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Fan motor replacement savings

asked 2016-01-29 09:36:51 -0500

FrontierAssoc104's avatar

updated 2020-03-08 11:26:43 -0500

Right now I am deciding between writing a hand calculation (for which sources are readily available) to calculate energy savings for replacing a supply fan electric motor with one of higher efficiency and of the same size. The energy savings would need to be general enough to have energy/demand savings for motor replacements of varying rated powers. Currently, I do not see any motor sizing functionality in Energy Plus, so I do not believe this could be done via modeling (since power-efficiency curves are not an available object for supply fan motors or motor sizing cannot be specified). Energy savings per horsepower would be a desirable outcome from the modeling effort, and if anyone could let me know if this is possible, that would be great. Additionally, if someone has a compelling argument to go with the hand calculation method as an alternative to the modeling approach, that would be equally beneficial.

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answered 2016-02-01 07:38:28 -0500

Archmage's avatar

The fan model in EnergyPlus is a a sort of whole-system model that includes the fan motor, blade/blower and drive. The fan's motor size is included in the design volume flow rate (and pressure rise) for the fan system. The total fan efficiency is an input which describes the overall fan system and this includes the motor efficiency, blade efficiency, and drive efficiency. In general you can model improved fan motor performance by raising the total fan efficiency. To determine a new value I would suggest as a starting point to form a ratio of the replacement motor's nominal efficiency divided by the original motor's nominal efficiency and multiply this ratio by the original fan system's total fan efficiency.

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For clarification, there are two inputs for the fan efficiency: Fan Total Efficiency and Motor Efficiency. As Fan Total Efficiency includes the Motor Efficiency, I am wondering if the fan electricity changes if you change just the Motor Efficiency? It seems to me that if Fan Total Efficiency includes the Motor Efficiency, then you would have to change the Fan Total Efficiency and not just the Motor Efficiency if you expect to see a difference in fan electricity?

Chris Jones's avatar Chris Jones  ( 2016-02-01 08:24:21 -0500 )edit

I appreciate the feedback. I understand that the Fan Total Efficiency is what really is read by the model for energy usage purposes, but I was wondering if there was another way to change the motor size rather than simply the efficiency of the system. The method you described Archmage is exactly what I was thinking I would have to do, but I wasn't sure if there was something I would be missing to take me from changing efficiency to savings per horsepower. Chris, your line of thought is correct, you would have to change fan total efficiency to see a difference.

FrontierAssoc104's avatar FrontierAssoc104  ( 2016-02-01 09:14:07 -0500 )edit

The table summary report called "Fans" in the "HVAC Sizing Summary" reports out the size of each fan in electric Watts (and Watts per m3/s). You could use those to categorize by motor horsepower. Savings of course comes from comparing two different runs.

Archmage's avatar Archmage  ( 2016-02-01 11:02:47 -0500 )edit

I saw that, and my only concern there is I have no documentation to validate what the Fan Rated Electric Power [W] really represents. If it is simply the rated power of the motor powering the fan, then it should be trivial enough to get a savings value for each fan to get what I need.

FrontierAssoc104's avatar FrontierAssoc104  ( 2016-02-05 09:52:23 -0500 )edit

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Asked: 2016-01-29 09:36:51 -0500

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Last updated: Feb 01 '16