Ever wondered about the difference between free air, airside economizers, direct and indirect evaporative and immersive cooling and how they might benefit your business? If so, then The Green Data Center Conference should be on your ‘to do’ list. Presented by the Global Strategic Management Institute in San Diego this month, the event and technology expo focused on data centre efficiency, organized into two tracks: development and regulatory issues in site selection, and power and cooling innovation. In the second track, speakers from across the efficiency landscape — including experts from the front line, and representatives from the academic and vendor communities — took a deep dive into power and cooling, explaining not only various technologies, but their potential cost benefits, adoption considerations and use cases. In this way, speakers provided the tools for audience members to evaluate implementation in their own particular circumstances — an increasingly important imperative given increasing server densities and the fact that cooling is generally reckoned to account for approximately 40 percent of data centre power consumption and resulting carbon emissions.
Day one of the event opened with a presentation by Tim Chadwick, president of Alfa Tech Consulting Enterprises, and designer of the cooling systems for several of the Facebook and other high profile data centers. Noting that facilities designers consider many factors when thinking about cooling systems (free cooling, HVAC, retrofitting, electrical considerations, airside economizers) Chadwick focused in his talk on the use of evaporative cooling, a technique involving the spray of water directly into a partitioned space in the facility between the outside and the server rooms. As the water evaporates in this type of system, it cools nearby air, which is then pumped directly into the data centre. As Chadwick explains in the accompanying video, "swamp cooling" is an ancient technique that was first deployed in data centre facilities as a means of introducing humidity in the early 2000s, but designers quickly discovered the cooling benefits of the approach.
Direct evaporative cooling comes in several flavours, including use of a cooling tower where moisture is added directly to the air, evaporation reduces air temperature and cooled air is released into the facility; and use of a specific media that is continuously doused to produce cooler air. While use of this media can reduce the cost of a direct evaporative cooling system, it provides less control. According to Chadwick, the spray technique offers a "pretty good level of precision and control" as an operator may use one mister nozzle or a bank of them, depending on temperature requirements; however, the systems require clean water, and purification can introduce additional expense.
When weighing the merits of these different direct cooling methods, Chadwick advised adopters to consider the potential downsides beyond expense. For example, there is a need to manage moisture in the air when the nozzle-based systems are used, while with evaporative media, you may need more force to push air through the media as it ages and becomes less permeable.
Ultimately, cooling solution decisions are based on business needs. In a large cloud-based data centre, where there is need to address temperature requirements associated with rapid load changes, control may be more critical. In other circumstances, the facility may be willing to experiment with operating outside of tightly-defined temperature and humidity guidelines — to be, as Chadwick explained, only "somewhere inside the ASHRAE box," the area sketched out in ASHRAE psychrometric charts as being safe for server operation (65°-80° F is current recommended temperature for high-end data centres, with relative humidity of 30%-60% to keep condensation and static electricity within reasonable levels). Discussing levels of play within these guidelines, Chadwick characterized the Facebook Open Compute project at Prineville, Oregon (for which he was the designer), which relies totally on direct evaporative cooling, as "an outlier that is trying to push the industry" towards more advanced solutions. As he noted, though, Facebook — with its "move fast and break things" motto — may be willing to take risks that a bank cannot entertain.
Other considerations in adoption of different cooling technologies are more site specific. Access to clean water is key, a lesson that eBay learned when Salt Lake city administrators managed water shortages by cutting water supply to its data centre, and situating data centres in locations away from areas susceptible to forest fires or polluted with other toxins is critical in managing issues with particulates in free air cooling. Proximity to salt water has also proved problematic, as salt in the air can introduce corrosion of equipment. And while there are ways to mitigate many of these issues, such as techniques to reduce water consumption (water holding tanks or use of grey water) or the deployment of misters as scrubbers to remove particulates, other factors indicate adoption of alternative systems.
A critical consideration in the selection of data centre cooling systems is outside air temperature — the average mean temperature in a region must be cool enough for a free air approach to make sense, and Chadwick recommends an hour by hour temperature and humidity analysis of the location as prerequisite to choosing the best system. Free cooling maps can determine how many hours in a year that it would be possible to use free air cooling: climate data, for example, has demonstrated that 8,500 hours a year would be available in Canada and northern regions of the US. When the outside air is cool enough, it may be brought in, filtered, and mixed with air in the data centre to reduce temperatures via airside economizer systems. Alternatively, when there are issues with particulates, this air can be used to cool pipes that circulate hot air drawn off the server room. When cooled air in this closed loop system is pushed back down to cool IT equipment, the system is described as indirect. In cases where the outside temperature differential is not great enough, an indirect cooling system that relies on water to cool air — or other heat transferring liquid — in the loop may be indicated. A final temperature dependent technology is the water-side economizer which uses cold air to cool an exterior water tower. Chilled water from the tower is then used in air conditioners inside the data center to replace energy-intensive mechanically-chilled water.
As compared with a chiller-based system, Chadwick estimates that direct evaporative cooling produces cost savings of 30 percent or more, though moving to in indirect system will entail loss of 10-20 percent of that overall benefit. With this kind of outcome, the direct approach is compelling. Cost may not be the only issue though, and while unique environmental conditions, including water quality, latitude, altitude, temperature and humidity, associated with a particular region can factor into the equation, server deployment and internal air flow and temperature conditions as measured with computational fluid dynamics modelling should be taken into account along with other business issues. For example, supplemental cooling (CRAC units) may be required as an insurance policy that supports resale value of the data centre. Greenfield or retrofit as well as facilities update budgets can also impact decision-making.
Space and design considerations may also play a role in determining the best cooling options - some cooling towers or louvers on the airside economizer may require large openings across a lot of wall space or in the roof, and as a result, it may not be possible to these systems in a data centre that is located in a high rise building. In this type of situation — or where business requirements introduce demand for high levels of precision cooling — yet another alternative technology, liquid cooling, may be the right solution. At the event, the benefits of liquid cooling were also addressed by Jon Summers, senior lecturer in the School of Mechanical Engineering at the University of Leeds. But that is subject for another post — stay tuned.