The Air To Water Heat Pump "Coil" described here models an air-to-water DX compression system (which includes a water heating coil, air coil, compressor, and water pump) to determine its air-side and water-side performance. It is used within the composite EnergyPlus air to water heat pump which also includes a water heater tank and a fan to provide air flow across the air coil associated with the DX compression system. These objects work together to model a system which heats water using zone air, outdoor air, or a combination of zone and outdoor air as the primary heat source.
This DX coil calculates the air-side sensible and latent cooling capacity at the specific operating conditions for each simulation timestep, as well as the condenser’s water-side temperature difference at a given condenser water flow rate. The heat pump water heater DX coil model performs the following major functions:
The input fields for this object are described below in detail.
Enter a unique name for the air to water heat pump coil.
This numeric field defines the DX coil heating capacity (in W or Btu/h) at the rated evaporator inlet air temperatures, rated condenser inlet water temperature, rated evaporator air flow rate, and rated condenser water flow rate specified below. Values must be greater than 0. This value represents water heating capacity, and it may or may not include the impact of condenser pump heat (see field Condenser pump heat included in rated heating capacity and rated COP below).
This numeric field defines the DX coil’s water heating coefficient of performance (COP=water heating capacity divided by electrical power input) at rated conditions (rated inlet temperatures and flow rates specified below). This input not only determines the electric energy use of the heat pump DX coil, but also the amount of total air cooling provided by the evaporator. The rated COP includes compressor power, and may or may not include condenser pump power or evaporator fan power (see field Evaporator fan power included in rated COP and field Condenser pump power included in rated COP). Values must be greater than 0.
This numeric field defines the air-side sensible heat ratio (SHR=sensible cooling capacity divided by total cooling capacity) of the DX coil at rated conditions (rated inlet temperatures and flow rates specified below). This value should not include the effect of evaporator fan heat. Values must be greater than or equal to 0.5, and less than or equal to 1.0. The default value is 0.85.
This numeric field defines the evaporator inlet air dry-bulb temperature (in °C or °F), that corresponds to rated coil performance (heating capacity, COP and SHR). Values must be greater than 5°C. The default value is 19.7°C.
This numeric field defines the evaporator inlet air wet-bulb temperature (in °C or °F) that corresponds to rated coil performance (heating capacity, COP and SHR). Values must be greater than 5°C. The default value is 13.5°C.
This numeric field defines the evaporator air volume flow rate (m3/s or ft3/min) at rated conditions. Values must be greater than 0. If this field is autocalculated (value = autocalculate), the default value is 5.035E-5 m3/s/W (31.25 cfm/MBH) multiplied by the Rated heating capacity specified above. When autocalculating the rated evaporator air volumetric flow rate, the zone sizing data is not used.
This choice field specifies if evaporator fan power is included in the rated COP defined above. This input impacts the calculation of compressor electric power and total air cooling provided by the evaporator for each simulation timestep. If this option is selected, the evaporator fan power is subtracted from the total electric heating power when calculating total evaporator cooling capacity. Otherwise it is assumed that the total heating power does not include evaporator fan power. See the Engineering Reference section for Coil:WaterHeating:AirToWaterHeatPump for further details.
This numeric field defines the condenser inlet water temperature (in °C or °F) that corresponds to rated coil performance (heating capacity, COP and SHR). Values must be greater than 25°C. The default value is 57.5°C.
This numeric field defines the condenser water volumetric flow rate (in m3/s or gal/min) at rated conditions. Values must be greater than 0. If this field is left blank or autocalculated (value = autocalculate), the default value is 4.487E-8 m3/s/W (0.208 gpm/MBH) multiplied by the Rated heating capacity specified above. When autocalculating the rated condenser water volumetric flow rate, the data in the zone sizing object is not used. A warning message will be issued if the ratio of Rated condenser water flow rate to Rated heating capacity is less than 1.79405E-8 m3/s/W (0.083 gpm/MBH) or greater than 8.97024E-8 m3/s/W (0.417 gpm/MBH), but the simulation will continue.
