Liquid cooling is cheaper to operate than air cooling at scale for one dominant reason: every megawatt moved from air to liquid escapes the chiller – the most expensive component in the cooling chain. But air cooling never disappears entirely. This guide explains the real economics of the air/liquid split in an AI-era data center, based on our experience planning new builds at DELSKA.
Key takeaways
- Direct-to-chip liquid cooling captures roughly 70–85% of rack heat; memory, NICs, power supplies and optics still reject to air, creating a structural floor of about 15–20% air cooling in any large deployment.
- Air and liquid are different thermodynamic worlds: the air loop runs on low-temperature water (about 7°C to low-20s°C) and needs chillers; the high-temperature liquid loop returns water at 60–70°C and can reject heat with dry coolers alone, year-round, almost anywhere in Europe.
- The “chiller tax” is the economic core: chillers carry the highest capex, maintenance, F-gas regulatory exposure and compressor energy – hitting PUE directly. Liquid megawatts do not pay it.
- At 60–70°C, return water is near-ready district heating supply – in Germany a permitting argument and a potential revenue line under the Energy Efficiency Act.
- Design conclusion: fix the power blocks, not the cooling ratio. Make power infrastructure technology-agnostic and let the cooling mix follow tenant demand.
Definitions
- Direct-to-chip (DLC) liquid cooling – cold plates on CPUs/GPUs transfer heat to a liquid loop, removing the majority of rack heat without moving air.
- Chiller – a compressor-based refrigeration machine producing chilled water for the air-cooling loop; the most expensive element of the chain in capex, maintenance and energy.
- Dry cooler – a heat exchanger rejecting heat to ambient air without compressors or refrigerants; works alone when loop temperatures are high enough.
- PUE (Power Usage Effectiveness) – total facility power divided by IT power; compressor energy is one of its biggest drivers.
- Chiller tax – our shorthand for the combined capex, opex, F-gas exposure and PUE penalty that every air-cooled megawatt carries and liquid-cooled megawatts avoid.
Air vs liquid: the comparison
| Dimension | Air cooling loop | High-temp liquid loop (DLC) |
|---|---|---|
| Working temperature | ~7°C to low-20s°C water | 60–70°C return water |
| Heat rejection | Chillers required (at least for peak trimming) | Dry coolers alone, 365 days, virtually anywhere in Europe |
| Compressors / refrigerants | Yes – capex, maintenance, F-gas exposure | None |
| PUE impact | Compressor energy hits PUE directly | Minimal mechanical cooling energy |
| Share of rack heat (AI hall) | Structural floor of ~15–20% | ~70–85% via direct-to-chip |
| Heat reuse readiness | Needs heat pump upgrade to be useful | Near district-heating grade as-is |
| Scaling behaviour | Share shrinks as facility grows | Share grows with rack density |
Why the air share shrinks as facilities scale
When we started planning our new builds we assumed roughly 20% air / 80% liquid. Then we noticed the larger the facility gets, the smaller the air percentage becomes – not by choice, but because physics and economics push it there. Every air-cooled megawatt pays the chiller tax; every liquid megawatt escapes it into dry cooler economics. At scale, that difference compounds.
And yet air never reaches zero. Even in a “fully liquid” AI hall, the residual heat from memory, NICs, power supplies and optics – plus network cores, storage and support infrastructure – lands at about 15–20% air. That is not a design choice. That is physics, at least until chip makers eliminate air-cooled components entirely.
Two loops, not one
One shared water system is not realistic: the loops live at completely different temperatures. What you can share is the top layer – the heat rejection field masterplan, water treatment, and BMS. The hydraulic circuits stay separate.
Waste heat: from cost to revenue
At high-temperature DLC return levels you are sitting on near-ready district heating supply. In Germany, where the Energy Efficiency Act requires heat reuse readiness for large data centers, 60–70°C return water stops being an ESG talking point and becomes a permitting argument – and potentially a revenue line. Lower-temperature loops need a heat pump in between. See the EU Data Centre Regulation Tracker for country-by-country heat reuse rules.
The design conclusion: fix the power blocks
Do not fix the air/liquid ratio in concrete. Design transformers, distribution and UPS topology to be technology-agnostic. Oversize pipes, headers and pump capacity in phase one – cheap now, brutally expensive later – and pre-reserve dry cooler positions in the field masterplan. Then let the cooling mix follow tenant demand, phase by phase. Only one thing in the building is permanent: power.
Frequently asked questions
Can a data center be 100% liquid cooled?
Not today. Direct-to-chip captures 70–85% of rack heat, but memory, power supplies, optics, network and storage still reject heat to air – a structural floor of roughly 15–20% until component design changes.
Does liquid cooling improve PUE?
Yes, primarily by removing compressor energy: high-temperature loops reject heat through dry coolers without chillers, and chiller compressor energy is one of the largest PUE drivers in air-cooled facilities.
Is liquid cooling worth it for existing facilities?
Retrofits are far more expensive than new builds designed for it. The economical path is hybrid: keep the air loop as a service layer and add liquid capacity where rack density demands it – if the pipes and pumps were sized for it in phase one.
Related articles
- The Only Constant in a Modern Data Center Is Power – the original analysis this guide expands
- Data Centre Waste Heat Reuse Regulation in the EU
- AI Inside AI: How Data Centers Can Use AI to Run AI Workloads Better
