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However, an “oversized” cooling tower will also

have a higher fan energy consumption

190 | Building Energy Efficiency Technical Guideline For Active Design

SIMULATION STUDIES

A set of simulation studies were developed to study the impact of cooling tower optimisation based on all the issues raised on the previous page.

Case Descriptions kWe/Hrt tower Leaving Water Design Cooling temperature (°C/°f)

Cooling tower Leaving Water temperature Set

Point (°C/°f)

C1 Constant Speed Fan, Base Case 0.045 29.4/85 21.1/70

C2 Constant Speed Fan 0.035 29.4/85 21.1/70

C3 Constant Speed Fan 0.025 29.4/85 21.1/70

C4 Constant Speed Fan 0.015 29.4/85 21.1/70

C5 C2 with 2 Speed Fan 0.035 29.4/85 21.1/70

C6 C2 with Variable Speed Fan 0.035 29.4/85 21.1/70

C7 C2 with lower design condenser return _{temperature (oversized)} 0.035 28.9/84 21.1/70 C8 C2 with lower design condenser return _{temperature (oversized)} 0.035 28.3/83 21.1/70 C9 C2 with lower design condenser return _{temperature (oversized)} 0.035 27.8/82 21.1/70 C10 C2 with lower design condenser return _{temperature (undersized)} 0.035 30.0/86 21.1/70 C11 C2 with lower design condenser return _{temperature (undersized)} 0.035 30.6/87 21.1/70 C12 C2 with lower design condenser return _{temperature (undersized)} 0.035 31.1/88 21.1/70 C13 C6 (VSD Fan) Condenser return _{temperature set point varied} 0.035 29.4/85 23.9/75 C14 C6 (VSD Fan) Condenser return _{temperature set point varied} 0.035 29.4/85 26.7/80 C15 C6 (VSD Fan) Condenser return _{temperature set point varied} 0.035 29.4/85 27.2/81 C16 C6 (VSD Fan) Condenser return _{temperature set point varied} 0.035 29.4/85 27.8/82 C17 C6 (VSD Fan) Condenser return _{temperature set point varied} 0.035 29.4/85 28.3/83 C18 C6 (VSD Fan) Condenser return _{temperature set point varied} 0.035 29.4/85 28.9/84 C19 C6 (VSD Fan) Condenser return _{temperature set point varied} 0.035 29.4/85 29.4/85 C20 C6 (VSD Fan) Condenser return _{temperature set point varied} 0.035 29.4/85 30.0/86 C21 C6 (VSD Fan) Condenser return _{temperature set point varied} 0.035 29.4/85 30.6/87 C22 C6 (VSD Fan) Condenser return _{temperature set point varied} 0.035 29.4/85 31.1/88 C23 C6 (VSD Fan) Condenser return _{temperature set point varied} 0.035 29.4/85 32.2/90 TABLE 8.12 | COOLING TOWER EffICIENCY TEST CASES

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RESULTS

Figure 8.23 shows that for an improvement of cooling tower fan efficiency by 0.01 kWe/HRT, the BEI will reduce by approximately 1.5 kWh/m2_{.year on a constant fan speed cooling tower.}

fIGURE 8.23 | BEI RELATIONSHIP TO COOLING TOWER EffICIENCY AT CONSTANT fAN SPEED

Figure 8.24 shows that there is almost no efficiency gain with the use of a 2-speed fan or variable speed fan when the cooling tower leaving water temperature set point was fixed at a low value of 21.11°C (70°F). This is because our climate does not have such a low wet-bulb temperature in the first place for this temperature set- point to be achieved; therefore, the cooling tower fan will be running at full speed for all conditions.

fIGURE 8.24 | BEI RELATIONSHIP TO COOLING TOWER fAN OPTIONS WITH LEAvING WATER TEMPERATURE SET POINT Of 21.11°C

y = 146.92x + 149.34 R2 _{= 1} 151 152 153 154 155 156 157 0.000 0.005 0.010 0.015 0.020 0.025 0.030 0.035 0.040 0.045 0.050 BEI (kWh/m 2.year) Cooling Tower, kWs/HRT

C1 - C4, CONSTANT FAN SPEED

O! 20 40 60 80 100 120 140 160 180 Constant Speed 154.48 154.46 154.39

2 Speed Fan Variable Speed Fan

BEI (kWh/m

2.year)

