reversal
Advanced
MassFlowRate	m_flow_small	1E-4*abs(m_flow_nominal)	Small mass flow rate for regularization of zero flow [kg/s]
Boolean	homotopyInitialization	true	= true, use homotopy method
Diagnostics
Boolean	show_T	false	= true, if actual temperature at port is computed
Flow resistance
Boolean	from_dp	false	= true, use m_flow = f(dp) else dp = f(m_flow)
Boolean	linearizeFlowResistance	false	= true, use linear relation between m_flow and dp for any flow rate
Real	deltaM	0.1	Fraction of nominal flow rate where flow transitions to laminar
Dynamics
Nominal condition
Time	tau	30	Time constant at nominal flow (if energyDynamics <> SteadyState) [s]
Equations
Dynamics	energyDynamics	Modelica.Fluid.Types.Dynamic...	Type of energy balance: dynamic (3 initialization options) or steady state
Dynamics	massDynamics	energyDynamics	Type of mass balance: dynamic (3 initialization options) or steady state
Initialization
AbsolutePressure	p_start	Medium.p_default	Start value of pressure [Pa]
Temperature	T_start	Medium.T_default	Start value of temperature [K]
MassFraction	X_start[Medium.nX]	Medium.X_default	Start value of mass fractions m_i/m [kg/kg]
ExtraProperty	C_start[Medium.nC]	fill(0, Medium.nC)	Start value of trace substances

Connectors

Type	Name	Description
FluidPort_a	port_a	Fluid connector a (positive design flow direction is from port_a to port_b)
FluidPort_b	port_b	Fluid connector b (positive design flow direction is from port_a to port_b)
output RealOutput	TLvg	Leaving water temperature
input RealInput	TAir	Entering air dry or wet bulb temperature [K]

Modelica definition

model FixedApproach "Cooling tower with constant approach temperature" extends Buildings.Fluid.HeatExchangers.CoolingTowers.BaseClasses.CoolingTower; parameter Modelica.SIunits.TemperatureDifference TApp(min=0, displayUnit="K") = 2 "Approach temperature difference"; Modelica.Blocks.Interfaces.RealInput TAir(min=0, unit="K") "Entering air dry or wet bulb temperature"; equation TAppAct=TApp; TAirHT=TAir; end FixedApproach;

Buildings.Fluid.HeatExchangers.CoolingTowers.YorkCalc

Cooling tower with variable speed using the York calculation for the approach temperature

Buildings.Fluid.HeatExchangers.CoolingTowers.YorkCalc

Information

Model for a steady-state or dynamic cooling tower with variable speed fan using the York calculation for the approach temperature at off-design conditions.

Thermal performance

To compute the thermal performance, this model takes as parameters the approach temperature, the range temperature and the inlet air wet bulb temperature at the design condition. Since the design mass flow rate (of the chiller condenser loop) is also a parameter, these parameters define the rejected heat.

For off-design conditions, the model uses the actual range temperature and a polynomial to compute the approach temperature for free convection and for forced convection, i.e., with the fan operating. The polynomial is valid for a York cooling tower. If the fan input signal y is below the minimum fan revolution yMin, then the cooling tower operates in free convection mode, otherwise it operates in the forced convection mode. For numerical reasons, this transition occurs in the range of y ∈ [0.9*yMin, yMin].

Fan power consumption

The fan power consumption at the design condition can be specified as follows:

The parameter fraPFan_nominal can be used to specify at the nominal conditions the fan power divided by the water flow rate. The default value is 275 Watts for a water flow rate of 0.15 kg/s.
The parameter PFan_nominal can be set to the fan power at nominal conditions. If a user does not set this parameter, then the fan power will be PFan_nominal = fraPFan_nominal * m_flow_nominal, where m_flow_nominal is the nominal water flow rate.

In the forced convection mode, the actual fan power is computed as PFan=fanRelPow(y) * PFan_nominal, where the default value for the fan relative power consumption at part load is fanRelPow(y)=y³. In the free convection mode, the fan power consumption is zero. For numerical reasons, the transition of fan power from the part load mode to zero power consumption in the free convection mode occurs in the range y ∈ [0.9*yMin, yMin].
To change the fan relative power consumption at part load in the forced convection mode, points of fan controls signal and associated relative power consumption can be specified. In between these points, the values are interpolated using cubic splines.

