[BLDG-SIM] Heat Pump COP
Xiaobing Liu
XLiu at climatemaster.com
Tue Oct 16 09:11:32 PDT 2007
Brandon:
Thanks for your continuous effort to discuss the issue of heat pump COP and the fan power. Just want to clarify several things cited in your e-mails.
1. The fan power included in the EER/COP calculation may not be the same as the fan actually demand when the heat pump runs. For water-source heat pumps, ANSI/ARI/ASHRAE ISO Standard 13256-1:1998 (Water-source heat pumps - testing and rating for performance - Part 1: Water-to-air and brine-to-air heat pump) states following requirements for fan power input in COP/EER calculations
4.1.2 Power input of fans for heat pumps without duct connection
In the case of heat pumps which are not designed for duct connection and which are equipped with an integral fan, all power consumed by the fans shall be included in the effective power input to the heat pump.
4.1.3 Power input of fans for heat pumps with duct connection
4.1.3.2 If a fan is an integral part of a heat pump, only the portion of the fan power required to overcome the internal resistance shall be included in the effective power input to the heat pump. The fraction which is to be excluded from the total power consumed by the fan shall be calculated using the following formula: ?fa = q x ?p / ?
where
?fa is the fan power adjustment, in watts;
? = 0.3 × 103 by convention;
?p is the measured external static pressure difference, in pascals;
q is the nominal airflow rate, in litres per second.
This value shall be subtracted from the heating capacity and added to the cooling capacity.
The reason for this is that, in a particular heat pump installation, depending on the air flow and external static pressure of the duct work, the fan power is different, which is unknown when the heat pump is tested in the lab. So, to isolate from the unknown effect of the external duct work, only the portion of the fan power required to overcome the internal resistance shall be included in the effective power input to the heat pump if the heat pump is designed for duct connection. Therefore, the fan power in the real operating condition should be specified in the simulation to more accurately predict the fan energy consumption.
2. The electric consumption of fan depends on not only the fan power (which may be less than 10% of the compressor power) but also the run time of the fan. In the case of mild climate, if fan runs continuously (to provide OA and air circulation in the conditioned space), the electric consumed by fan may be higher than the compressor power since fan runs 8760 hours a year but the cumulative run time of compressor over a year may be less than 1000 hours. Whether the fan power play an important role in energy analysis does not necessarily depends on the size of heat pump, it depends on what the system is (i.e. single small heat pump or multiple small heat pumps connected with a water loop) and how the indoor fan of the heat pump is operated (continuous or intermittently). For this reason, it is recommended to use DOAS for WLHP system so that the indoor fan of the heat pump can run intermittently to take care the space loads while the DOAS delivers the required amount of OA to the space.
Hope it helps,
Xiaobing
-----Original Message-----
From: BLDG-SIM at gard.com [mailto:BLDG-SIM at gard.com]On Behalf Of Brandon Nichols
Sent: Tuesday, October 16, 2007 12:54 AM
To: BLDG-SIM at gard.com
Subject: [BLDG-SIM] Heat Pump COP
OK All,
Has anyone looked at this <http://www.eere.energy.gov/buildings/appliance_standards/residential/pdfs/cac_tp_101105.pdf> lovely document, which is the Department of Energy's test procedure for air conditioners and heat pumps to determine EER and SEER? To this I turned to answer the burning question "what is the standard external fan static pressure drop specified to calculate EER and SEER?"
Seems to me after wading through the swamp of equations in the back of the document that fan energy in EER and SEER calculations is specified at a 0.05 inch external static pressure drop.
So then I started digging around for data to support my intuitive feeling that fan energy is going to be something like 1/10th that of compressor energy on small heat pumps.
In the process I ran across this nifty DOE/ORNL Heat <http://www.ornl.gov/~wlj/hpdm/doehpdm.shtml> Pump Design Model. Now we're cooking... see this <http://elcca-exchange.blogspot.com/2007/10/heat-pump-parametrics.html> post for a 3D plot of parametric runs (did I say 'nifty'?), varying supply airflow and static pressure, from 600 to 4200 and 0.05 to 0.65 respectively.
I'm standing by my assertion that supply fan energy does not need to be subtracted from heat pump EER as SEER and the new number recalculated -- playing those kinds of games can lead to an exercise in futility.
<http://www.ornl.gov/~wlj/hpdm/doehpdm.shtml>
Best regards,
Brandon Nichols, PE, LEED® AP
Mechanical
HARGIS ENGINEERS
600 Stewart Street
Suite 1000
Seattle, WA 98101
www.hargis.biz
d | 206.436.0400 c | 206.228.8707
o | 206.448.3376 f | 206.448.4450
_____
From: Fred Porter [mailto:fporter at archenergy.com]
Sent: Friday, October 12, 2007 5:22 PM
To: BLDG-SIM; Brandon Nichols
Subject: RE: [BLDG-SIM] Heat Pump COP
G3.1.2.1 Equipment Efficiencies. All HVAC equipment
in the baseline building design shall be modeled at the minimum
efficiency levels, both part load and full load, in accordance
with Section 6.4. 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 be
modeled separately.
G3.1.2.4 Fan System Operation. Supply and return fans
shall operate continuously whenever spaces are occupied and
shall be cycled to meet heating and cooling loads during unoccupied
hours. If the supply fan is modeled as cycling and fan
energy is included in the energy-efficiency rating of the equipment,
fan energy shall not be modeled explicitly.
