Unraveling the Efficiency, Performance and Sustainability Trade-Offs: Case Study of Liquid Assisted Air Cooling vs Traditional Air Cooling (OCP EMEA Summit 2025)
Open Compute Project Foundation · Leslye Paniagua, Sharon Alper
Cloudflare compares a liquid-assisted air-cooled (LAAC) 2U server, where a closed-loop cold plate dumps CPU heat to an in-chassis radiator, with the same server using a conventional heat sink. It is a useful data point for sites that cannot yet supply facility water.
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Overview
The presentation was given at the OCP EMEA Summit 2025 (Dublin, DC Sustainability track) by Leslye Paniagua and Sharon Alper. The test platform is a 2U1N chassis with one 500 W, 192-core AMD Turin CPU, 12 x 64 GB DDR5 and five 80 mm fans.
The team used a 49-workload benchmark suite and a 12-hour full-load test to measure performance, wall power and component temperatures. Across the 49 workloads all results were within 5%, with 80% of per-core tests within 1%. The LAAC system drew 1.6% (about 12 W) more power than the air-cooled unit, attributed to the pump; fan speeds were similar or lower.
In the reliability results, CPU, NIC and DC-SCM were cooler with LAAC, but DIMMs ran 3-6 C hotter and SSD and PSU temperatures rose because of radiator-preheated air and unbalanced impedance. As of early 2025 Cloudflare's fleet in 190+ cities was 100% air cooled; the study cites under 25% of global data centres having adopted any liquid cooling.
Improvement levers include pump selection and BMC speed control, fan algorithm tuning, radiator placement, air ducts and fillers. The study rests on a single server platform; results should not be transferred directly to other hardware.
Key points
- Test platform: 2U1N chassis with one 500 W, 192-core AMD Turin CPU, 12 x 64 GB DDR5, five 80 mm fans.
- Across 49 benchmark workloads all results were within 5%; 80% of per-core tests within 1%.
- The LAAC system drew 1.6% (about 12 W) more power than the air-cooled unit, attributed to the pump.
- CPU, NIC and DC-SCM were cooler; DIMMs ran 3-6 C hotter and SSD/PSU temperatures rose.
- Cloudflare's fleet in 190+ cities was 100% air cooled in early 2025; under 25% of global data centres use any liquid cooling.
- Improvement levers: pump selection and BMC speed control, fan algorithm tuning, radiator placement, air ducts and fillers.
Using it in design
- Distinguish LAAC (no facility water, heat still rejected to room air) from DLC with facility water in the server catalog, and do not reduce air-side cooling load for LAAC racks.
- Add a note or warning that LAAC adds pump power and preheats downstream components.
- Check DIMM, SSD and PSU temperatures separately when assessing LAAC.
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Who should read it
Designers and hardware engineers evaluating liquid-assisted cooling at sites that cannot yet supply facility water.
Publication details
- Authors
- Leslye Paniagua; Sharon Alper
- Publisher
- Open Compute Project Foundation
- Year
- 2025
- Document number
- OCP EMEA Summit 2025, DC Sustainability track
- Event
- OCP EMEA Summit 2025, Dublin
- 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
Leslye Paniagua; Sharon Alper (2025). Unraveling the Efficiency, Performance and Sustainability Trade-Offs: Case Study of Liquid Assisted Air Cooling vs Traditional Air Cooling (OCP EMEA Summit 2025). Open Compute Project Foundation. OCP EMEA Summit 2025, DC Sustainability track. OCP EMEA Summit 2025, Dublin. https://drive.google.com/file/d/1e70KiX7lqY4rgBzNLqE9Oprlm74TOuaW/view
Events on this subject
- ISK-SODEX Istanbul 2027
- SC26 – International Conference for High Performance Computing, Networking, Storage and Analysis
- ASHRAE Winter Conference
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