Warm Liquid Cooled Cloud Facilities: What Do We Achieve? (Microsoft submission to OCP)

Overview

This is a Microsoft white paper submitted to the Open Compute Project in 2022, arguing for warm-water liquid-cooled cloud facilities. It reviews efficiency trends and compares the heat-transfer capability of air, water and two-phase dielectric fluids. The volumetric heat transfer ratio of water to air is cited as about 1900:1. The PDF carries the notice 'Copyright Microsoft 2022.

The authors' processor tests compare air, cold-plate and two-phase immersion cooling. In those tests water-cooled cold plates gave about 41 percent higher CPU clock rates than air, and two-phase immersion close to 51 percent higher. These results come from the authors' own experiments, and applying them to other processors, workloads and hardware would need separate verification.

For design, the most valuable part of the paper is its discussion of facility water temperature classes (W27 to W45). Lower classes (W27, W32) may need chiller operation to reach peak performance, whereas warm classes (W40, W45) can match or beat air cooling and open the door to waste-heat reuse.

Section 7 explains that raising facility water temperature can remove chillers and adiabatic or evaporative cooling, along with the associated water use. The heat can serve district, space and water heating, or processes such as adsorption chillers, direct air capture (DAC) and desalination. The limit of the document is that it is a white paper expressing a manufacturer's view, and its results have not been independently verified.

Key points

1. The volumetric heat transfer ratio of water to air is cited as about 1900:1.
2. In the authors' tests, water-cooled cold plates gave about 41 percent higher CPU clock rates than air, and two-phase immersion close to 51 percent higher.
3. Lower water classes (W27, W32) may need chiller operation to reach peak performance.
4. Warm classes (W40, W45) can match or beat air cooling and open up waste-heat reuse.
5. Raising facility water temperature can remove chillers and adiabatic or evaporative cooling and the water use that goes with them.
6. Heat can serve district, space and water heating, and adsorption chillers, DAC and desalination.

Using it in design

• Default liquid-cooled designs to warm facility water (W40/W45) in chiller-free or dry-cooler-only plant options, citing this paper as supporting evidence.
• Show the WUE and chiller-energy penalty of lower water classes explicitly.
• Link warm-water designs to the heat-reuse module.
• Cross-check against the ASHRAE Liquid Cooling Guidelines W-classes and the OCP 30 C coolant roadmap.

Who should read it

Cooling designers planning liquid-cooled facilities, and sustainability and heat-recovery engineers.

Related standards

• ASHRAE Liquid Cooling Guidelines (W-classes, 2021)
• OCP 30 C Coolant - A Durable Roadmap

Publication details

Authors

Open Compute Project Foundation

Publisher

Open Compute Project Foundation

Year

2022

Document language

English

Source type

Industry body

Access

Free to read

Licence

Bibliographic reference only — the document is not redistributed here; confirm reuse terms at the original source

Citation