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fan efficiency vs. fan total efficiency issue?

asked 2021-12-28 16:25:07 -0500

mattkoch's avatar

updated 2022-02-19 08:03:09 -0500

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 -0500

updated 2021-12-28 18:50:06 -0500

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.

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.

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Thank you Matthew!

mattkoch's avatar mattkoch  ( 2021-12-30 09:18:58 -0500 )edit

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Asked: 2021-12-28 16:25:07 -0500

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Last updated: Dec 30 '21