Evaporative cooling is an integral part of many data center cooling systems as engineers know the benefits of the efficiency achieved by using them. Water is by far a more effective medium for removing heat than air, especially when using evaporation to keep data centers cool. Cooling towers are one of the main means to reject heat from a data center, and recently the argument of water scarcity has brought cooling towers and evaporative cooling under deeper scrutiny. Comparisons of water use of cooling towers against water used to generate power at fossil fuel plants show that water use at the data center is much more efficient overall.
When reviewing the data center efficiency, the PUE is the most quoted, no matter how skewed and inaccurate it may be. Behind this, Water Use Effectiveness (WUE) is next in line when reviewed by data center operators, owners, and end users. Water used by evaporative systems are dependent upon location, and no cooling technology works the same or as well for all regions, client criteria, and application. Each facility should be analyzed to determine or reinforce the best approach for the needs of the customer and optimizes the water and energy use.
The southern and western states in the United States has had drought issues on and off, short and extended, through the years, impacting the cooling systems available to cool data centers. With the shortages and demands for water for agriculture, commercial, and residential use threatened, the push for lowering water used for data centers becomes an obvious target. Of course, when evaluating how water is used holistically it should be critical to understand how it is spent supporting how the end power is made. When viewed in this manner, evaporative cooling for data centers are far more efficient than alternative dry systems.
The amount of water needed by a steam cycle of a fossil fuel based power plant to generate electricity is greater than the water used by a cooling tower to reject heat from a data center. As above, the location matters, but overall an air cooled chiller that uses no evaporative cooling locally will still require more power than an evaporative system, thereby requiring more water to be used at the regional power plant. This evaluation takes into account possible inefficiencies such as a low flush cycle (cycles of concentration) and a high amount of drift and other water losses.
The bigger picture is coming to light: demanding zero water use at a site can use more water than a data center with twice the capacity or more. Reflecting this fully is starting to become more apparent as ESG and other sustainability reports highlight such water and energy use.
Add in the free cooling aspects of a water-side economizer and the energy and water efficiency as compared with an air-cooled system are stretched even higher. Adding an economizer or dry coolers is required to create the same effect with a waterless system, but the amount of free cooling time and the efficiency of those systems is much lower, sometimes being too costly for the limited benefits they achieve.
Other complexities of the systems and operations, such as variable speed motors and that the evaporative water use evaluations are not likely taking diversity of workload and weather into account, and suddenly the data center water use is much lower than any dry type system that a local jurisdiction might demand due to pubic perception. Instead it would be more advantageous if a data center kept a generous amount of water used for evaporative cooling in tanks to help with peak periods.