Liquid cooling of data centers: A necessity facing challenges

The evolving data generation landscape requires faster and more efficient microprocessors, prompting innovative manufacturing methods for smaller and faster transistors. Transistor congestion and rising demand for parallel processing are pushing the thermal design power of microprocessors well beyond 280 W, a limit for air cooling, and are expected to surpass 700 W by 2025. Consequently, transitioning towards liquid cooling is necessary. This article is intended to serve as a comprehensive roadmap to understanding this shift. It covers four major liquid cooling techniques: indirect water cooling with rear door heat exchangers, direct liquid cooling using water blocks or evaporators, single-phase, and two-phase immersion cooling. Indirect water cooling with rear door heat exchangers is a simple water cooling adaptation for reducing the power consumption of existing air-cooled data centers, but it faces the same limitations as air cooling for high-power servers. With enhancements such as reduced hot air leakage, active rear door heat exchangers, and deployment in locations conducive to free cooling, this approach could provide highly efficient data centers for the foreseeable future. Direct liquid cooling is well suited to meet rising thermal design power demands with the highest heat transfer coefficient report of 25 W/cm2-K, using water-based manifold microjet impingement on die. Emerging technologies are new thermosyphon systems, on-die/on-lid refrigeration/impingement, two phase impingement, and on/in die microchannel cooling. Air cooling is still required for peripheral equipment in this method, adding to the complexity and power consumption. Immersion cooling has the potential of reducing infrastructure size by one-third of air cooled data centers. Single-phase immersion cooling, while the most simple to implement, is limited by the low thermophysical properties of the dielectric liquids, and lack of flow control mechanism. In contrast, two-phase immersion cooling faces significant challenges related to the use of engineered fluids with global warming potential, health hazards, and long term reliability.