# Re: [EnergyPlus_Support] Re: Equipment efficiencies according to ASHRAE

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: Friday, April 12, 2013 9:59 AM
Subject: [EnergyPlus_Support] Re: Equipment efficiencies according to ASHRAE

I am not sure that I have understood the implications of the following sentence: "Airflow is not dependent on ventilation ? ventilation aaffects the coil, chiller and boiler capacity, but airflow is set only by zone loads. (Infiltration IS a zone load!)".
Does it mean that in equation 5 (see previous message of Mr. Dirkes) as Airflow the value "User Design Air Flow [m3/s]" of the table "Zone cooling" of the report "HVAC Sizing Summary" (see the object Output:Table:SummaryReports) should be used?
Francesco

--- In mailto:EnergyPlus_Support%40yahoogroups.com, Jim Dirkes <jim@...> wrote:
>
> Francesco,
> You are correct! A Baseline is always autosized and that means you have no way of knowing in advance the cooling capacity or airflow. This is not iterative, however, since the capacity is fixed by your input specifications and is known and unchanging after a successful simulation (unless you change inputs).
> Airflow is not dependent on ventilation - ventilation affects the coil, chiller and boiler capacity, but airflow is set only by zone loads. (Infiltration IS a zone load!)
> Keep in mind that EER is an "artificial" rating in the sense that it is based on a fixed set of AHRI test conditions, so you are correct to note that the actual conditions are different. The AHRI rating conditions may never occur at your site! Nonetheless, it is useful as a reference for cooling equipment efficiency and is used by ASHRAE and LEED. For that reason, it is appropriate to use the default / reference value of 773 for fan power / flow in many cases. The AHRI test standard indicates that this value is assumed to be true for systems "which do not have indoor air-circulating fans furnished as part of the model, i.e., split systems with indoor coil alone, ...". In my thinking, that means a different value is probably appropriate for systems like PTAC or PTHP. I want to think about this a bit more, but 200W / m3/s seems about right for systems like PTAC which inherently have less fan power.
> Paragraphs G3.1.2.8, 9 do not address EER calculations; they are discussing how fan power must be defined for Baseline systems. Remember that EERs are "artificial" reference values for cooling efficiency, and are independent of the 90.1 fan power.
>
>
>
> James V Dirkes II, PE, BEMP, LEED AP
> www.buildingperformanceteam.com<http://www.buildingperformanceteam.com/>
> Energy Analysis, Commissioning & Training Services
> 1631 Acacia Drive, Grand Rapids, MI 49504 USA
> 616 450 8653
>
> From: mailto:EnergyPlus_Support%40yahoogroups.com [mailto:mailto:EnergyPlus_Support%40yahoogroups.com] On Behalf Of FrancescoP
> Sent: Friday, April 12, 2013 4:31 AM
> To: mailto:EnergyPlus_Support%40yahoogroups.com
> Subject: [EnergyPlus_Support] Re: Equipment efficiencies according to ASHRAE
>
>
>
> Thank you, Jim Dirkes and Alvaro Urrutia!
> Considering eq. 5, Mr. Dirkes, for a baseline model (autosizing) do you obtain the values of Qtotal and of Airflow from a preliminary simulation? I think it could be an iterative process, couldn't it?
> Airflow obtained from a simulation depends also by ventilation requirements (outdoor air) and it can be very different from rating conditions. Isn't this fact a problem?
> If I understood correctly, 773.3 is the default value, but the correct value varies case-by-case (see paragraphs G3.1.2.8 and G3.1.2.9), doesn't it?
> I'm sorry for my many doubts. Thank you again.
> Best Regards
> Francesco Passerini
>
> --- In mailto:EnergyPlus_Support%40yahoogroups.com<mailto:EnergyPlus_Support%40yahoogroups.com>, Jim Dirkes <jim@<mailto:jim@>> wrote:
> >
> > Francesco,
> > I am not able to stop and look carefully at your message, but I think the following (from my personal notes) may help....
> >
> > Equivalent COP calculation (How to specify a DX coil in EnergyPlus so a desired COP results)
> >
> > EnergyPlus calculates DX coil energy by using a COP value that is defined differently than that used by manufacturers of packaged air
> >
> > conditioning equipment.
> >
> > For all equipment, the definition of COP is (cooling capacity / power), with both terms expressed in the same units.
> >
> > Manufacturers and AHRI (their rating organization) define the terms as:
> >
> > Cooling capacity = total cooling capacity of the system
> >
> > Power = Total equipment power, including supply fan, compressor, condenser fans and controls
