Buildings.Fluid.Chillers

Package with chiller models

Information

This package contains component models for chillers.

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

Package Content

Name	Description
Carnot_TEva	Chiller with prescribed evaporator leaving temperature and performance curve adjusted based on Carnot efficiency
Carnot_y	Chiller with performance curve adjusted based on Carnot efficiency
ElectricEIR	Electric chiller based on the DOE-2.1 model
ElectricReformulatedEIR	Electric chiller based on the DOE-2.1 model, but with performance as a function of condenser leaving instead of entering temperature
Data	Performance data for electric chillers
Examples	Collection of models that illustrate model use and test models
Validation	Collection of models that validate the chiller models
BaseClasses	Package with base classes for Buildings.Fluid.Chillers

Buildings.Fluid.Chillers.Carnot_TEva

Chiller with prescribed evaporator leaving temperature and performance curve adjusted based on Carnot efficiency

Information

This is a model of a chiller 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 evaporator leaving temperature, which is met exactly at steady state if the chiller 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_eva,0 ⁄ (T_con,0-T_eva,0)).

On the Advanced tab, a user can specify the temperatures that will be used as the evaporator and condenser temperature.

During the simulation, the chiller 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 cooling 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 chiller 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 added to the condenser. If the mass flow rate is too small, very high outlet temperatures can result.

The evaporator heat flow rate QEva_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 QEva_flow_nominal is set to a reasonable value.

The maximum cooling capacity is set by the parameter QEva_flow_min, which is by default set to negative 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 heat pump, see Buildings.Fluid.HeatPumps.Examples.Carnot_TCon.

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.Chillers.Carnot_y

Chiller with performance curve adjusted based on Carnot efficiency

Information

This is model of a chiller 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_eva,0 ⁄ (T_con,0-T_eva,0)).

The chiller 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 cooling 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 chiller 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 evaporator heat flow rate QEva_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 QEva_flow_nominal is set to a reasonable value.

The maximum cooling capacity is set by the parameter QEva_flow_min, which is by default set to negative 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 heat pump, see Buildings.Fluid.HeatPumps.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.Chillers.ElectricEIR

Electric chiller based on the DOE-2.1 model

Information

Model of an electric chiller, based on the DOE-2.1 chiller model and the EnergyPlus chiller model Chiller:Electric:EIR.

This model uses three functions to predict capacity and power consumption:

• A biquadratic function is used to predict cooling capacity as a function of condenser entering and evaporator leaving fluid temperature.
• A quadratic functions is used to predict power input to cooling capacity ratio with respect to the part load ratio.
• A biquadratic functions is used to predict power input to cooling capacity ratio as a function of condenser entering and evaporator leaving fluid temperature.

These curves are stored in the data record per and are available from Buildings.Fluid.Chillers.Data.ElectricEIR. Additional performance curves can be developed using two available techniques (Hydeman and Gillespie, 2002). The first technique is called the Least-squares Linear Regression method and is used when sufficient performance data exist to employ standard least-square linear regression techniques. The second technique is called Reference Curve Method and is used when insufficient performance data exist to apply linear regression techniques. A detailed description of both techniques can be found in Hydeman and Gillespie (2002).

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:

1. The test

     PLR1 =min(QEva_flow_set/QEva_flow_ava, per.PLRMax);
   

   ensures that the chiller capacity does not exceed the chiller capacity specified by the parameter per.PLRMax.
2. 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 operate. Note that this model continuously operates 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.
3. The test

     PLR2 = max(per.PLRMinUnl, PLR1);
   

   computes the part load ratio of the compressor. The assumption is that for a part load ratio below per.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.

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

References

• Hydeman, M. and K.L. Gillespie. 2002. Tools and Techniques to Calibrate Electric Chiller Component Models. ASHRAE Transactions, AC-02-9-1.

Extends from Buildings.Fluid.Chillers.BaseClasses.PartialElectric (Partial model for electric chiller based on the model in DOE-2, CoolTools and EnergyPlus).

Parameters

Connectors

Modelica definition

Buildings.Fluid.Chillers.ElectricReformulatedEIR

Electric chiller based on the DOE-2.1 model, but with performance as a function of condenser leaving instead of entering temperature

Information

Model of an electric chiller, based on the model by Hydeman et al. (2002) that has been developed in the CoolTools project and that is implemented in EnergyPlus as the model Chiller:Electric:ReformulatedEIR. This empirical model is similar to Buildings.Fluid.Chillers.ElectricEIR. The difference is that to compute the performance, this model uses the condenser leaving temperature instead of the entering temperature, and it uses a bicubic polynomial to compute the part load performance.

This model uses three functions to predict capacity and power consumption:

• A biquadratic function is used to predict cooling capacity as a function of condenser leaving and evaporator leaving fluid temperature.
• A bicubic function is used to predict power input to cooling capacity ratio as a function of condenser leaving temperature and part load ratio.
• A biquadratic functions is used to predict power input to cooling capacity ratio as a function of condenser leaving and evaporator leaving fluid temperature.

These curves are stored in the data record per and are available from Buildings.Fluid.Chillers.Data.ElectricReformulatedEIR. Additional performance curves can be developed using two available techniques (Hydeman and Gillespie, 2002). The first technique is called the Least-squares Linear Regression method and is used when sufficient performance data exist to employ standard least-square linear regression techniques. The second technique is called Reference Curve Method and is used when insufficient performance data exist to apply linear regression techniques. A detailed description of both techniques can be found in Hydeman and Gillespie (2002).

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:

1. The test

     PLR1 =min(QEva_flow_set/QEva_flow_ava, per.PLRMax);
   

   ensures that the chiller capacity does not exceed the chiller capacity specified by the parameter per.PLRMax.
2. 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 operate. Note that this model continuously operates 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.
3. The test

     PLR2 = max(per.PLRMinUnl, PLR1);
   

   computes the part load ratio of the compressor. The assumption is that for a part load ratio below per.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.

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

References

• Hydeman, M., N. Webb, P. Sreedharan, and S. Blanc. 2002. Development and Testing of a Reformulated Regression-Based Electric Chiller Model. ASHRAE Transactions, HI-02-18-2.
• Hydeman, M. and K.L. Gillespie. 2002. Tools and Techniques to Calibrate Electric Chiller Component Models. ASHRAE Transactions, AC-02-9-1.

Extends from Buildings.Fluid.Chillers.BaseClasses.PartialElectric (Partial model for electric chiller based on the model in DOE-2, CoolTools and EnergyPlus).

Parameters

Connectors

Modelica definition