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70 TRG-TRC016-EN
notes
period three
System Variations
There is usually an efficiency penalty associated with the use of heat recovery
with a chiller. The cost of this efficiency penalty, however, is typically much less
than the energy saved by recovering the “free” heat.
The energy consumption of a heat-recovery chiller will be higher than that of
a cooling-only chiller, because of the higher pressure differential at which the
compressor must operate. In this example, the energy consumption of a
centrifugal chiller operating in heat-recovery mode (producing 105°F [40.6°C]
condenser water) is 0.69 kW/ton [5.1 COP]. The efficiency of the same chiller
operating in the cooling-only mode (no heat being recovered and producing
95°F [35°C] condenser water) is 0.60 kW/ton [5.9 COP]. A comparable cooling-
only chiller of the same capacity and operating at the same cooling-only
conditions consumes 0.57 kW/ton [6.2 COP]. In this example, the heat-recovery
chiller uses four percent more energy in the cooling-only mode than the chiller
designed and optimized for cooling-only operation. It is therefore important
to perform a life-cycle cost analysis to determine when heat recovery is a
viable option.
It should also be noted that the chiller can only recover the amount of heat
transferred into the evaporator plus the energy input to the compressor.
Therefore, the more load on the chiller evaporator, the more heat can be
recovered. To maximize the heat available for recovery, a heat-recovery chiller is
often piped in one of the preferential-loading configurations described earlier.
An advantage of the sidestream position is that the heat-recovery chiller is not
required to maintain a chilled-water temperature set point. It can be loaded just
enough to satisfy the heating load while the more efficient cooling-only chillers
provide the rest of the cooling. The heat removed from the chilled-water loop
benefits the system by precooling the return water.
Heat-Recovery Chiller Efficiency
chiller type
cooling
mode
cooling-only
centrifugal chiller
heat-recovery
centrifugal chiller
0.57 kW/ton
[6.2 COP]
heat-recovery
mode
0.60 kW/ton
[5.9 COP]
0.69 kW/ton
[5.1 COP]
cooling mode conditions:
• evaporator ∆T = 44°F to 54°F [6.7°C to 12.2°C]
• condenser ∆T = 85°F to 95°F [29.4°C to 35.0°C]
heat-recovery mode conditions:
• evaporator ∆T = 44°F to 54°F [6.7°C to 12.2°C]
• condenser ∆T = 85°F to 105°F [29.4°C to 40.6°C]
not
applicable
Figure 78