Water usage effectiveness (WUE) and water-free cooling

Data centers have long been judged on energy, but water is now just as visible. Cooling towers and evaporative systems can consume large volumes of water, and communities, regulators and customers increasingly ask how much a facility uses. Water usage effectiveness, or WUE, is the metric developed to answer that question.

This guide explains how WUE is calculated, where water goes in a data center, the trade-off between water and energy, and the options for water-free cooling, which are particularly practical in Canada's climate.

The definition

WUE was developed by The Green Grid and has since been standardized within the ISO/IEC 30134 series. The basic site metric is:

WUE = annual site water usage (litres) / annual IT equipment energy (kWh)

The result is expressed in litres per kilowatt-hour (L/kWh). A lower value means less water per unit of IT energy.

Source WUE

The Green Grid also defines a source WUE, which adds the water consumed in generating the electricity the data center uses. This matters because thermal power plants consume water for cooling. A site that switches from water-cooled to air-cooled equipment may lower site WUE while raising electricity use, and the net effect depends on the grid.

In provinces with predominantly hydroelectric generation, the water associated with generation is accounted for differently than for thermal plants, and the published data on it varies. Treat source WUE as an indicator that depends on assumptions, and state the assumptions used.

Where data center water goes

Clearly state what is included in your WUE figure. Some operators include only cooling water; others include everything on the site meter.

The energy and water trade-off

Evaporation is a very efficient way to reject heat. Systems that use water generally use less electricity, and systems that avoid water generally use more fan or compressor energy, at least on hot days. Choosing between them means weighing:

• Local water availability and stress, now and in future climates
• Water and sewer costs, including blowdown discharge
• Electricity cost and carbon intensity of the grid
• Regulatory and community expectations
• Legionella risk management obligations for wet systems

There is no single correct answer. A facility in a water-stressed region may accept higher energy use; one with abundant water and a carbon-intensive grid may choose the opposite.

Water-free cooling options

Canada's cold climate makes dry heat rejection practical for much of the year. Options include:

Dry coolers

Closed-circuit fluid coolers with glycol reject heat to outdoor air with no water consumption. They need larger coils and more fan power than towers, and their capacity drops on hot days. Pairing them with warm water systems, such as direct-to-chip liquid cooling, allows full dry operation in most Canadian climates.

Air-cooled chillers with free cooling coils

Air-cooled chillers with integral free cooling coils use outdoor air to precool water in cool weather and compressors in warm weather. No water is consumed.

Airside economizers without evaporative stages

Direct or indirect airside economizers cool the data hall with outdoor air. Without evaporative assist, mechanical cooling handles the warmest hours. See free cooling and economizers for data centers in Canada.

Hybrid and adiabatic systems

Adiabatic precooling on dry coolers uses water only on the hottest days, giving most of the water savings of dry systems with a peak-day boost. Hybrid cooling towers can run dry in cool weather and wet in hot weather.

Pumped refrigerant economizers

For DX systems, pumped refrigerant economizers provide compressor-free cooling in cold weather without water.

Reducing water use in existing wet systems

If a facility already uses towers or evaporative cooling, it can still reduce water:

1. Increase cycles of concentration with better water treatment, reducing blowdown.
2. Raise chilled water and supply air temperatures to extend dry economizer hours.
3. Switch to dry operation in winter where equipment allows.
4. Fix leaks, overflows and float valve problems at tower basins.
5. Meter make-up and blowdown separately to track performance.
6. Reduce humidification by adopting the wider ASHRAE humidity range. See humidity control.

Measuring WUE properly

• Install meters on cooling tower make-up, blowdown, evaporative cooling supply and humidifier supply.
• Measure IT energy at the same boundary used for PUE.
• Report annual values, since water use varies strongly by season.
• Publish what is included and excluded.

Frequently asked questions

What is a good WUE?

Values range from near zero for fully dry systems to much higher for facilities relying on evaporative cooling in warm climates. Compare against your own baseline over time, and against similar facilities in similar climates.

Does going water-free always raise PUE?

Not necessarily. In cold climates with warm supply temperatures, dry systems can achieve excellent PUE because compressors rarely run. The energy penalty is concentrated in the hottest hours of the year.

Should WUE replace PUE?

No. Use them together, along with carbon metrics. Optimizing one alone can make the others worse. See power usage effectiveness.

Does a closed chilled water loop count toward WUE?

Only the make-up water added to it. A closed loop is filled once and topped up occasionally for small leaks or maintenance, so its contribution is normally tiny. If make-up to a closed loop is significant, that usually points to a leak worth finding rather than a normal operating cost.

Key takeaways

• WUE is annual site water divided by annual IT energy, in L/kWh.
• Cooling towers, evaporative cooling and humidification are the main water users.
• Water and energy trade off; the right balance depends on local water, grid and cost.
• Canada's climate makes dry coolers, air-cooled free cooling and airside economizers practical.
• Meter water uses separately and report what your WUE includes.