.Buildings.Fluid.DXSystems.Heating.BaseClasses.DefrostCapacity

Information

Block that calculates the heat transfered to airloop QTotDef_flow, the total heating power consumption of the component PTot, the defrost input power consumption PDef and the crankcase heater input power consumption PCra.

The calculation are based on in Section 15.2.11.5 and 11.6 in the the EnergyPlus 22.2 engineering reference document, and are as

• the defrost cycle time fraction tDefFra, the heating capacity multiplier heaCapMul and the input power multiplier inpPowMul calculated by Buildings.Fluid.DXSystems.Heating.BaseClasses.CoilDefrostTimeCalculations.

• the total heat transfer QTot_flow and energy input ratio EIR calculated by Buildings.Fluid.DXSystems.BaseClasses.DryCoil.

• the measured temperature TConIn, humidity ratio per kg of total air XConIn and pressure pIn of the indoor coil inlet air temperature.

• the measured outdoor air temperature TOut.

Calculations

Based on the type of defrost method defOpe, different calculations are used:

• If defOpe = reverseCycle, the heat transfer rate from the airstream to the outdoor coil QDef_flow and the power consumption for defrost PDef are calculated as follows:

  QDef_flow = 0.01*tDefFra*(7.222 - TOut)*(QTot_flow/1.01667),
  PDef = defMod_T*(QTot_flow/1.01667)*tDefFra*RTF,

  where defMod_T is the defrost operation modifier calculated as

  defMod_T = defCur[1] + defCur[2]*TConInWetBul + defCur[3]*TConInWetBul² + defCur[4]*TOut + defCur[5]*TOut² + defCur[6]*TOut*TConInWetBul,

  and RTF is the runtime fraction of the heating coil, calculated from part load ratio PLR and part load fraction PLFra as

  RTF = PLR/PLFra.

  The part load ratio PLR is set to 1 since only a constant speed DX heating coil is implemented.

  The PLFra is calculated from the part load fraction curve defCur.PLFraFunPLR as

  PLFra = PLFraFunPLR[1] + PLFraFunPLR[2]*PLR + PLFraFunPLR[3]*PLR² + PLFraFunPLR[4]*PLR³.

• If defOpe = reverseCycle, the calculations are

  QDef_flow = 0,
  PDef = QDefResCap*tDefFra*RTF,

  where QDefResCap is the rated capacity of the resistive heating element on the outdoor coil.
  The remaining variable calculations for RTF, PLR and PLFra are the same as above.

The total power consumption PTot and total heat flowrate to the air stream QTotDef_flow (while accounting for defrost operation) is calculated as follows:

PTot = QTot_flow*EIR*PLR*inpPowMul/PLFra,
QTotDef_flow = QTot_flow*heaCapMul.

Contents

Revisions

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