Key takeaways
- Direct-to-chip liquid cooling captures 70–85% of rack heat; the rest still rejects to air – a structural floor of about 15–20% air in any large deployment.
- Air and liquid loops run at completely different temperatures (about 7–20°C vs 60–70°C) and cannot realistically share one water system.
- Every megawatt moved from air to liquid escapes the “chiller tax” – the capex, maintenance, F-gas exposure and PUE penalty of compressor-based cooling.
- Design conclusion: keep power infrastructure technology-agnostic and let the cooling mix follow tenant demand.
Everything else – especially cooling – is a variable.
When we started planning our new data center builds, we began with what felt like a safe assumption: roughly 20% air cooling, 80% liquid. A reasonable split for an AI-era facility.
Then we noticed something. The larger the facility gets, the smaller the air percentage becomes – not because we decided so, but because the physics and the economics push it there. And yet air never reaches zero. Here’s what we’ve learned designing around that tension.
Air cooling doesn’t disappear – it becomes a service layer.
Even in a “fully liquid” AI hall, direct-to-chip cooling captures roughly 70–85% of rack heat. The rest – memory, NICs, power supplies, optics – still rejects to air. Add network cores, storage, and support infrastructure, and you land at a structural floor of about 15–20% air in any large deployment. That’s not a design choice. That’s physics, at least until chip makers eliminate air-cooled components entirely.
These are two different thermodynamic worlds.
Here’s what gets glossed over in most “hybrid cooling” discussions: air and liquid loops don’t just differ in medium – they live at completely different temperatures.
The air-cooling loop is a low-temperature water system, typically operating anywhere from about 7°C up to the low-20s°C depending on facility design and economization strategy – which means chillers, at least for peak trimming. By contrast, the high-temperature liquid-cooling loop can return water at 60–70°C – and at those temperatures, dry coolers alone handle heat rejection year-round, virtually anywhere in Europe. No compressors. No refrigerants. Free cooling, 365 days.
One shared water system? Not realistically. What you can share is the top layer: the heat rejection field masterplan, water treatment, BMS. The hydraulic circuits themselves stay separate.
Every megawatt you move from air to liquid escapes the chiller tax. This is the economic insight hiding inside the ratio question. Chillers are the most expensive component of the cooling chain – capex, maintenance, F-gas regulatory exposure, and above all compressor energy that hits your PUE directly.
Shift a megawatt from air to liquid, and it doesn’t just change cooling technology. It moves from chiller economics to dry cooler economics. That’s why the air percentage naturally shrinks as facilities scale: every air-cooled megawatt carries a chiller tax that liquid megawatts don’t pay.
70°C return water isn’t waste – it’s an asset. At high-temperature DLC return levels, you’re sitting on near-ready district heating supply. In Germany, where the Energy Efficiency Act already requires heat reuse readiness for large data centers, this stops being an ESG talking point and becomes a permitting argument – and potentially a revenue line. Lower-temperature liquid loops need a heat pump in between; at 60–70°C, you’re much closer to plug-and-play.
So what does this mean for design? Our conclusion: don’t fix the air/liquid ratio in concrete. Fix the power blocks.
Design power infrastructure – transformers, distribution, UPS topology – to be technology-agnostic. Oversize the pipes, headers, and pump capacity in phase one (cheap now, brutally expensive later). Then let the cooling mix follow tenant demand, phase by phase, with dry cooler positions pre-reserved in the field masterplan.
Because in the end, only one thing in the building is permanent: power. Everything downstream of the busbar should be ready to change.
How are you approaching the air/liquid split in your new builds? Curious whether others are seeing the same structural floor around 15–20% air.
#DataCenters #LiquidCooling #AIInfrastructure #Sustainability #DistrictHeating #PUE
https://www.linkedin.com/pulse/only-constant-modern-data-center-power-andris-gailitis-jayrf
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