> >
> >
> > EnergyPlus defines the COP terms for a DX coil as:
> >
> > Cooling capacity = total cooling capacity of the system (same as manufacturers)
> >
> > Power = compressor and condenser fans (Note: No supply fan or controls!)
> >
> >
> > Most real-world packaged air conditioning equipment is not applied with the same operating conditions as exist in the AHRI test setup,
> >
> > so EnergyPlus is well justified to do things differently than AHRI. Nonetheless, because of the different definitions, an EnergyPlus user
> >
> > must derive an equivalent COP for the DX coil that will result in something close to the manufacturer's actual energy performance.
> >
> >
> > Conveniently, controls use very little power, the AHRI test conditions require a specific small fan pressure, and fan efficiency and system Ã?T
> >
> > fall in a relatively narrow range. This means that the power burden of fan and controls is very proportional to cooling capacity for this
> >
> > type of equipment. EnergyPlus defaults to a value of 0.365 W/cfm (773.3 W / m3/s) for the combination of fan and controls.
> >
> >
> > For an AHRI-rated packaged AC equipment manufacturer:
> >
> > COPahri =
> >
> > Qtotal
> >
> > Eqn #1; where both Q and P are both expressed in Watts or BTUH
> >
> >
> > Ptotal
> >
> > This can be expressed as:
> >
> > COPahri =
> >
> > Qtotal
> >
> > Eqn #2; where Pfan includes the supply fan and controls
> >
> >
> > Pdx + Pfan
> >
> >
> > For an EnergyPlus DX coil:
> >
> > COPe+ =
> >
> > Qtotal
> >
> > Eqn #3; where Pdx is the power of compressor and condenser fans (only)
> >
> >
> > Pdx
> >
> >
> > If we assume that Pfan can be considered a constant (K) and that K = airflow * 773.3 W / m3/s, Eqn #2 becomes:
> >
> > COPahri =
> >
> > Qtotal
> >
> >
> > Pdx + 773.3 * Airflow
> >
> > (airflow in m3/s)
> >
> >
> > Qtotal =
> >
> > COPahri * (Pdx + 773.3 * Airflow)
> >
> > Qtotal =
> >
> > COPahri*Pdx + COPahri*773.3*Airflow
> >
> >
> > AHRI fan power1:
> >
> > 773.3
> >
> > W / m3/s
> >
> > Qtotal - COPahri*773.3*Airflow =
> >
> > COPahri*Pdx
> >
> > Ref: AHRI 210/240-2008 pp4, 5
> >
> > 365.0
> >
> > W / 1000 cfm
> >
> >
> > for split systems
> >
> > Pdx =
> >
> > Qtotal - COPahri*773.3*Airflow
> >
> > Eqn #4
> >
> > 2,118.64
> >
> > cfm (1 m3/s)
> >
> >
> > COPahri
> >
> > 1.555
> >
> > delta P
> >
> >
> > 0.500
> >
> > fan efficiency
> >
> > Then, substituting Pdx into Eqn #3 :
> >
> > 773.3
> >
> > Power (watts)
> >
> > COPe+ =
> >
> > Qtotal
> >
> >
> > Qtotal - COPahri*773.3*Airflow
> >
> >
> > COPahri
> >
> >
> > AHRI fan power2:
> >
> > 200
> >
> > W / m3/s
> >
> > COPe+ =
> >
> > Qtotal * COPahri
> >
> > Eqn #5
> >
> > Ref: BPT estimate
> >
> > 94.4
> >
> > W / 1000 cfm
> >
> >
> > Qtotal - COPahri*773.3*Airflow
> >
> > for self-contained (e.g., PTAC) systems
> >
> >
> > 2,118.64
> >
> > cfm (1 m3/s)
> >
> > or (since most US manufacturers use EER instead of COP):
> >
> > 0.400
> >
> > delta P
> >
> > COPe+ =
> >
> > Qtotal * EERahri / 3.412
> >
> > Eqn #6
> >
> > 0.500
> >
> > fan efficiency
> >
> >
> > Qtotal - EERahri/3.412*773.3*Airflow
> >
> > 198.9
> >
> > Power (watts)
> >
> >
> > ASHRAE 90.1 minimum efficiency tables cite specific AHRI standards, so it is appropriate to assume the above COP derivation for Baseline
> >
> > systems. Proposed systems will use the manufacturer data for EER.
> >
> >
> > Note:
> >
> > For ASHRAE 90.1-compliant energy models, the "baseline" specified fan power is much greater than what is included for an AHRI-rated
> >
> > packaged air conditioning unit. Depending on the "baseline" system type, the ratio of fan power per volume flow rate is 1486 W / m3/s or
> >
> > more. This is almost twice the EnergyPlus default value of 773.3.
> >
> >
> >
> > James V Dirkes II, PE, BEMP, LEED AP
> > www.buildingperformanceteam.com<http://www.buildingperformanceteam.com/>
> > Energy Analysis, Commissioning & Training Services
> > 1631 Acacia Drive, Grand Rapids, MI 49504 USA
> > 616 450 8653
> >
> > From: mailto:EnergyPlus_Support%40yahoogroups.com<mailto:EnergyPlus_Support%40yahoogroups.com> [mailto:mailto:EnergyPlus_Support%40yahoogroups.com<mailto:EnergyPlus_Support%40yahoogroups.com>] On Behalf Of FrancescoP
> > Sent: Thursday, April 11, 2013 11:55 AM
> > To: mailto:EnergyPlus_Support%40yahoogroups.com<mailto:EnergyPlus_Support%40yahoogroups.com>
> > 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 with 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?
> > With Regards
> >
>

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