General

The multi zone office complex air emulator model emulating a large office building with multi-zone VAV systems, chilled water systems, and hot water systems. In the emulator, the Spawn-of-EnergyPlus (Spawn) supports the cosimulation of EnergyPlus and Modelica. EnergyPlus (V9.6) calculates the building’s thermal loads with the boundary conditions. Modelica is responsible for the airflow calculation (e.g., building infiltration) and HVAC system and controls. The geometry of the large offcie building is shown in the figure below.

\"BuildingGeometry.\"

Building Design and Use

Architecture

The layout is representative of the large commercial office building stock and is consistent with the building prototypes. The test case building is located in Chicago, IL and based on the DOE Reference Large Office Building Model (Constructed In or After 1980). The original model has 12 floors with a basement. For simplicity, the middle 10 floors are modeled as a single representative floor using floor multiplier ( i.e., mass flow rate multiplier in Modelica), resulting in three modeled floors (ground, middle, and top), each served by a dedicated AHU, representing 460,233 ft2 (42,757 m2) of total conditioned floor area. The ground floor is assumed to be adiabatic with the basement. EnergyPlus (V9.6) calculates the building’s thermal loads. The detailed input file is wholebuilding96_spawn.idf, containing building geometry, constructions, occupancy and internal schedules, internal mass, etc.

The represented floor has five zones, with four perimeter zones and one core zone. Each perimeter zone has a window-to-wall ratio of about 0.38. The height of each zone is 2.74 m and the areas are as follows:

The geometry of the floor is shown as the following figure:

\"Zones.\"

Constructions

Opaque constructions: Mass walls; built-up flat roof (insulation above deck); slab-on-grade floor.

Windows: Window-to-wall ratio = 38.0%, equal distribution of windows.

Occupancy and internal loads schedules

The design occupancy density is 0.05 people/m2. The people internal gain is calculated based on the activity level of 120 W. The number of occupants present in each zone at any time coincides with the internal gain schedule. The occupied time for the HVAC system is between 6:00 and 22:00 each weekday and 6:00 and 18:00 on Saturday. The unoccupied time is outside of this period.

The design internal gains include lighting, plug loads, and people. The lighting load is with a radiant-convective-visible split of 70%-10%-20%. The plug load is with a radiant-convective-latent split of 50%-50%-0%. The people sensible load is with a radiant-convective split of 30%-70%. The occupancy and the internal gains are activated according to the schedule in the figure below.

\"Schedules.\"

The power densities of the internal gains are listed in the following table.

Internal Gains Power Density [W/m2]
Lighting 16.14
Plug 10.76

Climate data

The weather data is from TMY3 for Chicago O'Hare International Airport.

HVAC System Design

The HVAC system of the test case can be categorized into the air-side systems (i.e., variable air volume (VAV) systems) and water-side systems (i.e., a chilled water systems and a hot water system).

Air-side system designs

The air-side systems are VAV systems with terminal reheat. Each floor is served by a dedicated AHU and each zone of the test case is served by a dedicated VAV box. The following figure depicts how the VAVs, AHU, and zones are connected on each floor in general. Each AHU has a supply and return fan. A mixing box carries out the economizer function of providing cooling and ventilation. The heating is provided by the water-based reheat coils in the VAV terminals, while an electric resistance freeze protection coil is located in the AHU.

\"AirSide.\"

Water-side system designs

The water-side systems of the test case include one chilled water system and one hot water system. The chilled water systems are composed of three chillers, three cooling towers, a primary chilled water loop with three constant speed pumps, a secondary chilled water loop with two variable speed pumps, and a condenser water loop with three constant speed pumps. The hot water system consists of two gas boilers and two variable speed pumps. The figure below shows the schematics of the chilled water and hot water systems.

\"ChilledWater.\"

\"HotWater.\"

Equipment specifications

The HVAC sizing in Modelica is determined by the annual simulation results of EnergyPlus. The pressure loop related sizing parameters are estimated using a common HVAC design procedure. The following tables list all the sizing parameters in detail. See later sections for specific control details.


