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

[Deepak Tewari]

Aaron 
Can you elaborate how the larger chilled water delta-T will decrease chiller 
lift and increase efficiency?





________________________________
From: Aaron Powers <caaronpowers at gmail.com>
To: "James V Dirkes II, PE" <jvd2pe at tds.net>
Cc: Deepak Tewari <dipaktwri at yahoo.co.in>; bldg-sim at lists.onebuilding.org
Sent: Mon, 21 March, 2011 8:41:21 PM
Subject: Re: [Bldg-sim] Difference in chiller energy for VAV and CAV system


There are several things going on here:
 
- Yes, the lower fan energy for VAV will add less heat to the chilled water 
loop.  This will propogate to the pumps, chillers, and heat rejection.
- Chilled water coils are complex heat-exchangers, which do not exibit linear 
behavior.  Reducing the air flow decreases the water-side coil effectiveness; 
therefore, at a given load, a CAV system will have a larger chilled water 
delta-T.
- With equivalent pumping schemes, this will result in pump savings for the CAV 
system (in the absence of 3-way valves).  I'm not sure about ePlus, but this can 
be demonstrated in the latest DOE2.2.
- In reality, the larger chilled water delta-T will decrease chiller lift and 
increase its efficiency.  Again I'm not sure about ePlus, but in DOE2.2, chiller 
curves are a function of a dT parameter which is the difference between 
condenser entering and chilled water leaving temperatures.  Its an attempt to 
account for chiller lift, but it does not give an efficiency credit for 
increasing the chilled water delta-T.  So, you will not see the chiller 
efficiency boost in DOE2 for CAV systems due to a greater chilled water delta-T.
 
In my experience, the VAV fan savings (and reduced chilled water load savings) 
usually outweigh the pump and chiller savings for CAV.  However, it varies from 
building to building.  For example, if you had a rare building with a low 
air-side static pressure drop to begin with and a long, high head pumping 
system, then its possible that the CAV system will be more efficient overall.
 
Aaron

On Mon, Mar 21, 2011 at 8:13 AM, James V Dirkes II, PE <jvd2pe at tds.net> wrote:

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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-- 
Sent from my DynaTAC 8000x


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