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RE: [EnergyPlus_Support] What should Heat Rejection reasonable value be?

Also, you may check suitable output variables and just find max for cooling rate and heat rate rejected. It should be in according with thermodynamicsJ.

I have had the similar issue and the reason was in temperature difference specified in idf and real dT in output variables. When E+ calculates the rated energy rates it takes the dT from idf and max flow, but in real dT may be higher or lower (see corresponding value in output variable). This is the reason of the difference.

Therefore when you specify the dT in idf anyway you need o correct it after investigation of dT in output varibles.







Artem Zhukov


HVAC Consultant/Building Simulation Engineer


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From: EnergyPlus_Support@xxxxxxxxxxxxxxx [mailto:EnergyPlus_Support@xxxxxxxxxxxxxxx] On Behalf Of Griffith, Brent
Sent: Monday, September 30, 2013 23:18
To: EnergyPlus_Support@xxxxxxxxxxxxxxx; EnergyPlus_Support@xxxxxxxxxxxxxxx
Subject: RE: [EnergyPlus_Support] What should Heat Rejection reasonable value be?



I think you are confusing the energy consumed by the operation of the cooling tower with the heat rejected by that cooling tower. 


From: EnergyPlus_Support@xxxxxxxxxxxxxxx [mailto:EnergyPlus_Support@xxxxxxxxxxxxxxx] On Behalf Of hisham.rashrash
Sent: Sunday, September 29, 2013 10:49 PM
To: EnergyPlus_Support@xxxxxxxxxxxxxxx
Subject: [EnergyPlus_Support] What should Heat Rejection reasonable value be?



I have done a simulation for a project and I got these results for baseline:
Cooling = 830,787.3 [kWh/year]
Heat Rejection = 50,417.5 [kWh/year]

My question is:
Shouldn't the Heat Rejection value be greater than Cooling? because what I remember from Thermodynamics is Qh=Ql+W
So, the heat rejection should be equal to the heat pumped out of spaces + the energy consumed in the compressor, right?

If the above discussion is correct, what could I have done wrong? because I modeled the HVAC system as System 8: Water cooled chiller and VAV with PFP boxes(ASHRAE 90.1-2007 Appendix G) by putting the following HVACTemplate Objects:
1. Thermostat.
2. Zone:VAV:FanPowered.
3. System:VAV.
4. Plant:ChilledWaterLoop.
5. Plant:Chiller.
6. Plant:Tower.
I couldn't find where to define that the cooling tower is connected to the chiller, so I assumed this is done automatically in HVACTemplate method, right?

Thank you.


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