Fan Options

kWe/HRT = 0.035, SET POINT 21.11°C/70°F

Constant Speed 2 Speed Fan Variable Speed Fan

192 | Building Energy Efficiency Technical Guideline For Active Design

Figure 8.25 is the simulated result of the testing an upsized or downsized cooling tower. The upsizing of a cooling tower is conducted via the design of a lower leaving water temperature from the cooling tower. The current standard of practice in the industry is to size the cooling tower for a water leaving temperature of 29.44°C (85°F). Sizing it for a lower leaving water temperature will a require larger cooling tower, while a sizing for a higher leaving water temperature will indicate a smaller cooling tower is used. The simulated results in Figure 8.25 indicate that reducing the cooling tower size reduces building energy consumption due to the smaller fan power used. The results show that the loss of chiller efficiency is minimal due to the use of a smaller cooling tower and fan. Finally, it is very important to note that the impact on energy efficiency due to the upsizing or downsizing of the cooling tower is very minimal. Therefore, designers should not be too concerned over upsizing or downsizing cooling tower to achieve better efficiency in a building.

fIGURE 8.25 | BEI RELATIONSHIP TO DESIGN COOLING TOWER WATER LEAvING TEMPERATURE

*Take note of the scale of BEI presented on the Y-axis, which is very small. y = -0.1002x + 163.01 R2 = 0.99539 154.0 154.1 154.2 154.3 154.4 154.5 154.6 154.7 154.8 154.9 155.0 81 82 83 84 85 86 87 88 89 BEI (kWh/m 2.year)

Design Cooling Tower Leaving Water Temperature to Chiller (°F)

UPSIZE COOLING TOWER

kWe/HRT = 0.035, SET POINT 21.11°C/70°F

CONSTANT SPEED FAN

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Figures 8.26 to 8.28 indicates the energy efficiency of using variable speed drive (VSD) fan on a cooling tower with different set points of water leaving temperature. The VSD will reduce the fan speed when the leaving water temperature meets the set point temperature to reduce the cooling tower fan energy consumption. The results of the simulation show that it is most efficient to run a VSD on a cooling tower with a water leaving temperature set point of 28.33 – 30.00°C (83 - 86°F). Somehow the results show that having the water leaving temperature set point at 27.78°C (82°F) and 30.56°C (87°F) increases chiller energy consumption significantly via a reduction of chiller efficiency. Further investigation showed that this result is largely due to the centrifugal chiller performance curve-fit used in this study, where the efficiency of the chiller is particularly low then the water leaving temperature set point is fixed at 27.78°C (82°F) and 30.56°C (87°F) .

Figure 8.27 shows that as the leaving water temperature from a cooling tower increases, the cooling tower fan energy reduces with the use of a VSD. Figure 8.28 shows that as the leaving water temperature from a cooling tower increases, the chiller energy increases with a particular spike at the water temperature set point of 27.78°C (82°F) and 30.56°C (87°F). The most ideal water leaving temperature set point for a cooling tower with a VSD is found between 29.44°C (85°F) and 30°C (86°F).

fIGURE 8.26 | BEI RELATIONSHIP TO SET POINT TEMPERATURE Of A vARIABLE fAN SPEED COOLING TOWER

fIGURE 8.27 | HEAT REJECTION ENERGY INDEx* RELATIONSHIP TO SET POINT TEMPERATURE Of A vARIABLE fAN SPEED COOLING TOWER

*Heat Rejection Energy = Condenser Pump Energy + Cooling Tower Fan Energy, since condenser pump is kept constant in this study, the change of heat rejection energy index is only attributed to the cooling tower leaving water temperature set point.

152 153 154 155 156 157 158 80 81 82 83 84 85 86 87 88 89 90 91 BEI (kwh/m 2.year)

Setpoint Temperature of Leaving Water Temperature (°F)

VARIABLE SPEED FAN, TEMPERATURE SETPOINT kWe/HRT = 0.035, DESIGN 29.4°C/85°F 0.0 2.0 4.0 6.0 8.0 10.0 12.0 14.0 16.0 78 80 82 84 86 88 90 92 Energy Index (kWh/m 2.year)

Cooling Tower Setpoint of Leaving Water Temperature (°F)

HEAT REJECTION ENERGY INDEX

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fIGURE 8.28 | CHILLER ENERGY INDEx RELATIONSHIP TO SET POINT TEMPERATURE Of A vARIABLE fAN SPEED COOLING TOWER