                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           
VAV terminal with Reheat CoilDesign Air Mass Flow Rate [kg/s]Minimum Cooling Air Mass Flow Rate Setpoint [kg/s]Heating Air Mass Flow Rate Setpoint [kg/s]Minimum Damper Position [0-1]Design Reheat Coil Water Mass Flow Rate [kg/s]
Bot Floor Core Zone VAV49.10.3 of design air mass flow rate0.3 of design air mass flow rate0.33.88
Mid Floor Core Zone VAV491.40.3 of design air mass flow rate0.3 of design air mass flow rate0.338.8
Top Floor Core Zone VAV49.10.3 of design air mass flow rate0.3 of design air mass flow rate0.33.88
Bot Floor South Zone VAV10.10.3 of design air mass flow rate0.3 of design air mass flow rate0.30.8
Bot Floor East Zone VAV6.70.3 of design air mass flow rate0.3 of design air mass flow rate0.30.53
Bot Floor North Zone VAV8.60.3 of design air mass flow rate0.3 of design air mass flow rate0.30.68
Bot Floor West Zone VAV7.80.3 of design air mass flow rate0.3 of design air mass flow rate0.30.62
Mid Floor South Zone VAV101.20.3 of design air mass flow rate0.3 of design air mass flow rate0.38
Mid Floor East Zone VAV670.3 of design air mass flow rate0.3 of design air mass flow rate0.35.3
Mid Floor North Zone VAV85.50.3 of design air mass flow rate0.3 of design air mass flow rate0.36.8
Mid Floor West Zone VAV77.90.3 of design air mass flow rate0.3 of design air mass flow rate0.36.2
Top Floor South Zone VAV10.10.3 of design air mass flow rate0.3 of design air mass flow rate0.30.8
Top Floor East Zone VAV6.70.3 of design air mass flow rate0.3 of design air mass flow rate0.30.53
Top Floor North Zone VAV8.60.3 of design air mass flow rate0.3 of design air mass flow rate0.30.68
Top Floor West Zone VAV7.80.3 of design air mass flow rate0.3 of design air mass flow rate0.30.62

                                                                                                                                                                        
AHU Cooling CoilsDesign Coil Capacity [W]Design Water Mass Flow Rate [kg/s]Design Air Mass Flow Rate [m3/s]
Bot Floor Cooling Coil750,0002670
Mid Floor Cooling Coil7,500,000260700
Top Floor Cooling Coil1,000,0003070

                                                                                                                 
AHU Freeze Protection CoilsDesign Coil Capacity [W]
Bot Floor Freeze Coil1,650,000
Mid Floor Freeze Coil16,500,000
Top Floor Freeze Coil1,650,000

                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                
FansTypeDesign Size Supply Fan Mass Flow Rate [kg/s]Design Size Supply Fan Pressure Head [Pa]Total EfficiencyDesign Power Consumption [W]
Bot Floor Supply FanVariable speed7010000.7297,222
Mid Floor Supply FanVariable speed70010000.72972,222
Top Floor Supply FanVariable speed7010000.7297,222
Bot Floor Return FanVariable speed703000.7229,167
Mid Floor Return FanVariable speed7003000.72291,667
Top Floor Return FanVariable speed703000.7229,167

                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        
PumpsTypeDesign Mass Flow Rate [kg/s]Design Size Supply Pump Pressure Head [Pa]Total EfficiencyDesign Power Consumption [W]
CHW Primary Pumps 1-3Constant speed105211,0000.8725,000
CHW Secondary Pumps 1-2Variable speed155478,0000.8890,000
CW Pumps 1-3Variable speed123571,1000.8781,000
HW Pumps 1-2Variable speed39478,2500.8622,900

                                                                                                                                                                                                                                                                                      
ChillersDesign Chilled Water Mass Flow Rate [kg/s]Design Condenser Water Mass Flow Rate [kg/s]Design Capacity [W]
Chillers 1-31051232,600,000

                                                                                                                                                                                                                                                                                                                                     
BoilersDesign Hot Water Mass Flow Rate [kg/s]Design Capacity [W]
Boiler 1-2393,300,000

                                                                                                                                                                                                                                                                                                                                                                                                   
Cooling TowersDesign Water Mass Flow Rate [kg/s]Fan Power at Design Air Flow Rate [W]
Cooling Towers 1-212343,000

Supervisory-level mode control

Zone air temperature setpoints and equipment enable/disable are determined by a schedule-based supervisory control scheme that defines a set of operating modes. The table below shows the zone air temperature setpoints, fan, and pump enable/disable.