Comparison the cooling tower model of EnergyPlus

This model is similar to the model Cooling Tower:Variable Speed that is implemented in the EnergyPlus building energy simulation program version 6.0. The main differences are

Not implemented are the basin heater power consumption, and the make-up water usage.
The model has no built-in control to switch individual cells of the tower on or off. To switch cells on or off, use multiple instances of this model, and use your own control law to compute the input signal y.

Assumptions and limitations

This model requires a medium that has the same computation of the enthalpy as Buildings.Media.Water, which computes

h = c_p (T-T₀),

where h is the enthalpy, c_p = 4184 J/(kg K) is the specific heat capacity, T is the temperature in Kelvin and T₀ = 273.15 Kelvin. If this is not the case, the simulation will stop with an error message. The reason for this limitation is that as of January 2015, OpenModelica failed to translate the model if Medium.temperature() is used instead of Water.temperature().

References

EnergyPlus 2.0.0 Engineering Reference, April 9, 2007.

Extends from Buildings.Fluid.HeatExchangers.CoolingTowers.BaseClasses.CoolingTower (Base class for cooling towers).

Parameters

Type	Name	Default	Description
replaceable package Medium		PartialMedium	Medium in the component
Real	fraPFan_nominal	275/0.15	Fan power divided by water mass flow rate at design condition [W/(kg/s)]
Power	PFan_nominal	fraPFan_nominal*m_flow_nominal	Fan power [W]
fan	fanRelPow	fanRelPow( r_V = {0, 0....	Fan relative power consumption as a function of control signal, fanRelPow=P(y)/P(y=1)
Real	yMin	0.3	Minimum control signal until fan is switched off (used for smoothing between forced and free convection regime)
Real	fraFreCon	0.125	Fraction of tower capacity in free convection regime
Nominal condition
MassFlowRate	m_flow_nominal		Nominal mass flow rate [kg/s]
PressureDifference	dp_nominal		Pressure difference [Pa]
Temperature	TAirInWB_nominal	273.15 + 25.55	Design inlet air wet bulb temperature [K]
TemperatureDifference	TApp_nominal	3.89	Design approach temperature [K]
TemperatureDifference	TRan_nominal	5.56	Design range temperature (water in - water out) [K]
Assumptions
Boolean	allowFlowReversal	true	= false to simplify equations, assuming, but not enforcing, no flow reversal
Advanced
MassFlowRate	m_flow_small	1E-4*abs(m_flow_nominal)	Small mass flow rate for regularization of zero flow [kg/s]
Boolean	homotopyInitialization	true	= true, use homotopy method
Diagnostics
Boolean	show_T	false	= true, if actual temperature at port is computed
Flow resistance
Boolean	from_dp	false	= true, use m_flow = f(dp) else dp = f(m_flow)
Boolean	linearizeFlowResistance	false	= true, use linear relation between m_flow and dp for any flow rate
Real	deltaM	0.1	Fraction of nominal flow rate where flow transitions to laminar
Dynamics
Nominal condition
Time	tau	30	Time constant at nominal flow (if energyDynamics <> SteadyState) [s]
Equations
Dynamics	energyDynamics	Modelica.Fluid.Types.Dynamic...	Type of energy balance: dynamic (3 initialization options) or steady state
Dynamics	massDynamics	energyDynamics	Type of mass balance: dynamic (3 initialization options) or steady state
Initialization
AbsolutePressure	p_start	Medium.p_default	Start value of pressure [Pa]
Temperature	T_start	Medium.T_default	Start value of temperature [K]
MassFraction	X_start[Medium.nX]	Medium.X_default	Start value of mass fractions m_i/m [kg/kg]
ExtraProperty	C_start[Medium.nC]	fill(0, Medium.nC)	Start value of trace substances

Connectors

Type	Name	Description
FluidPort_a	port_a	Fluid connector a (positive design flow direction is from port_a to port_b)
FluidPort_b	port_b	Fluid connector b (positive design flow direction is from port_a to port_b)
output RealOutput	TLvg	Leaving water temperature
input RealInput	TAir	Entering air wet bulb temperature [K]
input RealInput	y	Fan control signal [1]

