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98 TRG-TRC016-EN
notes
period four
Chiller-Plant Control
Lowering the temperature of the condenser water can also reduce the energy
consumption of the chiller. Depending on the system load and outdoor
conditions, cooling towers typically have the ability to supply colder condenser
water than at design conditions. This, however, increases the energy
consumption of the cooling tower fans. The key to maximizing energy savings
is knowing the relationship of cooling-tower energy consumption to chiller
energy consumption.
At design conditions, a chiller typically uses five to ten times more energy
than a cooling tower. This would suggest that it might be beneficial to use
more cooling-tower energy to save chiller energy. However, there is a point of
diminishing return where the chiller energy savings is less than the additional
energy used by the cooling tower. Figure 106 shows the combined annual
energy consumption of a chiller and cooling tower in a system that is controlled
to various condenser-water-temperature set points. The third column shows a
system that attempts to supply 55°F [12.8°C] water from the cooling tower at
all times. Of course, at design conditions, the cooling tower may not be able to
supply this temperature, but it will supply the water at the coldest temperature
possible.
The fourth column shows a system that uses a control system to dynamically
determine the optimal condenser-water temperature that minimizes the
combined energy use of the chiller plus cooling tower. It is obvious that this
method of optimal control minimizes overall system energy consumption.
Condenser-Water Temperature
annual energy consumption, kWh
annual energy consumption, kWh
cooling
tower
cooling
tower
300,000
300,000
chiller
chiller
condenser-water temperature set pointcondenser
condenser
-
-
water temperature set point
water temperature set point
85°F
[29.4°C]
85°F
85°F
[29.4°C]
[29.4°C]
70°F
[21.1°C]
70°F
70°F
[21.1°C]
[21.1°C]
55°F
[12.8°C]
55°F
55°F
[12.8°C]
[12.8°C]
optimal
control
optimal
control
200,000
200,000
100,000
100,000
Figure 106