Liquid Cooling - When performance meets sustainability: the data center cooling challenge

Overview

The introduction says NVIDIA Blackwell-class systems push rack densities from 70 kW toward 120 kW and that liquid is up to about 1,000 times more effective than air as a heat carrier. On regulation it cites the German Energy Efficiency Act: existing DCs PUE 1.5 from July 2027 and 1.3 from July 2030, DCs starting from 1 July 2026 must reach 1.2, and EU reporting applies from 500 kW IT.

Direct-to-chip removes 70-90 % of rack heat via liquid and the remaining 10-30 % by air. For temperatures, practical TCS regimes cited are 35/25 and 36/26 C; liquid-cooling return temperatures of up to about 60 C support heat reuse, and heat pumps can lift these to about 75 C.

Three worked cases are for a 1,000 kW CDU. Case 1: FWS 22/32 C water, TCS 36/26 C PG25, APT 4 K, heat-exchanger area 71.8 m2. Case 2: the same duty with APT 2 K needs 143.6 m2, double the area. Case 3: with APT 2 K but TCS delta-T raised to 15 K (39/24 C) the area stays 71.8 m2 and TCS flow falls 33 % (25.3 to 16.9 m3/h). The paper reflects a manufacturer view and its conclusions rest on these three cases.

Key points

1. Blackwell-class systems push rack densities from 70 kW toward 120 kW; liquid is up to about 1,000x more effective than air as a heat carrier.
2. German Energy Efficiency Act: existing DCs PUE 1.5 from July 2027 and 1.3 from July 2030; DCs starting from 1 July 2026 must reach 1.2; EU reporting from 500 kW IT.
3. Direct-to-chip removes 70-90 % of rack heat via liquid, 10-30 % by air.
4. Case 1: 1,000 kW CDU, FWS 22/32 C, TCS 36/26 C PG25, APT 4 K, heat-exchanger area 71.8 m2.
5. Case 2: same duty with APT 2 K needs 143.6 m2 (double).
6. Case 3: APT 2 K with TCS delta-T 15 K (39/24 C) keeps 71.8 m2 and cuts TCS flow 33 % (25.3 to 16.9 m3/h).
7. Return temperatures up to about 60 C support heat reuse; heat pumps can lift them to about 75 C.

Using it in design

• In the CDU selection tool let users trade approach temperature against heat-exchanger area and TCS delta-T, with Cases 1-3 as validation fixtures.
• Default the liquid fraction for D2C racks to 70-90 % and size the residual air load from the remainder.
• Show German EnEfG PUE targets by commissioning date, and flag sites of 500 kW IT or more for EU EED reporting.

Who should read it

Designers selecting CDUs for direct-to-chip AI facilities and tracking European efficiency rules.

Related standards

• German Energy Efficiency Act (EnEfG)
• EU Energy Efficiency Directive 2023/1791
• ASHRAE TC 9.9

Publication details

Authors

STULZ

Publisher

STULZ

Year

2025

Document number

Release 31, January 2025

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