A Study on the Energy Reduction Measures of Data Centers through Chilled Water Temperature Control and Water-Side Economizer

Overview

The study examines cooling energy reduction through chilled-water control and a WSE by simulation. The baseline central plant (7/12 °C chilled water, 20 °C CRAH supply, cooling tower 37/32 °C) used 97,404 MWh/yr for cooling: CRAH fans took about 43.9%, chillers 42.9%, cooling towers 6.4%, condenser pumps 4.6% and chilled-water pumps 2.3%. The study separates the energy of chillers, CRAH fans, cooling towers and pumps, which shows the trade-off between chiller savings and higher CRAH fan energy as water temperature rises.

Without a WSE, the optimum chilled-water setpoint was 11 °C, cutting total energy by 1.8%; at 15-17 °C, CRAH fan energy rose enough to increase total energy. With a WSE, the optimum setpoint was 17 °C: total energy fell to 78,343 MWh/yr (about 14.8% less than at 7 °C), with chiller energy down 59.6% and chilled-water pump energy down 40.1%. Overall reductions against the conventional model were 1.8% (higher setpoint), 19.6% (adding WSE) and 30.1% (WSE with delta-T control).

With delta-T control, return chilled water at an 11 °C setpoint reached 25 °C with a 14 K delta-T; a higher return temperature lengthens free-cooling time. The authors conclude that higher return chilled-water temperature, secured through delta-T, should be used to get the most from a WSE, while watching the increase in CRAH coil and fan use. A general central cooling system can be operated efficiently when supply chilled water is set to 11 °C.

Limits: a single large Seoul data centre and an hourly simulation, so results depend on that climate and plant layout. Efficient design of the WSE system is left to future work.

Key points

1. The baseline central plant (7/12 °C chilled water, 20 °C CRAH supply, cooling tower 37/32 °C) used 97,404 MWh/yr for cooling; CRAH fans about 43.9%, chillers 42.9%, cooling towers 6.4%, condenser pumps 4.6%, chilled-water pumps 2.3%.
2. Without a WSE, the optimum chilled-water setpoint was 11 °C, cutting total energy by 1.8%; at 15-17 °C, CRAH fan energy rose enough to increase total energy.
3. With a WSE, the optimum setpoint was 17 °C; total energy fell to 78,343 MWh/yr (about 14.8% less than at 7 °C), with chiller energy down 59.6% and chilled-water pump energy down 40.1%.
4. Overall reductions against the conventional model were 1.8% (higher setpoint), 19.6% (adding WSE) and 30.1% (WSE with delta-T control).
5. With delta-T control, return chilled water at an 11 °C setpoint reached 25 °C with a 14 K delta-T; a higher return temperature lengthens free-cooling time.
6. The authors conclude that higher return chilled-water temperature secured through delta-T should be used to get the most from a WSE, while watching the increase in CRAH coil and fan use.

Using it in design

• In a central plant without a WSE, raise the chilled-water supply setpoint to about 11 °C; at 15-17 °C, CRAH fan energy increased total energy.
• Add a WSE and raise chilled-water delta-T to raise return temperature (25 °C return with 14 K delta-T at an 11 °C setpoint); free-cooling time lengthens and savings reached 30.1%.
• Model the CRAH fan and coil energy increase as water temperature rises as a separate item alongside chiller savings.

Who should read it

Cooling and energy engineers setting setpoints and WSE design for central chilled-water plants.

Document contents

1. 1. Introduction
2. 2. Building Outline
3. 3. Analysis of Cooling Energy Consumption of Central Cooling System (Type 1)
4. 4. Supply Chilled Water Temperature Control (Type 2: system, simulation, simulation result)
5. 5. Water-Side Economizer Applied Model (Type 3: system modeling, simulation result)
6. 6. WSE Application and ΔT Variation Model (Type 4: system modeling, energy consumption and free cooling time, comparison of Type 4 energy saving rate)
7. 7. Conclusions

Related standards

• ASHRAE TC9.9 Thermal Guidelines
• ASHRAE 90.4

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Publication details

Authors

Yu-Jin Kim; Ju-Wan Ha; Kyung-Soon Park; Young-Hak Song

Publisher

Energies (MDPI AG)

Year

2021

Volume

14

Issue

12

Pages

3575

Document language

English

Source type

Scientific

Access

Open licence — downloadable

DOI

10.3390/en14123575

Licence

CC BY 4.0 — share with attribution

Licence evidence

www.mdpi.com

Citation