How Higher Chilled Water Temperature Can Improve Data Center Cooling System Efficiency

Overview

The paper centres on two levers: raising the CHW supply temperature and widening the CHW deltaT. With adiabatic heat rejection added as an option, the chiller acts only as a capacity booster on hot days. The weight of the strategy sits on the chiller: chillers account for about 60 to 85% of total cooling-system energy, so cutting the hours they run goes straight at the largest item.

An hourly model compares CHW setpoints from 7 C to 21 C, with 7 C (45 F) as the traditional value. Total cooling energy falls against the 7 C baseline by 11% at 10 C, 22% at 13 C, 30% at 15 C, 39% at 17 C, 48% at 20 C and 50% at 21 C. Combined strategies cut cooling opex by 41 to 64%. IT supply temperature is assumed at about 23 C. For fans and pumps, reducing flow to 80% cuts power by about 49%.

The gain is not unlimited. Above about 13 C the air-to-water deltaT is smaller, so more CRAH coil and fan capacity is needed, and savings level off once CRAH coil capacity limits the achievable supply air temperature. The paper lists three checks before adopting the strategy: chiller capability at high CHW temperature, CRAH coil selection, and low-cost water for adiabatic cooling. The paper contains an inconsistency: the capex increase is given as 13% in the executive summary but 15 to 20% in the conclusion. When costing, treat that range as uncertainty.

Key points

1. Combined strategies cut cooling opex by 41 to 64%; capex increase is reported as 13% (executive summary) but 15 to 20% (conclusion), an internal inconsistency.
2. Chillers account for about 60 to 85% of total cooling-system energy.
3. The traditional CHW supply setpoint is 7 C (45 F).
4. Cooling energy reduction versus 7 C: 10 C -11%, 13 C -22%, 15 C -30%, 17 C -39%, 20 C -48%, 21 C -50%.
5. Above about 13 C CHW the smaller air-to-water deltaT needs more CRAH coil and fan capacity; IT supply of about 23 C is assumed.
6. Fan and pump affinity: reducing flow to 80% cuts power by about 49%.

Using it in design

• Expose CHW supply temperature (default 15 C for new builds, range 7 to 21 C) and CHW deltaT as design inputs, and recompute economizer hours, chiller kW and CRAH coil selection when they change.
• Warn that CRAH counts or coil rows may increase above 13 C.
• Use the cube law for fan and pump part-load energy (power proportional to flow cubed).
• Before adopting, confirm the chiller's operating range at high CHW temperature and the water cost for adiabatic cooling.

Who should read it

Mechanical designers and energy-efficiency owners setting chilled-water plant setpoints and CRAH coil selections.

Related standards

• ASHRAE TC9.9 Thermal Guidelines
• ASHRAE 90.4

Publication details

Authors

Paul Lin; Victor Avelar; John Bean

Publisher

Schneider Electric

Year

2021

Document number

WP 227

Document language

English

Source type

Manufacturer

Access

Free to read

Licence

Copyright retained; citation only — the publisher’s terms apply to the original document

Citation