.Buildings.Fluid.Chillers.Carnot_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.

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.

Revisions

• May 8, 2017, by Michael Wetter:
  Replaced model that interfaces with fluid stream.
  This is for Buildings, #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.