Capital Cost Analysis of Immersive Liquid-Cooled vs. Air-Cooled Large Data Centers

Overview

The paper compares the capex of a 2 MW air-cooled data centre with a chassis-based immersive liquid-cooled design. At equal density, 10 kW per rack, the two cost about the same (reference build cost about $10/W): savings of about $0.91/W from removing chillers and CRAHs offset a liquid-cooling technology premium of about $0.77/W. The sensitivity range for the technology premium is $0.50 to $1.10/W.

Because liquid cooling allows compaction, the paper also models 20 kW and 40 kW per rack. Capex falls by about 10% at 20 kW per rack and about 14% at 40 kW, thanks to less building and white space. Assumptions drive the result: the liquid design uses 40 C/47.75 C facility water supply/return with 25% glycol, against 20/30 C for air. The air-cooled room supplies 24 C with a 14 C deltaT, and core and shell cost $90/ft2 ($969/m2).

On energy, IT fan savings are assumed at 9% of server energy (15 to 20% measured on high-intensity compute), the micro-pump penalty at 3% and rack heat loss to air at 5.5%. 40 C facility water is assumed to give full free cooling. Limits: costs are 2019 dollars and need escalation before use. The authors note the technology was not mature at the time and expect costs to fall. The result applies to chassis-based immersion only and does not transfer directly to other liquid-cooling methods.

Key points

1. At 10 kW per rack in a 2 MW site, air-cooled and immersion capex are roughly equal (reference about $10/W).
2. Compaction to 20 kW per rack saves about 10% capex; to 40 kW per rack about 14%.
3. At 10 kW per rack, chiller/CRAH removal saves about $0.91/W; the liquid-cooling technology premium is about $0.77/W (sensitivity $0.50 to $1.10/W).
4. Liquid design assumptions: 40 C/47.75 C facility water supply/return with 25% glycol, versus 20/30 C for air.
5. IT fan savings assumed at 9% of server energy (15 to 20% measured on high-intensity compute), micro-pump penalty 3%, rack heat loss to air 5.5%.
6. Air-cooled room supply 24 C with 14 C deltaT; core and shell cost $90/ft2 ($969/m2).

Using it in design

• Use as a seed for the TCO/capex comparison engine: liquidTechPremiumUsdPerW = 0.77 and chillerCrahSavingsUsdPerW = 0.91 (immersion).
• Compute compaction savings through smaller building area, and assume residual air heat of 5.5% of IT for immersion.
• Show the user that liquid cooling is roughly capex-neutral at equal density and cheaper when compacted.
• Label costs as 2019 USD and require escalation.

Who should read it

Finance, planning and design teams comparing the capital cost of liquid and air cooling.

Related standards

• ASHRAE TC9.9 Liquid Cooling Guidelines
• OCP Advanced Cooling Solutions

Publication details

Authors

Robert Bunger; Wendy Torell; Victor Avelar

Publisher

Schneider Electric

Year

2019

Document number

WP 282

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