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Sizing problem with Coil:WaterHeating:AirToWaterHeatPump

asked 2016-06-02 09:50:52 -0600

Urado gravatar image

updated 2016-06-02 15:44:45 -0600

I use a WaterHeater:HeatPump for heating and I have the following error:

** Severe  ** For object = `Coil:WaterHeating:AirToWaterHeatPump:Pumped`, name = "HP COIL"
Calculated outlet air relative humidity greater than 1. The combination of rated air volume flow rate, total cooling capacity and sensible heat ratio yields coil exiting air conditions above the saturation curve. Possible fixes are to reduce the rated total cooling capacity, increase the rated air volume flow rate, or reduce the rated sensible heat ratio for this coil. If autosizing, it is recommended that all three of these values be autosized.

I have already used this object without problem in another case so I think it's a sizing problem but I don't understand the proposed fix...

Some names slightly differ from what I have in this object (confusion between "total cooling capacity" and "rated heating capacity" ?) and mostly it does not seem possible to autosize these parameters (except for the "Rated Evaporator Air Flow Rate").

Did I miss something ?

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answered 2016-06-09 05:10:23 -0600

There has been apparently an autosizing problem with Air-to-Water HeatPumps in EnergyPlus, which may or may not have been resolved in latest edition. As a user I succeeded in getting around the autosizing problems by

  • setting the Rated Evaporator Air Flow Rate {m3/s} to an appropriate fixed numerical value
  • making sure that the associated HeatPump Fan is assigned the same Maximum Flow Rate {m3/s}, (avoiding the autosizing option, if necessary).

I also adjusted

  • Rated Sensible Heat Ratio .

In order to avoid troubles, make also sure the entries for

    * `Rated Evaporator Inlet Air Dry-Bulb Temperature {C}`
    * `Rated Evaporator Inlet Air Wet-Bulb Temperature {C}`
    * `Rated Condenser Inlet Water Temperature {C}`

are correct, and the COP- and Capacity curves are normalized at the rated temperatures.

An Example of how a solution may look like is as follows:

Coil:WaterHeating:AirToWaterHeatPump,
    AWHP_Coil_DHW,           !- Name
    700,                     !- Rated Heating Capacity {W}
    3.7,                     !- Rated COP {W/W}
    0.75,                    !- Rated Sensible Heat Ratio
    7,                       !- Rated Evaporator Inlet Air Dry-Bulb Temperature {C}
    6,                       !- Rated Evaporator Inlet Air Wet-Bulb Temperature {C}
    35,                      !- Rated Condenser Inlet Water Temperature {C}
    0.1,                     !- Rated Evaporator Air Flow Rate {m3/s}
    autocalculate,           !- Rated Condenser Water Flow Rate {m3/s}
    No,                      !- Evaporator Fan Power Included in Rated COP
    No,                      !- Condenser Pump Power Included in Rated COP
    No,                      !- Condenser Pump Heat Included in Rated Heating Capacity and Rated COP
    12,                      !- Condenser Water Pump Power {W}
    0.1,                     !- Fraction of Condenser Pump Heat to Water
    Fan_AWHP_DHW_OUT,        !- Evaporator Air Inlet Node Name
    ExhaustAirDHW_OUT,       !- Evaporator Air Outlet Node Name
    DHWTankSourceOUT,        !- Condenser Water Inlet Node Name
    DHWTankSourceIN,         !- Condenser Water Outlet Node Name
    0,                       !- Crankcase Heater Capacity {W}
    5,                       !- Maximum Ambient Temperature for Crankcase Heater Operation {C}
    WetBulbTemperature,      !- Evaporator Air Temperature Type for Curve Objects
    AWHP_DHW_rCAP,           !- Heating Capacity Function of Temperature Curve Name
    ,                        !- Heating Capacity Function of Air Flow Fraction Curve Name
    ,                        !- Heating Capacity Function of Water Flow Fraction Curve Name
    AWHP_DHW_rCOP,           !- Heating COP Function of Temperature Curve Name
    ,                        !- Heating COP Function of Air Flow Fraction Curve Name
    ,                        !- Heating COP Function of Water Flow Fraction Curve Name
    HPWHPLFFPLR_DHW;         !- Part Load Fraction Correlation Curve Name

and finally:

Fan:OnOff,
    Fan_AWHP_DHW,            !- Name
    Always On Discrete,      !- Availability Schedule Name
    0.6,                     !- Fan Total Efficiency
    250,                     !- Pressure Rise {Pa}
    0.1,                     !- Maximum Flow Rate {m3/s}
    0.8,                     !- Motor Efficiency
    1,                       !- Motor In Airstream Fraction
    OutdoorAirIN_DHW,        !- Air Inlet Node Name
    Fan_AWHP_DHW_OUT,        !- Air Outlet Node Name
    ,                        !- Fan Power Ratio Function of Speed Ratio Curve Name
    ,                        !- Fan Efficiency Ratio Function of Speed Ratio Curve Name
    DHW;                     !- End-Use Subcategory
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Asked: 2016-06-02 09:50:52 -0600

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Last updated: Jun 09 '16