10 Ways to Harness the Energy and Water Efficiencies of Direct Liquid Cooling

Overview

The paper measures which decision saves how much in a facility using single-phase direct-to-chip cooling. The main case is a 10 MW site in Paris, with other climates compared. The biggest lever is the TCS (technology cooling system) supply temperature. Liquid cooling cuts energy 30 to 60% against air cooling even without water-consuming adiabatic heat rejection, so efficiency can be gained without trading away water.

The effect of TCS temperature is not linear. Raising supply by 20 C, from 25 to 45 C, cuts energy 30 to 40%, and where adiabatic rejection is present it also cuts water use 40 to 60%. In the Paris 40 kW/rack case about half the savings come from the first 5 C step (25 to 30 C), while the last step (40 to 45 C) adds only about 1.1%. Server specifications reference 40, 45 and 50 C inlet water; the NVIDIA GB300 NVL72 specifies 40 C TCS supply and OCP promotes a 30 C minimum.

Other decisions are quantified too. Moving from 20 kW to 120 kW per rack lowers energy use by 4 to 16% because the heat-capture ratio improves. The most common TCS fluid is PG25, 25% propylene glycol in 75% deionized water. Liquid-to-liquid CDUs feeding dedicated high-temperature air-cooled chillers are recommended because they maximize economizer hours. The paper also covers heat reuse, component life and PEP/EPD disclosures. The scope is limited to single-phase direct-to-chip systems under the simulation assumptions; two-phase and immersion cooling are not assessed. Within that scope it gives designers a clear order of priority: TCS temperature first, then CDU type and heat-rejection equipment, then rack density.

Key points

1. Direct-to-chip liquid cooling cuts energy 30 to 60% versus air cooling without adiabatic (water-using) heat rejection.
2. Raising TCS supply from 25 to 45 C cuts energy 30 to 40% and, with adiabatic rejection, water use 40 to 60%.
3. In the Paris 40 kW/rack case half the savings come from the first 5 C; 40 to 45 C adds only about 1.1%.
4. A 120 kW/rack design uses 4 to 16% less energy than a 20 kW/rack design thanks to a higher heat-capture ratio.
5. PG25 (25% propylene glycol, 75% deionized water) is the most common TCS coolant.
6. Server specs reference 40/45/50 C inlet water; GB300 NVL72 specifies 40 C TCS supply and OCP promotes 30 C minimum.
7. Evaporative heat rejection uses about 43 L/min per 1 MW of cooling capacity.

Using it in design

• Default TCS supply to 40 C for liquid-cooled designs; warn below 30 C (efficiency loss) and above 45 C (server spec limit).
• Default to PG25 coolant, liquid-to-liquid CDUs and dedicated high-temperature dry coolers or chillers.
• Compute evaporative water at 43 L/min per MW (about 2.6 L per kWh of heat rejected while evaporating).
• Plot energy savings against TCS temperature to show diminishing returns, and add a heat-reuse output (supply/return temperature, available kW).

Who should read it

Mechanical engineers designing liquid-cooled AI/HPC sites, and sustainability leads who must budget the trade-off between water and energy.

Related standards

• ASHRAE TC9.9 Liquid Cooling Guidelines
• OCP 30C coolant guidance
• ISO 14025 (EPD)
• ISO/IEC 30134-9 (WUE)

Publication details

Authors

Maria A. Torres Arango; Wendy Torell; Stefano Lena; David McGlocklin

Publisher

Schneider Electric

Year

2025

Document number

WP 211

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