Can't control chilled water loop max temperature

Hello,

I try to build simple simulation for single zone temperature and humidity control in open studio, using air side with chilled water coil and hot water coil. The air loop is working fine, the water loop however still got problem. I want to simulate chiller water loop with CHWS/CHWR 5/10 C, but the simulation result got the maximum evaporator inlet water temperature at 6.4, makin the Chiller COP very low.

I've set in the plant loop settings the maximum loop temperature at 10 C and minimum loop temperature at 5 C. I got maximum and minimum flowrate autosized. I also set the design loop exit temperature at 5 C and Loop design temperature difference at 5 K. Still, the inlet evaporator temperature max always at 6.4 C.

In CHW loop, I only put pipe adiabatic and cooling coil in demand equipment and only chiller in supply equipment (and set point manager, set at 5 C)

this is the file for the model https://drive.google.com/file/d/1RbNi...

Please help, thank you!!

syauqi

asked 2024-02-25 22:05:30 -0500, updated 2024-02-26 19:15:49 -0500

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@syauqi can you upload your model to Google Drive, Dropbox, etc. and then share a URL for others to download? That would help others provide better solutions.

Aaron Boranian (2024-02-26 10:42:23 -0500) edit

I've just add the link to the model. Thanks!

syauqi (2024-02-26 19:16:47 -0500) edit

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This is only a partial diagnosis of your question.

Right now your chiller is set to constant flow, in the .idf:

  ConstantFlow,                           !- Chiller Flow Mode

And your setpoint manager is controlling the outlet temperature, keeping it fixed at 5oC (Actually I think the setpoint is set at 0.5oC but the minimun loop temperature forbids it from going lower).

With constant flow and fixed outlet (Impulsion) temperature, the only variable for the heat balance to satisfy the demand is changing the return temperature. (In Q=mSpDT; m=cte; Sp=cte; DT=T_return-5).

Here is the input output guide of the chiller for other modes of modulation: https://bigladdersoftware.com/epx/docs/24-1/input-output-reference/group-plant-equipment.html#field-chiller-flow-mode

Two possible solutions I can think of:

Change the HVAC system to use variable flow and somehow keep the DT fixed (I myself don't know how to make that work in EP). Keep in mind that also the pumps should change to variable flow in this case.
Keep the constant flow but control the return temperature instead of the evaporator outlet (Impulsion) temperature. I found degree of success with this approach using water-to-water heat pumps and the return temperature setpoint manager (https://bigladdersoftware.com/epx/docs/24-1/input-output-reference/group-setpoint-managers.html#setpointmanagerreturntemperaturechilledwater)

If you find a solution that works, I encorage you to post it here!

Good luck with your simulations, PMP