How to model Central Plant System using a Scheduled Load?

Introduction

Users can model a mechanical central plant system in IESVE, with no associated geometry, using a “Scheduled Load”; thus avoiding the need to include any model geometry. A Scheduled load is an hourly or sub-hourly representation of the cooling or heating demand load that can be assigned to a Chilled Water Loop (ChWL), Hot Water Loop (HWL) or Heat Transfer Loop (HTL) respectively without a need for associated geometry, templates, gains, constructions, etc.

In order to create a ‘geometry-less’ HVAC system, the user will need to complete four steps.

Step 1: Obtain the annual hourly (or sub-hourly) cooling and/or heating Load Data

If the user has obtained the annual hourly (or sub-hourly) cooling and/or heating Load from a BMS (or any other non-IESVE approach) then please skip to Step 2. Make sure that the loads are in Excel format.

Note that the sub-hourly data needs to be at 1 min, 2 mins, 6 mins, 10 mins, or 30 mins.

Otherwise, if the user wants to use the cooling/heating load of an existing model as a scheduled load, then they will need to run the following steps:

a) Run the simulation of the building that has been modeled in VE.
b) In the VistaPro application, select the appropriate loads, and ‘Table’; then copy. Alternatively, open the HTL, ChWL, or HWL (VistaPro’s HVAC view) to obtain the hydronic load on the loop.

Note that separate .fdd files will need to be created for Heating load and Cooling load for HTL.

c) Paste the cooling and/or heating load data to an excel sheet.
d) Make a note of the maximum hourly cooling/heating load for the year. This will be used to run sizing calculations. This can be done in VistaPro, using ‘Synopsis’ function. Close this model.

Step 2: Creating a new IESVE Model for modeling the Plant System Only

a) Create a new 1-zone VE model of any area, with no internal gains and default construction. Ensure a high cooling setpoint (150 F or 65 C) and a low heating setpoint (5 F or -15 C) is enabled so that the airside system remains inactive.
b) Assign the weather file associated with the model.
c) Select an HVAC system that best represents the water side of the user’s model.

Note this step needs to be done as the VE requires geometry (1 zone) and an airside HVAC system to run sizing calculation and whole year energy simulation.

Step 3: Create a Free-form Profile (.ffd file) that represents the load by creating an annual .csv file

a) Open “VE Python Scripts” Navigator in IESVE; see “IES Sample Scripts”.
b) Click on “Create Annual Data for Free-form Profile (.csv file).” This will open a dialog box called “Create FDD .csv.” make sure that:

         - The “Timestep” is the same as your cooling and/or heating load timestep.
         - Switch the “Type” to “Absolute.”
         - Make sure you have the correct “Unit” selected. If the loads are in kW, select “Metric” and if they are in BTU/hr then select “IP.”
         - Give an appropriate name to your .csv file. Note that this will be the name of your .fdd file.  Click “OK”. This will create a “fdd_data” folder in your project model. This folder will contain the generated .csv file.

c) Open the .csv file and copy the load from the excel sheet from Step 1 over to column E in the new CSV file. There should be 8,760 cells of copied data.

d)       To create the annual Free-form profile (.fdd file), click on “Create Annual Load Free-form Profile (.fdd file)” in the Navigator.

e)      Open the .csv file created in step 3 (b). You will get a message saying “Free-form profile added to project…” and an FFD profile will be added to your model.

f) Create and Assign an “Associated Profile” in ApPro, by creating an absolute constant weekly profile.  It is recommended that the value should be the same as the maximum (peak) value of the exported load. This associated profile will be used in the sizing calculation.

g)  Assign the Associated profile to the .fdd profile in ApPro.

Step 4: Assigning the .fdd file to the ChWL and/or HWL as a Scheduled Load.

a)       Open the HTL and/or ChWL loop and assign the respective .fdd file.

b) After setting up the appropriate Plant; perform system sizing and energy simulation with the associated central plant.  Room sizing is not necessary as there are no rooms or room-based coils to consider.

Possible Error Message:  If the user gets the following error message while running the sizing calculation, ensure that the Associated profile was created and assigned correctly. Refer to Step 1(d) and Step 3(f) and 3(g).