.Buildings.Fluid.Chillers.Carnot_y

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 input signal y is the control signal for the compressor.

Set use_eta_Carnot_nominal=true to specify directly the Carnot effectiveness ηCarnot,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

ηCarnot,0 = COP0 ⁄ (Teva,0 ⁄ (Tcon,0 + Tapp,con,0 - (Teva,0-Tapp,eva,0))),

where Teva,0 is the evaporator temperature, Tcon,0 is the condenser temperature, Tapp,eva,0 is the evaporator approach temperature and Tapp,con,0 is the condenser approach temperature.

The COP is computed as the product

COP = ηCarnot,0 COPCarnot ηPL,

where COPCarnot is the Carnot efficiency and ηPL is the part load efficiency, expressed using a polynomial. This polynomial has the form

ηPL = a1 + a2 y + a3 y2 + ...,

where y ∈ [0, 1] is the part load for cooling and the coefficients ai 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.

Revisions


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