Sustainability Sharpens the Focus on Data Center Water Usage Effectiveness

Last week in our special report series, we explored three IT equipment heat removal challenges, focusing on liquid cooling and the green grid metric. This week, as we continue to look at how to sustainably meet high density cooling challenges, we’ll continue our discussion of air cooling versus liquid cooling and discuss three external heat rejection systems. 

External Heat Rejection Systems, Part 1

We have been discussing the differences between air cooling and liquid cooling of IT equipment within the data center, which has recently gotten the most attention due to rising power densities. However, the various types of heat rejection systems also have a significant impact on the energy efficiency as well as the sustainability of the data center facility.


Generally speaking, the most common methods of mainstream data center external heat rejection systems fall into categories listed in the table below.

Liquid Cooling

Click the image to see a larger version of this chart.

Water is a critical global sustainability resource. According to the US EPA, at least 40 states are anticipating water shortages by 2024, and consider the need to conserve water is critical.

Water Usage Effectiveness

The Green Grid developed the Water Usage Effective- ness (WUE) metric in 2011. While many data center operators measure and report their PUEs publicly, very few disclose their water usage. For data centers, the consumption of water is not taken into consideration in the PUE metric. When discussing water consumption and WUE, it is important to note two types of WUE calculations: WUE site and WUE source.

WUE Site – Based on the water used by the data center; expressed as the annualized liters of water used per kWh of IT Energy (liters/IT kWh). This is the most commonly used reference and is easily measured.

WUE Source – Based on the annualized water used by the energy source (power generation), plus the site water usage, expressed as liters of water used per kWh of IT Energy. (liters/IT kWh).

Many people are unaware of the two types of WUE, and in most cases, data centers that cite their WUE are only referencing their site.


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Generally speaking, evaporating more water during heat rejection will normally reduce the energy used by mechanical cooling systems. However, in order to put this into perspective, the type of heat rejection used is a design decision, which is based on various factors. Some of those decisions are based on a business perspective, based on the cost of energy vs. the availability and cost of water. From a technical perspective, climate has a significant impact on water usage.


Moreover, large-scale and hyperscale data centers often use evaporative cooling solutions, and a significant number of sites are located in water- stressed areas. Multiple publications and sources have noted this.

According to a Virginia Tech June 2021 white paper, The environmental footprint of data centers in the United States, “one-fifth of data center servers direct water footprint comes from moderately to highly water-stressed watersheds, while nearly half of servers are fully or partially powered by power plants located within water-stressed regions.”

According to a NASA Earth Observatory March 2021 post: “Almost half of the United States is currently experiencing some level of drought, and it is expected to worsen in upcoming months.”

Climate Factors on Energy Efficiency and Water Usage

The effectiveness of evaporative cooling is based on climatic conditions. While this is well known, it was not taken into account in the WUE metric. However, this has been partially addressed in the ASHRAE 90.4 Energy Standard for Data Centers, which in the US has a table of mechanical energy adjustment factors based on the climate zone where the data center is located. Nevertheless, ASHRAE 90.4 does not account for water usage in its energy calculations.

While it is relatively easy to measure site water usage, the WUE source metric is more complex but helps provide a more holistic view of overall sustainability.

However, power production water usage varies widely based on the fuel source. It, therefore, requires a clear understanding of the type of energy source that is actually supplied to a data center, which can differ by location. While more and more data centers are committed to using renewable energy, calculating the WUE source requires knowing the water consumption of the actual source used to power the data center. This information can sometimes be obtained from their energy provider; however, the validity of this information can be muddied when the energy is acquired via a power purchase agreement (PPA) or especially via a Renewable PPA (RPPA). In most cases, a RPPA is simply a paper transaction rather than the actual renewable energy source. PPAs and Renewable Energy Certificates (RECs) are used by many organizations and some data centers.

Liquid cooling

Source: EPA

For reference, the U.S. Energy Information Administration (EIA) recognizes and tracks water usage by power plant generation.

Water Treatment Chemicals Impact Sustainability

It is important to note that in addition to the water used by evaporation, cooling towers must be properly maintained and cleaned for operational efficiency, as well as for health safety to prevent legionella growth. This requires water treatment chemicals that have a negative impact on environmental sustainability. The amount of chemicals used increases with water usage and the energy consumption of the data center. The use of water treatment chemicals has a double impact on sustainability, both by the amount used by the facility and the chemical waste and water used in the manufacturing processes of the water treatment chemicals.

Download the full report, “Sustainably Meeting High Density Cooling Challenges: When, Where, and How,” courtesy of Nautilus Data Technologies to learn more about liquid cooling in high density data centers. In our next article, we’ll look at three additional external heat rejection systems. Catch up on the previous articles in the series here and here.

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