Name Condition TZonHeaSet [oC] TZonCooSet [oC] Fan Pump TSupSet [oC] Economizer Min OA Flow
Occupied In occupied period. 20 24 Enabled Enabled 12.8 Enabled Ventilation
Unoccupied off In unoccupied period, all TZon within setback deadband. 15.6 26.7 Disabled Enabled (With the lowest flow rate) 12.8 Disabled Zero
Unoccupied, night setback In unoccupied period, triggered by minimum TZon below unoccupied TZonHeaSet. Minimum state time is 60 min. 15.6 26.7 Enabled Enabled 12.8 Disabled Zero
Unoccupied, night setup In unoccupied period, triggered by maximum TZon above unoccupied TZonCooSet. Minimum state time is 60 min. 15.6 26.7 Enabled Enabled 12.8 Disabled Zero

Once the operating mode is determined, a number of low-level, local-loop controllers are used to maintain the desired setpoints using the available actuators. The primary local-loop controllers are specified in the HVAC system control section.

HVAC system control

Air-side system control

\"AirSideControl.\"

VAV terminal control

VAV reheat control

The VAV reheat is controlled based on the "single maximum VAV reheat control logic".

  • When the Zone State is Cooling, the cooling-loop output is mapped to the active airflow setpoint, ranging from the Minimum Cooling Air Mass Flow Rate Setpoint to the Design Air Mass Flow Rate. The heating coil remains disabled.
  • When the Zone State is Deadband, the active airflow setpoint is set to the Minimum Cooling Air Mass Flow Rate Setpoint, and the heating coil remains disabled.
  • When the Zone State is Heating, the active airflow setpoint is set to the constant Heating Air Mass Flow Rate Setpoint. The reheat valve position is modulated from between 0-1 to maintain the zone air temperature at the heating setpoint.
  • VAV air flow rate control

    VAV damper position is controlled by a PI controller to maintain the air flow rate at setpoint. It takes the zone air flow rate measurements and setpoints as inputs. It takes the VAV damper position as the output.

    VAV supply air temperature control

    Heating coil valve position is controlled by a PI controller to maintain the supply air temperature at setpoint. It takes the supply air temperature measurements and setpoint as inputs. It takes the heating coil valve position as the output.

    AHU control

    AHU duct static pressure control

    The supply fan speed is regulated by a PI controller to maintain the duct static pressure at its setpoint of 400 Pa (or 1.61 in. w.c.). It takes the static pressure measurements and setpoints as inputs. It takes the supply fan speed as the output. The AHU return fan speed is set as a constant ratio (0.9) of the supply fan speed.

    During unoccupied hours, the night-cycle control is activated. If any zone air temperature falls outside the setback temperature bounds, the fans are switched on to bring the zone air temperature back within the allowable range.

    AHU supply air temperature control

    Cooling coil valve position is controlled by a PI controller to maintain the AHU supply air temperature at setpoint. It takes the supply air temperature measurements and setpoints as inputs. It takes the cooling coil valve position as the output.

    Mixing box damper and economizer control

    The minimum outdoor air damper position is 0.3 in order to provide ventilation. On top of that, an economizer control based on the fixed dry-bulb outdoor air temperature-based is adopted. The economizer higher temperature limit is set as 15.58 oC (60 oF), while the lower temperature limit is set as 0 oC (32 oF). Under economizer control, the outdoor air damper position is regulated by a PI controller to maintain the mixed air temperature at its setpoint, with a minimum damper position of 0.3. The controller uses the mixed air temperature, outdoor air temperature, and the mixed air temperature setpoint as inputs. Its output is the commanded outdoor air damper position. The return air damper is interlocked with the outdoor air damper as the inverse, while the exhaust air damper is interlocked with the outdoor air damper as sharing the same opening position. Note, however, that overwriting any of these damper positions bypasses the interlocks, thus they should be controlled as a group.

    AHU freeze protection coil control

    The freeze protection coil is turned on if the mixed air temperature is measured to be below the freeze protection temperature set point (3.33 oC) and the supply airflow rate is at or above 10% of the design value. Once activated, the coil remains on for at least 5 minutes and until the mixed air temperature raises above the set point. Overwriting the freeze coil control signal will bypass the minimum on-time, but will not bypass the airflow switch as a safety.