This option specifies whether condenser pump power is included in the rated COP defined above. This input impacts the calculation of compressor electric power which then impacts the total air cooling provided by the evaporator for each simulation timestep. If it is selected, the condenser pump power is subtracted from the total electric heating power when calculating total evaporator cooling capacity. Otherwise it is assumed that the total heating power does not include the condenser pump. See Engineering Reference section for Coil:WaterHeating:AirToWaterHeatPump for further details.
This option specifies whether condenser pump heat is included in the rated heating capacity and rated COP defined above. It impacts the calculation of compressor electric power and total air cooling provided by the evaporator for each simulation timestep. If it is selected, the condenser pump heat is already included in the rated heating capacity and rated COP. If it isn't selected, it is assumed that the rated heating capacity and rated COP do not include the condenser pump heat, and pump heat is added to the total water heating capacity based on the Condenser water pump power and Fraction of condenser pump heat to water fields below. See Engineering Reference section for Coil:WaterHeating:AirToWaterHeatPump for further details.
This numeric field defines the DX coil’s condenser pump power (in W or Btu/h). This is the operating pump power as installed. Values must be greater than or equal to 0. A warning message will be issued if the ratio of Condenser water pump power to Rated heating capacity exceeds 0.1422 W/W (41.67 Watts/MBH), but the simulation will continue.
This numeric field defines the fraction of condenser pump heat that is transferred to the condenser water. The pump is assumed to be downstream of the condenser water coil, and this field is used to determine the water temperature at the condenser outlet node when the Condenser pump power included in rated heating capacity and COP option is unchecked. Values must be greater than or equal to 0 and less than or equal to 1. The default value is 0.2.
This numeric field defines the compressor’s crankcase heater capacity (in W or Btu/h). The crankcase heater only operates when the compressor is off and the air surrounding the compressor is below the Maximum ambient temperature for crankcase heater operation specified below.
This numeric field defines the maximum ambient temperature for crankcase heater operation (in °C or °F). The crankcase heater only operates when the air surrounding the compressor is below this maximum temperature value and the compressor is off. The ambient temperature surrounding the compressor is set by the Air to water heat pump parent object (Compressor location).
You can use curves to modify the rated capacity and rated COP defined above based on one or more performance curves. To apply a curve first check its associated check box and then select the required curve.
Note: to use the rated capacity and rated COP without modification you should uncheck all of the check boxes in this section.
This drop list selection specifies the air temperature type used for the heating capacity and COP modifier curve objects below. The valid selections are:
If the curves are not defined then this field is not used.
This Bi-quadratic or Cubic performance curve defines the variation in DX coil heating capacity as a function of inlet fluid (air and water) temperatures. The Bi-quadratic curve uses evaporator inlet air temperature (dry-bulb or wet-bulb temperature based on the field Evaporator air temperature type for curves defined above) and condenser inlet water temperature as the independent variables. The cubic curve uses evaporator inlet air (dry-bulb or wet-bulb) temperature as the independent variable. The output of this curve is multiplied by the rated heating capacity to give the heating capacity at specific operating conditions (i.e., at temperatures different from the rating point temperatures). The curve should be normalized to have the value of 1.0 at the rating point temperatures.
If the associated checkbox is unchecked, the heating capacity remains constant (curve value assumed to be 1.0 for all conditions).
This Quadratic or Cubic performance curve defines the variation in DX coil heating capacity as a function of the ratio of actual air flow rate across the evaporator coil to the rated evaporator air flow rate. The output of this curve is multiplied by the Rated heating capacity and the Heating capacity modifier curve (function of temperature) to give the DX coil heating capacity at the specific inlet fluid temperatures and air flow rate at which the coil is operating. The curve should be normalized to have the value of 1.0 at the rated evaporator air flow rate (air flow fraction of 1.0).
