[EnergyPlus_Support] Re: Equipment efficiencies according to ASHRAE

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)
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 Î?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 =	COPahriPdx + COPahri773.3*Airflow
				AHRI fan power1:	773.3	W / m3/s
Qtotal - COPahri773.3Airflow =	COPahri*Pdx			Ref: AHRI 210/240-2008 pp4, 5	365.0	W / 1000 cfm
				for split systems
Pdx =	Qtotal - COPahri773.3Airflow		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 - COPahri773.3Airflow
	COPahri
				AHRI fan power2:	200	W / m3/s
COPe+ =	Qtotal * COPahri		Eqn #5	Ref: BPT estimate	94.4	W / 1000 cfm
	Qtotal - COPahri773.3Airflow			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/3.412773.3Airflow				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