Buildings.Fluid.Storage

Information

Package Content

Name	Description
ExpansionVessel	Pressure expansion vessel with fixed gas cushion
Stratified	Model of a stratified tank for thermal energy storage
StratifiedEnhanced	Stratified tank model with enhanced discretization
Examples	Collection of models that illustrate model use and test models
BaseClasses	Package with base classes for tank models

Buildings.Fluid.Storage.ExpansionVessel

Information

This is a model of a pressure expansion vessel. The vessel has a fixed total volume. A fraction of the volume is occupied by a fixed mass of gas, and the other fraction is occupied by the liquid that flows through the port. The pressure p in the vessel is

 VGas0 * p_start = (VTot-VLiquid) * p

Optionally, a heat port can be activated by setting use_HeatTransfer=true. This heat port connects directly to the liquid. The gas does not participate in the energy balance.

Extends from Buildings.Fluid.Interfaces.PartialLumpedVolume (Lumped volume with mass and energy balance).

Parameters

Type	Name	Default	Description
replaceable package Medium		PartialMedium	Medium in the component
Volume	fluidVolume	VLiq	Volume [m3]
Volume	VTot		Total volume of vessel (water and gas side) [m3]
Volume	VGas0	VTot/2	Initial volume of gas [m3]
Pressure	pMax	5E5	Maximum pressure before simulation stops with an error [Pa]
Assumptions
Dynamics
Dynamics	energyDynamics	Modelica.Fluid.Types.Dynamic...	Formulation of energy balance
Dynamics	massDynamics	Modelica.Fluid.Types.Dynamic...	Formulation of mass balance
Dynamics	substanceDynamics	Modelica.Fluid.Types.Dynamic...	Formulation of substance balance
Dynamics	traceDynamics	Modelica.Fluid.Types.Dynamic...	Formulation of trace substance balance
Heat transfer
Boolean	use_HeatTransfer	false	= true to use the HeatTransfer model
Initialization
AbsolutePressure	p_start	system.p_start	Start value of pressure [Pa]
Boolean	use_T_start	true	= true, use T_start, otherwise h_start
Temperature	T_start	if use_T_start then system.T...	Start value of temperature [K]
SpecificEnthalpy	h_start	if use_T_start then Medium.s...	Start value of specific enthalpy [J/kg]
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 port
HeatPort_a	heatPort

Modelica definition

model ExpansionVessel 
  "Pressure expansion vessel with fixed gas cushion"
 extends Buildings.Fluid.Interfaces.PartialLumpedVolume(
   fluidVolume = VLiq,
   energyDynamics=Modelica.Fluid.Types.Dynamics.DynamicFreeInitial,
    massDynamics=Modelica.Fluid.Types.Dynamics.FixedInitial,
    substanceDynamics=Modelica.Fluid.Types.Dynamics.FixedInitial,
    traceDynamics=Modelica.Fluid.Types.Dynamics.FixedInitial);

 parameter Modelica.SIunits.Volume VTot 
    "Total volume of vessel (water and gas side)";
 parameter Modelica.SIunits.Volume VGas0 = VTot/2 "Initial volume of gas";

  Modelica.Fluid.Interfaces.FluidPort_a port_a(redeclare package Medium = 
        Medium) "Fluid port";

  // Heat transfer through boundary
  parameter Boolean use_HeatTransfer = false 
    "= true to use the HeatTransfer model";
  parameter Modelica.SIunits.Pressure pMax = 5E5 
    "Maximum pressure before simulation stops with an error";
  replaceable model HeatTransfer = 
      Modelica.Fluid.Vessels.BaseClasses.HeatTransfer.IdealHeatTransfer (
        surfaceAreas={4*Modelica.Constants.pi*(3/4*VTot/Modelica.Constants.pi)^(2/3)}) 
    constrainedby 
    Modelica.Fluid.Vessels.BaseClasses.HeatTransfer.PartialVesselHeatTransfer 
    "Wall heat transfer";
  HeatTransfer heatTransfer(
    redeclare final package Medium = Medium,
    final n=1,
    final states = {medium.state},
    final use_k = use_HeatTransfer);
  Modelica.Thermal.HeatTransfer.Interfaces.HeatPort_a heatPort if use_HeatTransfer;

protected 
 Modelica.SIunits.Volume VLiq "Volume of liquid in the vessel";
equation 
  assert(medium.p < pMax, "Pressure exceeds maximum pressure.\n" +
       "You may need to increase VTot of the ExpansionVessel.");
  // Water content and pressure
  port_a.p * (VTot-fluidVolume) = p_start * VGas0;
  port_a.p = medium.p;

