With all its benefits, liquid cooling would appear to be a default solution for much of what ails today’s data center. But combined with its downsides and cost premiums, it still hasn’t seen the kind of adoption one might expect. Will liquid cooling, whatever the particular mode, ever replace air as the default option for data centers?
Shades of Green
Because liquid-based cooling systems typically deliver greater efficiency than air-based systems, they score environmental points by reducing energy consumption. But data center managers must be more concerned about another “green”: money. An environmentally friendly but unprofitable company is soon an empty building that is soon replaced by someone else with more financial sense, so energy-saving measures must also fit in the budget. To be sure, these two greens are not always at odds: greater energy efficiency does mean lower energy costs. According to Martin LaMonica at IEEE Spectrum, “Given computing’s growing energy cost and environmental footprint, proponents say it’s just a matter of time before some form of liquid cooling wins out.” But lower energy costs are not the only fiscal consideration.
Liquid Cooling in Brief
Liquid cooling comes in a number of flavors, each with numerous variations to fit different data center applications and operating conditions. The first step from simple air cooling (through, for instance, mechanical computer room air conditioners, or CRACs) is liquid as a heat-transfer medium from computer room air handlers (CRAHs) to some form of “heat disposal” into the environment. Such infrastructure could include cooling towers, mechanical chiller plants or even passive cooling using ambient outside temperatures. Another step up is delivery of water to the rack or server level, reducing the volume of air that must be cooled to a given temperature to protect the IT equipment. The ultimate in liquid cooling is immersion, which bypasses air and some liquid delivery infrastructure (such as pipes to the server or even processor) and brings the thermal benefits of liquid directly to the electronics.
The benefits of submersion cooling are tremendous. According to TechTarget, “Combined with a simple pump or a heat-exchanger system, immersion cooling, which uses mineral oil baths and other industrial cooling mixes, can save money not only on power bills but also computer room air conditioner maintenance. These immersive systems eliminate the need for air cooling and, due to their efficiency, also allow equipment to run at higher temperatures.” Asicminer in Hong Kong, for instance, claims a 97% savings on cooling its 500 kW Bitcoin-mining system compared with conventional methods, thanks to immersion cooling. Yet, IEEE Spectrum notes that liquid cooling is “a rarity in the corporate data centers that run bank transactions and the cloud facilities that serve data to smartphones.” Are companies, therefore, just stubborn?
More Than Culture
As companies face an uncertain economic environment but rising demand for IT resources, it’s unlikely that cultural factors are the main limitation preventing adoption of liquid cooling. Consider the case of immersion cooling specifically: although it can all but eliminate energy spent on cooling, it has a number of drawbacks that impose cost premiums. The following are a few:
- Lower profiles. Unlike a rack, a tub for immersion of servers is only accessible from the top, meaning the potential for vertical scaling of infrastructure is extremely limited. Traditional racks, however, effectively enable stacking of servers floor to ceiling in a given floor space. Thus, this approach can reduce the power density per square foot. To be fair, however, this consideration only applies if an equivalent power density can be achieved using other cooling methods; very high-density deployments preclude the use of air cooling, for instance.
- Mess. Any maintenance, changing of cables or other activity involving contact with the servers requires contact with the liquid—mineral oil, for instance. These liquids are chosen to avoid toxicity, but a spill can create a hazard for employees, not to mention requiring significant effort to clean.
- Supporting infrastructure. Immersion cooling requires vats to hold the servers, as well as a large supply of the liquid.
- Special HDDs. Hard-disk drives (HDDs) immersed in liquid must be designed to prevent leakage or otherwise sealed, as the spinning disks must operate in a gas.
- Retrofitting costs. Designing a new data center from scratch to accommodate liquid cooling is less troublesome than retrofitting an existing data center. Thus, investment in an existing deployment creates a barrier for many facilities.
In addition, liquid cooling systems—whether immersion or otherwise—require filtration of the liquid to avoid problems like buildup of contaminants, excessive sediment and biological growth. For water-based systems such as those that employ cooling towers or other evaporative measures, the amount of sediment in a given volume increases as vapor is removed, requiring separation and disposal of this “blowdown.” Even this disposal can create environmental concerns. Furthermore, water usage—particularly in dry areas—is a concern, with regard to both utility capacity (in the case of large data centers) and the limited local supply.
What Does Liquid Cooling Need?
Liquid cooling is a necessity for very high-density data center deployments, such as those performing high-performance computing. In these cases, organizations who want to exceed a certain power density simply have no choice, regardless of the costs or hassles. The question is whether liquid will become a mainstay in lower-density facilities.
The total cost of ownership (TCO) for servers already owes more than half to operating expenses—read: energy. At a certain point, rising energy prices can make even small efficiency improvements worthwhile, so conceivably, a continued trend of higher prices could make liquid cooling more attractive even to smaller data centers that may otherwise be deterred by capital costs.
Regulations are another potential factor. Governments may or may not impose energy-efficiency regulations directly on data centers, but regulations on power suppliers can have nearly the same effect. The costs of compliance and more-expensive power sources (alternative instead of coal, for instance) are passed to consumers, so companies running data centers will face the financial brunt of regulations either way.
Adoption of liquid cooling, like any technology, can accelerate when it sees greater adoption. Being a pioneer involves risks—something that many companies wish to avoid when it comes to their data centers. But once the trail is blazed and more heavily traveled, others will find it easier to follow.
Liquid cooling still faces a number of hurdles to broad adoption, particularly in smaller data centers and facilities with relatively low power density. At high densities, liquid cooling is a necessity. Energy prices and regulations make efficiency improvements more attractive to data center operators, so at some point, they may drive a much faster move to liquid instead of air (or in conjunction with air). The key to its adoption is the color green: but more the color of money than the color of environmental stewardship. When all the considerations align—costs, use of floor space, maintenance, convenience and so on—liquid will have a better chance of becoming the dominant cooling method.