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Why the cooling electricity usage increase a lot after install pump VFD

asked 2016-02-25 14:44:50 -0500

lyw0211 gravatar image

updated 2016-02-25 15:08:59 -0500

There is very interesting output of cooling electricity usage after changing from constant volume to variable volume for chilled water pump by using OpenStudio modeling. The output showed that the cooling electricity peak demand increased a lot on May compared with the baseline model. At the same time, the heating peak demand also increased a lot on the same month. Does anyone know why it happens like that?

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More information will be required to find the root cause. For example, what kind of valves do you have on your chilled water coils. Did you put the VFD on the primary or secondary chilled water pump? The results could potentially be entirely reasonable depending on your inputs and what you're trying to model. As with most energy modeling tasks - the devil is in the details.

Lincoln gravatar image Lincoln  ( 2016-02-25 15:29:48 -0500 )edit

@Lincoln Thanks for your comments. Actually, this is a very complicated model with both secondary and primary CHW and HHW loops there. There are a lot of chilled water coils in the model and it is hard to show all the values for clarifying my question. I'll focus on the details to find the answer.

lyw0211 gravatar image lyw0211  ( 2016-02-26 09:46:25 -0500 )edit

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answered 2016-03-05 13:36:52 -0500

301_Hours gravatar image

The inefficiency of the VFD is the only thing that should increase the pump power, which would be ~5%. If you have a load that generally requires constant volume, then a VFD would be worse performing. If it's a huge increase in energy then something else is going on in the model.

An improper schedule that wasn't active with a constant volume pump, but became active with the VFD might be a possible explanation.

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Thank you for your comments! That will be helpful!

lyw0211 gravatar image lyw0211  ( 2016-03-09 11:23:08 -0500 )edit

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Asked: 2016-02-25 14:44:50 -0500

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Last updated: Mar 05 '16