Adrian,
1. I think I answered some of your questions in my response to Adam Barker (particularly the spreadsheet attachment). That spreadsheet does not address the VSD power losses, but that is easy to add into the “Actual” column. (I forgot to do that before sending L)
2. VSD manufacturers are normally different than the pump manufacturers. In addition, pumps need not be applied with a VSD. That means the data comes from two sources.
3. Bless you for actually using the Engineering Reference! Too many new users do not use the documentation. It’s rather intimidating because there is so much of it, but it is definitely an important tool for understanding E+.
4. For pump power, I think I need to refresh my understanding with a new reading of the reference information, but here is my understanding:
o Several years ago, I read in the I/O Reference or Engineering Reference that auto-sized pump mechanical efficiency was a constant 78%. I cannot find that reference this evening, but I’m pretty sure that it’s still true. In order to calculate power for auto-sized pumps, it is also necessary to input pump head.
o For a “real” pump, I think you can just input the Rated Power Consumption from the manufacturer � tthat value will include the impact of pump mechanical efficiency.
5. The power reported in manufacturer literature takes no account of motor efficiency; the manufacturer reports the power needed for the pump work alone. Motor inefficiencies add to that value for what is needed from the power utility.
6. I think you are absolutely correct that Shaft Power should be Power / MotorEffic. I recommend that you report this to the Help Desk for confirmation and correction.
7. (http://energyplus.helpserve.com/) EnergyPlus is very good, but not perfect and small contributions by everyone will help make it even better!
o I’m not perfect either, but take pride that I’ve found a few E+ bugs in the past few years. My loving wife is the only one who normally hears about my discovery of various bugs, since the rest of my friends don’t know much about energy modeling. She decided a few years ago to give me some small adhesive labels with images of insects (“bugs”) and I place one on my monitor each time a bug is discovered and verified. (see photo below) I’m up to 14 so far! You can also take pride in this! Catch me if you can�
James V Dirkes II, PE, BEMP, LEED AP
www.buildingperformanceteam.com
Energy Analysis, Commissioning & Training Services
1631 Acacia Drive, Grand Rapids, MI 49504 USA
616 450 8653
From: EnergyPlus_Support@xxxxxxxxxxxxxxx [mailto:EnergyPlus_Support@xxxxxxxxxxxxxxx] On Behalf Of adrianchong85@xxxxxxxxx
Sent: Wednesday, January 08, 2014 4:46 PM
To: EnergyPlus_Support@xxxxxxxxxxxxxxx
Subject: [EnergyPlus_Support] RE: Cubic Curve in variable speed pump
Jim,
Thanks for the insights. They are definitely informative especially for a new user to EPlus such as myself.
However, out of curiosity, If you were to derive these pump coefficients, how would you go about doing this? Would it be like how I have mentioned earlier whereby we assume a constant head or rather the rated head of the pump and based on that, obtain the various pump brake horsepower and flow rate from pump performance curves usually provided by manufacturers.
sorry if I sound very inexperienced but you mentioned both pump and VSD data, what do you mean by VSD data shouldn't VSD efficiencies be accounted for in the pump curves provided by manufacturers?
On a side note, I was looking through the Eplus Engineering reference and came across the following which is part of the pseudo code for pump:variable speed
Power = FracFullLoadPower * PumpEquip(PumpNum)%NomPowerUse
ShaftPower = Power * PumpEquip(PumpNum)%MotorEffic
From the above it seems that Power refers to the theoretical power provided by the pump.
Does this mean that if the FracFullLoadPower is calculated from pump performance curves provided by manufacturers' which usually is the shaft power, the Motor Efficiency that we use in this module should be 1 otherwise we would be double counting and underestimating the Pump's energy consumption.
Also, shouldn't the ShaftPower be calculated by Power / MotorEffic instead of Power * MotorEffic as indicated by the manual?
Regards,
Adrian
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