[Bldg-sim] Difference in chiller energy for VAV and CAV system

[James V Dirkes II, PE]

Deepak,

Here are some thoughts:

·         Less fan energy  = less cooling load, since the fan energy is a part of the total cooling load.

·         If the pump is variable volume, the pump energy required for VAV fans will be slightly less due to less fan heat to cool.

·         If you are using the identical chiller for each system (VAV, CAV), then the chiller should use less energy also, due to less fan and pump heat.

·         A more common comparison would be to contrast a VAV chiller system with a CAV packaged rooftop system.  For that comparison, the part load efficiencies of chiller and compressor / DX coil will be a major factor. Dehumidification will also be different for DX vs. chilled water coils.

 

The Building Performance Team
James V. Dirkes II, P.E., LEED AP
1631 Acacia Drive NW
Grand Rapids, MI 49504
616 450 8653

 

From: bldg-sim-bounces at lists.onebuilding.org [mailto:bldg-sim-bounces at lists.onebuilding.org] On Behalf Of Deepak Tewari
Sent: Monday, March 21, 2011 2:31 AM
To: bldg-sim at lists.onebuilding.org
Subject: [Bldg-sim] Difference in chiller energy for VAV and CAV system

 

Dear All,

I am comparing the energy performance of a constant volume and variable volume system for a composite climate of India (New Delhi) in EnergyPlus. The building area is 7500 sq m. The chiller capacity is same for both the cases. The chilled water to the cooling coils is supplied by a constant speed pump. 

The savings in the fan energy is evident due to variable speed of the supply fan in case of VAV. However i am getting energy saving in cooling energy (chiller energy) also, in VAV compared to CAV, which i feel is due to higher delta T (chilled water) across cooling coil for CAV compared to VAV, this in turns increases the chiller electricity consumption. However while discussions with some consultant, it is their feeling that the cooling energy would remain same for both type of systems.

I want to ask has someone else tried this simulation and would there be any difference in cooling energy or not? 
Thanks in advance.
Deepak



 

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