classAutoSizing

AutoSizing

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Information

AutoSizing

Autosizing in Spawn of EnergyPlus is implemented to help users automatically size system equipment and components modeled in Modelica using sizing capabilities in EnergyPlus. The general workflow is to specify sizing objects in the idf file that are typically used in EnergyPlus workflows and propagate values from pre-defined sizing parameters in Modelica, whose values are obtained from the EnergyPlus sizing process during initialization, throughout the model as-needed. More information about setting up autosizing and the pre-defined sizing parameters is described below.

Autosizing for Zones and Systems in Modelica

The process for setting up autosizing in Modelica is as follows, with related notes:

  1. Instantiate an instance of class Buildings.ThermalZones.EnergyPlus_24_2_0.SystemSizing.
    • Name the autosize system using the string parameter hvacSystemName.
    • Toggle the boolean parameter autosizeHVAC=true.
  2. Add thermal zones to be autosized as part of the autosize system instantiated in the previous step by specifying the string parameter hvacSystemName in the thermal zone object so that it matches the name given to the system in the previous step.
    • You may assign multiple zones to the same autosize system.
    • The value of the boolean parameter autosizeHVAC in the autosize system object will apply to all thermal zones specified as part of that system.
    • The sizing values returned for each zone are for the design condition of each zone individually, while the sizing values returned for each system are for the design condition of the system considering the coincident load from each zone that is part of that system.
    • Any zone not assigned to a system will not be autosized.
Pre-defined Sizing Parameters in Modelica

The results of autosizing from EnergyPlus are populated into Modelica records that are accessible at the zone and system levels. Two records exist that contain the same parameters: one for heating and one for cooling.

The zone level records are:

The system level records are:

The parameters available within these records are:

  • QSen_flow: Design sensible load [W] (if not sized, equals 0)
  • QLat_flow: Design latent load [W] (if not sized, equals 0)
  • TSet: Indoor temperature set point at the design load [K] (if not sized, equals 0°C for zone heating and cooling, -273.15°C for system group heating and cooling)
  • XSet: Indoor humidity ratio set point at the design load per total air mass [kg/kg] (if not sized, equals 0)
  • TOut: Outdoor drybulb temperature at the design load [K] (if not sized, equals 0°C for zone heating and cooling, 21°C for system group heating and cooling)
  • XOut: Outdoor humidity ratio at the design load per total air mass [kg/kg] (if not sized, equals 0)
  • mOut_flow: Minimum outdoor air flow rate during the design load [kg/s] (if not sized, equals 0)
  • t: Time within the sizing day at which the design load occurred [s] (if not sized, equals 0)
Other Notes

Infiltration: All zone air infiltration for thermal zones connected to EnergyPlus is implemented in Modelica, and any infiltration information for these zones in the .idf is ignored during autosizing. For zones in the .idf not connected to Modelica thermal zones, infiltration information is still utilized during autosizing. For autosizing zones in Modelica connected to EnergyPlus, zone air infiltration can be considered using the parameter ThermalZone.airChaRatInf, which will add sensible and latent infiltration loads to the design zone heating and cooling loads at the specified air exchange rate using the temperature and humidity ratio zone set points and outdoor air conditions at the time of the zone design loads. The infiltration loads will also be added to the system level from each zone that is part of the system, with each zone's contribution calculated using the outdoor air conditions for the system level design load and each zone's zone level set points.

Internal Gains: Internal gain objects for people, lights, and equipment in the .idf are considered by EnergyPlus during autosizing, and are thus reflected in the sizing results returned to Modelica. If internal gain inputs are specified in Modelica, they are ignored during autosizing, but used in place of .idf objects during simulation.

Interzonal Air Exchange: All interzonal air exchange is implemented in Modelica, and any interzonal air exchange information in the .idf is ignored during both autosizing and simulation.

Thermostat and Humidistat Controls: Spawn uses thermostat and humidistat controls specified in the .idf for autosizing thermal zones connected to Modelica. For thermal zones connected to Modelica that do not have thermostat nor humidistat controls specified in the .idf, Spawn automatically adds a dual-setpoint thermostat with heating and cooling set points of 20°C and 22°C respectively and a dual-set point humidistat with humidifying and dehumidifying relative humidity set points of 45% and 55% respectively.

References for Autosizing in EnergyPlus: Autosizing objects in the .idf are used to direct the autosizing in Spawn, and the EnergyPlus algorithms are followed for the sizing calculations. However, note that Spawn replaces the SimulationControl object specified in the .idf to invoke autosizing on its own. Some useful references for working with those objects and setting up sizing in the .idf are as follows:

Example Models