Closed-Loop vs. Cooling Tower: How to Tell Which One Your Data Center Will Actually Use

Two machines, one word

The reason the comment war never ends is that people argue "data centers and water" like it's one thing. It's two. A closed-loop system is sealed — the same water runs the circuit over and over, and you only top it off a little when some leaks out. An evaporative cooling tower is basically the building sweating. Your body dumps water onto your skin and lets it evaporate to pull heat off you; the tower dumps water and lets it boil off to pull heat off the chips. Once it evaporates, it's gone from that town, up into the air as vapor.

That's why the numbers people throw at each other look insane side by side. A true closed-loop barely moves the water meter after it's filled. An evaporative campus can go through north of a billion gallons in a single year — I found one Meta site in Iowa that did exactly that in 2025 — which is less "data center" and more "small city nobody voted for." Same industry, completely different machine.

"Closed-loop" is doing a lot of work on that slide

So I figured, fine — find out which machine it is and we're done. That's where it got muddy. "Closed-loop" on a project slide is doing the same job "all-natural" does on a bag of chips: technically true, tells you almost nothing. A building can run a sealed, closed loop to the chips inside and still have a row of evaporative towers out back rejecting all that heat the old way. The loop's closed. The water's still evaporating.

Then there's the in-between machine nobody brings up at the town hall: the hybrid, or "adiabatic," cooler. It runs dry most of the year — no water at all — and then, on the handful of brutal days when dry air can't keep up, it starts spraying itself down and evaporating like a regular tower. So a place can honestly call itself mostly water-free and still hit the local supply hardest on the exact July afternoon everyone else's lawn is going brown.

What I couldn't nail down from the outside is how often that wet mode actually kicks in for any specific project, because that rides on the local climate and how the operator tunes it, and that's not usually in the press release. Microsoft's newer design does appear to skip evaporation for real — fill it once during construction, then recirculate — which is the closed-loop everyone pictures in their head. But a developer saying "we're using closed-loop" with no other detail could mean any of these.

What to actually ask before you vote

Here's the part that reframed the whole thing for me. Saving water isn't the clean win it sounds like. Going fully dry — air-cooled, no evaporation — usually burns noticeably more electricity. And a lot of electricity gets made by, you guessed it, boiling water at a power plant. So the water doesn't vanish; it moves. Dry-cool the data center and you might just be evaporating that water thirty miles away at the generating station instead. Planners have a name for this tug-of-war, the water-energy nexus, and it means "does it use water" and "does it use power" are really the same question wearing two coats.

Which is why the questions worth bringing to the meeting aren't "will it use water" — everything uses water somewhere. They're sharper. Which cooling design, exactly: fully closed-loop, evaporative towers, or hybrid? If it evaporates, how much, and is it pulling from the drinking supply or from recycled water? And ask for the water-withdrawal permit numbers, not the brochure — a lot of that is already public record through the utility or the county if someone just asks.

The thing I keep circling back to is that most of that comment-section fight isn't really about water. Water's just the part that's easy to picture. Underneath it, people are arguing about who pays — for the water, sure, but mostly for the extra power that moves it and the bill that lands on everyone else's utility statement whether the loop is open or closed.

If you want to see the machines, not the slide

I went down this whole rabbit hole because the words kept hiding the hardware. A "closed-loop" and a "cooling tower" can sit on the same site plan and behave like opposites, and the only way I could keep them straight was to actually watch how each one moves heat and water around.

That's basically what the channel is for — pulling one of these systems apart on screen until the thing that sounded like a spec-sheet shouting match turns into something you can picture before you walk into the meeting. If this is the kind of rabbit hole you fall into too, come watch the data center breakdowns on Byte Bungalow and subscribe on YouTube so the next one lands in your feed.

Common questions

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Sources

• sensorex.com — Closed-loop recirculates in sealed pipes with minimal loss + occasional makeup water; can reduce freshwater use ~70% vs open evaporative. **"Closed-loop does not automatically mean
• flatwaterfreepress.org — Meta Sarpy County NE withdrew 26.7–37.5M gal/yr 2020–2024, then **~732.1M gallons in 2025**; **Meta Council Bluffs, Iowa consumed ~1.3 billion gallons in 2025** (most of any Meta s
• latitudemedia.com — Google uses **evaporative cooling OR closed-loop air-cooled chillers depending on water availability**; Google water use in The Dalles OR quintupled 2012→2025 to ~550M gallons, ~40
• iol.co.za — **A 1-MW data center on traditional evaporative cooling ≈ 6.75M gallons/year**, evaporated to the air; indirect water use via electricity generation is often **80%+** of total wate
• media — Microsoft's AI-optimized design consumes **zero water for cooling in operation**; chip-level/direct-to-chip closed loop, **filled once during construction then recirculates**; roll