Densifying Modular Data Centers with Hybrid Cooling

Author: Carl Woodward, Wooden Data Center; John (MAA, Project Lead for RDHx); David Gaz, Thermacon Collection: 2023 Global Summit - Cooling Environments

Summary

This presentation documents a collaborative OCP effort to dramatically increase the power density of modular data centers (MDCs) by combining direct-to-chip cold plate cooling with rear door heat exchangers (RDHx) — a hybrid approach. The baseline challenge: existing OCP MDC specifications top out at 90 kW per container, while emerging GPU and AI workloads require far higher rack densities. The team set a target of at least 500 kW total capacity in the same footprint, requiring rack powers exceeding 50 kW with at least 75% of heat rejected via liquid.

The analysis modeled a configuration using the ORv3 rack frame with a hypothetical 4OU GPU box (62 kW per rack, 77% liquid cooled) — leveraging data from an OAI white paper contributed by Meta's Chen Chen and Albert Lee from Wiwynn. Sixteen such racks in a 3.3m × 13.7m module achieve approximately 1 MW of IT load. Two climate locations — Frankfurt, Germany and Dubai, UAE — were modeled to compare cooling infrastructure requirements and energy metrics. Frankfurt enables a near-unity PUE of 1.01 using purely adiabatic cooling in dry mode 99% of the time. Dubai requires a dual-loop system (air-cooled chiller for the door HX loop plus adiabatic cooler for cold plates) achieving PUE 1.07.

The work demonstrates that native OCP technologies — ORv3 frame, blind-mate liquid cooling, cold plates, and door HX — are sufficient to build 1 MW modular solutions in the same footprint as a 90 kW container, with heat reuse return temperatures reaching 55–56°C.