Iceotope Engineering Review - Desktop Study Report: Liquid and Air-Cooling Compared

Overview

Prepared by Neil Edmunds and dated 2021, this report (ICTP-CDL-XX-XX-RP-Z-90200) is a desktop study by the engineering consultancy Cundall for the liquid-cooling manufacturer Iceotope. The study compares chassis-level precision immersion cooling with best-practice air cooling for hyperscale, colocation, edge and tropical scenarios. The report is freely accessible, but because the client is a vendor, its savings figures should be read as best-case.

The report reviews the ASHRAE W1-W5 facility water classes, direct-to-chip versus full immersion, dielectric oils versus fluorocarbons, and Uptime continuous-cooling classes. ASHRAE liquid classes range from W1 (FWS 2-17 C) to W5 (above 45 C); liquid systems typically supply 40-48 C versus 16-25 C for air cooling. Dielectric oils are lower cost, combustible but not flammable, with a 10-20 year life; fluorocarbons are non-flammable with a life of about 30 years, but cost more and carry a higher GWP if they evaporate.

Energy and water modelling with ASHRAE weather data produces savings figures per kW of IT. Versus the air baseline, water use is down by up to 96%, power about 40% lower and CO2 about 39.5% lower. Typical air-cooled racks run at 7-10 kW, and raising density from 8 to 60 kW per rack cut the data hall footprint by about 86% in the study.

One important argument of the report is that PUE and WUE hide server-level parasitic loads. Server and PSU fans use about 10% of rack power versus about 3% for dielectric pumps, a hidden saving of about 7% of IT power that does not show in PUE. The report therefore recommends ITUE and TUE, presenting TUE = ITUE x PUE as a fairer whole-facility efficiency metric.

Key points

1. ASHRAE liquid classes range from W1 (FWS 2-17 C) to W5 (>45 C); liquid systems typically supply 40-48 C versus 16-25 C for air cooling.
2. Server/PSU fans use about 10% of rack power versus about 3% for dielectric pumps, a hidden ~7% IT saving not visible in PUE.
3. Typical air-cooled racks are 7-10 kW; raising density from 8 to 60 kW/rack cut data hall footprint by about 86% in the study.
4. Per kW IT versus air baseline: water use down up to 96%, power about 40% lower, CO2 about 39.5% lower.
5. Dielectric oils: lower cost, combustible not flammable, 10-20 year life; fluorocarbons: non-flammable, ~30-year life, higher cost and GWP if they evaporate.
6. Recommends TUE = ITUE x PUE as a fairer whole-facility efficiency metric.
7. Because the client is a vendor, savings figures should be read as best-case.

Using it in design

• Catalogue liquid-cooled IT and CDUs with ASHRAE W-class and supply temperature fields, and use these to decide whether compressor cooling can be omitted for the site climate.
• Offer an optional ITUE/TUE readout when users enter server fan power.
• Label vendor-sourced savings as indicative.
• Show the effect of density on data hall footprint, for example about 86% reduction going from 8 to 60 kW/rack, as a scenario comparison.

Who should read it

Mechanical designers, sustainability engineers and investment decision makers comparing liquid and air cooling.

Related standards

• ASHRAE TC 9.9 Liquid Cooling Guidelines
• ISO/IEC 30134-2 (PUE)

Publication details

Authors

Neil Edmunds

Publisher

Cundall (for Iceotope Technologies)

Year

2021

Document number

ICTP-CDL-XX-XX-RP-Z-90200

Document language

English

Source type

Expert / community

Access

Free to read

Licence

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

Citation