--
Fred
_____
From: Brandon Nichols [mailto:BrandonN at Hargis.biz]
Sent: Friday, October 12, 2007 6:14 PM
To: Fred Porter; BLDG-SIM at gard.com
Subject: RE: [BLDG-SIM] Heat Pump COP
Good point Fred, for larger unitary packages and for dialing-in energy savings rebate studies. I did say 'arguably', and yours is an insightful observation. Scanned all 90.1-2004 but didn't see the reference you alluded to -- if you can be more specific please advise.
Given the small heatpumps my firm is accustomed to installing, coupled with the fuzzy level of design at which we're doing our preliminary energy studies, the simplification of discounting fan operation when compressors aren't running I think is well within the error bars of the overall study.
Which doesn't provide any comfort from the realization that this business gives one plenty of cause to ask where picking the theoretical flyspecs out of manufacturer's published pepper has crossed a threshold of mind-numbingly diminished returns!
Regards
Brandon Nichols, PE, LEED® AP
Mechanical
HARGIS ENGINEERS
600 Stewart Street
Suite 1000
Seattle, WA 98101
www.hargis.biz
d | 206.436.0400 c | 206.228.8707
o | 206.448.3376 f | 206.448.4450
_____
From: BLDG-SIM at gard.com [mailto:BLDG-SIM at gard.com] On Behalf Of Fred Porter
Sent: Friday, October 12, 2007 3:31 PM
To: BLDG-SIM at gard.com
Subject: [BLDG-SIM] Heat Pump COP
The method cited below only gives correct results if the heat pump fan operates intermittently on a call for heat/cool, and if the fan as installed runs at its ARI-tested power. Otherwise this method greatly under-estimates fan energy, at least for constant speed fans. This is mentioned specifically for App G/ECB models either in the Standard itself or in the UM.
--
Fred
_____
From: Brandon Nichols [mailto:BrandonN at Hargis.biz]
Sent: Friday, October 12, 2007 3:47 PM
To: Fred Porter; BLDG-SIM at gard.com
Subject: RE: [BLDG-SIM] Heat Pump COP
Good point, John -- but there may be an arguably easier way to not double-count fan energy...following is a reprint from the DOE2 documentation:
COOLING-EIR
The Electric Input Ratio (EIR), or 1/(Coefficient of Performance), for the cooling unit at ARI rated conditions. The program defines EIR to be the ratio of the electric energy input to the rated capacity, when both the energy input and rated capacity are expressed in the same units. This EIR is at ARI rated conditions, i.e., without correction for different temperature or part load.
Note: If you include fan electric energy consumption in your value of COOLING-EIR, then you should set SUPPLY-KW/FLOW to zero (and SUPPLY-STATIC, SUPPLY-EFF and SUPPLY-DELTA-T should be omitted). Otherwise, the supply fan electrical energy will be double counted. For commercial systems the default value of COOLING-EIR includes compressor and outdoor fan energy, but not indoor fan energy.
HEATING-EIR
Electric Input Ratio, or 1/(heating Coefficient of Performance), for the heat pump. This EIR is at ARI rated conditions, i.e., without corrections for temperature or part load. The program-calculated HEATING-EIR does not include fan power and heat. This keyword is appropriate only to HP, RESYS, and PTAC systems.
Mysteriously, the HEATING-EIR documentation makes no mention of the fan energy double counting issue. But if you're consistent and use 3.41 divided by the manufacturers published EER for the cooling EIR, the inverse of the published COP for the heating EIR, and zero-out the supply fan energy as described in the documentaion ... then Bob's your uncle, right?
FWIW, that's the way we do it...
Brandon Nichols, PE, LEED® AP
Mechanical
HARGIS ENGINEERS
600 Stewart Street
Suite 1000
Seattle, WA 98101
www.hargis.biz
d | 206.436.0400 c | 206.228.8707
o | 206.448.3376 f | 206.448.4450
_____
From: BLDG-SIM at gard.com [mailto:BLDG-SIM at gard.com] On Behalf Of Fred Porter
Sent: Friday, October 12, 2007 10:23 AM
To: BLDG-SIM at gard.com
Subject: [BLDG-SIM] Heat Pump COP
Yes and no. The energy is "removed," but the COP improves because the fan heat had a COP of 1.0, and is included in the rated gross kW in and Btuh out. I think the T24 ACM may have a "method" or formula.
_____
From: Aulbach, John
Sent: Thu, 10/11/2007 3:25pm
To: BLDG-SIM at gard.com
Subject: [BLDG-SIM] Heat Pump COP
Folks:
In eQuest, the Energy Input Ratio (EIR) can be derived from EER or COP given for a unit or from a Standard. To correctly model such a unit, one must subtract the fan energy for the unit to obtain the EER (EIR) of the compressor only. This improves the input from, say, a COP of 12.3 to 15.
What about in the case of the heat pump heating COP? Must one subtract out the fan energy here as well, thus "worsening" the COP for the heating side?
I cannot find anything written on this subject.
Thanks.
John R. Aulbach, PE, CEM
Project Manager
Nexant, Inc.
100 North Barranca, Suite 820
West Covina, CA 91791 USA
Phone: 626-430-9054
Fax: 626-430-9060
email: jaulbach at nexant.com
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