Liquid cooling efficiently manages AI-driven heat in data centers, but water scarcity poses challenges. Water restrictions force data centers to find alternative cooling methods.
Liquid cooling has become necessary as AI continues increasing IT heat densities to unimaginable levels. Climate change, rising temperatures, deforestation and shifts in weather patterns and population densities have left many parts of the world water-stressed, leading to advancements in cooling systems.
Water is the most efficient way to transfer heat as it has about 3,500 times the heat exchange capacity of air at standard conditions. The volume of water necessary to cool a large computing facility has brought attention to all data center water use. Unfortunately, much of the water used to move heat outside a building is lost to evaporation.
Areas impacted by water scarcity are typically prime locations for mega-data centers, due to low energy costs and land availability. Liquid cooling is the best system for keeping up with AI workloads and heat production. However, water scarcity has resulted in restrictions and, in some cases, outright prohibition of water use.
This article explains different data center cooling systems, liquid and dry, as well as the concern of water demand in water-stressed locations and how data centers adapt to cooling methods.
Liquid cooling technology
Liquid cooling systems have two separate cooling loops: the technology cooling system (TCS) and the facility water system (FWS).
Technology cooling system (TCS) vs. facility water system (FWS).
The TCS runs through a heat exchanger, which transfers its heat to the FWS. This keeps the liquid circulating through computing equipment separate from the liquid carrying heat to the atmosphere -- or into the ground with geothermal cooling.
Technology cooling system
The requirements for these two liquid loops differ in terms of flow rates, pressures and cleanliness. Liquids in the TCS, water or refrigerant, must pass through small openings in the computing hardware that can easily clog with foreign matter. The same liquid recirculates as the loop is contained within the computing area. No liquid is lost in the process, so restrictions don't apply here.
Since energy use is a concern in data center design, this makes for a challenging trade-off between power and water usage, particularly in water-stressed areas.
Facility water system
Liquid in the FWS mostly consists of water that is mixed with glycol in cold environments to keep it from freezing. It's filtered and treated to reduce corrosion and scale buildup in pipes, pumps and heat exchangers, but not to the level of the TCS. The concern with water use in the FWS is how the heat is transferred from this loop to the atmosphere. Water evaporates from the heat transfer, which leads to water loss.
Conventional cooling towers
Towers have long been the norm in large cooling systems. Warm FWS water cascades down a series of cooling tower surfaces, like large radiator fins. Fans pull air through the water, which transfers heat to the air through a combination of air flow and evaporation. The white plumes found in cooler weather are water vapor that's evaporating from cooling towers on rooftops.
How conventional cooling towers work.
Modern towers evaporate less water than in the past, but lost water must still be made up. The actual amount of water depends on location and weather conditions. Water loss is greater on hot, dry days than on cool, humid ones. Despite water loss, this remains the most energy-efficient cooling method available.
Direct evaporative or adiabatic cooling
Direct evaporation systems introduce water vapor into the airstream or the free chlorine loop through a large tube, spraying water from the outside. Rapid water evaporation cools the air or liquid inside the tube. Evaporative cooling methods increase humidity levels inside the data center, which must be monitored to ensure equipment safety.
How direct evaporative cooling works.
Evaporative cooling is most effective and highly energy efficient in hot, dry climates, but it is prohibited in areas where water use is restricted. It is commonly used in large data centers in areas like Phoenix, Ariz., where relatively inexpensive and sustainable hydroelectric power exists. However, increasing global temperatures have classified Phoenix and many other popular data center locations as water-scarce.
Dry cooling
Air flow is the only remaining way to transfer heat to the outside, other than geothermal. Data centers use dry coolers for smaller systems where large mechanical cooling installations are either impractical or overly expensive. However, with an increase in water scarcity and restrictions, dry cooling is now the primary option for many large data centers.
How dry cooling works.
Scale and density are two differentiators of dry and liquid cooling use. IT hardware is compact, so heat is packed into small spaces, which increases heat densities. With limited liquid cooling options, data centers must use outdoor air coolers that can efficiently move large quantities of air past liquid coils. Note that water does not evaporate in liquid coils, so they would not fall under restrictions.
Moving large volumes of air requires fan power, which follows a cube law curve. When the outside temperature rises, and a fan doubles its speed to move enough air to maintain cooling, it draws eight times the power. Since energy use is a concern in data center design, this makes for a challenging trade-off between power and water usage, particularly in water-stressed areas.
Cooling power vs. water usage for two types of heat rejection.
The chart above illustrates the significant trade-off between power and water consumption for cooling tower and dry cooler systems in three different U.S. climate zones. Dry coolers discharge no water into the atmosphere but use significantly more power to dissipate the heat. While not ideal, this trend will continue across the data center spectrum as concerns about water waste increase. Though still far from the energy efficiency of water-cooled systems, these newer dry coolers have become the "new normal" for many modern data centers.
Robert McFarlane is senior principal in charge of data center design for the international consulting firm Shen Milsom and Wilke LLC. McFarlane has spent more than 40 years in communications consulting and has experience in every segment of the data center industry.
