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[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 EnergyPlus_Support@xxxxxxxxxxxxxxx, Jim Dirkes <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 ÄT
>
> 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: EnergyPlus_Support@xxxxxxxxxxxxxxx [mailto:EnergyPlus_Support@xxxxxxxxxxxxxxx] On Behalf Of FrancescoP
> Sent: Thursday, April 11, 2013 11:55 AM
> To: EnergyPlus_Support@xxxxxxxxxxxxxxx
> 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?
> Any suggestion, please?
> With Regards
>
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