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

[Bill]

I have a bit different view of this:
 
The load seen by the chiller is not just the load in the zones, but the load
from the system.  The system load is what is required to cool the supply
flow down to the supply air temperature (SAT) setpoint.  The SAT is either
fixed, or cold enough to satisfy the zone requiring the most cooling (the
"critical" zone), assuming the zone loads are met.  All zones except for the
critical zone don't need as much cooling.  In a VAV system, the cooling is
reduced via reduced flow.  This reduced flow means less system load to the
chiller plant.  In contrast, the CAV system load does not get reduced
because some zones have lower loads.  The full fan flow needs to be cooled
to the SAT.  In this case, the non-critical zones are kept from being
overcooled by the addition of reheat.
 
Regards,
Bill

  _____  

From: bldg-sim-bounces at lists.onebuilding.org
[mailto:bldg-sim-bounces at lists.onebuilding.org] On Behalf Of Aaron Powers
Sent: Monday, March 21, 2011 8:11 AM
To: James V Dirkes II, PE
Cc: Deepak Tewari; bldg-sim at lists.onebuilding.org
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 <tel:616%20450%208653> 

 

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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