modelCarnot

Extends from IBPSA.Fluid.Interfaces.PartialFourPortInterface.

Information

This is the base class for the Carnot chiller and the Carnot heat pump whose coefficient of performance COP changes with temperatures in the same way as the Carnot efficiency changes.

The model allows to either specify the Carnot effectivness η_Carnot,0, or a COP₀ at the nominal conditions, together with the evaporator temperature T_eva,0 and the condenser temperature T_con,0, in which case the model computes the Carnot effectivness as

η_Carnot,0 = COP₀ ⁄ (T_use,0 ⁄ (T_con,0-T_eva,0)),

where T_use is the temperature of the the useful heat, e.g., the evaporator temperature for a chiller or the condenser temperature for a heat pump.

The COP is computed as the product

COP = η_Carnot,0 COP_Carnot η_PL,

where COP_Carnot is the Carnot efficiency and η_PL is the part load efficiency, expressed using a polynomial. This polynomial has the form

η_PL = a₁ + a₂ y + a₃ y² + ...

where y ∈ [0, 1] is either the part load for cooling in case of a chiller, or the part load of heating in case of a heat pump, and the coefficients a_i are declared by the parameter a.

Implementation

To make this base class applicable to chiller or heat pumps, it uses the boolean constant COP_is_for_cooling. Depending on its value, the equations for the coefficient of performance and the part load ratio are set up.

Parameters

Type	Name	Default	Description
Nominal condition
Modelica.SIunits.HeatFlowRate	QEva_flow_nominal		Nominal cooling heat flow rate (QEva_flow_nominal < 0)
Modelica.SIunits.HeatFlowRate	QCon_flow_nominal		Nominal heating flow rate
Modelica.SIunits.TemperatureDifference	dTEva_nominal	-10	Temperature difference evaporator outlet-inlet
Modelica.SIunits.TemperatureDifference	dTCon_nominal	10	Temperature difference condenser outlet-inlet
Modelica.SIunits.Pressure	dp1_nominal		Pressure difference over condenser
Modelica.SIunits.Pressure	dp2_nominal		Pressure difference over evaporator
Efficiency
Boolean	use_eta_Carnot_nominal	true	Set to true to use Carnot effectiveness etaCarnot_nominal rather than COP_nominal
Real	etaCarnot_nominal	COP_nominal/(TUseAct_nominal/(TCon_nominal + TAppCon_nominal - (TEva_nominal - TAppEva_nominal)))	Carnot effectiveness (=COP/COP_Carnot) used if use_eta_Carnot_nominal = true
Real	COP_nominal	etaCarnot_nominal*TUseAct_nominal/(TCon_nominal + TAppCon_nominal - (TEva_nominal - TAppEva_nominal))	Coefficient of performance at TEva_nominal and TCon_nominal, used if use_eta_Carnot_nominal = false
Modelica.SIunits.Temperature	TCon_nominal	303.15	Condenser temperature used to compute COP_nominal if use_eta_Carnot_nominal=false
Modelica.SIunits.Temperature	TEva_nominal	278.15	Evaporator temperature used to compute COP_nominal if use_eta_Carnot_nominal=false
Real[:]	a	{1}	Coefficients for efficiency curve (need p(a=a, yPL=1)=1)
Modelica.SIunits.TemperatureDifference	TAppCon_nominal	if cp1_default < 1500 then 5 else 2	Temperature difference between refrigerant and working fluid outlet in condenser
Modelica.SIunits.TemperatureDifference	TAppEva_nominal	if cp2_default < 1500 then 5 else 2	Temperature difference between refrigerant and working fluid outlet in evaporator
Advanced
Boolean	homotopyInitialization	true	= true, use homotopy method
Flow resistance › Condenser
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
Flow resistance › Evaporator
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 › Condenser
Modelica.SIunits.Time	tau1	60	Time constant at nominal flow rate (used if energyDynamics1 <> Modelica.Fluid.Types.Dynamics.SteadyState)
Modelica.SIunits.Temperature	T1_start	Medium1.T_default	Initial or guess value of set point
Dynamics › Evaporator
Modelica.SIunits.Time	tau2	60	Time constant at nominal flow rate (used if energyDynamics2 <> Modelica.Fluid.Types.Dynamics.SteadyState)
Modelica.SIunits.Temperature	T2_start	Medium2.T_default	Initial or guess value of set point
Dynamics › Evaporator and condenser
Modelica.Fluid.Types.Dynamics	energyDynamics	Modelica.Fluid.Types.Dynamics.SteadyState	Type of energy balance: dynamic (3 initialization options) or steady state

Components

Type	Name	Default	Description
Modelica.Blocks.Interfaces.RealOutput	QCon_flow		Actual heating heat flow rate added to fluid 1
Modelica.Blocks.Interfaces.RealOutput	P		Electric power consumed by compressor
Modelica.Blocks.Interfaces.RealOutput	QEva_flow		Actual cooling heat flow rate removed from fluid 2
Real	yPL	if COP_is_for_cooling then QEva_flow/QEva_flow_nominal else QCon_flow/QCon_flow_nominal	Part load ratio
Real	etaPL	if evaluate_etaPL then 1 else IBPSA.Utilities.Math.Functions.polynomial(a = a, x = yPL)	Efficiency due to part load (etaPL(yPL=1)=1)
Real	COP	etaCarnot_nominal_internalCOPCaretaPL	Coefficient of performance
Real	COPCar	TUseAct/IBPSA.Utilities.Math.Functions.smoothMax(x1 = 1, x2 = TConAct - TEvaAct, deltaX = 0.25)	Carnot efficiency
Modelica.SIunits.Temperature	TConAct	Medium1.temperature(staB1) + QCon_flow/QCon_flow_nominal*TAppCon_nominal	Condenser temperature used to compute efficiency, taking into account pinch temperature between fluid and refrigerant
Modelica.SIunits.Temperature	TEvaAct	Medium2.temperature(staB2) - QEva_flow/QEva_flow_nominal*TAppEva_nominal	Evaporator temperature used to compute efficiency, taking into account pinch temperature between fluid and refrigerant

Revisions

June 16, 2017, by Michael Wetter:
Added temperature difference between fluids in condenser and evaporator for computation of nominal COP and effectiveness.
This is for #698.
March 28, 2017, by Felix Buenning:
Added temperature difference between fluids in condenser and evaporator. The difference is based on discussions with Emerson Climate Technologies.
This is for #698.
January 2, 2017, by Filip Jorissen:
Removed option for choosing what temperature should be used to compute the Carnot efficiency. This is for issue 497.
January 26, 2016, by Michael Wetter:
First implementation of this base class.