Buildings.Fluid.HeatPumps

Package with models for heat pumps

Information

This package contains component models for heat pumps.

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

Package Content

Name	Description
Carnot_TCon	Heat pump with prescribed condenser leaving temperature and performance curve adjusted based on Carnot efficiency
Carnot_y	Reversible heat pump with performance curve adjusted based on Carnot efficiency
EquationFitReversible	Model for a reversable heat pump based on the equation fit method
ReciprocatingWaterToWater	Model for a reciprocating water to water heat pump
ScrollWaterToWater	Model for a scroll water to water heat pump
Calibration	Package for calibration of heat pump models
Compressors	Package with compressor models
Data	Package with model parameters for heat pumps
Examples	Collection of models that illustrate model use and test models
Validation	Collection of models that validate the heat pump models
BaseClasses	Package with base classes for Buildings.Fluid.HeatPumps

Buildings.Fluid.HeatPumps.Carnot_TCon

Heat pump with prescribed condenser leaving temperature and performance curve adjusted based on Carnot efficiency

Information

This is a model of a heat pump whose coefficient of performance COP changes with temperatures in the same way as the Carnot efficiency changes. The control input is the setpoint of the condenser leaving temperature, which is met exactly at steady state if the heat pump has sufficient capacity.

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_con,0 ⁄ (T_con,0-T_eva,0)).

The heat pump 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 a polynomial in heating part load ratio y_PL that can be used to take into account a change in COP at part load conditions. This polynomial has the form

η_PL = a₁ + a₂ y_PL + a₃ y_PL² + ...

where the coefficients a_i are declared by the parameter a.

On the Dynamics tag, the model can be parametrized to compute a transient or steady-state response. The transient response of the model is computed using a first order differential equation for the evaporator and condenser fluid volumes. The heat pump outlet temperatures are equal to the temperatures of these lumped volumes.

Typical use and important parameters

When using this component, make sure that the condenser has sufficient mass flow rate. Based on the evaporator mass flow rate, temperature difference and the efficiencies, the model computes how much heat will be removed by to the evaporator. If the mass flow rate is too small, very low outlet temperatures can result, possibly below freezing.

The condenser heat flow rate QCon_flow_nominal is used to assign the default value for the mass flow rates, which are used for the pressure drop calculations. It is also used to compute the part load efficiency. Hence, make sure that QCon_flow_nominal is set to a reasonable value.

The maximum heating capacity is set by the parameter QCon_flow_max, which is by default set to infinity.

The coefficient of performance depends on the evaporator and condenser leaving temperature since otherwise the second law of thermodynamics may be violated.

Notes

For a similar model that can be used as a chiller, see Buildings.Fluid.Chillers.Examples.Carnot_TEva.

Extends from Buildings.Fluid.Chillers.BaseClasses.PartialCarnot_T (Partial model for chiller with performance curve adjusted based on Carnot efficiency).

Parameters

Connectors

Modelica definition

Buildings.Fluid.HeatPumps.Carnot_y

Reversible heat pump with performance curve adjusted based on Carnot efficiency

Information

This is model of a heat pump whose coefficient of performance COP changes with temperatures in the same way as the Carnot efficiency changes. The input signal y is the control signal for the compressor.

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_con,0 ⁄ (T_con,0-T_eva,0)).

The heat pump 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 a polynomial in the heating part load ratio y_PL that can be used to take into account a change in COP at part load conditions. This polynomial has the form

η_PL = a₁ + a₂ y_PL + a₃ y_PL² + ...

where the coefficients a_i are declared by the parameter a.

On the Dynamics tag, the model can be parametrized to compute a transient or steady-state response. The transient response of the model is computed using a first order differential equation for the evaporator and condenser fluid volumes. The heat pump outlet temperatures are equal to the temperatures of these lumped volumes.

Typical use and important parameters

When using this component, make sure that the evaporator and the condenser have sufficient mass flow rate. Based on the mass flow rates, the compressor power, temperature difference and the efficiencies, the model computes how much heat will be added to the condenser and removed at the evaporator. If the mass flow rates are too small, very high temperature differences can result.

