Pull request overview

• Fixes Super high heating cooling COP in VRFFluidTCtrl #10751
• Fixes Many "Refrigerant is saturated at the given conditions" warnings for VRF FluidTCtrl model #10764
• Fixes This warning should not happen: Unable to find a unit conversion from Cooling Coil Type [1] into IP units #10767

1. Fix of the super high COP problem

The following shows the distribution of heating and cooling COP before vs after the fix

eplusout_defect_output.csv
eplusout_fix.csv

COP is calculated as this->TotalHeatingCapacity divided by this->ElecHeatingPower and other auxiliary power.

this->OperatingHeatingCOP = (this->TotalHeatingCapacity) /
                            (this->ElecHeatingPower + this->CrankCaseHeaterPower + this->EvapCondPumpElecPower + this->DefrostPower);

this->ElecHeatingPower is calculated as compressor power and outdoor fan power this->ElecHeatingPower = this->Ncomp + this->OUFanPower;

Previously there were some problems of compressor power and fan electricity rate not considering cycling ratio, see #10427, #10105. Now that they're fixed, these terms have cycling ratios in them, while, the numerator term seems to have missed the multiplication of cycling ratio.

See the following comparison

// FluidTCtrl model
    this->TotalCoolingCapacity = TotalCondCoolingCapacity * CoolingPLR;
    this->TotalHeatingCapacity = TotalCondHeatingCapacity * HeatingPLR;
// curve-based model
    vrf.TotalCoolingCapacity = TotalCondCoolingCapacity * CoolingPLR * CyclingRatio;
    vrf.TotalHeatingCapacity = TotalCondHeatingCapacity * HeatingPLR * CyclingRatio;

The issue of super high COP is fixed by multiplying CyclingRatio in this->TotalCoolingCapacity and this->TotalHeatingCapacity.

2. Fix of the refrigerant warning

The issue is originated from the change of this line in PR#10416 the following line. The intention is to

The intention of the change is to impose a lower bound in solving for an evaporating temperature when the heating load is small. Such a lower bound is necessary as otherwise, in many time steps when the system is cycling, the OU evaporating temperature will be -72C, which is too low to be feasible. However, using the value of the input field "Variable Evaporating Temperature Minimum for Indoor Unit" as the lower bound will trigger a bunch of refrigerant warnings.

To fix the refrigerant flow problem, it is observed that the following line might have been a mistake CapMinPe = min(Pdischarge - this->CompMaxDeltaP, RefMinPe);. Pdischarge is the compressor discharge pressure,
this->CompMaxDeltaP is the maximum compressor pressure rise. Then Pdischarge - this->CompMaxDeltaP is minimum compressor suction pressure considering pressure rise bound. The RefMinPe is minimum refrigerant evaporating pressure, which is a pressure lower bound considering physical feasibility. CapMinPe should be no less than both of them, so it should be max of them, not min.

In the code base, there are three places where CapMinPe is calculated (shown in the following). Two of them are max, one is min, hinting that the min might be a bug.

After imposing the right pressure bound, the refrigerant warnings are mostly gone. The following are the eplusout.err files from the develop and the feature branch. A difference can be seen just from the file size.

fix_eplusout.err.zip
develop_eplusout.err.zip

3. Fix of the recurring warning issue

The warning message keep showing up is the non-recurring warning (which should only be shown once or a few times).

The excessive amount of warning is produced by this chunk of code. The condition this->errors[(int)RefrigError::SatSupDensity].count <= df->RefrigErrorLimitTest decides whether it prints this warning. The right hand side of the inequality is 1, while the left hand side overflows to negative at some point to -2147450880. From then on, the condition just keeps evaluates to true and the warning keeps showing up. It overflows as it is not a counter, but a sum of all previous counter values. This makes it get large very quickly. When it overflows, the counter value of df->SatErrCountGetSupHeatDensityRefrig is 65536. This this->errors[(int)RefrigError::SatSupDensity].count <= df->RefrigErrorLimitTest is 1 + 2 + ... + 65536, which overflowed.

