Hybrid Cooling: The Bridge to Full Liquid Cooling in Data Centers

Session after session at this year’s Data Center World extolled the glories of various forms of liquid cooling. Keynote speakers and panelists offered tips about how and when to implement it, while analysts produced impressive figures related to the sector’s projected growth.

Such is the hype around liquid cooling that one might think it was about to replace traditional air cooling in data centers. But as it stands, nothing could be further from the truth.

While liquid cooling is indeed coming fast, it will be part of a hybrid cooling arrangement whereby air and liquid techniques are harnessed together and optimized depending on the application, the amount of heat being generated, the cost, and the capabilities of the facility.

“Cooling innovation enables us to do more computing for less power,” said Shen Wang, principal analyst at Omdia. “But is hybrid cooling that will enable the best balance of cost and performance.”

The Rise of Liquid Cooling

As IT equipment manufacturers turn to liquid cooling to remove heat from high-powered chips, it is important to remember that many components in the data center will remain air-cooled – and they are likely to remain cooled in that manner for many years to come.

“Even if all cabinets are cooled with liquid, there will still be a whole lot of heat dissipated into the room,” said Lars Strong, senior engineer at Upsite Technologies, during this year’s Data Center World in Washington, DC. “The most advanced liquid cooled data centers will probably still need at least 20% to 30% of air cooling.”

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“Air cooling will gradually be replaced by liquid cooling techniques over the next few decades,” he explained. “As liquid cooling and other more efficient cooling technologies become more widespread and cost-effective, more and more data centers will switch to these technologies.”

The Hybrid Approach

Despite the ongoing advances in liquid cooling technologies, the concept of the 100% liquid cooled data center is still a pipe dream, Strong said.

Once liquid is used to cool equipment, heat is transferred to it. Some of that heat is dissipated into the surrounding space, and air cooling is required to remove it. Hybrid facilities, therefore, are emerging that can maximize the benefit of both air and liquid cooling. After all, each cooling technology has its distinct advantages and disadvantages. Some are more efficient at cooling, but difficult to implement and require heavy upfront investment. Others are cheap but struggle once density levels exceed a certain point.

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“Data center managers must understand the complementary nature of the different cooling solutions,” said Strong.

Carrie Goetz, Principal/CTO of StrategITcom, added: “Many data centers will deploy liquid to cool hot spots and high-density areas, while the main room will be air or cooled due to its lower power density.”

The Many Stages of Hybrid Cooling

Some take a limited view of hybrid cooling. They look upon it as a facility that has implemented liquid cooling that also used air cooling to keep room and rack temperatures down. However, there is a lot more to it.

“As well as having a hybrid data center space, there can be a hybrid approach to a computing enclosure or within a cabinet,” said Strong. “One example is a rear door heat exchanger that introduces doors that contain chilled water that can fit onto the back of an otherwise air-cooled rack.”

Direct-to-chip (DtC) cooling, too, may channel away a lot of heat by circulating a cool liquid through a cold plate in direct contact with CPUs and other hot spots. However, some of the heat transferred to the liquid would be dissipated into the room, and that still has to be removed via air cooling.

Similarly, immersion cooling – submerging a server, rack, or entire row of equipment into a dielectric liquid – removes heat by direct contact with the liquid. But the tank containing the IT equipment is constantly emitting heat into its surroundings.  

“Immersion cooling doesn’t remove all the heat as the liquid vat gives off a lot of heat into the room that needs removed by air cooling,” said Strong. 

Continue to Work on Air Cooling Efficiency

Strong urged those keen to implement data center liquid cooling to continue to work on air cooling, too. The basics of air-cooling efficiency remain true regardless of how much liquid is introduced, he said. And in an emerging hybrid cooling environment, the value of a liquid cooling investment is greatly enhanced by implementing air cooling best practices.

“It took decades for PUE numbers to come down significantly through the use of air cooling and other efficiency measures,” said Strong. “But PUE numbers have stagnated. There is still plenty of room for improvement in air cooling.”  

He advised data center managers to remain true to the fundamentals. Separating the supply air from the exhaust air was vital 30 years ago and remains vital today. Computer room air conditioning (CRAC) and computer room air handler (CRAH) units use fans, cooling coils, chillers, and other components to remove heat. Each element should be running optimally. Air flows should be investigated to ensure that cold air flows where it is most needed and hot air is properly contained. Thus, aisle containment systems, floor tiles, blanking panels, and various sealing solutions to prevent the escape of air must be well implemented.

“Air gets a bad rap as being inefficient, but that is not the case if it is handled correctly,” said Strong. “Intel has documented air cooling of 40 KW racks.”

He gave an example of a CRAH chiller that was wasting $190,000 a year due to inefficient operation. By calibrating water value position sensors, chiller water pumps, and chiller set points, the cooling infrastructure load was slashed by 20%. But even then, the facility had not implemented relatively inexpensive rack containment that could have further lowered the cooling load.

A Hybrid Future

Liquid cooling is going to steadily advance into data centers in the coming years.

A recent Uptime Institute survey found that 16% of data center managers believe liquid cooling will become the primary cooling method within 1-3 years and 41% think it will take 4-6 years.

Despite that, Strong asserts that a hybrid cooling approach is likely to apply to almost all data centers in the foreseeable future. Some will do a lot of liquid cooling, while others will continue to rely mainly on traditional air-cooling systems and gradually add some liquid.

“No one is going to eradicate air cooling entirely,” said Strong.