Does the spread of direct liquid cooling make PUE less relevant?

Overview

This article discusses how the spread of direct liquid cooling (DLC) affects the meaning of the PUE metric. Its core argument is that DLC changes a facility's energy profile, so PUE understates the benefit of the technology. It is a conceptual argument rather than a measurement study and was published in the Uptime Institute Journal. The argument centres on server fans. Server fan power is typically estimated at 5% to 10% of total IT power and is counted as part of IT load in PUE.

When DLC removes that fan power and reduces leakage power, IT energy falls. Because part of the facility load stays the same while the IT share shrinks, PUE can fail to improve or may even worsen, and partial PUE approaches its limit. The article states that most DLC implementations reach a partial PUE of 1.02 to 1.03, so the cooling side is already close to its floor. It notes that cold-plate systems still need some fans in power supplies and for low-power electronics.

It also recalls that PUE was developed by The Green Grid in 2007. It also cites survey data: for sites above 1 MW, only one in four respondents think air cooling will remain dominant beyond the next decade. As a limit, the piece is an opinion article; its message is that comparing DLC and air designs needs metrics beyond PUE.

Key points

1. Most DLC implementations reach a partial PUE of 1.02 to 1.03.
2. Server fan power is typically estimated at 5% to 10% of total IT power.
3. DLC removes fan power (counted as IT) and reduces leakage power, so IT energy falls.
4. Cold-plate systems still need some fans in power supplies and for low-power electronics.
5. Only one in four respondents (>1 MW sites) think air cooling will remain dominant beyond the next decade.
6. PUE was developed by The Green Grid in 2007.

Using it in design

• When comparing air and DLC options, report total facility energy and IT energy, not PUE alone.
• In DLC scenarios, subtract the server fan share (5-10%) from IT load before comparing.
• Note in reports that PUE loses discriminating power as partial PUE nears 1.02-1.03.
• In cold-plate designs, remember remaining fan loads such as power supplies.

Who should read it

Designers choosing between liquid and air cooling and engineers who report energy efficiency metrics.

Publication details

Authors

J. Davis

Publisher

Uptime Institute Journal (Uptime Institute)

Year

2021

Document language

English

Source type

Industry body

Access

Free to read

Licence

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

Citation