.Buildings.Fluid.FMI.ExportContainers.HVACZones

Information

Model that is used as a container for an HVAC system that is to be exported as an FMU and that serves multiple zones.

Typical use and important parameters

To use this model as a container for an FMU, simply extend from this model, rather than instantiate it, and add your HVAC system. By extending from this model, the top-level signal connectors on the right stay at the top-level, and hence will be visible at the FMI interface. The example Buildings.Fluid.FMI.ExportContainers.Examples.FMUs.HVACZones shows how a simple HVAC system that serves two rooms can be implemented and exported as an FMU. The example Buildings.Fluid.FMI.ExportContainers.Validation.RoomHVAC shows how such an FMU can be connected to a room model that has signal flow.

The following two parameters need to be assigned by the user: Set nZon to the number of thermal zones to which the FMU will be connected. Set nPorts to the largest number of fluid ports that the thermal zones has. For example, if nZon=2 and zone 1 has one inlet and one outlet (hence it has 2 ports), and zone 2 has one inlets and two outlets (hence it has 3 ports), then set nPorts=3. This will add more fluid ports than are needed for zone 1, but this causes no overhead if they are not connected.

The conversion between the fluid ports and signal ports is done in the HVAC adapter hvacAda. This adapter has a vector of fluid ports called ports. The supply and return air ducts, including any resistance model for the inlet diffusor or exhaust grill, need to be connected to these ports. Also, if a thermal zone has interzonal air exchange or air infiltration, these flows need to be connected to ports. This model outputs at the port fluPor the mass flow rate for each flow that is connected to ports, together with its temperature, water vapor mass fraction per total mass of the air (not per kg dry air), and trace substances. These quantities are always as if the flow enters the room, even if the flow is zero or negative. If a medium has no moisture, e.g., if Medium.nXi=0, or if it has no trace substances, e.g., if Medium.nC=0, then the output signal for these properties are removed. These quantities are always as if the flow enters the room, even if the flow is zero or negative. Thus, a thermal zone model that uses these signals to compute the heat added by the HVAC system need to implement an equation such as

Q_sen = max(0, ṁ_sup)   c_p   (T_sup - T_air,zon),

where Q_sen is the sensible heat flow rate added to the thermal zone, ṁ_sup is the supply air mass flow rate from the port fluPor (which is negative if it is an exhaust), c_p is the specific heat capacity at constant pressure, T_sup is the supply air temperature and T_air,zon is the zone air temperature. Note that without the max(·, ·), the energy balance would be wrong.

The input signals of this model are the radiative temperature of each zone. The the zone air temperatures, the water vapor mass fractions per total mass of the air (unless Medium.nXi=0) and trace substances (unless Medium.nC=0) are obtained from the connector fluPor.backward. The outflowing fluid stream(s) at the port ports will be at the states obtained from fluPor.backward. For any given i_zon ∈ {1, ..., n_zon}, for each i_ports ∈ {1, ..., n_ports} all fluid streams at port ports[izon, iports] are at the same pressure. For convenience, the instance hvacAda also outputs the properties obtained from fluPor.backward. These can be used to connect a controller. The properties are available for each flow path in fluPor.backward. For a thermal zone with mixed air, these are all equal, while for a stratified room model, they can be different.

See Buildings.Fluid.FMI.ExportContainers.Examples.FMUs.HVACZones for a model that uses this model.

For models that only have one thermal zone connected to the HVAC system, use the simpler model Buildings.Fluid.FMI.ExportContainers.HVACZone.

Assumption and limitations

The mass flow rates at ports sum to zero, hence this model conserves mass for each thermal zone.

This model does not impose any pressure, other than, for any given i_zon ∈ {1, ..., n_zon} and for each j,k ∈ {1, ..., n_ports}, setting the pressure of ports[izon, j].p = ports[izon, k].p to be the same. The reason is that setting a pressure can lead to non-physical system models, for example if a mass flow rate is imposed and the HVAC system is connected to a model that sets a pressure boundary condition such as Buildings.Fluid.Sources.Outside. Also, setting a pressure would make it impossible to use multiple instances of this model (one for each thermal zone) and build in Modelica an airflow network model with pressure driven mass flow rates.

The model has no pressure drop. Hence, the pressure drop of an air diffuser or of an exhaust grill needs to be modelled in models that are connected to ports.

Contents

Name	Description
Medium	Medium in the component

Revisions

• January 18, 2019, by Jianjun Hu:
  Limited the media choice to moist air only. See #1050.
• May 25, 2016, by Michael Wetter:
  First implementation.