  // Balance equations
  mb_flow = port_a.m_flow;
  mbXi_flow = port_a.m_flow * actualStream(port_a.Xi_outflow) 
    "Component mass flow";
  mbC_flow  = port_a.m_flow * actualStream(port_a.C_outflow) 
    "Trace substance mass flow";
  Hb_flow   = port_a.m_flow * actualStream(port_a.h_outflow) "Enthalpy flow";
  Qb_flow   = heatTransfer.Q_flows[1];
  Wb_flow   = 0;

  // Outflowing quantities
  port_a.h_outflow  = medium.h;
  port_a.Xi_outflow = medium.Xi;
  port_a.C_outflow  = C;
end ExpansionVessel;

Buildings.Fluid.Storage.Stratified

Information

This is a model of a stratified storage tank. The tank uses several volumes to model the stratification. Heat conduction is modeled between the volumes through the fluid, and between the volumes and the ambient. The port heaPorVol may be used to connect a temperature sensor that measures the fluid temperature of an individual volume. It may also be used to add heat to individual volumes.

The tank has nSeg fluid volumes. The top volume has the index 1. Thus, to add a heating element to the bottom element, connect a heat input to heaPorVol[nSeg].

The heat ports outside the tank insulation can be used to specify an ambient temperature. Leave these ports unconnected to force adiabatic boundary conditions. Note, however, that all heat conduction elements through the tank wall (but not the top and bottom) are connected to the heat port heaPorSid. Thus, not connecting heaPorSid means an adiabatic boundary condition in the sense that heaPorSid.Q_flow = 0. This, however, still allows heat to flow through the tank walls, modelled by conWal, from one fluid volume to another one.

For a model with enhanced stratification, use Buildings.Fluid.Storage.StratifiedEnhanced.

Extends from Buildings.Fluid.Interfaces.PartialStaticTwoPortInterface (Partial model transporting fluid between two ports without storing mass or energy), Buildings.BaseClasses.BaseIcon (Base icon).

Parameters

Type	Name	Default	Description
replaceable package Medium		PartialMedium	Medium in the component
Volume	VTan		Tank volume [m3]
Length	hTan		Height of tank (without insulation) [m]
Length	dIns		Thickness of insulation [m]
ThermalConductivity	kIns	0.04	Specific heat conductivity of insulation [W/(m.K)]
Integer	nSeg	2	Number of volume segments
Time	tau	1	Time constant for mixing due to temperature inversion [s]
Nominal condition
MassFlowRate	m_flow_nominal		Nominal mass flow rate [kg/s]
Initialization
MassFlowRate	m_flow.start	0	Mass flow rate from port_a to port_b (m_flow > 0 is design flow direction) [kg/s]
Pressure	dp.start	0	Pressure difference between port_a and port_b [Pa]
Assumptions
Boolean	allowFlowReversal	system.allowFlowReversal	= true to allow flow reversal, false restricts to design direction (port_a -> port_b)
Dynamics
Dynamics	energyDynamics	system.energyDynamics	Formulation of energy balance
Dynamics	massDynamics	system.massDynamics	Formulation of mass balance
Advanced
MassFlowRate	m_flow_small	1E-4*m_flow_nominal	Small mass flow rate for regularization of zero flow [kg/s]
Diagnostics
Boolean	show_V_flow	false	= true, if volume flow rate at inflowing port is computed
Initialization
AbsolutePressure	p_a_start	p_start	Guess value for inlet pressure [Pa]
AbsolutePressure	p_b_start	p_start	Guess value for outlet pressure [Pa]
AbsolutePressure	p_start	system.p_start	Start value of pressure [Pa]
Boolean	use_T_start	true	= true, use T_start, otherwise h_start
Temperature	T_start	if use_T_start then system.T...	Start value of temperature [K]
SpecificEnthalpy	h_start	if use_T_start then Medium.s...	Start value of specific enthalpy [J/kg]
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)
replaceable package Medium
HeatPort_a	heaPorVol[nSeg]	Heat port of fluid volumes
HeatPort_a	heaPorSid	Heat port tank side (outside insulation)
HeatPort_a	heaPorTop	Heat port tank top (outside insulation)
HeatPort_a	heaPorBot	Heat port tank bottom (outside insulation). Leave unconnected for adiabatic condition
output RealOutput	Ql_flow	Heat loss of tank (positive if heat flows from tank to ambient)

