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Fan motor efficiency has no impact on energy consumption

asked 2015-09-03 06:42:10 -0600

dpud12 gravatar image

updated 2017-08-05 13:10:21 -0600

In OpenStudio, when I change the motor efficiency of the OS:Fan:OnOff object for a 4-Pipe fan coil there is no alteration in the fan energy consumption. I have changed the default from 0.9 all of the way down to 0.2, but there is still no impact. Is this a bug or is there something that I am missing?

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answered 2015-09-03 10:39:21 -0600

updated 2015-09-03 11:15:55 -0600

In EnergyPlus and OpenStudio the "Fan Efficiency" field represents the total fan efficiency including all components (e.g. the motor). As a result when you change the motor efficiency the over all fan efficiency doesn't change. in reality you need to also ratchet down the "Fan Efficiency" value.

Here is an example of this using our API. You could request a change in this behavior to the EnergyPlus or OpenStudio user voice pages. We could expose the impeller efficiency and hide "Fan Efficiency" and that can be calculated when needed.

 fan_impeller_eff = fan.fanEfficiency/fan.motorEfficiency
 fan.setFanEfficiency(fan_impeller_eff * fan.motorEfficiency)

Here is an example calculation

Fan Efficiency = 0.2
Motor Efficiency = 0.29
Impeller Efficiency = Fan Efficiency/Motor Efficiency = 0.2/0.29 = 0.69

If Motor Efficiency is changed to 90% here are updated values
New Motor Efficiency = 0.9
Impeller Efficiency = 0.69 (no change from original system)
New Fan Efficiency = Impeller Efficiency * New Motor Efficiency = 0.69 * 0.9 = 0.621

Credit to @aparker who taught me this.

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I've hit this same issue myself. I'm not a huge fan of the way that E+ asks for these inputs. It seems like they should instead ask for motor efficiency and impeller efficiency, and calculate total efficiency as the product of these two behind the scenes. FYI here is the link to the description of this field

aparker gravatar image aparker  ( 2015-09-03 11:11:33 -0600 )edit

ANSWERED BY READING APARKER POST ABOVE. Would it impact the heat gain to the air stream if the motor efficiency became a referenced input along with the impeller eff? According to the OP simulation results, it appears that the current input for motor eff does not influence the rate of heat gain. (Correct me if I'm wrong.)

nfonner gravatar image nfonner  ( 2015-09-03 11:14:02 -0600 )edit

Does EnergyPlus differentiate between the fan motor being in the air stream or outside of the air stream?

Chris Jones gravatar image Chris Jones  ( 2015-09-03 11:45:00 -0600 )edit

There is a "Motor In Airstream Fraction" value, which I assume is for this purpose.

David Goldwasser gravatar image David Goldwasser  ( 2015-09-03 12:02:46 -0600 )edit

If in reference to my post, sorry for any confusion. I was referring to a variance in heat gain based on the efficiency of the motor, not whether the motor was in/ out of the air stream. It does not appear to matter either way in David's answer since motor eff is already accounted for in Fan Total eff. It's just confusing now that it appears to prompt the user to account for motor eff twice. 1x in Tot eff and 1x in Mot eff.

nfonner gravatar image nfonner  ( 2015-09-03 14:57:50 -0600 )edit

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Asked: 2015-09-03 06:42:10 -0600

Seen: 323 times

Last updated: Sep 03 '15