A cooling label tells you how heat moves near the equipment, not how much water a facility uses.
To understand water use, trace how the facility rejects heat and check for other water-using equipment. WUE answers a different question: how reported water use relates to IT energy use.
Data center fundamentals series
What does “liquid cooling” tell you?
Direct liquid cooling moves heat into a fluid at or near where the heat is generated, rather than first moving it into room air. That describes an equipment-side heat-transfer method—not the facility’s water use. The U.S.
Department of Energy’s Best Practices Guide for Energy-Efficient Data Center Design distinguishes rear-door heat exchangers, which transfer heat from rack exhaust air to liquid, from cold plates and immersion systems, which it classifies as direct liquid cooling.
The equipment-side fluid may be treated water, a glycol-based solution or a nonconductive fluid. In many designs, a cooling distribution unit (CDU) interfaces between that equipment-side circuit and the facility cooling loop. Some liquid-cooled designs also leave part of the IT heat load to air cooling.
“Liquid cooled” therefore does not identify the fluid, eliminate every air-cooling path or tell you what happens to the heat outdoors.
U.S. Department of Energy — Best Practices Guide for Energy-Efficient Data Center Design
Where might the facility use water?
The next question is how the facility moves and rejects heat—and whether it operates other equipment that uses water. This misconception map separates possible paths; it is not a list of equipment present at every data center.
Water flowing in a cooling circuit must be evaporated outdoors
- More precise distinction
- A water-cooled CRAC condenser circuit can connect to an outdoor dry cooler; a cooling tower is a different heat-rejection arrangement that can use water through evaporation. Water in a circuit does not, by itself, establish evaporative water use.
- What to check at a particular facility
- What outdoor equipment receives heat from the facility loop?
A water-side economizer or separated water loops rule out cooling-tower evaporation
- More precise distinction
- In the guide’s cooling-tower-based water-side economizer, a heat exchanger can keep the chilled-water and cooling-tower-water loops separate. During suitable outdoor conditions, the economizer may reduce or bypass chiller operation while still using the tower’s evaporative cooling capacity.
- What to check at a particular facility
- Does this economizer rely on a cooling tower?
Air cooling rules out water use
- More precise distinction
- An air-cooled DX condenser can have an optional device that sprays water over its coils. That option is not present in every air-cooled installation.
- What to check at a particular facility
- Is a water-spray device installed and operated?
Liquid cooling requires a cooling tower
- More precise distinction
- Where higher-temperature liquid-cooling operation permits it, a dry cooler may reject heat to the atmosphere and offer a water-saving option. This is a conditional possibility, not a result of every liquid-cooled design.
- What to check at a particular facility
- What temperatures and outdoor heat-rejection equipment does the design use?
Heat rejection is the only path worth checking
- More precise distinction
- Humidification can be another water-use path if the equipment is installed and operated. The guide also says most data centers do not need humidification to maintain its cited recommended minimum humidity.
- What to check at a particular facility
- Is humidification equipment present and operating?
Heat reuse can change the heat-rejection path too: the guide says it can reduce or, in some cases, eliminate the use of cooling towers that evaporate water. It does not make that outcome universal.
For a specific facility, trace the equipment-side fluid and any remaining air cooling, the facility loop, the outdoor heat-rejection equipment, and any optional spray or humidification system before drawing a conclusion about where water is used.
What does WUE report instead?
The DOE guide defines site-based water use effectiveness (WUE) as annual site water use divided by the energy drawn by all IT equipment, expressed in L/kWh. It is a ratio, not a diagram of the cooling system. A WUE value alone cannot tell you whether water was used by a cooling tower, condenser spray or humidification equipment.
The guide also describes a separate source-based WUE definition that considers water used off-site to produce energy consumed on-site, as well as on-site water use. Before interpreting or comparing values, ask which boundary is reported, what annual water use is in the numerator and what IT equipment energy is in the denominator.
The guide lists ISO/IEC 30134 Part 9 as a WUE-related standard; its discussion of site- and source-based WUE should not be treated as a complete account of the standard’s provisions.
What is the useful question to ask next?
For any particular data center, ask three separate questions: How does heat leave the IT equipment? Where, if anywhere, does the facility use water as it rejects heat or conditions the air? What water and energy boundaries does its WUE figure cover? The answers may differ even between facilities with the same cooling label.
Without the equipment path and reporting boundary, neither “liquid cooled” nor a WUE number establishes that facility’s water-use path.