Modelica definition

model Stratified 
  "Model of a stratified tank for thermal energy storage"

  extends Buildings.Fluid.Interfaces.PartialStaticTwoPortInterface(final 
      p_a_start =                                                                     p_start,
                                                                    final 
      p_b_start =                                                                     p_start);
  extends Buildings.BaseClasses.BaseIcon;
  replaceable package Medium = 
      Modelica.Media.Interfaces.PartialSimpleMedium;
  import Modelica.Fluid.Types;
  import Modelica.Fluid.Types.Dynamics;
  parameter Modelica.SIunits.Volume VTan "Tank volume";
  parameter Modelica.SIunits.Length hTan "Height of tank (without insulation)";
  parameter Modelica.SIunits.Length dIns "Thickness of insulation";
  parameter Modelica.SIunits.ThermalConductivity kIns = 0.04 
    "Specific heat conductivity of insulation";
  parameter Integer nSeg(min=2) = 2 "Number of volume segments";

  ////////////////////////////////////////////////////////////////////
  // Assumptions
  parameter Types.Dynamics energyDynamics=system.energyDynamics 
    "Formulation of energy balance";
  parameter Types.Dynamics massDynamics=system.massDynamics 
    "Formulation of mass balance";

  // Initialization
  parameter Medium.AbsolutePressure p_start = system.p_start 
    "Start value of pressure";
  parameter Boolean use_T_start = true "= true, use T_start, otherwise h_start";
  parameter Medium.Temperature T_start=
    if use_T_start then system.T_start else Medium.temperature_phX(p_start,h_start,X_start) 
    "Start value of temperature";
  parameter Medium.SpecificEnthalpy h_start=
    if use_T_start then Medium.specificEnthalpy_pTX(p_start, T_start, X_start) else Medium.h_default 
    "Start value of specific enthalpy";
  parameter Medium.MassFraction X_start[Medium.nX] = Medium.X_default 
    "Start value of mass fractions m_i/m";
  parameter Medium.ExtraProperty C_start[Medium.nC](
       quantity=Medium.extraPropertiesNames)=fill(0, Medium.nC) 
    "Start value of trace substances";

  ////////////////////////////////////////////////////////////////////

  MixingVolumes.MixingVolume[nSeg] vol(
    redeclare each package Medium = Medium,
    each energyDynamics=energyDynamics,
    each massDynamics=massDynamics,
    each p_start=p_start,
    each use_T_start=use_T_start,
    each T_start=T_start,
    each h_start=h_start,
    each X_start=X_start,
    each C_start=C_start,
    each V=VTan/nSeg,
    each nPorts=nPorts,
    each m_flow_small = m_flow_small,
    each use_HeatTransfer=true,
    redeclare each model HeatTransfer = 
        Modelica.Fluid.Vessels.BaseClasses.HeatTransfer.IdealHeatTransfer) 
    "Tank segment";
  Sensors.EnthalpyFlowRate hA_flow(redeclare package Medium = Medium) 
    "Enthalpy flow rate at port a";
  Sensors.EnthalpyFlowRate[nSeg-1] hVol_flow(redeclare package Medium = Medium);
  Sensors.EnthalpyFlowRate hB_flow(redeclare package Medium = Medium) 
    "Enthalpy flow rate at port b";
  BaseClasses.Buoyancy buo(
    redeclare package Medium = Medium,
    V=VTan,
    nSeg=nSeg,
    tau=tau) "Model to prevent unstable tank stratification";
  parameter Modelica.SIunits.Time tau=1 
    "Time constant for mixing due to temperature inversion";
  Modelica.Thermal.HeatTransfer.Components.ThermalConductor[
                                                 nSeg - 1] conFlu(each G=
        conFluSeg) "Thermal conductance in fluid between the segments";
  Modelica.Thermal.HeatTransfer.Components.ThermalConductor[
                                                 nSeg] conWal(
     each G=2*Modelica.Constants.pi*kIns*hSeg/Modelica.Math.log((rTan+dIns)/rTan)) 
    "Thermal conductance through tank wall";
  Modelica.Thermal.HeatTransfer.Components.ThermalConductor conTop(
     G=conTopSeg) "Thermal conductance through tank top";
  Modelica.Thermal.HeatTransfer.Components.ThermalConductor conBot(
     G=conTopSeg) "Thermal conductance through tank bottom";
  Modelica.Thermal.HeatTransfer.Interfaces.HeatPort_a[nSeg] heaPorVol 
    "Heat port of fluid volumes";
  Modelica.Thermal.HeatTransfer.Interfaces.HeatPort_a heaPorSid 
    "Heat port tank side (outside insulation)";
  Modelica.Thermal.HeatTransfer.Sensors.HeatFlowSensor heaFloSid[
                                                         nSeg] 
    "Heat flow at wall of tank (outside insulation)";
  Modelica.Thermal.HeatTransfer.Interfaces.HeatPort_a heaPorTop 
    "Heat port tank top (outside insulation)";
  Modelica.Thermal.HeatTransfer.Sensors.HeatFlowSensor heaFloTop 
    "Heat flow at top of tank (outside insulation)";
  Modelica.Thermal.HeatTransfer.Interfaces.HeatPort_a heaPorBot 
    "Heat port tank bottom (outside insulation). Leave unconnected for adiabatic condition";
  Modelica.Thermal.HeatTransfer.Sensors.HeatFlowSensor heaFloBot 
    "Heat flow at bottom of tank (outside insulation)";
  Modelica.Blocks.Interfaces.RealOutput Ql_flow 
    "Heat loss of tank (positive if heat flows from tank to ambient)";

