Cooling in most data centers uses air, and it does so for many practical reasons. Air is ubiquitous, it generally poses no danger to humans or equipment, it doesn’t conduct electricity, it’s fairly easy to move and it’s free. But air also falls short on a number of counts: for instance, it’s not very thermally efficient (i.e., it doesn’t hold much heat relative to liquids), so it’s impractical for cooling high-density implementations. Naturally, then, some companies are turning to water (or other liquids) as a means to increase efficiency and provide more-targeted cooling for high-power computing and similar situations. But will water wash away air as competition for cooling in the data center?
Why Water?
One of the main draws of liquid cooling is the greater ability of liquids (relative to air) to capture and hold heat—it takes more heat to warm a certain amount of water to a given temperature than to warm the same amount of air to the same temperature. Thus, a smaller amount of water can accomplish the same heat capture and removal as a relatively large amount of air, enabling a targeted cooling strategy (the entire data center need not be flooded to keep it sufficiently cool). And with the rising cost of energy and the growing power appetite of data centers, the greater energy efficiency of water is a temptation to companies.
For high-density implementations, air cooling is often simply insufficient—particularly when a whole-room cooling approach is used. In such cases, water (or, more generally, liquid) cooling offers an alternative in which not only can greater cooling efficiency be applied, it can be applied only where it is needed. That is, why cool an entire data center room when you can just cool, say, a particular high-density cabinet? And when individual cabinets are kept cool, they can also be placed much closer together, since air flow is less of a concern. Thus, liquid cooling can also enable more-efficient use of precious floor space.
Of course, no solution is ideal. Liquid cooling has its drawbacks, just as air does, and they’re worth noting. But it’s helpful to first review some liquid cooling solutions that are now in use in data centers.
Liquid Cooling Solutions
Liquids can serve as a medium for transporting heat in a number of different ways, ranging from broader cooling of the entire computer room to targeted cooling of particular racks/cabinets or even particular servers. The most basic option is to use water just as the means of moving heat from the computer room to the outside environment. In a whole-room approach to cooling, computer room air handlers (CRAHs) use chilled water to provide the necessary cooling; the water then moves the collected heat through pipes out of the building, where it is released to the environment (in part through evaporation, which is helpful but also can consume large amounts of water). This cooling approach is similar to the use of computer room air conditioners (CRACs).
A more targeted approach involves supplying cool water to the rack or cabinet. In the case of enclosed cabinets, only the space surrounding the servers need be cooled—the remainder of the room is largely unexposed to the heat produced by the IT equipment. This approach is similar to the whole-room case using CRAHs, except that only small spaces (the interiors of the cabinets) are cooled.
Cooling can be targeted even more directly by integrating the liquid system into the servers themselves. For instance, Asetek provides a system in which the CPU is cooled by a self-contained liquid apparatus inside the server. Essentially, it’s a miniature version of the larger data center cooling system, except the interior of the server is the “inside” and the exterior is the “environment.” Of course, the heat must still be removed from the rack or cabinet—which can be handled by any of the above systems.
At the extreme end of liquid cooling are submersion-based systems, where servers are actually immersed in a dielectric (non-conducting) liquid. For example, Green Revolution Cooling offers an immersion system, CarnotJet, that uses refined mineral oil as the coolant. Servers are placed directly in the liquid, yielding the benefits of water cooling without the hassles of containment, since mineral oil is non-conducting and won’t short out electronics. Using this system requires several fairly simple modifications, including use of a special coating on hard drives (since they cannot function in liquid) and removal of fans.
Going a step further, some solutions even use liquid inside the server only, avoiding the need for vats of liquid. In either case, however, the heat must still be removed from the server or liquid vat.
Now for the Downsides
Depending on the particular implementation, liquid cooling obviously poses some risks and other downsides. Air is convenient, partially because of its ubiquity; no one worries about an air leak. A water leak, on the other hand—particularly in a data center—is a potential disaster. Moving water requires its own infrastructure, and although moving air does as well, a leak from an air duct is much less problematic than a leak from a water pipe. Furthermore, water pipes can produce condensation, which can be a problem even in a system with no leaks. And with more-stringent requirements on infrastructure comes greater cost: infrastructure for liquid cooling requires greater capital expenses compared with air-based systems.
Another concern is water consumption. Evaporative cooling converts liquid water to water vapor, meaning the data center must have a steady supply. Furthermore, blowdown (liquid water released to the environment) can be problematic—not because it is polluted (it shouldn’t be), but simply because it’s warm. When warm water drains into a river or stream, for instance, it can damage the existing ecosystem.
Proponents of liquid cooling approaches cite the energy efficiency improvements that their systems can provide—estimates range as high as a 50% cut in total data center energy consumption. These numbers, of course, depend on the situation (where the particular data center starts and what type of system it installs), but the returns only begin after the infrastructure has been paid off. Also, given the greater infrastructure needs of liquid cooling, maintenance may be more of a concern. Furthermore, in cases where water is consumed by the cooling process, some energy efficiency comes at the cost of a high water bill.
Liquid Cooling to Stay
In some cases, particularly in lower-density data centers, air cooling may be the best option, if for no other reason than it is simpler and lower in cost to implement. Questions linger regarding at what point liquid cooling becomes financially beneficial (“Data Center Myth Disproved—Water Cooling Cost-effective Below 6kW/rack”). For high-density configurations, however, liquid may be the only viable option, and as high-power computing grows, so will an emphasis on liquid cooling. Air cooling simply has too many benefits—many of which center on its simplicity—to expect that liquid cooling will one day be the only cooling approach. Nevertheless, liquid/water cooling has an established position in the data center (particularly the high-density data center) that will grow over time. The only question is how much of the market will implement some form of liquid cooling, and which types of liquid cooling solutions will be the most prevalent.
