Modelica Cooling Tower Energy Balance

asked 2023-01-01 20:27:41 -0600

glinn07
285 ●4 ●6 http://www.hablab.llc

updated 2023-01-04 10:19:50 -0600

Aaron Boranian

14250 ●86 ●24 http://bigladdersoftware.com/

Hello,

I am running a District Energy System in Modelica/Dymola using the geojson to modelica translator and Modelica buildings library. My model has 150 buildings attached to the District HW and CHW loops.

I am having an issue with cooling plant operation.

Whenever the cooling plant is enabled, I get massive reduction in Integrator time step. I have traced the issue down to the calculation of energy balance for the fluid volume inside the cooling tower.

From what I can tell, the solver hits major errors whenever the value of the energy balance in the cooling tower fluid volume changes significantly. The strongest driver of changes to the energy balance is the outside air temperature.

I cannot figure out how to deal with this issue... theres nothing I can do the reduce the oscillations in outside air temperature. How can I reduce the sensitivity of the energy balance equation to these changes?

Thanks, Graham

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answered 2023-01-04 13:51:46 -0600

Michael Wetter
1695 ●12 ●4 http://simulationresea...

It is not uncommon that one or a few variables dominate the integration error. This typically happens if they have fast dynamics, such as because of low storage capacity or fast changing input. For the cooling tower, this can be the water mass flow rate and water temperature that flows through the tower, or the outdoor temperature but the latter only changes hourly. If these inputs change faster than you expect, it may indicate a controller that can be tuned better, such as the controller of a chiller that is connected to the cooling tower, or the controller of the condenser water pump (or condenser water valve if present).

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