protected 
  constant Integer nPorts = 2 "Number of ports of volume";
  parameter Modelica.SIunits.Length hSeg = hTan / nSeg 
    "Height of a tank segment";
  parameter Modelica.SIunits.Area ATan = VTan/hTan 
    "Tank cross-sectional area (without insulation)";
  parameter Modelica.SIunits.Length rTan = sqrt(ATan/Modelica.Constants.pi) 
    "Tank diameter (without insulation)";
  parameter Modelica.SIunits.ThermalConductance conFluSeg = ATan*Medium.lambda_const/hSeg 
    "Thermal conductance between fluid volumes";
  parameter Modelica.SIunits.ThermalConductance conTopSeg = ATan*kIns/dIns 
    "Thermal conductance from center of top (or bottom) volume through tank insulation at top (or bottom)";

protected 
  Modelica.Blocks.Routing.Multiplex3 mul(
    n1=1,
    n2=nSeg,
    n3=1);
  Modelica.Blocks.Math.Sum sum1(nin=nSeg + 2);
public 
  Modelica.Thermal.HeatTransfer.Components.ThermalCollector theCol(m=nSeg) 
    "Connector to assign multiple heat ports to one heat port";
equation 
  connect(hA_flow.port_b, vol[1].ports[1]);
  connect(vol[nSeg].ports[2], hB_flow.port_a);
  connect(hB_flow.port_b, port_b);
  for i in 1:(nSeg-1) loop

  connect(vol[i].ports[2], hVol_flow[i].port_a);
  connect(hVol_flow[i].port_b, vol[i+1].ports[1]);
  end for;
  connect(port_a, hA_flow.port_a);
  connect(buo.heatPort, vol.heatPort);
  for i in 1:nSeg-1 loop
  // heat conduction between fluid nodes
     connect(vol[i].heatPort, conFlu[i].port_a);
    connect(vol[i+1].heatPort, conFlu[i].port_b);
  end for;
  connect(vol[1].heatPort, conTop.port_a);
  connect(vol.heatPort, conWal.port_a);
  connect(conBot.port_a, vol[nSeg].heatPort);
  connect(vol.heatPort, heaPorVol);
  connect(conWal.port_b, heaFloSid.port_a);
  for i in 1:nSeg loop

  end for;

  connect(conTop.port_b, heaFloTop.port_a);
  connect(conBot.port_b, heaFloBot.port_a);
  connect(heaFloTop.port_b, heaPorTop);
  connect(heaFloBot.port_b, heaPorBot);
  connect(heaFloTop.Q_flow, mul.u1[1]);
  connect(heaFloSid.Q_flow, mul.u2);
  connect(heaFloBot.Q_flow, mul.u3[1]);
  connect(mul.y, sum1.u);
  connect(sum1.y, Ql_flow);
  connect(heaFloSid.port_b, theCol.port_a);
  connect(theCol.port_b, heaPorSid);
end Stratified;

Buildings.Fluid.Storage.StratifiedEnhanced

Information

This is a model of a stratified storage tank for thermal energy storage. The model is identical to Buildings.Fluid.Storage.Stratified, except that it adds a correction that reduces the numerical dissipation. The correction is identical to the one described by Wischhusen (2006).

References

Wischhusen Stefan, An Enhanced Discretization Method for Storage Tank Models within Energy Systems, Modelica Conference, Vienna, Austria, September 2006.

Extends from Stratified (Model of a stratified tank for thermal energy storage).

