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

[Aaron Powers]

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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