The condenser heat flow rate QCon_flow_nominal is used to assign the default value for the mass flow rates, which are used for the pressure drop calculations. It is also used to compute the part load efficiency. Hence, make sure that QCon_flow_nominal is set to a reasonable value.

The maximum heating capacity is set by the parameter QCon_flow_max, which is by default set to infinity.

The coefficient of performance depends on the evaporator and condenser leaving temperature since otherwise the second law of thermodynamics may be violated.

Notes

For a similar model that can be used as a chiller, see Buildings.Fluid.Chillers.Carnot_y.

Extends from Buildings.Fluid.Chillers.BaseClasses.PartialCarnot_y (Partial chiller model with performance curve adjusted based on Carnot efficiency).

Parameters

Connectors

Modelica definition

Buildings.Fluid.HeatPumps.EquationFitReversible

Model for a reversable heat pump based on the equation fit method

Information

Model for a reversable heat pump using the equation fit method and that takes as an input the set point for the leaving fluid temperature.

This reversable heat pump can be operated either in heating mode or in cooling mode. It typically is used for a water to water heat pump, but if the performance data per are set up for other media, such as glycol, it can also be used for such applications. Note that if used with air, the results will only be valid if there is no humidity condensation or frost build up. The heat exchanger at medium 1 is to be connected to the building load, and the other heat exchanger to the heat source or sink, such as a geothermal loop. If in heating mode, the heat exchanger at medium 1 operates as a condenser, and in cooling mode it operates as an evaporator.

The model is based on the model described in the EnergyPlus 9.1.0 Engineering Reference, Section 16.6.1: Water to water heat pump model and the model based on C.Tang (2005).

The model takes the following control signals:

• The integer input uMod which controls the heat pump operational mode. If per.reverseCycle = true the signal can take on the values -1 for cooling mode, 0 for off and +1 for heating mode.
  If per.reverseCycle = false and uMod = -1, the model stops with an error message.
• The input TSet is the set point for the leaving fluid temperature at port port_b1.

The heating and cooling performance coefficients are stored in the data record per and are available from Buildings.Fluid.HeatPumps.Data.EquationFitReversible.

The electric power only includes the power for the compressor, but not any power for pumps, as the pumps must be modeled outside of this component.

Main equations

The performance of the heat pump is computed as follows: Let α be the set of heat load performance coefficients determined by the data record per.hea.coeQ and let β be the set of electrical power performance coefficients determined by the data record hea.coeP. Then, the performance is computed as

• If uMod = 1, the heat pump is in heating mode and the load side available heat is

  Q̇_ava = ( α₁ + α₂ T_loa,ent/T_RefHeaLoa + α₃ T_sou,ent/T_RefHeaSou + α₄ ṁ_loa,ent/(ṁ_loa,0   s) + α₅ ṁ_sou,ent/(ṁ_sou,0   s) )   Q̇₀   s,

  where Q̇₀ is the design capacity as specified by the parameter per.hea.Q_flow and s is the scaling factor specified by the parameter scaling_factor. The corresponding power consumption is

  P= ( β₁ + β₂ T_loa,ent/T_RefHeaLoa + β₃ T_sou,ent/T_RefHeaSou + β₄ ṁ_loa,ent/(ṁ_loa,0   s) + β₅ ṁ_sou,ent/(ṁ_sou,0   s) )   P₀   s,

  where P₀ is the design power consumption as specified by the parameter per.hea.P. The actual heat provided at the load side is

  Q̇ = min(Q̇_ava , Q̇_set),

  where Q̇_set is the heat required to meet the temperature setpoint for the leaving fluid on the load side.

• If uMod = -1, the heat pump is in cooling mode, and the governing equations are as above, but with per.coo rather than per.hea used for the performance data, and the min(· ·) function replaced with max(· ·).
• If uMod = 0, the model sets Q̇ = 0 and P = 0.

The coefficient of performance COP is computed as

COP = Q̇ ⁄ P.

References

C. Tang Equation fit based models of water source heat pumps. Master Thesis. Oklahoma State University, Oklahoma, USA. 2005.

