blockCoilDefrostTimeCalculations
Extends from Modelica.Blocks.Icons.Block (Basic graphical layout of input/output block).
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).
Parameters
| Type | Name | Default | Description |
|---|---|---|---|
| Buildings.Fluid.DXSystems.Heating.BaseClasses.Types.DefrostTimeMethods | defTri | Buildings.Fluid.DXSystems.Heating.BaseClasses.Types.DefrostTimeMethods.timed | Type of method to trigger the defrost cycle |
| Real | tDefRun | 0.5 | If defrost operation is timed, timestep fraction for which defrost cycle is run |
| Modelica.Units.SI.ThermodynamicTemperature | TDefLim | Maximum temperature at which defrost operation is activated | |
| Advanced | |||
| Modelica.Units.SI.TemperatureDifference | dTHys | 0.5 | Temperature comparison hysteresis difference |
Connectors
| Type | Name | Default | Description |
|---|---|---|---|
| Modelica.Blocks.Interfaces.RealInput | TOut | Temperature of outdoor air | |
| Modelica.Blocks.Interfaces.RealInput | XOut | Humidity ratio of outdoor air | |
| Modelica.Blocks.Interfaces.RealOutput | tDefFra | Defrost operation timestep fraction | |
| Modelica.Blocks.Interfaces.RealOutput | heaCapMul | Heating capacity multiplier | |
| Modelica.Blocks.Interfaces.RealOutput | inpPowMul | Input power multiplier |
Components
| Type | Name | Default | Description |
|---|---|---|---|
| Modelica.Units.SI.MassFraction | delta_XCoilOut | Difference between outdoor air humidity ratio and the saturated air humidity ratio at estimated outdoor coil temperature | |
| Modelica.Units.SI.ThermodynamicTemperature | TCoiOut | Outdoor coil temperature | |
| Modelica.Units.SI.MassFraction | XOutDryAir | Outdoor air humidity ratio per kg total air | |
| Buildings.Utilities.Psychrometrics.ToDryAir | toDryAir | Convert outdoor air humidity ratio from total air to dry air | |
| Buildings.Controls.OBC.CDL.Reals.Hysteresis | hysTOut | Check if outdoor air temperature is below maximum limit for defrost operation |
Revisions
-
April 2, 2023, by Karthik Devaprasad:
First implementation.