    Water-side system control

    \"ChillerControl.\"

    \"BoilerControl.\"

    Chiller control

    Chiller plant staging control

    The number of operating chillers is determined via a state machine based on the thermal load (Q, kW), rated chiller cooling capacity of chiller k (cck, kW), threshold to start chiller k+1 (ξk = 0.9), and waiting time (15 min). The maximum operating chiller number is N, which is equal to 3.

    The stage control logic is shown as the following figure.

    \"ChillerStage.\"

    Chilled water supply temperature control

    The model takes as an input the set point for the leaving chilled water temperature, which is met if the chiller has sufficient capacity. Thus, the model has a built-in, ideal temperature control.

    Secondary chilled water pump staging control

    The number of secondary chilled water pump is determined via a state machine based on the pump speed (S, rpm) and waiting time (30 min). The maximum operating pump number is M, which is equal to 2.

    The stage control logic is shown as the following figure.

    \"ChillerPumpStage.\"

    Secondary chilled water loop static pressure control

    Secondary chilled water pump speed is controlled by a PI controller to maintain the static pressure of the secondary chilled water loop at setpoint. It takes the chilled water loop static pressure measurements and setpoints as inputs. It takes the pump speed as the output. The operating secondary chilled water pumps share the same speed.

    Cooling tower control

    Cooling tower supply water temperature control

    Cooling tower fan speed is controlled by a PI controller to maintain the cooling tower supply water temperature at setpoint. It takes the cooling tower supply water temperature measurements and setpoints as inputs. It takes the cooling tower fan speed as the output. All the operating cooling towers share the same fan speed.

    Minimum condenser supply water temperature control

    Three-way valve position is controlled by a PI controller to maintain the temperature of the condenser water leaving the condenser water loop to be larger than 15.56 ℃. It takes the condenser supply water temperature measurements and setpoints as inputs. It takes the three-way valve position as the output.

    Boiler control

    Boiler staging control

    The number of operating boilers is determined via a state machine based on the thermal load(Q, kW), rated heating capacity of boiler k (hck, kW), threshold to start boiler k+1 (ξk = 0.9), and waiting time (30 min). The maximum operating boiler number is N, which is equal to 2.

    The stage control logic is shown as the following figure.

    \"BoilerStage.\"

    Boiler water temperature control

    Boiler heating power is controlled by a PI controller to maintain the temperature of the hot water leaving each boiler to be 80 ℃. It takes the hot water measurements and set points as inputs. It takes the heating power as the output.

    Boiler hot water loop static pressure control

    Boiler pump speed is controlled by a PI controller to maintain the static pressure of the boiler water loop at setpoint. It takes the heat water loop static pressure measurements and setpoints as inputs. It takes the pump speed as the output. All the boiler pumps share the same speed.

    Model IO's

    Inputs

    The model inputs are:

    Outputs

    The model outputs are:

    Forecasts

    The model forecasts are:

    Additional System Design

    Lighting

    Lighting heat gain is included in the internal heat gains and is not controllable.

    Shading

    There is no shading on this building.

    Onsite Generation and Storage

    There is no onsite generation or storage on this building site.

    Model Implementation Details

    Internal mass

    Building internal mass is modeled in EnergyPlus using two approaches: the InternalMass object and the Zone Air Capacitance Multiplier. Internal mass surface area values are taken from the DOE Reference Buildings, and a zone air capacitance multiplier of 8 is applied to represent typical internal-mass levels (Chen et al., 2022).

    Chen, Z. , Wen, J. , Bushby, S. , Lo, L. , O'Neill, Z. , Payne, V. , Pertzborn, A. , Calfa, C. , Fu, Y. , Grajewski, G. , Li, Y. and Yang, Z. (2022), An Analysis of the Hybrid Internal Mass Modeling Approach in EnergyPlus, Proceedings of eSim 2022: 12th Conference of IBPSA-Canada, Ottawa, CA, [online] , https://tsapps.nist.gov/publication/get_pdf.cfm?pub_id=934193 (Accessed September 12, 2025)

    Moist vs. dry air

    A moist air model is used. Relative humidity is tracked based on latent heat gain from occupants, outside air relative humidity, and a cooling coil model that includes condensation.

    Pressure-flow models

    The duct airflow and chilled/hot water hydronic loop are modeled using a pressure-flow network. No air exchange between zones.

