I'm trying to make base and change cases for energy efficiency comparisons for HVAC optimization. One measure I plan to implement is premium efficiency motors on HVAC fans. The only thing I see that ties into this would come from the fan efficiency field. Can anyone explain what goes into the fan efficiency, and how I would be able to directly change the motor efficiency of the fan in Open Studio or energy plus?
Fan objects (Fan:ConstantVolume, Fan:VariableVolume, Fan:OnOff) in EnergyPlus/OpenStudio have two inputs related to fan motor efficiency: Fan Total Efficiency and Motor Efficiency.
From the EnergyPlus Input/Output reference...
Fan Total Efficiency represents the product of motor efficiency and the fan efficiency. The motor efficiency is the power delivered to the shaft divided by the electrical power input to the motor. The fan efficiency is power delivered to the fluid (air) divided by the shaft power.
Motor Efficiency is the shaft power divided by the electrical power consumed
To understand the impact of premium efficiency motors you will need to adjust both input fields. The Motor Efficiency field is used in conjunction with the Motor In Airstream Fraction field to determine the amount of motor heat loss that goes into the airstream as heat. The Motor Efficiency field will not impact the amount of fan power directly but it will impact the amount of heating or cooling used by your HVAC system.
The Fan Total Efficiency field (simply 'Fan Efficiency' in OpenStudio) will need to be adjusted to represent the change in motor efficiency and fan power consumed. Fan Total Efficiency = Motor Efficiency * Fan Efficiency, so you can adjust your fan total efficiency accordingly if you know the change in motor efficiency.
We have a measure that changes the fan motor efficiency, but it only changes the motor field, so as @Lyle K suggests it only changes heat to the air stream. We need to update that measure. Code would look something like this
fan_impeller_eff = fan.fanEfficiency/fan.motorEfficiency
fan.setMotorEfficiency(motor_eff_arg_value)
fan.setFanEfficiency(fan_impeller_eff * fan.motorEfficiency) # we don't do this in current measure
Could be feature request to add "Fan impeller efficiency" field to EnergyPlus and/or OpenStudio and then have an auto-calculate option for "Fan Efficiency". This would probably have to support current method where you can leave impeller efficiency empty and just provide motor and total efficiency; but could be confusing if user tried to enter all three with conflicting values. If you are interested add to user voice.
The Fan:ConstantVolume and Fan:VariableVolume objects both have a field for motor efficiency separate from the fan efficiency (circled in red below). The difference between the two efficiencies is that the motor losses can be specified as out of the airstream and therefore not add any heat to the supply flow. So any losses that you don't want to contribute to a temperature rise (e.g. drive losses outside the AHU) will have to be included in the motor efficiency.
Comments
Okay, this gives me a few follow up questions. First, I'm assuming changing to a VFD controlled fan would change the fan efficiency component of the total fan efficiency (not the motor efficiency since this is a nameplate value?). Secondly, the pressure rise would also potentially change, in a manner that is not immediately obvious to me. Additionally, the fan power curve coefficients that are in the variable volume fan class that are not in the constant volume fan class could be set at default values, or would they need to be modified? If the coefficients would need to be modified, how and wh
Check out the Fan Part-Flow Power Curve table in the COMNET MGP. It's on page 104 of the PDF I have. This lists typical part-load curves for different types of fans.