We were just talking about this the other day.
The defrost power is calculated as:
DX Heating Coil:
OutdoorCoilT = 0.82d0 * OutdoorDryBulb - 8.589d0
OutdoorCoildw = MAX(1.0d-6,(OutdoorHumRat - PsyWFnTdpPb(OutdoorCoilT,OutdoorPressure)))
IF (DefrostControl .EQ. Timed) THEN
FractionalDefrostTime = DefrostTime
ELSE !else defrost control is on-demand
FractionalDefrostTime = 1.0d0 / (1.0d0 + 0.01446d0 / OutdoorCoildw)
END IF
DefrostEIRTempModFac = CurveValue(DefrostEIRFT,MAX(15.555d0,InletAirWetbulbC),MAX(15.555d0,OutdoorDryBulb))
DefrostPower = DefrostEIRTempModFac * RatedTotCap/1.01667d0) * FractionalDefrostTime
You want to calculate the power the HP uses while in defrost. Since the HP is in cooling mode during defrost, the DefrostEIRTempModFac performance curve should provide the cooling EIR at the defrost temperatures. The standard cooling EIR curve is a good place to start. Plot it out down to these low temperatures and see how it looks. But also notice that the RatedTotCap referred to here is the Rated Heating Capacity, not the cooling capacity. So you may have to manipulate the cooling EIR temperature curve a little to get a proper DefrostEIR function of temperature curve (since power will be CoolingEIR multiplied by heating capacity).
And no, we do not a performance curve for this. Although I have seen a DOE curve used for the HP model. Note that below 15.555 C for either curve input (indoor WB or outdoor DB), the impact on the curve result due to that input is constant even if you try to reduce the limits in the curve object (i.e., see MAX argument in equation above) . This shouldn't matter much since the HP should be shutting down on low ambient temperature soon after this limit is reached.
CURVE:BIQUADRATIC,
HPACDefrostEIRFT, ! name,
DOE-2.1E,
COOL-EIR-FT for RESYS w/ SI temps (SDL-C11)
0.297145,
0.0430933,
-0.000748766,
0.00597727,
0.000482112,
-0.000956448,
15.555, 23.88889, ! extended limits 15.555 C
- 23.889 C
(60F-75F) wet bulb
15.555, 46.11111; ! extended limits 15.555 C
- 46.111 C
(60F-115F) outdoor dry bulb
On 10/20/2011 7:06 AM, jvd2pe wrote:
Dear forum,
I'm tired of guessing about this.
Does anyone have "typical" coefficients for the EIR when a heat pump goes into a ReverseCycle defrost mode?
How about a rule of thumb for the energy of a resistive defrost heater?
Thanks in advance!
-- Richard A. Raustad Senior Research Engineer Florida Solar Energy Center University of Central Florida 1679 Clearlake Road Cocoa, FL 32922-5703 Phone: (321) 638-1454 Fax: (321) 638-1439 or 1010 Visit our web site at: http://www.fsec.ucf.edu UCF - From Promise to Prominence: Celebrating 40 Years
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