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to test the impact of reducing flow rate through
a high ∆T design on various conditions of supply
chilled water temperature
172 | Building Energy Efficiency Technical Guideline For Active Design
SIMULATION STUDIES
The following set of simulation cases were conducted to study the energy efficiency impact of these various ∆T chilled water flow options:
Case Description
Cases below are for a Primary Constant System. Specific pump power remains at 545 W per l/s, assuming that pipe size will be reduced to maintain same pressure at lower flow rates.
1
ΔT = 6.67°C (12°F)
Supply Temperature: 6.67°C (44°F) Return Temperature: 13.33°C (56°F) Primary Constant System @ 545 W per l/s
2
ΔT = 7.78°C (14°F)
Supply Temperature: 6.67°C (44°F) Return Temperature: 14.44°C (58°F) Primary Constant System @ 545 W per l/s
3
ΔT = 8.89°C (16°F)
Supply Temperature: 6.67°C (44°F) Return Temperature: 15.56°C (60°F) Primary Constant System @ 545 W per l/s
4
ΔT = 10.00°C (18°F)
Supply Temperature: 6.67°C (44°F) Return Temperature: 16.67°C (62°F) Primary Constant System @ 545 W per l/s 5
ΔT = 8.89°C (16°F)
Supply Temperature: 5.56°C (42°F) Return Temperature: 14.44°C (58°F) Primary Constant System @ 545 W per l/s
Cases below changes from a Primary Constant System to a Primary Variable System. Specific pump power remains at 545 W per l/s, assuming that pipe size will be reduced to maintain same pressure at lower flow rates.
6
ΔT = 6.67°C (12°F)
Supply Temperature: 6.67°C (44°F) Return Temperature: 13.33°C (56°F) Primary Variable System @ 545 W per l/s
7
ΔT = 7.78°C (14°F)
Supply Temperature: 6.67°C (44°F) Return Temperature: 14.44°C (58°F) Primary Variable System @ 545 W per l/s
8
ΔT = 8.89°C (16°F)
Supply Temperature: 6.67°C (44°F) Return Temperature: 15.56°C (60°F) Primary Variable System @ 545 W per l/s
9
ΔT = 10.00°C (18°F)
Supply Temperature: 6.67°C (44°F) Return Temperature: 16.67°C (62°F) Primary Variable System @ 545 W per l/s
10
ΔT = 8.89°C (16°F)
Supply Temperature: 5.56°C (42°F) Return Temperature: 14.44°C (58°F) Primary Variable System @ 545 W per l/s
TABLE 8.4 | SIMULATION CASE STUDIES Of vARIOUS ΔT SCENARIOS
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Cases below reduces Specific Pump Power. This is assuming pipe sizes remain the same as base design. i.e. no reduction in pipe size, pressure will be lower at lower flow rates.
11
ΔT = 7.78°C (14°F)
Supply Temperature: 6.67°C (44°F) Return Temperature: 14.44°C (58°F)
Primary Variable System @ 409 W per l/s, pipe sizes same as case 1
12
ΔT = 8.89°C (16°F)
Supply Temperature: 6.67°C (44°F) Return Temperature: 15.56°C (60°F)
Primary Variable System @ 327 W per l/s, pipe sizes same as case 1
13
ΔT = 10.00°C (18°F)
Supply Temperature: 6.67°C (44°F) Return Temperature: 16.67°C (62°F)
Primary Variable System @ 259 W per l/s, pipe sizes same as case 1
14
ΔT = 8.89°C (16°F)
Supply Temperature: 5.56°C (42°F) Return Temperature: 14.44°C (58°F)
Primary Variable System @ 327 W per l/s, pipe sizes same as case 1 Cases below test conditions with a low specific pump power of 280 W per l/s.
15
ΔT = 6.67°C (12°F)
Supply Temperature: 6.67°C (44°F) Return Temperature: 13.33°C (56°F) Primary Constant System @ 280 W per l/s
16
ΔT = 8.89°C (16°F)
Supply Temperature: 5.56°C (42°F) Return Temperature: 14.44°C (58°F) Primary Constant System @ 280 W per l/s
17
ΔT = 6.67°C (12°F)
Supply Temperature: 6.67°C (44°F) Return Temperature: 13.33°C (56°F) Primary Variable System @ 280 W per l/s
18
ΔT = 8.89°C (16°F)
Supply Temperature: 5.56°C (42°F) Return Temperature: 14.44°C (58°F)
Primary Variable System @ 280 W per l/s, pipe sizes reduced to maintain pressure
19
ΔT = 8.89°C (16°F)
Supply Temperature: 5.56°C (42°F) Return Temperature: 14.44°C (58°F)
Primary Variable System @ 140 W per l/s, pipe sizes same as case 17
RESULTS
Regardless of the chilled water distribution system in use, increasing the ∆T increases energy efficiency. This indicates that it is possible to reduce energy consumption while reducing pipe size, providing a reduction in capital costs as well as running costs. However, designers are cautioned that there will be an increase in the size of the cooling coil to provide a high ∆T return temperature. A lifecycle analysis between the reduction of costs in pipes and an increase in costs of cooling coil should be studied alongside the reduction of energy consumption to derive the optimum solution for a building.
