Hisham,
I think the method is applicable, but not using the 773 W / m3/s fan power. There is no guidance for this in technical literature that I am aware of, but I use 300 W / m3/s as an alternate value based on the rough calculations I show below. I have not checked the VRF standard for VRFs however. That is worth checking to see what fan power is assumed during VRF testing. I think it’s AHRI Standard 1230
AHRI fan power2: | W / m3/s | ||
Ref: BPT estimate | 141.6 | W / 1000 cfm | |
for self-contained | |||
(e.g., PTAC) systems | 2,119 | cfm / (1 m3/s) | |
0.20 | in. w.c. delta P / (50 Pa) | ||
16.6% | fan efficiency | ||
299.6 | Power (watts) | ||
From: EnergyPlus_Support@xxxxxxxxxxxxxxx [mailto:EnergyPlus_Support@xxxxxxxxxxxxxxx]
Sent: Tuesday, September 9, 2014 2:43 AM
To: EnergyPlus_Support@xxxxxxxxxxxxxxx
Subject: [EnergyPlus_Support] Re: Equipment efficiencies according to ASHRAE
Dear Jim,
Is this method of calculation also valid for VRF systems? In e+ output I couldn't find VRF COPs reported, neither nominal nor AHRI. How can I get them in output?
Thank you for your helpful contributions.
Hisham
---In EnergyPlus_Support@xxxxxxxxxxxxxxx, <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) |
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EnergyPlus calculates DX coil energy by using a COP value that is defined differently than that used by manufacturers of packaged air |
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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: | 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 | |||||||
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Posted by: Jim Dirkes <jim@xxxxxxxxxxxxxxxxxxxxxxxxxxx>
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