Francesco
i have the same problem with energy modeling in some buildings that pursue the LEED certification.
my aproach to the problem after some projects and many observations by the LEED review team using energyplus as modeling software is:
1. using the equations showed in ASHRAE 90.1 users guide G-A, G-B and G-C help me to direct the conversion of EER at ARI condition exposed in ASHRAE 90.1 to nominal efficiency that is the data required to define in energyplus.
2. using a preliminar simulation in energyplus with a any efficiency (EER 2.7 by example), the Gross Cooling Capacity and Net Cooling Capacity recorded in the Equipment Summary are used to estimate the Suply Fan Power at ARI conditions (based on equations G-A and G-C):
G-A = EER [rated 90.1] = Net Cooling Cap [W] / Total Power Input[W] = (Gross Cooling Capacity [W] - Supply Fan Power [W]) / Total Power Input [W]
3 using these results i can calculate the supply fan power at rated conditions
Supply Fan Power [W] = Nominal Total Cap [W] - Std. Rating (net) Cooling Capacity [W]
Total Power Input [W] = Net Cooling Capacity [W] / EER at rated conditions of 90.1 [SI]
4 to calculate the final nominal efficiency for use in energyplus simulation:
Nominal Efficiency [Energyplus] = Gross Cooling Capacity [W] / (Total Power Input[W] - Supply Fan Power[W])
tip:
i review the equipment summary from energyplus final simulation and compare the EER at ARI condition (not nominal EER used as input) with ASHRAE 90.1 to confirm that the procedure is correct.
I hope the information wil be helpful.
Alvaro Urrutia Astorga
Architect - LEED AP BD+C
Santiago - Chile.
From: FrancescoP <direzionecontraria@xxxxxxxx>
To: EnergyPlus_Support@xxxxxxxxxxxxxxx
Sent: Thursday, April 11, 2013 11:54 AM
Subject: [EnergyPlus_Support] Equipment efficiencies according to ASHRAE
Paragraph G3.1.2.1 of AHRAE 90.1-2007 states: "Where efficiency ratings, such as EER and COP, include fan energy, the descriptor shall be broken down into its components so that supply fan energy can modeled separately".
As baseline model I have to model System 6 ? Packaged VAV withh PFP boxes (Packaged rooftop with reheat).
I started modelling the system in DesignBuilder as you can see in this picture (in DesignBuilder the systems of annex G are proposed as template):
http://imageshack.us/f/547/system6.jpg/
As efficiency of the DX cooling coils I would consider the values indicated in Table 6.8.1B.
Thanks to the EnergyPlus developers, the rating conditions of EnergyPlus are the same conditions indicated in ARI 340/360 (the test procedure for ASHRAE).
I found in the "Input/output reference" of EnergyPlus, as explanation of the field "Rated COP " of the object Coil:Cooling:DX:SingleSpeed, that "The input power includes electric
power for the compressor(s) and condenser fan(s) but does not include the power consumption of the supply air fan".
While according to the User's manual of the ASHRAE 90.1-2007 the EER "compares the net cooling effect in Btu/h to the total input power of the packaged system in watts to drive the supply and condenser fans and the refrigerant compressor".
A part the units of measurement, it seems that there is a difference between how EnergyPlus and ASHRAE evaluate the performance of a DX cooling coil: the first one doesn't consider the supply fan while ASHRAE does. Do you agree?
Consider "air cooled (cooling mode)" in Table 6.8.1B and suppose that a DX cooling coil results between 19 kW and 40 kW. I have to consider "heating section type" = "Electric resistance (or none)" and therefore as minimum efficiency 3.22 COPc, right?
Since in order to calculate the performance of a DX cooling coil EnergyPlus doesn't consider the supply fan while
ASHRAE does consider it, I would do in the following way:
3.22 = cooling effect / ( compressor input power + condenser fan power + supply fan power )
where the "supply air fan" is the fan that is in the AHU (see the picture taken from DesignBuilder).
Then I would calculate the supply fan power according to paragraphs G3.1.2.8 and G3.1.2.9.
If I understood correctly, the "Rated COP" of EnergyPlus is
cooling effect / ( compressor input power + condenser fan power )
How could I calculate it?
Any suggestion, please?
With Regards