modelCarnot_TEva

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.

Set use_eta_Carnot_nominal=true to specify directly the Carnot effectiveness η_Carnot,0, in which case the value of the parameter COP_nominal will not affect the simulation. If use_eta_Carnot_nominal=false, the model will use the value of the parameter COP_nominal together with the specified nominal temperatures to compute the Carnot effectiveness as

η_Carnot,0 = COP₀ ⁄ (T_eva,0 ⁄ (T_con,0 + T_app,con,0 - (T_eva,0-T_app,eva,0))),

where T_eva,0 is the evaporator temperature, T_con,0 is the condenser temperature, T_app,eva,0 is the evaporator approach temperature and T_app,con,0 is the condenser approach temperature.

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 the part load for cooling and 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 AixLib.Fluid.HeatPumps.Examples.Carnot_TCon.

Parameters

Components

Revisions

• February 3, 2023, by Michael Wetter:
  Changed in base class the parameter binding etaCarnot_nominal(unit="1") = COP_nominal/(TUseAct_nominal/(TCon_nominal+TAppCon_nominal - (TEva_nominal-TAppEva_nominal))) to etaCarnot_nominal(unit="1") = 0.3 to avoid a circular assignment.
  Improved documentation.
  This is for Buildings, #3226.
• May 8, 2017, by Michael Wetter:
  Replaced model that interfaces with fluid stream.
  This is for AixLib, #763.
• January 2, 2017, by Filip Jorissen:
  Removed parameters effInpEva and effInpCon and updated documentation. This is for issue 497.
• August 8, 2016, by Michael Wetter:
  Changed default temperature to compute COP to be the leaving temperature as use of the entering temperature can violate the 2nd law if the temperature lift is small.
  This is for Annex 60, issue 497.
• November 25, 2015 by Michael Wetter:
  First implementation.