Ability to use a basic ESP-r HVAC model to manipulate conditions within a room.

Task

Instructions

1. Access and explore the HVAC model.

Start up ESP-r and choose open existing and then exemplar options and select the base case cellular model with component-based HVAC from the technical features category. Copy the model into your home directory. Minimise the plant schematic diagram that appears for future reference. Explore the basic features of the model by selecting browse/edit/simulate and Networks - plant & systems.

Using the menu interface look at components and connections and see how an HVAC network consists of a group of HVAC component models connected together. Compare what you see on the menus to the schematic diagram. Selecting Link plant to zone(s) shows how the plant network is coupled into the rest of the building model with a supply and/or extract component linking into a building zone - again compare this with the schematic diagram.

2. Manipulate the HVAC control settings for the plant system and examine the output of the HVAC model.

Return to the Browse/Edit/Simulate menu and select Controls - plant and systems. Notice that there are several control loops defined. Each loop controls an individual HVAC component. Select loop 6, which controls the humidifier and edit the period data. Choose period 2 PID flow control - select controller type l, which senses relative humidity (RH) and actuates the flow of water to the humidifier, and control law c (P,PI,PID). Proportional control will be used in this case.

Enter data as follows:

1. Set start time to 7.0.
2. Set PID mode to 1 - proportional only.
3. Set number of misc data items to 7.
4. Change the control values from 0. 0. 50.0 10. to 0.001 0.0 70.0 10.0
5. The integral action flag and derivative action flags should be set to 0.

Run the results analysis and read in /tmp/cellular_hvac_winter.res. Select plant results and read in the plant results file /tmp/cellular_hvac_winter.plr. Select output -> psychrometric chart -> generate chart. ESP-r will display a psychrometric chart for plotting the state of plant components on this. The x-axis shows the temperature of the air in the HVAC plant and the y-axis shows the amount of moisture in the air.

Choose select components -> all items. Then choose to display one timestep - look at timestep 660. The chart now shows what is happening inside the HVAC plant - can you identify mixing; humidification and re-heating processes? Looking at the plant network schematic diagram - can you identify which components perform these processes? Finally look at the humidification process - can you describe what is happening to temperature and moisture content?

Return to the Psychrometric menu and select output -> time series plot -> 2nd phase flow -> humidifier and draw the graph. This shows how the flow of water into the humidifier is controlled to obtain the 70% RH setpoint. Clear this graph and select output -> time series plot -> relative humidity% -> ex_duct_1_7 and plot the graph. Component ex_duct_1_7 draws air from zone manager_a and so this shows the zone humidity. How successful is the HVAC system at holding RH to 70%? Also look at the space temperature: select output -> time series plot -> temperature ex_duct_1_7. This shows the temperature in office manager_a. How successful is the HVAC system at holding temperature to the set point of 19°C? Select output -> summary statistics -> relative humidity% -> ex_duct_1_7 and for additional output -> re-heat_coil -> heating output again noting the values. This shows the amount of heat needed to maintain the room at the desired temperature.

3. Manipulate the HVAC control settings and investigate the change in performance of the HVAC system.

Repeat the process of editing the humidifier control loop 6. Keep all the control settings the same as before, except that when prompted set the control values to 0.0 0.0 70.0 10.0. This effectively closes down the humidifier by setting the maximum allowable water flow rate to 0 kg/s. Save the new control settings and re-run the simulation. Select the results analysis and read in the new results file e.g. /tmp/cellular_hvac_winter.res.

Select plant results and read in the new plant results file e.g. /tmp/cellular_hvac_winter.plr. Select output -> psychrometric chart -> generate chart and plot the state of all of the plant components as before for timestep 660. Can you see a difference between this and the last simulation? Which process (humidification, re-heat or mixing) is now missing? Clear this graph and return to the Psychrometric menu and select output -> time series plot -> relative humidity% -> ex_duct_1_7 and plot the graph. What is the difference between this and the humidity plot from the previous example? Select output -> summary statistics -> additional output -> re-heat_coil -> heating output and then generate the report. What has happened to the amount of heating energy supplied by the heating coil? Also look at the space temperature. Select output -> time series plot -> temperature ex_duct_1_7. How does this compare to the temperature plot from the previous simulation?

What conclusions can you draw from changing the humidifer control settings, looking at the psychrometric plots and the time-series plots of RH and temperature?

4. Run a summer simulation and examine the operation of the plant in cooling mode.

Return to the project manager Browse/Edit/Simulate menu and select browse/edit/simulate -> simulation -> simulation presets -> summer. Run an integrated simulation. Analyse the results from /tmp/cellular_hvac_summer.res and look at plant results /tmp/cellular_hvac_summer.plr. Select output -> time series plot -> temperature -> ex_duct_1_7 and temperature -> supply_duct7. Can you explain why the supply duct temperature profile is as shown (hint: remember that we are cooling the space and think about the level of heat gains during the day!). How successful is the HVAC system at maintaining temperatures at the set point of 23°C?

Select output -> psychrometric chart -> generate chart and select components -> all items. Now display the HVAC system state at timestep 780. There are two processes occurring - mixing and cooling/dehumidification. Can you identify them from the chart?

Finally clear all previous selections and select output -> summary statistics -> additional output -> cooling coil and look at the load on the cooling coil. Note down the values.

5. Alter the amount of re-circulation in the plant system and examine the impact on the cooling load.

Return to the Project Manager and select plant and systems under the networks section. Using the menu interface select connections. Select b (the second connection) and choose to edit the connection.

Select the mixing box as the receiving component then select option c - from another component then select fan2 as the sending component. Set the mass diversion ratio to 0.5. This has the effect of reducing the amount of cool extract air recirculated and mixed with the incoming supply air from 80% to 50%.

Can you identify this recirculation on the plant system schematic?

Also edit connection t (the last connection) to change the mass diversion ratio from 0.2 to 0.5. Again choose to edit the connection. In this case the receiving component is supply_duct1, the connection type is e - from ambient air and the sending component is fan2. Set the mass diversion ratio to 0.5. This increases the amount of air rejected to outside from 20% to 50%.

Update the plant configuration file and then return to the main menu and re-run the simulation, remembering to use the summer simulation pre-sets as before. Analyse the results from /tmp/cellular_hvac_summer.res and look at plant results /tmp/cellular_hvac_summer.plr. (If you have changed the results file names for these new simulations use these new names instead.)

Look at the resulting room temperature by selecting output -> time series plot -> temperature -> ex_duct_1_7. Is there much difference from the previous simulation? Now look at the load on the cooling coil. Select output -> summary statistics -> additional output -> cooling coil -> cooling output and then generate the report. What effect has reducing the recirculation had?