This is the liquid cooling technology Graham sees most often, with the biggest challenge being the additional plumbing and piping that’s required. “It hits the sweet spot of cooling data center racks without doing any modifications to the servers,” he says. “That’s what makes it very appealing to folks implementing high density racks. It does require new piping, but in terms of relative effort, it’s the path of least resistance.”
Direct-to-chip liquid cooling
Direct-to-chip cooling systems are even more efficient, according to Graham, able to handle rack densities from 50 to 100 kilowatts.
This approach to liquid cooling brings the liquid all the way inside the individual server, but it requires changes to the computing hardware and is complex to set up and to maintain. “You take out the heat sink and put a plate on it, and the plate is fed by a liquid mechanism,” says Smith.
The liquid can stay a liquid, or it can change into steam. The latter is known as a dual-phase system.
Different liquids have different boiling points, Smith says. In a single-phase system, the liquid is pumped through a loop in order to remove the heat from the servers. In a double-phase system, it moves by itself.
“Gas travels faster than liquid, creating a self-regulating pressure,” says Smith. “If you have a leak, it won’t flow the other way – and it uses less energy because you’re not running pumps.” And the hot steam can do double duty for generating energy, for heating, or other uses in the building, he says.
Immersion cooling
Another approach to liquid cooling is immersion cooling. “That’s where you take the computer and submerse it in a vat of liquid, a non-conductive liquid like oil,” says Smith. “It looks like a deep fryer. You’re putting a server into a deep fryer.”
The specific liquids used are ones that conduct heat but aren’t flammable, he adds. “This got popularized with cryptominers,” he says. “But it’s very heavy — a lot of data centers aren’t meant to handle this much weight.”
There’s also the problem of what to do if there’s a leak, he says. “And how would you plug in the network cable? And where does the switch go? All kinds of other things make it a niche application right now.”
There’s a lot of opportunity for manufacturers to rethink the fully immersed form factor. “Maybe they could look like Nintendo cartridges that are encased in liquid, but you never see the liquid,” Smith says. “There are new form factor designs that are fully immersed without vats of oil sitting around.”
According to IDC’s Graham, immersion cooling is the most efficient of the three approaches, able to handle densities from 50 to 250 kilowatts per rack.
For brand new, fully liquid-cooled data centers, all the air cooling infrastructure can be eliminated, saving space and money, says Joe Capes, CEO at LiquidStack, an immersion cooling vendor. In a hybrid approach, the immersion cooling systems can be used in a new high-density zone or in a modular deployment, he says. The benefit is three times the amount of compute density in the same amount of space.
Maxim Serezhin, founder and CEO at Standard Power, a colocation company, says that liquid cooling is now a key differentiator in the market for his firm. Standard Power uses technology from LiquidStack, he says, for both immersion and direct-to-chip cooling.
Customers using liquid cooling include those with AI, high-performance computing, crypto and other data-intensive workloads, he says. “We help them adopt the liquid cooling technology that best suits their needs.”
Obstacles to adoption
There are still some substantial obstacles to the adoption of liquid cooling, the lack of standards being one of the primary ones, in addition to safety worries and lack of training.
“There’s a lot of opportunity here for standards, and that is what we’re working with at SSAI [the Sustainable and Scalable Infrastructure Association],” says Smith.
For example, a particular chemical might be permitted in data center cooling in the United States, but not in Germany. And different vendors might have couplers in different sizes and different mechanisms.
“Everybody can put anything they want in data centers,” says Smith. “There are no standards about shape, size, form factor, where you put cables. There’s no standard that server manufacturers can plug into in an efficient way.”
The same is true for liquid cooling, he says. “Is it on the left side or the right side? Would it snap it or does it screw in?” Instead, there are multiple incompatible vendor solutions, he says. The industry needs a common set of standards to accelerate the deployment of liquid cooling.
That’s not the only obstacle to adoption. Another is a lack of trained data center staffers who can handle the technology.
“From a data center operator perspective, the big question isn’t ‘Do we support liquid?’ but ‘Are we comfortable with the operational side?’” Smith says. “What is the regulatory side where you are? How do you communicate with customers about what types of liquids you allow in? What happens if it breaks – is it your SLA or their SLA?”
Another issue is that of physical space, says Holger Mueller, analyst at Constellation Research. “Traditionally, you have rows of servers open in the front, and all the important stuff is in the back, where you can put in some infrastructure and do some cooling,” he says. “But the space there wasn’t set up to put in another cooling system.”
Under the sea
Some companies have experimented with full immersion. Literal full immersion – in the ocean. Since more than half of the world’s population lives within 120 miles of a coast, putting data centers underwater could, potentially, revolutionize the industry.
In 2018, Microsoft sank an entire data center 117 feet deep into the sea off the coast of Scotland. Two years later, Microsoft pulled the data center back up and discovered that its underwater servers were eight times more reliable than those on land, possibly due to the fact that the atmosphere in the data center was filled with nitrogen rather than air, and there weren’t any people around to bump into things and jostle components.
Since then, though, there’s been no further news from Microsoft on the subject, possibly because of the logistical challenges of putting a data center under water.
Other companies, including Subsea Cloud and Chinese company Highlander, are also getting into the game. Subsea planned to have its first commercial data centers running at the end of 2022, but, as of this October, it still hadn’t sunk any racks.
Highlander, however, opened its first commercial undersea data center in Hainan in late 2022, with China Telecom one of its first customers.
The problem with underwater data centers, says Smith, is that people have to be able to get into them to switch out equipment. “So it hasn’t been too popular. Whether the data center is under the ocean or up in space – or even just in Wisconsin – we don’t have good mechanisms as an industry for efficiently installing and removing equipment.”
