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fan efficiency vs. fan total efficiency issue?

asked 2021-12-28 16:25:07 -0600

mattkoch's avatar

updated 2022-02-19 08:03:09 -0600

So, in the OpenStudio 1.2.1 Application (with OpenStudio 3.2.1), for an OS:Fan:VariableVolume, I just entered 0.65 for "Fan Total Efficiency", 5.37 for "Pressure Rise" and 0.93 for "Motor Efficiency". Then I did a model_run_sizing_run() and looked at various outputs, which look plausible.

However, I was also curious to see what the fan outputs would be, so I used the following:

  fan_volume_flow_rate = supply_fan.autosizedMaximumFlowRate().get
  fan_eta = supply_fan.fanEfficiency()
  motor_eta = supply_fan.motorEfficiency()
  total_eta = supply_fan.fanTotalEfficiency()
  runner.registerInfo("#{supply_fan_name} has SA = #{fan_volume_flow_rate/0.3048**3*60.0} [cfm], FE = #{fan_eta}, ME = #{motor_eta} ,TE = #{total_eta}.")

Much to my surprise, both fan_eta and total_eta report as 0.65, when I would have expected fan_eta to report as 0.65/0.93 = 0.698925, so that 0.93*0.698925 = 0.65. Am I missing something? Shouldn't total_eta = motor_eta * fan_eta?

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answered 2021-12-28 18:27:18 -0600

updated 2021-12-28 18:50:06 -0600

The methods fanEfficiency and fanTotalEfficiency are the same. See the Pull Request and associated issue below for background. Impeller efficiency is not an explicit input in EnergyPlus for fans (or pumps), thus OpenStudio does not have a method for it.

https://github.com/NREL/OpenStudio/pu...

So, this code should work...

fan_tot_eff = fan.fanTotalEfficiency
fan_mtr_eff = fan.motorEfficiency
fan_imp_eff = fan_tot_eff / fan_mtr_eff

For pumps, I believe EnergyPlus assumes a constant impeller efficiency of 0.78.

https://bigladdersoftware.com/epx/doc...

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Thank you Matthew!

mattkoch's avatar mattkoch  ( 2021-12-30 09:18:58 -0600 )edit

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Asked: 2021-12-28 16:25:07 -0600

Seen: 224 times

Last updated: Dec 30 '21