Research firm IHS, in its Data Center Cooling Report—2014, is predicting the market for liquid cooling infrastructure in data centers to grow by 40% this year. At that rate, this market will double in size in just two years. The efficiency gains and capabilities of liquid cooling compared with more-traditional air cooling have helped this approach gain traction in applications like high-performance computing (HPC), but the high capital costs may hinder broad adoption in the data center industry.
Playing It Cool
A major contributor to data center energy consumption is cooling. According to a Digital Realty Trust survey from 2013, the average power usage effectiveness (PUE) of data centers in North America is 2.9—a dismal number indicating that for every watt that goes to IT tasks, nearly two watts go to non-IT uses. With energy prices consistently rising, companies have pursued all manner of strategies to reduce consumption, including more-efficient cooling methods. Although free cooling and similar air-based methods are beneficial, they become insufficient for high-density deployments when rack power exceeds certain levels.
Liquid cooling can pick up the slack in such cases. Although air cooling has the benefit of using a substance that’s everywhere, air has a relatively low heat capacity compared with water and other liquids, so it is less efficient when acting as a cooling medium. As the amount of power consumed in the limited space of a rack increases, liquid-based cooling eventually becomes a necessity. Combined with greater efficiency, that reality is driving growth in this market.
IHS forecasts 40% growth in the market for liquid cooling products this year, indicating solid uptake of this approach in data centers. Liz Cruz, senior analyst of data center and critical infrastructure at IHS, expects to see continued strong growth for at least five years. “Obviously, the rate of growth will slow as the annual market size increases, but I do believe that the current adoption of liquid cooling is not just a blip in the market. There are real use cases and true benefits to adoption that make it a worthwhile investment for some applications.” Currently, however, Cruz notes that this approach constitutes less than 1% of the market for data center cooling infrastructure by revenue.
Paying the Cost
Naturally, the benefits of liquid cooling come with a cost: unlike air, the liquid must be carefully contained (particularly when using water) if it is delivered to racks or ever particular components, requiring extensive infrastructure beyond what air cooling typically involves. And the time to recoup those costs through greater efficiency varies, depending in large part on the power densities involved in the deployment.
Given these high initial costs, liquid cooling isn’t for every data center. “Rack densities would need to be above the 15kW/rack range before it makes sense to start looking at liquid cooling,” said Cruz. “It’s a significantly more expensive product, at roughly five times the cost of a chilled-water solution, so the densities must be very high in order to justify the additional investment cost.” She identifies HPC data centers focusing on applications ranging from scientific computing to Bitcoin mining as candidates for liquid-based cooling.
Mixing Water and Electricity
Liquid cooling may bring to mind the dangers (to people and equipment) of combining water with electronics, but it often uses other media—particularly for the case of immersion. As long as the liquid is nonconductive and noncorrosive, electronic equipment can be safely submerged in it. A number of vendors offer such infrastructure; the main caveat is that certain components (specifically, hard drives) must be sealed to prevent the liquid from entering. “Green Revolution Cooling uses a mineral oil called GreenDEF, and Icetope is using the 3M Novac solution. There seems to be a few different varieties, but they are all nonconductive,” notes Cruz.
Water can still have a place, however. “There are direct-to-chip manufacturers that use water in the heat exchanger, but that’s a different beast since the chip is not actually placed in the water. The chip is simply next to the heat exchanger.” In other words, as long as the water is contained, the danger is mitigated—but in such cases, the infrastructure must avoid the sorts of small flaws that air cooling can tolerate, increasing the cost.
Immersion cooling versus directed cooling (delivering the liquid to specific components—specifically, processor chips) naturally involves tradeoffs. Immersion is messier but easier in the sense that liquid need not be contained as rigorously; direct-to-chip cooling is cleaner and doesn’t preclude traditional racks, but it can also be less efficient. Cruz sees the segmentation in this case being roughly equal, with neither approach holding a strong market lead over the other.
The Future of Liquid Cooling
Because of the costs and inconveniences of liquid cooling, it will likely remain a niche solution. Even given a continued strong growth rate, the market will still pale in comparison with more-traditional cooling solutions. Cruz said, “Even five years from now, my study forecasts that liquid cooling will still only account for less than 2% of total data center cooling revenues.” But for extreme deployments that involve high rack densities, the prospect of nearly halving electricity costs by using a liquid-based approach will be an increasingly tantalizing prospect, especially given rising electricity costs, environmental concerns regarding energy production and the fact that server lifetime operating costs now exceed their capital costs.
Even if the market doubles in size over the next two years, it will remain small. Factors that might accelerate adoption or broaden it to more applications and lower power densities include potential government efficiency regulations, rising energy costs (especially with the ongoing turmoil in the Middle East) and brighter perceptions of the economy—whether ill-founded or otherwise. The relatively high capital investments associated with liquid cooling, however, will remain a barrier to smaller companies and those for whom air cooling is sufficient. In many cases, for example, free cooling/air-side economization serves the purpose, so the value of the added infrastructure and hassle is minimal at best. But as data center space becomes increasingly precious, designers will seek to cram more computing power into racks, which can mean higher power densities. Beyond a certain point, the economic and technical case for liquid cooling becomes compelling. The only question is how much of the market will reach this point.