Modelica definition

model YorkCalc "Cooling tower with variable speed using the York calculation for the approach temperature" extends Buildings.Fluid.HeatExchangers.CoolingTowers.BaseClasses.CoolingTower; import cha = Buildings.Fluid.HeatExchangers.CoolingTowers.BaseClasses.Characteristics; parameter Modelica.SIunits.Temperature TAirInWB_nominal = 273.15+25.55 "Design inlet air wet bulb temperature"; parameter Modelica.SIunits.TemperatureDifference TApp_nominal(displayUnit="K") = 3.89 "Design approach temperature"; parameter Modelica.SIunits.TemperatureDifference TRan_nominal(displayUnit="K") = 5.56 "Design range temperature (water in - water out)"; parameter Real fraPFan_nominal(unit="W/(kg/s)") = 275/0.15 "Fan power divided by water mass flow rate at design condition"; parameter Modelica.SIunits.Power PFan_nominal = fraPFan_nominal*m_flow_nominal "Fan power"; parameter cha.fan fanRelPow( r_V = {0, 0.1, 0.3, 0.6, 1}, r_P = {0, 0.1^3, 0.3^3, 0.6^3, 1}) "Fan relative power consumption as a function of control signal, fanRelPow=P(y)/P(y=1)"; parameter Real yMin(min=0.01, max=1) = 0.3 "Minimum control signal until fan is switched off (used for smoothing between forced and free convection regime)"; parameter Real fraFreCon(min=0, max=1) = 0.125 "Fraction of tower capacity in free convection regime"; Modelica.Blocks.Interfaces.RealInput TAir( min=0, unit="K", displayUnit="degC") "Entering air wet bulb temperature"; Buildings.Fluid.HeatExchangers.CoolingTowers.Correlations.BoundsYorkCalc bou "Bounds for correlation"; Modelica.Blocks.Interfaces.RealInput y(unit="1") "Fan control signal"; Modelica.SIunits.TemperatureDifference TRan(nominal=1, displayUnit="K") "Range temperature"; Modelica.SIunits.MassFraction FRWat "Ratio actual over design water mass flow ratio"; Modelica.SIunits.MassFraction FRAir "Ratio actual over design air mass flow ratio"; Modelica.SIunits.Power PFan "Fan power"; protected package Water = Buildings.Media.Water "Medium package for water"; parameter Real FRWat0(min=0, start=1, fixed=false) "Ratio actual over design water mass flow ratio at nominal condition"; parameter Modelica.SIunits.Temperature TWatIn0(fixed=false) "Water inlet temperature at nominal condition"; parameter Modelica.SIunits.Temperature TWatOut_nominal(fixed=false) "Water outlet temperature at nominal condition"; parameter Modelica.SIunits.MassFlowRate mRef_flow(min=0, start=m_flow_nominal, fixed=false) "Reference water flow rate"; Modelica.SIunits.TemperatureDifference dTMax(nominal=1, displayUnit="K") "Maximum possible temperature difference"; Modelica.SIunits.TemperatureDifference TAppCor(min=0, nominal=1, displayUnit="K") "Approach temperature for forced convection"; Modelica.SIunits.TemperatureDifference TAppFreCon(min=0, nominal=1, displayUnit="K") "Approach temperature for free convection"; final parameter Real fanRelPowDer[size(fanRelPow.r_V,1)](each fixed=false) "Coefficients for fan relative power consumption as a function of control signal"; Modelica.SIunits.Temperature T_a "Temperature in port_a"; Modelica.SIunits.Temperature T_b "Temperature in port_b"; initial equation TWatOut_nominal = TAirInWB_nominal + TApp_nominal; TRan_nominal = TWatIn0 - TWatOut_nominal; // by definition of the range temp. TApp_nominal = Buildings.Fluid.HeatExchangers.CoolingTowers.Correlations.yorkCalc( TRan=TRan_nominal, TWetBul=TAirInWB_nominal, FRWat=FRWat0, FRAir=1); // this will be solved for FRWat0 mRef_flow = m_flow_nominal/FRWat0; // Derivatives for spline that interpolates the fan relative power fanRelPowDer = Buildings.Utilities.Math.Functions.splineDerivatives( x=fanRelPow.r_V, y=fanRelPow.r_P, ensureMonotonicity=Buildings.Utilities.Math.Functions.isMonotonic(x=fanRelPow.r_P, strict=false)); // Check