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?
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.setMotorEfficiency(0.29)
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.
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
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.)
Does EnergyPlus differentiate between the fan motor being in the air stream or outside of the air stream?
There is a "Motor In Airstream Fraction" value, which I assume is for this purpose.
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.
I had to revisit this today.
E+ never even calculates impeller efficiency explicitly.
It calculates fan power (Ptot) as static pressure * volumetric flow rate / fan total efficiency.
The Design Heat Gain of the fan is calculated as
Source: Engineering Reference Design Fan Heat
This can be seen for eg for the FanVariableVolume here: Fans.cc#L1801-L1804
If you start playing around with the equation and trying to reduce/express it differently, you'll notice that
given that total_efficiency = motor_efficiency * impeller_efficiency (I'm calling Ptot = Pfan,des)
Pshaft = motor_efficiency * Ptot = total_efficiency / impeller_efficiency * DeltaP * V_dot / total_efficiency
= DeltaP * V_dot / impeller_efficiency
Makes sense.
Fan heat gain is then:
Q_dot_fan_heat = Pshaft + (Ptot - Pshaft) * motorInAirFrac.
In other words, the fan heat gain is the sum of
If your motor in air stream fraction is 1, then no matter what you put in the "Motor Efficiency" field, you'll always have the same values.
Asked: 2015-09-03 06:42:10 -0500
Seen: 533 times
Last updated: May 19
