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 ΔT | |||||||
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: | 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: | 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) | ||||||
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
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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