Question-and-Answer Resource for the Building Energy Modeling Community
Get started with the Help page

# Variable Air Volume Fan Annual Energy Usage

I am trying to calculate energy savings for implementing a VFD and premium efficiency motor as an RTU retrofit. I am thinking to use the Variable Air Volume object to model the VFD, and changing the fan efficiency since it is a product of motor and fan efficiency. My question comes in to the Load Factor and % full rated speed of the fan air flow rate. Since I am going to make a lump system energy savings due to the interactive energy savings between the VFD and efficient motor, I need the % full rated speed (cubed) due to the fan affinity laws. Additionally I would need the motor load factor. I know that there is a design max air flow field in the variable air volume fan class, but I have not seen an output for hourly air flow rate to get the average % flow rate. Additionally I'm not sure about the load factor on the motor, although I may just assume the same load factor for both baseline and proposed cases. Any feedback on my line of thought or about the air volume fraction would be greatly appreciated.

edit retag close merge delete

The question is two fold. Is Energy+ capable of outputting the hourly fan speed? Also is there a roundabout way of finding the load factor from the fan motor via the hourly fan energy usage?

Sort by » oldest newest most voted

The only outputs available for variable volume fans are:

If by "fan speed", you mean the rotational speed of the impeller, this is not available. Though, you might be able to approximate it from the power factor.

You can calculate the load factor based on the mass flow rate of air through the fan (using the system node mass flow rate output) and the inputs for maximum flow rate, and the fan power coefficients. The algorithms used in EnergyPlus are defined here in the Engineering Reference.

more

The power factor is essentially what I'm after, because I was going to use (average fan speed/rated fan speed)^3 to estimate the power factor. Alternatively, the fan Rated electric power is an output seen via the HTML file produced by an energy plus simulation, and the hourly fan electric power output (specific to each fan) could be averaged over the year and normalized by the fan's rated electric power. I'm just not sure if this is a valid method of collecting the fan motor power factor.

Can you clarify, do you mean (1) Power Factor, (2) Load Factor, or (3) the Part Load Factor (described in the Engineering Reference for fans) defined by the flow fraction curve:

$PLF = c_1 + c_2 f_{flow} + c_3 f_{flow}^2 + c_4 f_{flow}^3 + c_5 f_{flow}^4$

Really I am looking for load factor, not power factor.

Thanks, @FrontierAssoc104. Sorry for the confusion. You should be able to use the rated fan power and the average electric power as you described to calculate the load factor.