modelSystem3

Information

This part of the system model modifies Buildings.Examples.Tutorial.SpaceCooling.System2 to use the actual outside temperature for a summer day, and it adds closed loop control. The closed loop control measures the room temperature and switches the chilled water flow rate on or off.

Implementation

This section describes how we modified Buildings.Examples.Tutorial.SpaceCooling.System2 to build this model.

1. The first step was to copy the model Buildings.Examples.Tutorial.SpaceCooling.System2.

2. Next, we changed in weaDat the parameter that determines whether the outside dry bulb temperature is used from the weather data file or set to a constant value. This can be accomplished in the GUI of the weather data reader as follows:

   

With this change to using real weather data, we also change the simulation time to be one day during the summer, where the start time is 4320 h (15552000 s) and the stop time is 4344 h (15638400 s).

If the model is now simulated, the following plot could be generated that shows that the room is cooled too much due to the open loop control:

To add closed loop control, we proceeded as follows.

3. First, we made an instance of Buildings.Controls.OBC.CDL.Reals.Subtract and set its name to sub. It calculates the difference between the set point and the measured temperature.

4. For the set point, we made the instance TRooSetPoi to feed a constant set point into the instance sub.

5. The output of the instance sub was then fed as the input to con, which is an instance of Buildings.Controls.OBC.CDL.Reals.Hysteresis. For the instance con, we set the parameter for the lower limit to -0.5 and the upper limit to 0.5.

6. The instance senTemRoo has been added to measure the room air temperature. Note that we decided to measure directly the room air temperature. If we would have used a temperature sensor in the return air stream, then its temperature would never change when the mass flow rate is zero, and hence it would not measure how the room temperature changes when the fan is off.

7. Since the controller output is a boolean signal, but the instance souWat needs a real signal as an input for the water mass flow rate, we needed to add a conversion block. We therefore replaced the instance mWat_flow from a constant block to the block Buildings.Controls.OBC.CDL.Conversions.BooleanToReal. Because the cooling control has a reverse action, i.e., if the measured value exceeds the set point, the system should switch on instead of off, we configured the parameters of the conversion block as follow:

     realTrue=0
     realFalse=mW_flow_nominal
   

   This will output mW_flow_nominal when the room temperature is above the set point, and 0 otherwise.

This completes building the model shown in the figure on Buildings.Examples.Tutorial.SpaceCooling. When simulating the model, the response shown below should be seen.

Notes

To add a continuous controller for the coil water flow rate, we could have used the model Buildings.Controls.OBC.CDL.Reals.PID.

Parameters

Connectors

Components

Contents

Name	Description
MediumA	Medium for air
MediumW	Medium for water

Revisions

• April 9, 2024, by Hongxiang Fu:
  Specified nominalValuesDefineDefaultPressureCurve=true in the mover component to suppress a warning. This is for #3819.
• December 11, 2023, by Jianjun Hu:
  Reimplemented on-off control to avoid using the obsolete OnOffController. This is for #3595.
• September 20, 2021 by David Blum:
  Correct supply and return water parameterization.
  Use design conditions for UA parameterization in cooling coil.
  Use explicit calculation of sensible and latent load to determine design load on cooling coil.
  This is for #2624.
• January 28, 2015 by Michael Wetter:
  Added thermal mass of furniture directly to air volume. This avoids an index reduction.
• December 22, 2014 by Michael Wetter:
  Removed Modelica.Fluid.System to address issue #311.
• January 11, 2012, by Michael Wetter:
  First implementation.