With server racks, CPUs, and GPUs increasing in power density, mechanical and electrical engineers are tasked to alleviate the heat found in data centers.
Many commercial and industrial facilities require computer room air-conditioning (CRAC), a computer room air handler (CRAH), chillers, and server rack fans to cool new or existing data centers. Some data centers even include raised flooring with a 2–4 foot crawl space for distributed cold air.
It is not until the end of the preliminary structural design that electrical design engineers are tasked with powering the units.
Engineering teams have gotten creative with the heat issue, too. For instance, Microsoft recently submerged a small data center 117 feet down off the coast of Scotland to cool approximately 864 computer servers. Alternatively, researchers in Switzerland have devised a design process that involved gas-etching slits of a GaN layer coated on a silicon material to cool chips.
Project Natick included the construction of a 40-foot long Northern Isles data center. Image used courtesy of Scott Eklund/Red Box Pictures
As data centers are increasingly strained, however, two established ways that design teams have addressed heat dissipation is direct-to-chip cooling and immersive cooling.
Direct-to-Chip Cooling or Immersive Cooling?
Direct-to-chip cooling can capture 50% to 80% of the heat that is dissipated in data centers while immersive cooling methods remove around 95% of heat, reducing the need for traditional cooling systems such as CRAC and CRAH units.
According to Schneider Electric, immersive cooling (or the “tub method”) involves submerging the full hardware into a large bath of leak-proof dielectric fluid. The fluid absorbs the heat and in some cases turns to vapor. Otherwise, it cools or condenses before returning to a fluid once more at its cooling point.
For immersive liquid cooling, the server lays vertically and can be placed back-to-back, a layout advantage that is not possible with servers following the direct-to-chip liquid cooling method. Image used courtesy of Schneider Electric
On-chip liquid cooling may involve direct-to-chip cold plates, evaporators, or immersion cooling. When design engineers utilize cold plates, the solution sits directly on board processors to draw away heat. There are two groups of cold plates: single- and dual-phase evaporators.
Single- vs. Dual-Phase Evaporators
Single-phase evaporators use cold water in a looped system that absorbs heat into the plate. Dual-phase evaporators, on the other hand, allow a low-pressure dielectric liquid to flow through the cold plates. Dielectric fluid is used to eliminate the risk of water exposure to IT equipment.
The architecture is shown above for a dual-phase immersive liquid cooling setup, complete with a dielectric coolant that changes from liquid form to a gas, taking away the heat. Image used courtesy of Schneider Electric
The heat released from the evaporator as a vapor is then transferred into a heat rejection unit. This will also require additional control units to help the fluid circulate evenly as it switches states.
Choosing the Most Effective Heat Management Method
While these circuit-level cooling methods have many advantages, they also include some drawbacks that warrant considerations.
If a liquid-cooled IT system is installed into an existing air-cooled facility, the electrical design cost will increase—even though it will cut down on energy costs by an average of 15%. The immersive cooling approach drives the cost up as well because it adds fluids to the design mix. However, it doesn’t negate the need for chillers or any air-cooled units.
For new systems, direct cooling may be the most cost-effective route since it reduces the need for traditional facility cooling units.
In data centers, one tradeoff of power density is thermal management concerns. Image used courtesy of Rittal
Direct cooling is similar to air-cooled systems in terms of serviceability, but there are components that can be added to the server racks to make it easier to maintain. Immersive cooling systems will require new equipment and procedures to service and maintain the components, but it will add more reliability to the system.
One key benefit in choosing immersive liquid cooling is the design flexibility. Because it doesn’t require airflow through the IT equipment, this method can make the most of a data center’s layout. And unlike direct cooling, immersive cooling can be used in harsh environments where airflow may not be feasible.
Choosing a Cooling Method by Application
Engineers can ultimately determine the best way to beat the heat depending on the application.
If developers are looking to retrofit a facility’s data center, a direct-to-chip liquid cooling approach would be the easiest and most cost-effective solution. With new data centers, developers might opt for a non-air conditioned space that uses immersive liquid cooling to capture heat and save on air-cooled equipment.