Data centers are described as consumers of electricity and water in roughly equal measure. The second follows from the first, because the water is spent removing the heat the power created.
Computing is a heating appliance
Essentially all electricity delivered to a server leaves it as heat. A rack drawing a large load is, thermally, a space heater running continuously.
Chips have a temperature ceiling above which they slow themselves or fail. Keeping a room full of them within that ceiling is the central engineering task of the building.
The heat must be moved from the chip to the air, from the air to a coolant, and from the coolant to the outdoors. Each transfer needs a temperature difference to drive it.
Evaporation is what uses water
Many facilities reject heat through cooling towers, where warm water is exposed to moving air and a portion evaporates. Evaporation carries away far more heat than warming the air alone.
The evaporated water leaves the system permanently. What remains grows more concentrated in dissolved minerals and must periodically be drained and replaced.
That combination, evaporation plus blowdown, is the consumption figure that appears in local water permits. It rises on hot days precisely when regional water is scarcest.
Air cooling trades water for power
A facility can reject heat to the air without evaporation, using large chillers and dry coolers. Water use falls sharply and electricity use rises.
Because much American electricity is generated at plants that themselves consume water for cooling, the saving is partly transferred rather than eliminated.
Operators therefore choose based on local conditions. Dry, temperate regions favor air cooling; hot, humid regions have historically favored evaporative systems despite the water cost.
Liquid cooling changes the arithmetic again
Newer high-density hardware generates more heat per rack than moving air can remove economically. Coolant is brought to the chip itself through cold plates or immersion.
Liquid carries heat far more effectively than air, allowing rejection at higher temperatures. Higher rejection temperatures make dry cooling workable in more climates.
The retrofit is substantial, involving plumbing, leak detection and different rack designs, so adoption follows new construction more than upgrades.
Siting decisions now turn on this
Communities evaluate proposed facilities partly on water draw, and several have imposed conditions or refused permits on that basis.
Operators respond by using reclaimed municipal water, treated wastewater unsuitable for drinking, or by committing to designs that consume little. The disclosure practices vary considerably between companies.