COOLING EXPLAINED / CHECKED 19 OCTOBER 2026
Why do data centres use water if it is recycled?
A closed loop can reuse coolant inside a building while a separate cooling tower evaporates water outside. Other designs release heat to air without that evaporative step. Ask what happens after the loop.
Two simplified liquid-cooling examples. Not a named facility or a scale measurement.
INSIDE / CLOSED COOLANT CIRCUIT
Coolant returns to the servers
Heat crosses.
The circuits do not mix.
EXAMPLE A / EVAPORATIVE
Cooling tower
Heat reaches a separate water circuit
Water circulates
Make-up water enters
Some water evaporates
Some is drained as blowdown
EXAMPLE B / DRY
Dry cooler
Heat reaches a separate cooling circuit
Heat leaves to outdoor air
No evaporative water step in this example.
Arrows label heat or water separately. Pipe lengths and box sizes show no quantities.
“Closed” answers only part of the question.
In the tower example, water evaporates to carry heat away; draining some remaining water helps control mineral build-up. Both require replacement water. Recirculation does not mean nothing leaves.
Design and outdoor conditions matter. DOE's May 2019 NREL case combines a closed internal loop, dry cooling when temperatures permit, and an evaporative tower for remaining heat. These are not two universal categories or a ranking.
Three checks before sharing a claim.
- Which circuit is closed? Inside the building, or the whole cooling system?
- How does heat get outside? Does operation change with the weather?
- What is counted? On-site water, or also electricity and manufacturing?
No evaporative cooling water at a site does not establish a zero total water footprint. Electricity supply and chip manufacturing are separate questions in the dated global accounting boundary.
Check the sources.
- DOE/FEMP: cooling water efficiency ↗ — typical systems, tower losses and direct liquid cooling.
- DOE: NREL hybrid cooling case, May 2019 ↗ — page 2, “How It Works”. A historical example, not a current facility survey.
Original explanatory diagram; technical stages omitted for clarity. Sources checked 19 October 2026. No facility water rate is inferred. Corrections record.