ASHRAE Journal - August 2024 - 34

TECHNICAL FEATURE
FIGURE 2 Water-cooled chiller performance profile for different cooling load
LCHWT, ECWT and CW flow.
Chiller Model Efficiency
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
20%
40%
- CHWT=42°F/CWT=77°F/100% CW Flow
- CHWT=53°F/CWT=77°F/100% CW Flow
- CHWT=42°F/CWT=68°F/100% CW Flow
- CHWT=48°F/CWT=77°F/50% CW Flow
- CHWT=42°F/CWT=84°F/50% CW Flow
60%
Cooling Load Ratio (%)
- CHWT=48°F/CWT=77°F/100% CW Flow
- CHWT=42°F/CWT=84°F/100% CW Flow
- CHWT=42°F/CWT=77°F/50% CW Flow
- CHWT=53°F/CWT=77°F/50% CW Flow
- CHWT=42°F/CWT=68°F/50% CW Flow
understood. Often for new buildings the manufacturer
may be able to provide factory testing data during the
design phase, although it is often not as comprehensive
as data captured during operation over time and
may not be available for existing buildings. For other
equipment, knowing exact part-load performance may
not be straightforward. For instance, comprehensive
part-load data for cooling towers' power use for a
broad range of heat rejection, wet-bulb temperature,
water flow and leaving water temperature is often not
available during the design phase. During operation, it
can be learned from operating data, however.
FIGURE 3 Optimal load balancing between two chillers.
80%
100%
Real-Time Optimal Control
The proposed optimal control strategy attempts
to tackle several challenges. The first one is to find
the most efficient target load for each chiller while
delivering the overall plant load. The optimal target
loads are applied by setting a different LCHWT setpoint
on each chiller, centered around the plant LCHWT
setpoint. The plant still produces the required cooling
at the required leaving water temperature. A simple
example illustrating this concept with only two chillers
is shown in Figure 3. For a specific scenario of set plant
load requirement (represented by the dotted black line)
and water temperatures, combined power use of both
chillers is displayed for all combinations of chillers' load.
Optimal load balancing (represented by a yellow star) is
located at the lowest point that meets both the required
total load and load limits on each chiller.
Then, the optimal combination of chillers that would
reduce the overall power use of the plant at any given
time can be found. Figure 4 shows an example with one
small, one medium and two large chillers. For a building
cooling request of approximately 850 tons (3,000 kW),
Chillers 2 and 3 are the most efficient to run together.
Loads are distributed between chillers to ensure they
run as close as possible to their efficiency sweet spots.
For water-cooled chillers with variable speed drives
on the condenser pumps, the condenser water flow
setpoint can be varied to reduce the overall power use
Total
Power
(kW)
700
600
500
400
300
200
100
200
400
600
800
1,000
1,200
1,400
1,600
1,800
Chiller 2 Load
2,000
Chiller 1 Load
34
ASHRAE JOURNAL ashrae.org A U G UST 2024
Efficiency (kW/ton)
0 100 200 300 400 500 600 700 800 900 1,000 1,100 1,200 1,300 1,400 1,500 1,600 1,7001,800 1,900 2,000

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ASHRAE Journal - August 2024

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