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[EnergyPlus_Support] Re: COP of AirLoopHVAC:UnitaryHeatPump:AirToAir

I used curves that are used as default in DesignBuilder: DXClgCoilTotalClgCapFuncTemperature and DXClgCoilEnergyInputRatioFuncTemperature
They are Curve:Biquadratic
I have plotted them (hopefully correctly)considering the condenser temperature 31°C:
http://imageshack.us/f/580/dbcurves.jpg/
It is a "baseline model" therefore it doesn't correspond to a real HVAC system. 
I have seen that in some examples of EnergyPlus as curves for "Total Heating Capacity Function of Temperature Curve" and for "Energy Input Ratio Function of Temperature Curve" objects Curve:Cubic are used (see AirflowNetwork_MultiZone_House.idf  in that case if external temperature increases COP increases). Do you think that neglecting the influence of the condenser temperature on the COP is an acceptable approximation? 
I simulated using those curves. Here the graph with the curves proposed by AirflowNetwork_MultiZone_House.idf :
http://imageshack.us/f/560/copu.jpg/
(comparing with the previous graph consider that in the meantime also the peak power of the heat pump was changed)
Probably I will develop curves using Curve:Biquadratic in order to consider both the condenser temperature and the evaporator temperature. I have found some interesting graphs in H. Ludwig von Cube et al. (1981) "Heat pump technology", Butterworths and in (1979) "Heat pump technology for saving energy", Noyes data corporation. 



--- In EnergyPlus_Support@xxxxxxxxxxxxxxx, "FrancescoP"  wrote:
>
> I am modeling System 4 - PSZ-HP according to the definition of Annex G of ASHRAE 90.1-2007. In excel I calculated the COP as ratio between "DX Coil Total Heating Rate" and "DX Heating Coil Electric Power" and I plotted the graph (outdoor air temperature ; COP) that you can see here:
> http://imageshack.us/f/442/copg.jpg/
> (when the "DX Heating Coil Electric Power" is equal to 0 I imposed the COP equal to 0)
> In the object AirLoopHVAC:UnitaryHeatPump:AirToAir the field "Maximum Outdoor Dry-Bulb Temperature for Supplemental Heater Operation" is equal to 4°C. I think that is the reason why the COP decreases between 3°C and 5°C: the electric heating coil switches off and the DX coil has to heat the supply air to a greater temperature.
> The COP decreases also when the outside temperatures are greater than 10°C. I don't understand why.
> Any comment, please?
> Best Regards
> Francesco Passerini
>



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