The energy saved on a primary constant system from the use of a high ∆T chilled water system is higher than a primary variable system. This is because a primary variable system is already a more efficient system than a primary constant system.
174 | Building Energy Efficiency Technical Guideline For Active Design
fIGURE 8.5 | BEI RELATIONSHIP WITH DIffERENT ΔT Of CHILLED WATER fOR A PRIMARY CONSTANT SYSTEM
fIGURE 8.6 | BEI RELATIONSHIP WITH DIffERENT ΔT Of CHILLED WATER fOR A PRIMARY vARIABLE SYSTEM
Figure 8.5 shows that with a higher ∆T of chilled water, the BEI reduction is higher. It was also interesting note that reducing the supply chilled water temperature while maintaining the same ∆T reduces the BEI marginally due to the lower chiller efficiency.
It should also be highlighted that the cooling coil size becomes larger when the temperature of chilled water supplied is higher, even for the same rate of heat transfer (same ∆T).
Therefore the design selection of supply chilled water temperature and ∆T is a choice between the cooling coil size (and cost increment) and the savings that are provided by it due to a reduced flow rate, reduced pump size, pipe size and a reduced BEI.
Base 0.7% 1.3% 1.7% 1.2% 150 152 154 156 158 160 162 DT 12F (44/56F) DT 14F (44/58F) DT 16F (44/60F) DT 18F (44/62F) DT 16F (42/58F) BEI (kwh/m 2.year)
Chilled Water Temperature Difference
150 152 154 156 158 160 162 Base 0.25% 0.45% 0.57% 0.40% DT 12F (44/56F) DT 14F (44/58F) DT 16F (44/60F) DT 18F (44/62F) DT 16F (42/58F) BEI (kwh/m 2.year)
Chilled Water Temperature Difference
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fIGURE 8.7 | BEI RELATIONSHIP WITH DIffERENT ΔT Of CHILLED WATER fOR A PRIMARY vARIABLE SYSTEM WITH PIPE SIZE DESIGNED fOR A STANDARD ΔT
TABLE 8.5 | SIMULATION RESULTS Of vARIOUS ΔT SCENARIOS
Case (kWh/mbEI2.year)
bEI reduction
(kWh/m2.year) % reduction remarks
C1 160.2 Base Base Base Case: Primary Constant System.Specific pump power @ 545 W per l/s ΔT = 6.67°C (12°F)
C2 159.0 1.2 0.7% ΔT = 14°F, Supply T = 44°F
C3 158.1 2.0 1.3% ΔT = 16°F, Supply T = 44°F
C4 157.5 2.7 1.7% ΔT = 18°F, Supply T = 44°F
C5 158.2 1.9 1.2% ΔT = 16°F, Supply T = 42°F
C6 155.1 5.0 3.1% Base Case Changed to Primary Variable System.Specific pump power @ 545 W per l/s ΔT = 6.67°C (12°F)
C7 154.8 5.4 3.4% ΔT = 14°F, Supply T = 44°F
C8 154.4 5.7 3.6% ΔT = 16°F, Supply T = 44°F
C9 154.3 5.9 3.7% ΔT = 18°F, Supply T = 44°F
C10 154.5 5.6 3.5% ΔT = 16°F, Supply T = 42°F
C11 154.0 6.1 3.8% C11 = C7 with pipe size = C1
C12 153.4 6.7 4.2% C12 = C8 with pipe size = C1
C13 153.1 7.1 4.4% C13 = C9 with pipe size = C1
C14 153.6 6.6 4.1% C14 = C10 with pipe size = C1
C15 156.2 4.0 2.5% C15 = C1 with Specific Pump Power of 280 W per l/s
C16 155.2 4.9 3.1% C16 = C5 with Specific Pump Power of 280 W per l/s
C17 153.6 6.6 4.1% C17 = C6 with Specific Pump Power of 280 W per l/s
C18 153.4 6.8 4.2% C18 = C10 with Specific Pump Power of 280 W per l/s
C19 152.7 7.4 4.6% C19 = C18 with Specific Pump Power of 140 W per l/s (pipe size is not reduced when flow rate is reduced)
Base 0.7% 1.1% 1.3% 1.0% 150 152 154 156 158 160 162 DT 12F (44/56F) DT 14F (44/58F) DT 16F (44/60F) DT 18F (44/62F) DT 16F (42/58F) BEI (kwh/m 2.year)
Chilled Water Temperature Difference
176 | Building Energy Efficiency Technical Guideline For Active Design