validity of relative fan power consumption at y=yMin and y=1 assert(cha.normalizedPower(per=fanRelPow, r_V=yMin, d=fanRelPowDer) > -1E-4, "The fan relative power consumption must be non-negative for y=0." + "\n Obtained fanRelPow(0) = " + String(cha.normalizedPower(per=fanRelPow, r_V=yMin, d=fanRelPowDer)) + "\n You need to choose different values for the parameter fanRelPow."); assert(abs(1-cha.normalizedPower(per=fanRelPow, r_V=1, d=fanRelPowDer))<1E-4, "The fan relative power consumption must be one for y=1." + "\n Obtained fanRelPow(1) = " + String(cha.normalizedPower(per=fanRelPow, r_V=1, d=fanRelPowDer)) + "\n You need to choose different values for the parameter fanRelPow." + "\n To increase the fan power, change fraPFan_nominal or PFan_nominal."); // Check that a medium is used that has the same definition of enthalpy vs. temperature. // This is needed because below, T_a=Water.temperature needed to be hard-coded to use // Water.* instead of Medium.* in the function calls due to a bug in OpenModelica. assert(abs(Medium.specificEnthalpy_pTX(p=101325, T=273.15, X=Medium.X_default) - Water.specificEnthalpy_pTX(p=101325, T=273.15, X=Medium.X_default)) < 1E-5 and abs(Medium.specificEnthalpy_pTX(p=101325, T=293.15, X=Medium.X_default) - Water.specificEnthalpy_pTX(p=101325, T=293.15, X=Medium.X_default)) < 1E-5, "The selected medium has an enthalpy computation that is not consistent with the one in Buildings.Media.Water Use a different medium, such as Buildings.Media.Water."); equation // States at the inlet and outlet if allowFlowReversal then if homotopyInitialization then T_a=Water.temperature(Water.setState_phX(p=port_a.p, h=homotopy(actual=actualStream(port_a.h_outflow), simplified=inStream(port_a.h_outflow)), X=homotopy(actual=actualStream(port_a.Xi_outflow), simplified=inStream(port_a.Xi_outflow)))); T_b=Water.temperature(Water.setState_phX(p=port_b.p, h=homotopy(actual=actualStream(port_b.h_outflow), simplified=port_b.h_outflow), X=homotopy(actual=actualStream(port_b.Xi_outflow), simplified=port_b.Xi_outflow))); else T_a=Water.temperature(Water.setState_phX(p=port_a.p, h=actualStream(port_a.h_outflow), X=actualStream(port_a.Xi_outflow))); T_b=Water.temperature(Water.setState_phX(p=port_b.p, h=actualStream(port_b.h_outflow), X=actualStream(port_b.Xi_outflow))); end if; // homotopyInitialization else // reverse flow not allowed T_a=Water.temperature(Water.setState_phX(p=port_a.p, h=inStream(port_a.h_outflow), X=inStream(port_a.Xi_outflow))); T_b=Water.temperature(Water.setState_phX(p=port_b.p, h=inStream(port_b.h_outflow), X=inStream(port_b.Xi_outflow))); end if; // Air temperature used for the heat transfer TAirHT=TAir; // Range temperature TRan = T_a - T_b; // Fractional mass flow rates FRWat = m_flow/mRef_flow; FRAir = y; TAppCor = Buildings.Fluid.HeatExchangers.CoolingTowers.Correlations.yorkCalc( TRan=TRan, TWetBul=TAir, FRWat=FRWat, FRAir=Buildings.Utilities.Math.Functions.smoothMax( x1=FRWat/bou.liqGasRat_max, x2=FRAir, deltaX=0.01)); dTMax = T_a - TAir; TAppFreCon = (1-fraFreCon) * dTMax + fraFreCon * Buildings.Fluid.HeatExchangers.CoolingTowers.Correlations.yorkCalc( TRan=TRan, TWetBul=TAir, FRWat=FRWat, FRAir=1); // Actual approach temperature and fan power consumption, // which depends on forced vs. free convection. // The transition is for y in [yMin-yMin/10, yMin] [TAppAct, PFan] = Buildings.Utilities.Math.Functions.spliceFunction( pos=[TAppCor, cha.normalizedPower( per=fanRelPow, r_V=y, d=fanRelPowDer) * PFan_nominal], neg=[TAppFreCon, 0], x=y-yMin+yMin/20, deltax=yMin/20); end YorkCalc;