Parameters

Type	Name	Default	Description
replaceable package Medium		PartialMedium	Medium in the component
Volume	VTan		Tank volume [m3]
Length	hTan		Height of tank (without insulation) [m]
Length	dIns		Thickness of insulation [m]
ThermalConductivity	kIns	0.04	Specific heat conductivity of insulation [W/(m.K)]
Integer	nSeg	2	Number of volume segments
Time	tau	1	Time constant for mixing due to temperature inversion [s]
Real	a	1E-4	Tuning factor. a=0 is equivalent to not using this model
Nominal condition
MassFlowRate	m_flow_nominal		Nominal mass flow rate [kg/s]
Initialization
MassFlowRate	m_flow.start	0	Mass flow rate from port_a to port_b (m_flow > 0 is design flow direction) [kg/s]
Pressure	dp.start	0	Pressure difference between port_a and port_b [Pa]
Assumptions
Boolean	allowFlowReversal	system.allowFlowReversal	= true to allow flow reversal, false restricts to design direction (port_a -> port_b)
Dynamics
Dynamics	energyDynamics	system.energyDynamics	Formulation of energy balance
Dynamics	massDynamics	system.massDynamics	Formulation of mass balance
Advanced
MassFlowRate	m_flow_small	1E-4*m_flow_nominal	Small mass flow rate for regularization of zero flow [kg/s]
Diagnostics
Boolean	show_V_flow	false	= true, if volume flow rate at inflowing port is computed
Initialization
AbsolutePressure	p_start	system.p_start	Start value of pressure [Pa]
Boolean	use_T_start	true	= true, use T_start, otherwise h_start
Temperature	T_start	if use_T_start then system.T...	Start value of temperature [K]
SpecificEnthalpy	h_start	if use_T_start then Medium.s...	Start value of specific enthalpy [J/kg]
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)
HeatPort_a	heaPorVol[nSeg]	Heat port of fluid volumes
HeatPort_a	heaPorSid	Heat port tank side (outside insulation)
HeatPort_a	heaPorTop	Heat port tank top (outside insulation)
HeatPort_a	heaPorBot	Heat port tank bottom (outside insulation). Leave unconnected for adiabatic condition
output RealOutput	Ql_flow	Heat loss of tank (positive if heat flows from tank to ambient)

Modelica definition

model StratifiedEnhanced 
  "Stratified tank model with enhanced discretization"
  extends Stratified(nPorts=3);
  BaseClasses.Stratifier str(
    redeclare package Medium = Medium,
    nSeg=nSeg,
    a=a) "Model to reduce numerical dissipation";
  parameter Real a=1E-4 
    "Tuning factor. a=0 is equivalent to not using this model";
  Modelica.Blocks.Sources.RealExpression mTan_flow(y=port_a.m_flow) 
    "Mass flow rate at port a";
equation 
  connect(vol[1:nSeg].ports[3], str.fluidPort[2:nSeg+1]);
  connect(hA_flow.H_flow, str.H_flow[1]);
  connect(hVol_flow[1:nSeg-1].H_flow, str.H_flow[2:nSeg]);
  connect(hB_flow.H_flow, str.H_flow[nSeg + 1]);
  connect(str.heatPort, vol.heatPort);
  connect(port_a, str.fluidPort[1]);
  connect(port_b, str.fluidPort[nSeg + 2]);
  connect(mTan_flow.y, str.m_flow);
end StratifiedEnhanced;

Buildings.Fluid.Storage.ExpansionVessel.HeatTransfer

Parameters

Type	Name	Default	Description
Area	surfaceAreas[n]	{4*Modelica.Constants.pi*(3/...	Heat transfer areas [m2]
Ambient
CoefficientOfHeatTransfer	k	0	Heat transfer coefficient to ambient [W/(m2.K)]
Temperature	T_ambient	system.T_ambient	Ambient temperature [K]
Internal Interface
replaceable package Medium		PartialMedium	Medium in the component
Integer	n	1	Number of heat transfer segments
Boolean	use_k	false	= true to use k value for thermal isolation

Connectors

Type	Name	Description
HeatPorts_a	heatPorts[n]	Heat port to component boundary

Modelica definition

replaceable model HeatTransfer = 
    Modelica.Fluid.Vessels.BaseClasses.HeatTransfer.IdealHeatTransfer (
      surfaceAreas={4*Modelica.Constants.pi*(3/4*VTot/Modelica.Constants.pi)^(2/3)}) 
  constrainedby 
  Modelica.Fluid.Vessels.BaseClasses.HeatTransfer.PartialVesselHeatTransfer 
  "Wall heat transfer";