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

[Nick Caton]

I too cannot claim to be capable of tearing a chiller apart and putting it back together, but I can affirm Aaron's fundamentals appear on-cue to my understanding.  Less cooling à less compressor work à $$$ savings.

 

The attached email exchange from a few months back may be of interest to those following/participating in this discussion, though it is a bit lengthy.  In it, I attempt to "reverse engineer" the three default curves found in eQuest/DOE2 which together define behavior of a centrifugal chiller, observing and discussing the isolated effects of all the variables that are taken into account on an hourly basis... One following along can see I was a bit puzzled along the way, but arrived at an "ah-ha" moment when I followed through and applied the curves against each other to observe their net effects on efficiency under set conditions.

 

My hope then and now is that sharing this thought process may help others arrive at their own "eureka" moments =).

 

~Nick

 

 

 

NICK CATON, E.I.T.

PROJECT ENGINEER

Smith & Boucher Engineers

25501 west valley parkway

olathe ks 66061

direct 913 344.0036

fax 913 345.0617

www.smithboucher.com 

 

From: bldg-sim-bounces at lists.onebuilding.org [mailto:bldg-sim-bounces at lists.onebuilding.org] On Behalf Of Aaron Powers
Sent: Tuesday, March 22, 2011 2:41 PM
To: Deepak Tewari
Cc: bldg-sim at lists.onebuilding.org
Subject: Re: [Bldg-sim] Difference in chiller energy for VAV and CAV system

 

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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