Liquid Cooling Technologies for Data Centers and Edge Applications

Overview

The paper first explains why liquids move heat far better than air, and that this saves fan energy and enables warm-water heat rejection. Liquid cooling can use water up to 45 C (113 F), which allows compressor-free heat rejection in most climates and reduces evaporative water use. Removing or reducing server fans saves about 4 to 15% of IT energy.

It describes five main methods: two direct-to-chip (DTC) variants (single-phase and two-phase cold plates) and three immersion variants (chassis-based, tub single-phase, tub two-phase). On heat capture, DTC puts about 50 to 80% of IT heat into the liquid and the rest still needs air cooling; in typical servers 70 to 80% of heat comes from CPUs, GPUs and memory, which cold plates target. Immersion removes more than 95% of heat via liquid.

On density, an average above 20 kW per rack is possible with air but difficult; immersion handles 20 kW per rack easily and can exceed 100 kW per rack. The selection advice depends on context: for retrofit sites the paper favours rack-based direct-to-chip or chassis immersion, and for new or harsh-environment sites, immersion. This is an introductory paper: it classifies and compares methods but does not give a detailed sizing method for a specific product. The heat-capture fraction is given with different ranges in later Schneider papers (WP 279: 70 to 80%, WP 133: 60 to 90%), so it should be re-verified for the chosen architecture.

Key points

1. Direct-to-chip captures about 50 to 80% of IT heat in the liquid; the rest still needs air cooling.
2. Immersion removes more than 95% of heat via liquid.
3. Removing or reducing server fans saves about 4 to 15% of IT energy.
4. Liquid cooling can use water up to 45 C (113 F), enabling compressor-free heat rejection in most climates and less evaporative water use.
5. In typical servers 70 to 80% of heat comes from CPUs, GPUs and memory, the components cold plates target.
6. An average rack density above 20 kW is possible with air but difficult; immersion handles 20 kW per rack easily and can exceed 100 kW per rack.
7. Five main methods: DTC single-phase, DTC two-phase, chassis immersion, tub single-phase, tub two-phase.

Using it in design

• Create a coolingMethod enum (air, rdhx, dtc_1p, dtc_2p, immersion_chassis, immersion_tub_1p, immersion_tub_2p).
• Give each method a default liquid heat capture fraction (DTC 0.7, range 0.5 to 0.8; immersion 0.95 or more) and size residual air cooling as (1 - fraction) x rack kW.
• Apply a default IT-energy credit of 8% (range 4 to 15%) for fan removal in PUE and energy models.
• Default facility water for liquid loops to 35 to 45 C, which unlocks dry coolers.

Who should read it

Designers and decision makers evaluating liquid cooling for the first time, or choosing between retrofit and new-build methods.

Related standards

• ASHRAE TC9.9 Liquid Cooling Guidelines (W-classes)
• OCP Advanced Cooling Solutions

Publication details

Authors

Tony Day; Paul Lin; Robert Bunger

Publisher

Schneider Electric

Year

2019

Document number

WP 265

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