Pump mass flow rate for Coil:Heating(or Cooling):WaterToAirHeatPump:VariableSpeedEquationFit

[yzhou601]

Issue overview

Found pump mass flow rate look suspicious when we model the two speed/variable speed geothermal heat pump with Coil:Heating:WaterToAirHeatPump:VariableSpeedEquationFit, Coil:Cooling:WaterToAirHeatPump:VariableSpeedEquationFit and Pump:VariableSpeed objects.
We compared the model with single speed system using Coil:Heating:WaterToAirHeatPump:EquationFit , Coil:Cooling:WaterToAirHeatPump:EquationFit and Pump:VariableSpeed, the pump looks reasonable:

Dark green: two speed GHP pump mass flow rate,
Dark red: single speed GHP pump mass flow rate
Blue: two speed GHP pump electricity rate
Orange: single speed GHP pump electricity rate

The pump mass flow rate looks more like an on-off operation rather than "perate and select a flow somewhere between the minimum and maximum limits", which looks like a bug in E+. Not sure if the higher energy use is also related to the suspicious mass flow rate, but there're curves: Total Cooling Capacity Function of Water Flow Fraction Curve Name, Energy Input Ratio Function of Water Flow Fraction Curve Name in the coils that are using these water flow rates as inputs. Checked those curve input variables, the same as the suspicious mass flow rate reporting.

Pump inputs (the same between these two models):

Pump:VariableSpeed,
  ground source heat pump pump,           !- Name
  Node 3,                                 !- Inlet Node Name
  Node 13,                                !- Outlet Node Name
  0.01125,                                !- Design Maximum Flow Rate {m3/s}
  20000,                                  !- Design Pump Head {Pa}
  300,                                    !- Design Power Consumption {W}
  0.85,                                   !- Motor Efficiency
  0,                                      !- Fraction of Motor Inefficiencies to Fluid Stream
  0,                                      !- Coefficient 1 of the Part Load Performance Curve
  1,                                      !- Coefficient 2 of the Part Load Performance Curve
  0,                                      !- Coefficient 3 of the Part Load Performance Curve
  0,                                      !- Coefficient 4 of the Part Load Performance Curve
  0,                                      !- Design Minimum Flow Rate {m3/s}
  Intermittent,                           !- Pump Control Type
  ,                                       !- Pump Flow Rate Schedule Name
  ,                                       !- Pump Curve Name
  ,                                       !- Impeller Diameter {m}
  ,                                       !- VFD Control Type
  ,                                       !- Pump RPM Schedule Name
  ,                                       !- Minimum Pressure Schedule {Pa}
  ,                                       !- Maximum Pressure Schedule {Pa}
  ,                                       !- Minimum RPM Schedule {rev/min}
  ,                                       !- Maximum RPM Schedule {rev/min}
  ,                                       !- Zone Name
  0.5,                                    !- Skin Loss Radiative Fraction
  PowerPerFlowPerPressure,                !- Design Power Sizing Method
  348701.1,                               !- Design Electric Power per Unit Flow Rate {W/(m3/s)}
  1.282051282,                            !- Design Shaft Power per Unit Flow Rate per Unit Head {W/((m3/s)-Pa)}
  0,                                      !- Design Minimum Flow Rate Fraction
  General;                                !- End-Use Subcategory

Operating System (Multiple choices)

Windows

Operating System Version

11

Version of EnergyPlus

24.2.0

Unmethours link or helpdesk ticket number

N/A

Defect file

in-twospeed.idf.txt

[yzhou601]

Just realized that there's a pump power EMS program in the defect IDF file, which has the unitary system part load ratio as sensor and actuates the pump pressure rise, this will probably makes the pump energy use more reasonable, need to run a test file without it to see how it looks like naturally.

EnergyManagementSystem:Program,
  ground_source_heat_pump_pump_power_program, !- Name
  Set heating_plr = ground_source_heat_pump_unitary_system_plr_s, !- Program Line 1
  Set pump_total_eff = ground_source_heat_pump_pump_rated_mfr / 1000 * 20000.0 / 300.0, !- Program Line 2
  Set pump_vfr = ground_source_heat_pump_pump_mfr_s / 1000, !- Program Line 3
  If pump_vfr > 0,                        !- Program Line 4
  Set ground_source_heat_pump_pump_pressure_rise_act = 300.0 * heating_plr * pump_total_eff / pump_vfr, !- Program Line 5
  Else,                                   !- Program Line 6
  Set ground_source_heat_pump_pump_pressure_rise_act = 0, !- Program Line 7
  EndIf;                                  !- Program Line 8