++df->SatErrCountGetSupHeatDensityRefrig;
// send warning
this->errors[(int)RefrigError::SatSupDensity].count += df->SatErrCountGetSupHeatDensityRefrig;
// send warning
if (this->errors[(int)RefrigError::SatSupDensity].count <= df->RefrigErrorLimitTest) {
    ShowWarningMessage(
        state,
        format("{}: Refrigerant [{}] is saturated at the given conditions, saturated density at given temperature returned. **",
                routineName,
                this->Name));
    ShowContinueError(state, fmt::format("...Called From:{}", CalledFrom));
    ShowContinueError(state, format("Refrigerant temperature = {:.2R}", Temperature));
    ShowContinueError(state, format("Refrigerant pressure = {:.0R}", Pressure));
    ShowContinueError(state, format("Returned Density value = {:.3R}", saturated_density));
    ShowContinueErrorTimeStamp(state, "");
}

4. Cooling Coil Type [1] IP to SI unit conversion warning

The warning is produced because the code assumes there is a unit of measure inside the bracket and will attempt to perform unit conversion. Here this is a categorical variable and should not attempt to do unit conversion, so [] is not appropriate. The problem if fixed by changing the "[]" into "()" in this "Cooling Coil Type [1]".

Regression diffs

Table big diff and Table string diffs

There are 733 files have "ColumnHeadingDifference", leading to table big diffs and string diffs. This is caused by changing "Cooling Coil Type [1]" to "Cooling Coil Type (1)" in this feature branch. The diffs are expected.

meter diff

Meter diff happens in test idf "VariableRefrigerantFlow_FluidTCtrl_5Zone", in the following two fields.

• Electricity:Facility
• Electricity:HVAC
  The meter diff is caused by this change. The changes here will cause differences in compressor power, which will lead to changes in Electricity:HVAC and Electricity:Facility.
  

eso diff

in US+SF+CZ4A+hp+crawlspace+IECC_2006_VRF, VariableRefrigerantFlow_FluidTCtrl_5Zone, and VariableRefrigerantFlow_FluidTCtrl_HR_5Zone, the following fields have eso diffs

• VRF HEAT PUMP:VRF Heat Pump Total Cooling Rate
• VRF HEAT PUMP:VRF Heat Pump Total Heating Rate
• VRF HEAT PUMP:VRF Heat Pump Heating Electricity Rate
• VRF HEAT PUMP:VRF Heat Pump Heating Electricity Energy
• VRF HEAT PUMP:VRF Heat Pump Compressor Electricity Rate
• VRF HEAT PUMP:VRF Heat Pump Outdoor Unit Evaporating Temperature

These diffs are expected as the numerator of the COP term (heating/cooling output) is changed in this feature: it is multiplied by the cycling ratio now. The changes in COP and heating cooling rate are both expected. Changes in Outdoor Unit Evaporating Temperature is due to the correction in refrigerant pressure lower bound, which is used in calculation of OU evaporating temperature. Heating electricity rate and energy difference happen in 1/21. Heating electricity rate is the sum of compressor power and outdoor unit fan power. As a result of the changing MinOutdoorUnitTe argument from max(this->IUEvapTempLow, CapMinTe) to CapMinTe in function VRFOU_CalcCompH, compressor power will change. So diffs in heating electricity rate is also expected.

err diffs

11 files have error diffs. This is because the following warning is removed as a result of the column header change of "Cooling Coil Type".

** Warning ** Unable to find a unit conversion from Cooling Coil Type [1] into IP units

NOTE: ENHANCEMENTS MUST FOLLOW A SUBMISSION PROCESS INCLUDING A FEATURE PROPOSAL AND DESIGN DOCUMENT PRIOR TO SUBMITTING CODE

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