If the associated checkbox is unchecked the heating capacity remains constant (curve value assumed to be 1.0 for all air flow rates).
This Quadratic or Cubic performance curve defines the variation in DX coil heating capacity as a function of the ratio of actual water flow rate through the condenser to the rated condenser water flow rate. The output of this curve is multiplied by the rated heating capacity and the output from the two other heating capacity modifier curves (function of temperature and function of air flow fraction) to give the DX coil heating capacity at the specific inlet fluid temperatures and flow rates at which the coil is operating. The curve should be normalized to have the value of 1.0 at the rated condenser water flow rate (water flow fraction of 1.0).
If the associated checkbox is unchecked the heating capacity remains constant (curve value assumed to be 1.0 for all water flow rates).
This Bi-quadratic or Cubic performance curve defines the variation in DX coil heating COP as a function of inlet fluid (air and water) temperatures. The biquadratic curve uses evaporator inlet air temperature (dry-bulb or wet-bulb temperature based on the field Evaporator air temperature type for curves defined above) and condenser inlet water temperature as the independent variables. The cubic curve uses evaporator inlet air (dry-bulb or wet-bulb) temperature as the independent variable. The output of this curve is multiplied by the rated COP to give the heating COP at specific operating conditions (i.e., at temperatures different from the rating point temperatures). The curve should be normalized to have the value of 1.0 at the rating point temperatures.
If the associated checkbox is unchecked the COP remains constant (curve value assumed to be 1.0 for all conditions).
This Quadratic or Cubic performance curve defines the variation in DX coil heating COP as a function of the ratio of actual air flow rate across the evaporator coil to the rated evaporator air flow rate. The output of this curve is multiplied by the rated COP and the heating COP modifier curve (function of temperature) to give the heating COP at the specific inlet fluid temperatures and air flow rate at which the coil is operating. The curve should be normalized to have the value of 1.0 at the rated evaporator air flow rate (air flow fraction of 1.0).
If the associated checkbox is unchecked the heating COP remains constant (curve value assumed to be 1.0 for all air flow rates).
This Quadratic or Cubic performance curve defines the variation in DX coil heating COP as a function of the ratio of actual water flow rate through the condenser to the rated condenser water flow rate.. The output of this curve is multiplied by the rated COP and the output from the two other heating COP modifier curves (function of temperature and function of air flow fraction) to give the DX coil heating COP at the specific inlet fluid temperatures and flow rates at which the coil is operating. The curve should be normalized to have the value of 1.0 at the rated condenser water flow rate (water flow fraction of 1.0).
If the associated checkbox is unchecked the heating COP remains constant (curve value assumed to be 1.0 for all water flow rates).
This Quadratic or Cubic performance curve parameterizes the variation of electrical power input to the DX unit as a function of the part load ratio (PLR, sensible cooling load/steady-state sensible cooling capacity). The product of the rated EIR and EIR modifier curves is divided by the output of this curve to give the “effective” EIR for a given simulation timestep. The part load fraction (PLF) correlation accounts for efficiency losses due to compressor cycling.
The part load fraction correlation should be normalized to a value of 1.0 when the part load ratio equals 1.0 (i.e., no efficiency losses when the compressor(s) run continuously for the simulation timestep). For PLR values between 0 and 1 (0 <= PLR < 1), the following rules apply:
PLF >= 0.7 and PLF >= PLR
If PLF < 0.7 a warning message is issued, the program resets the PLF value to 0.7, and the simulation proceeds. The runtime fraction of the coil is defined as PLR/PLF. If PLF < PLR, then a warning message is issued and the runtime fraction of the coil is limited to 1.0.
A typical part load fraction correlation for a conventional, single-speed DX cooling coil (e.g., residential unit) would be:
PLF = 0.85 + 0.15(PLR)
If the user wishes to model no efficiency degradation due to compressor cycling, the part load fraction correlation should be defined as follows:
PLF = 1.0 + 0.0(PLR)