General

This is the multi zone office air simple emulator model of BOPTEST, emulating a 5-zone single-duct VAV system. It is based on the Modelica model Buildings.Examples.VAVReheat.ASHRAE2006. with the addition of CO2 concentration tracking, variable efficiency heat pump model, variable efficiency chiller model, and BOPTEST signal exchange blocks. The emphasis is on the control of the air distribution system and not on the control of the central plant equipment providing hot and chilled water to the distribution system.

Building Design and Use

Architecture

The test case represents the middle floor of an office building located in Chicago, IL, as described in the set of DOE Commercial Building Benchmarks for new construction (Deru et al, 2009) and shown in the figure below. The represented floor has five zones, with four perimeter zones and one core zone. Each perimeter zone has a window-to-wall ratio of 0.33. The height of each zone is 2.74 m and the areas are as follows:

The upper and lower boundaries of the zones (top of zone air volume and bottom of 0.1 m floor concrete slab) assume the corresponding zones of the floor above and the floor below are at the same conditions as the middle floor. Therefore, the roof and the ground floor with exposure to ambient conditions are not explicitly modeled. The zones on the floor are assumed to be paritioned by internal walls with 10 m by 2.1 m openings, through which air is exchanged.

\"Floor

Deru M., K. Field, D. Studer, K. Benne, B. Griffith, P. Torcellini, M. Halverson, D. Winiarski, B. Liu, M. Rosenberg, J. Huang, M. Yazdanian, and D. Crawley. DOE commercial building research benchmarks for commercial buildings. Technical report, U.S. Department of Energy, Energy Efficiency and Renewable Energy, Office of Building Technologies, Washington, DC, 2009.

Constructions

The envelope thermal properties meet ASHRAE Standard 90.1-2004.

Occupancy schedules

The design occupancy density is 0.05 people/m2. The number of occupants present in each zone at any time coincides with the internal gain schedule defined in the next section. The occupied time for the HVAC system is between 6 AM and 7 PM each day. The unoccupied time is outside of this period.

Internal loads and schedules

The design internal gains including lighting, plug loads, and people are combined 20 W/m2 with a radiant-convective-latent split of 40%-40%-20%. The internal gains are activated according to the schedule in the figure below.

\"Internal

Climate data

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

HVAC System Design

Primary and secondary system designs

The HVAC system is a multi-zone single-duct Variable Air Volume (VAV) system with pressure-independent terminal boxes with reheat. A schematic of the system is shown in the figure below. The cooling and heating coils are water-based served by an air-cooled chiller and air-to-water heat pump respectively. The available sensor and control points, marked on the figure below and described in more detail in the Section Model IO's, are those specified as required by ASHRAE Guideline 36 2018 Section 4 List of Hardwired Points, specifically Table 4.2 VAV Terminal Unit with Reheat and Table 4.6 Multiplie-Zone VAV Air Handling Unit, as well as some that are specified as application specific or optional.

\"Schematic

Equipment specifications and performance maps

The terminal box dampers have exponential opening characteristics with design airflow rates defined in the table below. The design system airflow rate includes a 0.7 load diversity factor and is defined in the table below. The minimum outside airflow for each zone is calculated using outside airflow rates of 0.3e-3 m3/s-m2 and 2.5e-3 m3/s-person. The limiting zone air distribution effectiveness is 0.8 and the occupant diversity ratio is 0.7. This leads to the minimum outside airflow rates for each zone and system defined in the table below.

Table 1: Zone Terminal Unit and System Specifications Summary
Name Design Airflow [m3/s] Min OA Airflow [m3/s] Design Heating Load [kW]
North 0.947948667 0.1102769 6.87
South 0.947948667 0.1102769 6.87
East 0.9001996 0.0698148 6.52
West 0.700155244 0.0698148 5.07
Core 4.4966688 0.5231070 32.6
System 5.595044684 0.8590431

The supply fan hydraulic efficiency is constant at 0.7 and the motor efficiency is constant at 0.7. The cooling coil is served by an air-cooled chiller supplying 6 degC water with varying COP according to a York YCAL0033EE chiller as modeled by the ElectricEIR model with coefficients defined in EnergyPlus v9.4.0. The peak design load on the chiller is 101 kW, equal to the design load on the cooling coil. A chilled water distribution pump circulates water from the chiller with a design head of 45 kPa and design mass flow equal to that of the chiller of 4.3 kg/s. The heating coil and terminal box reheat coils are served by a single air-to-water heat pump supplying 45 degC water with varying COP as 0.3 of the carnot COP. The peak design load on the heat pump is 122 kW, equal to sum of design loads on the heating coil in the AHU plus zone terminal box reheat coils multiplied by a load diversity factor of 0.85. A hot water distribution pump circulates water from the heat pump with a design head of 45 kPa and design mass flow equal to that of the heat pump of 2.9 kg/s.

