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Data Center Cooling: How to Reduce Water Usage in Cooling Towers



Q: How can we reduce the water usage of data center cooling towers?


A: Elevate the temperature of the coolant to enable non-evaporative ambient cooling to be used a higher percentage of the year.


Many data centers use cooling towers to reject heat from their IT equipment into the ambient atmosphere. Cooling towers operate on the principle of evaporative cooling, which involves spraying a liquid coolant into the ambient air stream to evaporate a portion of the coolant stream. This acts to dissipate heat and provide cooling to the remaining liquid stream to then re-enter the data center cooling loop and continue to accept heat from the IT equipment.


While cooling towers offer considerable energy savings compared to chiller systems by avoiding the use of compressors in the refrigeration cycle, they consume large quantities of water in the evaporation process. In unique cases, data centers may be located in an area with an abundance of water. But, in many locations, water is a limited resource that may require significant processing, transportation, or environmental strain to have available in large quantities.


Is there an alternative? In an ideal world, a data center would simply dump heat to the surrounding atmosphere using a non-evaporative ambient heat exchanger (e.g. thermosiphons, finned heat exchangers, etc.). This has many advantages: it requires minimal infrastructure, requires low energy input, utilizes minimal water, and avoids environmentally unfriendly materials or refrigerants. However, often times the surrounding dry bulb air temperature is too high to use a non-evaporative ambient heat exchanger, requiring the cooling tower to lower the coolant temperature down near the wet bulb value of the evaporation process.


How big of a problem is this? Just how much water is at stake? This depends on many important design features of the cooling tower system, such as water entrainment / recapture efficiency, water dissolved solids composition, and wet bulb and coolant temperatures. However, the two most important determinants of cooling tower water usage are the data center heat load and the percentage of the year that the cooling tower is used.


To illustrate this, let us consider an example data center with a nominal 100MW computational load. We can see the annual required water usage versus the percentage of the year the cooling tower is used in the figure below:

Figure 1: Annual water usage for a 100MW data center plotted against cooling tower usage. This assumes that when the cooling tower is not in use, a non-evaporative ambient cooling technique requiring minimal water expenditure is used.


Therefore, a 100MW data center can use upwards of 375 million gallons if the cooling tower is used year-round. Thus, the goal should be to maximize the amount of time that the non-evaporative ambient heat exchanger is used, in order to minimize the water usage from the cooling tower. Let’s take a look at an example of an annual temperature cycle in Albuquerque, New Mexico, to illustrate how often cooling towers may need to be used for various data center coolant temperatures:




Figure 2: (Left) Example annual temperature cycle for Albuquerque, NM. Note the daily temperature variation produces an apparent continuous band. (Right) Plot displaying how often a given data center coolant temperature is below the dry bulb ambient in Albuquerque, NM, and therefore may require a cooling tower. Examples are provided at 15°C and 25°C, showing how elevating the coolant temperature by just 10°C can drastically reduce the amount of time a cooling tower is needed from ~50% to ~15%.


As seen in Figure 2, a data center coolant temperature of 15°C may require a cooling tower up to ~50% of the year, while a data center coolant temperature of 25°C may only require a cooling tower up to ~15% of the year. Looking back at Figure 1, this could result in water savings of around 120 million gallons each year. Of course, this is just one hypothetical example; every data center is unique and water usage and coolant temperature numbers may vary.


Conclusion


Cooling towers are a low energy option for cooling data centers, but they require high water usage. By elevating the temperature of the data center coolant, a non-evaporative ambient heat exchanger can be used more frequently throughout the year, and therefore limit water expenditure. Using high effectiveness liquid cooled heat sinks enables a higher coolant temperature and therefore more water savings.


Interested in reducing your annual water usage? Contact JETCOOL Technologies to learn more about their ultra-low thermal resistance cooling plates for promoting low energy, low water usage data center cooling systems.

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