[EnergyPlus_Support] Heat Pump Water Heaters (doubts subsist)

["anarui.oliveira" <anarui.oliveira@xxxxxxxxx>]

Hello again.

In the sequence of the recent posts, I've been studying the Example 
File "HeatPumpWaterHeater" in more detail...

The Heat Pump I'm trying to model is indeed located outdoor, and I 
see now that, below 5ºC (or below the "Minimum Inlet Air Temperature 
for Heat Pump Compressor Operation"), the Heat Pump simply doesn't 
work and it has to be the water system "internal" system to respond.

BUT...
I still seem to find the same values in the Example File that I 
couldn't explain in my model...
- In the Example File, only the five zone heating coils and the main 
heating coil are consuming energy from this water heater (Plant Water 
Heater), right?
- and the Plant Water Heater has zero ambient losses defined, right?

SO...
- Why is the "Use energy" usually quite higher than the sum of 
heating coils demand? Particularly, it seems that, the higher above 
the HW Loop setpoint are the Water Heater and Heat Pump setpoints, 
the more the supply energy departs drom the demand energy, which 
doesn't make sense to me if losses are set to zero?...
- When only the internal Water Heater energy supply is working it 
seems to follow quite closely the "use energy",
- But, when the Heat Pump is working, "source energy" and "DX Ccoil 
energy" are usually much higher than "use energy"?...
- Also, when the Water Heater and the Heat Pump temperature setpoints 
are very close, an error regarding "part-load ratio" appears, 
recommending that it be sent to EnergyPlus Support Group...

If it facilitates, I uploaded the series of tests I've made with the 
Example File as a Zip file: "WaterHeater tests":
- Tests 1 to 3 use exactly the original Example File, only simulating 
the Weather file in three different moments (instead of the 
DesignDays), to see times with T<5ºC, 5ºC<T<10ºC and T>10ºC;
- Teste 4 to 7 experiment with different Water Heater and Heat Pump 
Temperature setpoints;
- The Zip file includes some excel files (with the same name of the 
test files) already showing some graphics and analysis of the 
relation between the demand and supply energies reported.

My ultimate aim was to model a Heat Pump producing water at 45ºC 
(with an inertia tank) to serve a HW Loop also at 45ºC with a nominal 
5ºC drop along the loop. Because my Weather file as several moments 
with temperatures below 5ºC, I was thinking of using Purchased 
Heating in the Water Heater to account for these moments, and then 
estimate "manually" (by post-processing) an electricity consumption 
correspondent to this moments, but I'd like to reach a reasonable 
estimate of Heat Pump consumptions whenever T>5ºC.
Since the purpose is to perform a (legaly bound) Energy Building 
Certification, I can't really move on without understanding properly 
the simulation behaviour and results, especially because I'm getting 
such higher Heating consumptions than expected and usual.

Could you help me understand the behaviour of the Example File and 
how to set up my model in order to achieve a behaviour that is as 
close as posible to reality?

Thanks in advance,
Ana Rui.



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