Rule-based or local-loop controllers (if included)

The baseline control emulates a typical scheme seen in practice and is based on the ASHRAE VAV 2A2-21232 of the Sequences of Operation for Common HVAC Systems 2006 as well as that which is implemented as baseline control in the Modelica Buildings Library model Buildings.Examples.VAVReheat.ASHRAE2006. Setpoints and equipment enable/disable are determined by a schedule-based supervisory control scheme that defines a set of operating modes. This scheme is summarized in Table 2 below. An addition is made in this implementation from the one in Modelica Buildings Library to add an Unoccupied Night Set Up mode, which allows for the system turning on during unoccupied hours to maintain a cooling set point.

Table 2: HVAC Operating Mode Summary
Name Condition TZonHeaSet [degC] TZonCooSet [degC] Fan [degC] TSupSet [degC] Economizer Min OA Flow
Occupied In occupied period. 20 24 Enabled 12 Enabled Ventilation
Unoccupied off In unoccupied period, all TZon within setback deadband. Minimum state time is 1 min. 12 30 Disabled 12 Disabled Zero
Unoccupied, night setback In unoccupied period, minimum TZon below unoccupied TZonHeaSet. Minimum state time is 30 min. 12 30 Enabled 35 Disabled Zero
Unoccupied, night setup In unoccupied period, maximum TZon above unoccupied TZonCooSet. Minimum state time is 30 min. 12 30 Enabled 35 Enabled Zero
Unoccupied, warm-up In unoccupied period, within 30 minutes of occupied period, average TZon below occupied TZonHeaSet 20 30 Enabled 35 Disabled Zero
Unoccupied, pre-cool In unoccupied period, within 30 minutes of occupied period, outside TDryBul below limit of 13 degC, average TZon above occupied TZonCooSet 12 24 Enabled 12 Enabled 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 on the diagram above as C1 to C3.

C1 is responsible for maintaining the zone temperature setpoints as determined by the operating mode of the system and implements dual-maximum logic, as shown in the Figure below. It takes as inputs the zone temperature heating and cooling setpoints and zone temperature measurement, and outputs the desired airflow rate of the damper and position of the reheat valve. Seperate PI controllers (both k = 0.1 and Ti = 120 s) are used for control of the damper airflow for cooling and reheat valve position for heating. If the zone requires heating, the desired airflow rate of the damper is mapped to the specified maximum value for heating.

\"Controller

C2 is responsible for maintaining the duct static pressure setpoint and implements a duct static pressure reset strategy. The first step of the controller takes as input all of the terminal box damper positions and outputs a duct static pressure setpoint using a PI controller (k = 0.1 and Ti = 60 s) such that the maximum damper position is maintained at 0.9. The second step then maintains this setpoint using a PI controller (k = 0.5 and Ti = 15 s) and measured duct static pressure as input to output a fan speed setpoint.

\"Controller

C3 is responsible for maintaining the supply air temperature setpoint as well as the minimum outside air flow rate as determined by the operating mode of the system. It takes as inputs the supply air temperature setpoint, supply air temperature measurement, outside drybulb temperature measurement, and supply fan speed. The first part of the controller uses a PI controller (k = 0.01, Ti = 120 s) for supply air temperature setpoint tracking to output a signal that is then mapped to position setpoints for the heating coil valve, cooling coil valve, and outside air damper position. If the heating valve is commanded to open and the supply fan is determined to be enabled (either by schedule or by supply air flow detection of at least 5% design air flow), the heating coil pump is enabled. Similar for cooling coil. The second part of the controller uses a map to determine the minimum outside air damper position for the given fan speed to provide minimum outside air requirements. Interpolation points for the map are assumed to be provided during commissioning and are given as follows. For a fan speed of 0.44, the minimum outside air damper position is 0.47. For a fan speed of 1.0, the minimum position is 0.32. The maximum of the two outside air damper position signals is finally output to ensure at least enough enough airflow is delivered for ventilation when needed. The economizer is enabled only if the outside drybulb temperature is lower than the return air temperature.

\"Controller

Also present, but not depicted in the diagrams above, is a freeze stat controller. This controller detects potentially freezing conditions by measuring the mixed air temperature and determining if it is less than a limit, 3 degC. If true, the controller will fully open the heating coil valve and enable the heating coil pump.

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

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 is modeled using a pressure-flow network. Air exchange between zones is modeled through an open door model implemented with Buildings.Airflow.Multizone.DoorOpen.

Infiltration models

Airflow due to infiltration is calculated based on time-varying wind pressure coefficients for each facade using Buildings.Fluid.Sources.Outside_CpLowRise.

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].

    • 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/