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

[Aaron Powers]

Bill, you're right about that.  A central CAV system with reheat is limited
by the most loaded zone.  My statements before were simply for one air path
and one zone, which is probably rare.  The degree to which this hurts a CAV
system depends on the diversity of the loads.  If all zone loads remain
similar, then the supply air temperature off the CAV coils will start to
float upwards at reduced loads, and you'll see an increased chilled water
delta-T.

As for the increased chiller efficiency, here's my thinking.  At a constant
load with no mixing valves, an increased delta T across the cooling coils
allows for a lower gpm.  Slowing the water flow rate and raising the
entering water temperature increases the effectiveness of the evaporator
barrel (i.e. the enthalpy of the refrigerant now has a greater ability to
approach the enthalpy of the higher enthalpy water).  Running the compressor
as before would over-cool the water.  To maintain a constant chilled water
supply temperature, the mass flow of the refrigerant must be reduced.
Depending on the type of compressor unloading mechanism, this should produce
some energy savings.

This is my understanding simply based on theory, and I'm sure it's much more
complicated in reality.  Again, I know that DOE2 does not account for
varying inlet conditions to the chiller evaporator barrel, but I'm not sure
about ePlus.

Aaron

On Tue, Mar 22, 2011 at 5:30 AM, Deepak Tewari <dipaktwri at yahoo.co.in>wrote:

> 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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Sent from my DynaTAC 8000x
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