In short
- Start by metering: without Category 2 or 3 data you cannot see where the overhead is.
- The cheapest gains are in airflow: blanking, sealing and containment.
- The biggest structural gains come from free cooling, warmer water and, for dense racks, direct liquid cooling.
- Fan and pump speed matters cubically: 20 % slower uses about half the power.
- Each 0.1 of PUE on 1 MW of IT is about 876 MWh a year.
Where the overhead goes
In a typical air-cooled hall at PUE 1.5, most of the 0.5 of overhead is cooling. An illustrative split:
| Overhead at PUE 1.50 | Share of IT energy | Main drivers |
|---|---|---|
| Cooling plant and air movement | ≈ 35 % | Chillers, CRAH fans, pumps, low set points, bypass air |
| Power chain losses | ≈ 10 % | UPS at low load, transformers, distribution |
| Lighting and building services | ≈ 5 % | Lighting, offices, security |
| Total overhead | ≈ 50 % | PUE 1.50 |
The shares are indicative. Measure your own split before choosing measures.
The twelve measures, ranked by effort
| # | Measure | Effort | Typical PUE gain |
|---|---|---|---|
| 1 | Submeter to Category 2 or 3 | Low | Enables the rest |
| 2 | Blanking panels, sealed cable cut-outs | Low | 0.02 – 0.05 |
| 3 | Hot- or cold-aisle containment | Low–medium | 0.05 – 0.15 |
| 4 | Raise supply air towards 27 °C | Low | 0.03 – 0.08 |
| 5 | Raise chilled water temperature | Medium | 0.03 – 0.08 |
| 6 | Variable-speed fans and pumps | Medium | 0.03 – 0.10 |
| 7 | Optimised plant controls, including AI | Medium | 0.02 – 0.08 |
| 8 | Modern UPS and efficient operating modes | Medium | 0.02 – 0.05 |
| 9 | Fewer conversions, higher distribution voltage | High | 0.01 – 0.03 |
| 10 | Consolidate IT, raise utilisation | Medium | 0.02 – 0.10 |
| 11 | Free cooling / economisers | High | 0.10 – 0.30 |
| 12 | Direct liquid cooling for dense racks | High | 0.05 – 0.20 |
Gains are indicative ranges for an air-cooled hall starting around PUE 1.5. They overlap and do not simply add up; the climate and the starting point decide the real result.
Quick wins: stop the air going the wrong way
1. Meter first
Install meters at PDU or server level (Category 2 or 3) and on the main cooling loads. Without them, every later measure is a guess, and the EU and German rules require the data anyway.
2. Blank and seal
Empty rack units and open cable cut-outs let cold air bypass the servers and hot air recirculate. Blanking panels and brush grommets are the cheapest fix in the hall.
3. Contain the aisles
Hot- or cold-aisle containment separates supply and return air. It lets you raise set points and slow fans without creating hot spots.
4. Raise the supply air temperature
ASHRAE's recommended envelope for IT inlets runs to 27 °C. Many halls still supply at 18 to 20 °C. Raise it in steps and watch server fan power, which counts as IT energy. More on envelopes at allinCOOLING.
Plant and controls: make the machines work less
5. Warmer chilled water
Every degree of warmer chilled water raises chiller efficiency and adds free-cooling hours. Containment is what makes higher water temperatures possible.
6. Variable speed
Fan and pump power falls with the cube of speed: running at 80 % speed uses about 51 % of the power. EC fans and variable-speed drives pay back quickly where fans run at constant speed today.
7. Smarter controls
Coordinated set points across chillers, towers, pumps and fans often save more than new equipment. Google reported that machine learning cut the energy used for cooling in its data centers by up to 40 %. A digital twin helps test changes first; see allinTWIN.
8. Modern UPS
Older double-conversion UPS units lose more at the low loads most halls run at. Modern units and efficient operating modes cut these losses; check them against your availability requirements.
Structural changes: the big steps
9. Fewer power conversions
Each transformer and conversion stage loses energy. Higher distribution voltages and busway instead of long cable runs reduce the losses; see allinBUSBAR.
10. Consolidate and raise utilisation
Idle and forgotten servers use energy and keep the hall at a low load, where PUE is worst. Decommissioning and consolidation cut IT energy and improve PUE at the same time.
11. Free cooling
Air- or water-side economisers use outside conditions instead of chillers for part of the year. In a temperate climate this is usually the single largest gain, and it is how new sites reach 1.2.
12. Direct liquid cooling
For AI and high-density racks, cold plates take most of the heat out at warm water temperatures, cutting fan energy and chiller hours. Sites built for it report PUE in the 1.1 range.
A plan for a legacy hall at 1.6
- Months 0–3: meter, map hot spots, blank and seal, fix bypass air.
- Months 3–9: contain the aisles, raise supply air and chilled water in steps, retrofit variable-speed fans and pumps.
- Months 9–18: tune plant controls, consolidate IT, review the UPS operating mode.
- Next investment cycle: add free cooling or liquid cooling where the climate and density justify it.
This path typically takes a legacy hall from around 1.6 towards 1.4. Reaching 1.3, the German limit for existing sites from 2030, usually needs the structural steps.
Questions about lowering PUE
Which measure pays back fastest?
Blanking panels, sealing and containment. They cost little, need no downtime and make every later step, such as higher set points, possible.
Does heat reuse improve PUE?
No. Exported heat does not reduce the energy a data center uses, so PUE stays the same. It is reported as ERF instead. See allinHEATREUSE.
Does liquid cooling always lower PUE?
Usually, because it removes fan energy and allows warm water. But server fans and pumps move between the IT and facility sides of the ratio, so compare total energy, not PUE alone.
How far can an old site realistically go?
Airflow and control measures typically move a legacy hall from about 1.6 to around 1.4. Lower values usually need new cooling plant.
Sources
- ASHRAE TC 9.9, Thermal Guidelines for Data Processing Environments.
- ISO/IEC 30134-2:2016 (PUE); EN 50600-4-6 (energy reuse factor).
- Google DeepMind, machine learning for data center cooling (2016).
- Fan and pump affinity laws; typical gains from industry practice, indicative only.