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

Parameters

Connectors

Modelica definition

Buildings.Fluid.HeatPumps.ReciprocatingWaterToWater

Model for a reciprocating water to water heat pump

Information

Model for a water to water heat pump with a reciprocating compressor, as described in Jin (2002). The thermodynamic heat pump cycle is represented below.

The rate of heat transferred to the evaporator is given by:

Q̇_Eva = ṁ_ref ( h_Vap(T_Eva) - h_Liq(T_Con) ).

The power consumed by the compressor is given by a linear efficiency relation:

P = P_Theoretical / η + P_{Loss,constant}.

Heat transfer in the evaporator and condenser is calculated using an ε-NTU method, assuming constant refrigerant temperature and constant heat transfer coefficient between fluid and refrigerant.

Variable speed is acheived by multiplying the full load piston displacement by the normalized compressor speed. The power and heat transfer rates are forced to zero if the resulting heat pump state has higher evaporating pressure than condensing pressure.

Options

Parameters TConMax and TEvaMin may be used to set an upper or lower bound for the condenser and evaporator. The compressor is disabled when these conditions are not satisfied, or when the evaporator temperature is larger than the condenser temperature. This mimics the temperature protection of heat pumps and moreover it avoids non-converging algebraic loops of equations, or freezing of evaporator medium. This option can be disabled by setting enable_temperature_protection = false.

Assumptions and limitations

The compression process is assumed isentropic. The thermal energy of superheating is ignored in the evaluation of the heat transferred to the refrigerant in the evaporator. There is no supercooling.

References

H. Jin. Parameter estimation based models of water source heat pumps. PhD Thesis. Oklahoma State University. Stillwater, Oklahoma, USA. 2002.

Extends from Buildings.Fluid.HeatPumps.BaseClasses.PartialWaterToWater (Partial model for water to water heat pumps and chillers).

Parameters

Connectors

Modelica definition

Buildings.Fluid.HeatPumps.ScrollWaterToWater

Model for a scroll water to water heat pump

Information

Model for a water to water heat pump with a scroll compressor, as described in Jin (2002). The thermodynamic heat pump cycle is represented below.

The rate of heat transferred to the evaporator is given by:

Q̇_Eva = ṁ_ref ( h_Vap(T_Eva) - h_Liq(T_Con) ).

The power consumed by the compressor is given by a linear efficiency relation:

P = P_Theoretical / η + P_{Loss,constant}.

Heat transfer in the evaporator and condenser is calculated using an ε-NTU method, assuming constant refrigerant temperature and constant heat transfer coefficient between fluid and refrigerant.

Variable speed is achieved by multiplying the full load suction volume flow rate by the normalized compressor speed. The power and heat transfer rates are forced to zero if the resulting heat pump state has higher evaporating pressure than condensing pressure.

The model parameters are obtained by calibration of the heat pump model to manufacturer performance data. Calibrated model parameters for various heat pumps from different manufacturers are found in Buildings.Fluid.HeatPumps.Data.ScrollWaterToWater. The calibrated model is located in Buildings.Fluid.HeatPumps.Calibration.ScrollWaterToWater.

Options

Parameters TConMax and TEvaMin may be used to set an upper or lower bound for the condenser and evaporator. The compressor is disabled when these conditions are not satisfied, or when the evaporator temperature is larger than the condenser temperature. This mimics the temperature protection of heat pumps and moreover it avoids non-converging algebraic loops of equations, or freezing of evaporator medium. This option can be disabled by setting enable_temperature_protection = false.

Assumptions and limitations

The compression process is assumed isentropic. The thermal energy of superheating is ignored in the evaluation of the heat transferred to the refrigerant in the evaporator. There is no supercooling.

References

H. Jin. Parameter estimation based models of water source heat pumps. PhD Thesis. Oklahoma State University. Stillwater, Oklahoma, USA. 2002.

Extends from Buildings.Fluid.HeatPumps.BaseClasses.PartialWaterToWater (Partial model for water to water heat pumps and chillers).

Parameters

Connectors

Modelica definition