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

["FrancescoP" <direzionecontraria@xxxxxxxx>]

Thank you, Mr. Raustad.
You are right: the function proposed by DesignBuilder is valid only for x values (i.e. indoor coil entering air temperature) between 12.77 and 23.88°C. I was wrong because I plotted the graph for indoor coil entering air temperature equal to 31°C. Now I have plotted the graph for indoor coil entering air temperature equal to 23°C:
http://imageshack.us/f/844/db23.jpg/
Yes, I divided heating capacity factor by EIR (energy input  factor). I think that the result is the factor that multiplies the COP, isn't it? I don't understand why you wrote the "The COP is 1/EIR" (energy input  factor). COP is defined as "the ratio of the rate of heat delivered to the rate of energy input".
I have plotted also come curves from RooftopPackagedHeatPump.idf
http://imageshack.us/f/716/jcip.jpg/
Yes, there are examples where both heating capacity and power increase as outdoor temps increase but capacity increases faster than power leading to an increasing COP:
http://www.rheem.co.nz/UserFiles/1/image/commercial-heat-pump-cop-vs-temp.jpg
Actually, I'm wondering whether for a baseline model for LEED certification I could use constant curves. It would be unrealistic but the baseline model has not to correspond to a real situation.
With Regards
Francesco Passerini


--- In EnergyPlus_Support@xxxxxxxxxxxxxxx, Richard Raustad  wrote:
>
> What is the range (min/max) on the x (indoor coil entering air temp) and 
> y (outdoor coil entering air temp) curve inputs if used? These limits 
> can influence the results (the curves output, which also has limits if 
> used). Also, 31C is rather high for the indoor coil entering air temp. 
> How does this look for other lower temps. You should plot at least 2 
> indoor coil entering air temps on the chart to ensure the curves act 
> accordingly.
> 
> It looks like you divided heating capacity factor by EIR (energy input 
> factor). The COP is 1/EIR which would show an increase in COP as outdoor 
> temp increased.
> 
> I see a downward trend in heating capacity as outdoor air temp increases 
> at higher outdoor temps. Find some manufacturers data and verify this is 
> correct (or look in the data sets file RooftopPackagedHeatPump.idf). 
> This curve appears incorrect to me (i.e., capacity should increase as 
> outdoor air temp increases). Heating capacity and power should both 
> increase as outdoor temps increase. But capacity increases faster than 
> power leading to an increasing EER and COP.
> 
> Always question the performance curves since these are what is actually 
> simulated.
> 
> 
> On 2/13/2013 6:35 AM, FrancescoP wrote:
> > 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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> -- 
> Richard A. Raustad
> Senior Research Engineer
> Florida Solar Energy Center
> University of Central Florida
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