    Infiltration models

    Air infiltration features have been incorporated into the exterior zones on all the floors. The specified infiltration rate (m_flow_infAir) is based on an air leakage rate of 1 cfm/ft² of exterior surface area, measured at a constant building pressure differential of 75 Pa. This value is then converted to a wind-driven infiltration rate at a reference wind speed of 4.47 m/s, following the methodology outlined in ASHRAE Standard 90.1-2022, Section G3.2.1.7. During the occupied hours, the infiltration schedule uses a fraction of 0.25 to approximate the reduced infiltration rate resulting from mechanical ventilation being active. This assumption aligns with the modeling rules in Appendix C of ASHRAE Standard 90.1-2022. Additionally, the infiltration rate is dynamically adjusted to account for variations in wind speed.

    CO2 models

    CO2 generation is 0.0048 L/s per person (Table 5, Persily and De Jonge 2017) and density of CO2 assumed to be 1.8 kg/m3, making CO2 generation 8.64e-6 kg/s per person. Outside air CO2 concentration is 400 ppm.

    Persily, A. and De Jonge, L. (2017). Carbon dioxide generation rates for building occupants. Indoor Air, 27, 868–879. https://doi.org/10.1111/ina.12383.

    Scenario Information

    Time Periods

    The Peak Heat Day (specifier for /scenario API is 'peak_heat_day') period is:

    The Typical Heat Day (specifier for /scenario API is 'typical_heat_day') period is:

    The Peak Cool Day (specifier for /scenario API is 'peak_cool_day') period is:

    The Typical Cool Day (specifier for /scenario API is 'typical_cool_day') period is:

    The Mix Day (specifier for /scenario API is 'mix_day') period is:

    Energy Pricing

    Constant electricity prices are based on those from ComEd [1], the utility serving the greater Chicago area. The price is based on the Basic Electricity Service (BES) rate provided to the Watt-Hour customer class for applicable charges per kWh. This calculation is an approximation to obtain a reasonable estimate of price. The charges included are as follows:

    The total constant electricity price is $0.094/kWh

    Dynamic electricity prices are based on those from ComEd [1], the utility serving the greater Chicago area. The price is based on the Residential Time of Use Pricing Pilot (RTOUPP) rate for applicable charges per kWh. This calculation is an approximation to obtain a reasonable estimate of dynamic price. The charges included are the same as the constant scenario (using BES) except for the following change:

  • Retail Purchased Electricity Charge:

    Summer (Jun, Jul, Aug, Sep):

    Winter:

  • Highly Dynamic electricity prices are based on those from ComEd [1], the utility serving the greater Chicago area. The price is based on the Basic Electric Service Hourly Pricing (BESH) rate for applicable charges per kWh. This calculation is an approximation to obtain a reasonable estimate of highly dynamic price. The charges included are the same as the constant scenario (using BES) except for the following change:

  • PJM Services Charge: $0.00836
  • Retail Purchased Electricity Charge: Based on Wholesale Day-Ahead Prices for the year of 2019 based on [2].
  • Gas price is assumed constant and of 0.024 $/kWh as obtained from the Nicor Gas for Jan. 2023 https://www.icc.illinois.gov/natural-gas-choice/purchased-gas-adjustment-rates (accessed on Aug 2023).

    References:

  • [1] https://www.comed.com/MyAccount/MyBillUsage/Pages/CurrentRatesTariffs.aspx
  • [2] https://secure.comed.com/MyAccount/MyBillUsage/Pages/RatesPricing.aspx
  • Emission Factors

    The Electricity Emissions Factor profile is based on the average annual emissions from 2019 for the state of Illinois, USA per the EIA. It is 752 lbs/MWh or 0.341 kgCO2/kWh. For reference, see https://www.eia.gov/electricity/state/illinois/

    The Gas Emissions Factor profile is based on the kgCO2 emitted per amount of natural gas burned in terms of energy content. It is 0.18108 kgCO2/kWh (53.07 kgCO2/milBTU). For reference, see: https://www.eia.gov/environment/emissions/co2_vol_mass.php

    Weather Forecast Uncertainty: Temperature

    Options for /scenario API are 'low', 'medium', or 'high'. Empty or None will lead to deterministic forecasts. See the BOPTEST design documentation for more information.

    Weather Forecast Uncertainty: Global Horizontal Irradiation (GHI)

    Options for /scenario API are 'low', 'medium', or 'high'. Empty or None will lead to deterministic forecasts. See the BOPTEST design documentation for more information.