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Big gap between cooling energy from zone and from chiller

asked 2018-06-12 02:31:27 -0500

katsuya.obara gravatar image

updated 2018-06-12 02:51:16 -0500

I am modelling VAV system with water cooled chiller.
When I look at Sensible Heat Gain Summary in OutputTable.html, total cooling removed by HVAC Terminal Unit Sensible Air Cooling[kWh] is 815MWh. On the other hand, when I sum up the Chiller Evaporator Cooling Rate[W] exported from Output:Variable, it is 1345MWh which is much bigger. Does anyone know what is causing this situation?

When I export Chiller False Load Heat Transfer Rate, found that sum of them is 296MWh. However, still there is gap between cooling from zone and chiller. Does anyone know which data I should check?

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The difference is likely coming for different factors: mostly latent cooling in the zone, pump heat (on the chilled water side), and fan heat (on the AirLoopHVAC)

Julien Marrec gravatar imageJulien Marrec ( 2018-06-12 06:36:55 -0500 )edit

Is it possible to export those three factor through output:variable?

katsuya.obara gravatar imagekatsuya.obara ( 2018-06-12 20:56:16 -0500 )edit

Also look at the water coil connected to the chiller.

Output:Variable,*,Cooling Coil Total Cooling Rate, Hourly;
Output:Variable,*,Pump Fluid Heat Gain Rate, Hourly;

Output:Variable,*,Cooling Coil Total Cooling Energy, Hourly;
Output:Variable,*,Pump Fluid Heat Gain Energy, Hourly;
rraustad gravatar imagerraustad ( 2018-06-12 21:05:05 -0500 )edit

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answered 2018-06-20 18:08:17 -0500

updated 2018-06-20 18:48:30 -0500

Collecting the answers from above . . . The difference is latent cooling (at the coil), fan heat, pump heat, and outdoor air. Chiller False Load Heat Transfer is not included in Chiller Evaporator Cooling Rate, so that does not apply here. And if there is reheat, then you need to add the reheat coil output (whenever the cooling coil is active).

Working backwards

Chiller Evaporator Cooling Rate = Cooling Coil Total Cooling Rate + Pump Fluid Heat Gain Rate

Cooling Coil Sensible Cooling Rate = Zone Air System Sensible Cooling Rate + Fan Heat Gain to Air + Heating Coil Heating Rate (for reheat coils) + Net outdoor air load

The Net Outdoor Air Load = Zone Mechanical Ventilation Cooling Load Increase Energy + Zone Mechanical Ventilation Cooling Load Increase Due to Overheating Energy - Zone Mechanical Ventilation Cooling Load Decrease Energy

Outdoor air is messy, because free cooling in cool weather is included in the Zone Air System Sensible Cooling Rate which represents the cooling delivered to the zone. (same thing as HVAC Terminal Unit Sensible Air Cooling in the heat gain report).

And the difference in Cooling Coil Total vs Sensible load, you just have to accept. There aren't any other simple outputs that can add up to that unless you have no outdoor air and you report all of the zone internal latent gains.

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Asked: 2018-06-12 02:31:27 -0500

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Last updated: 2 days ago