.Buildings.Fluid.DXSystems.Heating.BaseClasses.CoilDefrostTimeCalculations

Information

Block that calculates the defrost cycling time fraction tDefFra, the heating capacity multiplier heaCapMul and the input power multiplier inpPowMul.

The inputs are the measured temperature TOut and humidity ratio per kg total air XOut of the outdoor air. The calculation is based on Section 15.2.11.4 in the EnergyPlus 22.2 engineering reference document and is as decribed below.

Calculations

The model first estimates the outdoor coil temperature TCoiOut from the outdoor air drybulb temperature TOut using

TCoiOut = 0.82*TOut - 8.589.

The difference between the outdoor air humidity ratio XOutDryAir and the saturated air humidity ratio at outdoor coil temperature is used to determine frost formation on the outdoor coil as

delta_XCoilOut = max(1e-6, (XOutDryAir - XSatOutCoil)).

The block then calculates the time period fraction tDefFra for which the defrost operation is assumed to run, based on defrost calculation mode specified by the parameter defTri.
In the EnergyPlus model, tDefFra represents the fraction of the constant timestep in the simulation model for which the defrost operation is assumed to be active. This is calculated to be a higher value when the outdoor air temperature is lower and the relative humidity is higher. This results in a higher proportion of the consumed power going towards the defrost operation and lower heating of the condenser's air stream.

In this Modelica implementation of the model, while the timestep may not be constant and may vary based on the solver for the simulation, the same assumption is used for calculating the proportion of energy consumed for defrost mode operation. The coil does not actually enter defrost operation (with reverse flow of refrigerant) during this timestep fraction.

If defTri is set to timed, tDefFra is set to the user input parameter for defrost operation timestep fraction tDefRun. The heating capacity multiplier heaCapMul and input power multiplier inpPowMul are calculated as

tDefFra = tDefRun,
heaCapMul = 0.909 - 107.33*delta_XCoilOut,
inpPowMul = 0.9 - 36.45*delta_XCoilOut.

If defTri is set to onDemand, tDefFra, heaCapMul and inpPowMul are calculated as

tDefFra = 1/(1 + (0.01446/delta_XCoilOut)),
heaCapMul = 0.875*(1 - tDefFra),
inpPowMul = 0.954*(1 - tDefFra).

Revisions

• April 2, 2023, by Karthik Devaprasad:
  First implementation.