Engineering Reference — EnergyPlus 22.1

Engineering Reference

Whenever an EnergyPlus model includes anything that consumes electricity the program will do some basic modeling of what can be thought of as the main panel where utility grid connects to the building and HVAC loads. The following output variables and meters are created and filled to report on the overall, whole-facility power flows at the main panel:

 Ppurch Facility Total Purchased Electric Power (W) Qpurch Facility Total Purchased Electric Energy (J) Pnet,purch Facility Net Purchased Electric Power (W) Qnet,purch Facility Net Purchased Electric Energy (J) Psurp Facility Total Surplus Electric Power (W) Qsurp Facility Total Surplus Electric Energy (J) Pbldg,dmd Facility Total Building Electric Demand Power (W) PHVAC,dmd Facility Total HVAC Electric Demand Power (W) Pdmd Facility Total Electric Demand Power (W) Pprod Facility Total Produced Electric Power (W) Qprod Facility Total Produced Electric Energy (J)

The models for electric power service use the meter reporting system inside EnergyPlus to obtain data from all the other models in EnergyPlus for the electric power demand (and maybe power production). The ElectricPowerServiceManager uses a query routine called “GetInstantMeterValue” which returns the sum for all the metered equipment for the specified meter. Values for Pbldg,dmd and PHVAC,dmd are both filled from the “Electricity:Facility” meter. Pbldg,dmd is filled by passing an argument that triggers to return the “Building” electric load, which is at the zone timestep, while PHVAC,dmd is filled by passing an argument that returns the “HVAC” electric load, which is at the system timestep. The modeling is based on average power levels for the duration of the timestep. The return values are converted from energy in Joules to power in Watts by dividing by the number of seconds in the zone timestep for building loads and by the system timestep for the HVAC loads. Then the total facility power demand is the sum of the building loads and the HVAC loads.

Pdmd=Pbldg,dmd+PHVAC,dmd

When the electric power service device models described below are being used, they also make use of the meter reporting system using the components of the ElectricityProduced:Facility ( ResourceType=ElectricityProduced) meter to determine aggregate on-site electricity production rate, Pprod. This value rolls up all the power generated and consumed by the on-site generators, storage, and power conversion equipment:

• Pprod,cogen is the power from fuel-fired generators, from meter Cogeneration:ElectricityProduced.

• Pprod,pv is the power from solar photovoltaics, from meter Photovoltaic:ElectricityProduced.

• Pprod,wind is the power from wind generators, from meter WindTurbine:ElectricityProduced.

• Pprod,storage is the power from electric storage, from meter ElectricStorage:ElectricityProduced, with storage charging negative and storage discharging positive.

• Pprod,conversion is the power loss by converting between different voltages or AC and DC, from meter PowerConversion:ElectricityProduced which are metered as negative values that decrease the amount of electricity produced.

Pprod=Pprod,cogen+Pprod,pv+Pprod,wind+Pprod,storage+Pprod,conversion

Then the net purchased electricity, which can be either positive or negative (for export), is calculated using,

Pnet,purch=PdmdPprod

Only the positive portion of the net purchased power is the purchased power.

Ppurch=max(Pnet,purch,0.0)

The surplus electric power is exported out to the grid and is only positive.

Psurp=max((PprodPdmd),0.0)

The energy versions of the power variables are calculated by multiplying the power level by the number of seconds in the system timestep.