Free Cooling for Data Centers
STULZ
This 2015 STULZ document compares five free-cooling (economiser) options for data centres by their pros and cons. Its main message is that the move to supply-air control and higher ASHRAE temperatures has extended free-cooling hours.
On this page
Overview
The paper surveys five topologies: direct outside air, indirect air/air plate exchangers, thermal wheels, CRAC units with glycol dry coolers, and CRAH units with chillers. Each comes with a short list of pros and cons. Every free-cooling variant listed keeps 100 % mechanical cooling as backup.
The shift in temperature level is the heart of the explanation. Older rooms controlled return air at 22-24 C, which produced supply air around 12 C. Current practice controls supply air at 18-27 C per ASHRAE TC 9.9 2011, giving return air of 25-40 C; overall the level is about 10 C higher, which lengthens free-cooling hours.
The differences between options are quantified. Thermal-wheel indirect systems need 4-6 times more indoor footprint than two-stage systems. With a CRAC plus dry cooler, mixed (partial free-cooling) mode begins when outdoor air is about 5 C below the room return-air temperature. Chiller-based CRAH systems run relatively low, typically glycol-mixed chilled-water temperatures, which limits their free-cooling hours. Humidity issues with direct outside air are also noted.
Key points
- Older practice: return air at 22-24 C, supply air around 12 C.
- Current practice: supply air 18-27 C (ASHRAE TC 9.9 2011), return air 25-40 C; overall about 10 C higher.
- All free-cooling variants keep 100 % mechanical cooling backup.
- Thermal-wheel indirect systems need 4-6x more indoor footprint than two-stage systems.
- CRAC plus dry cooler enters mixed mode when outdoor air is about 5 C below return-air temperature.
- Chiller-based CRAH runs low, glycol-mixed chilled water, limiting free-cooling hours.
- Humidity control is flagged as an issue with direct outside air.
Using it in design
- Model the cooling-topology choice with five options (direct air, indirect plate, thermal wheel, CRAC with glycol dry cooler, CRAH with chiller economiser), each with a footprint multiplier and an economiser start condition.
- Recompute free-cooling hours whenever the user changes the ASHRAE inlet target, because the hours depend on the supply setpoint.
- Assume full mechanical backup by default.
- When a thermal wheel is chosen, carry the 4-6x indoor footprint into the layout check.
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Who should read it
HVAC and data-centre designers comparing cooling topologies and free-cooling potential at an early stage.
Related standards
- ASHRAE TC 9.9 (2011)
- EU Code of Conduct on Data Centres
- EN 50600
Publication details
- Authors
- STULZ
- Publisher
- STULZ
- Year
- 2015
- Document number
- Release 1, September 2015
- Document language
- English
- Source type
- Manufacturer
- Access
- Free to read
- Licence
- Copyright retained; citation only — the publisher’s terms apply to the original document
Citation
STULZ (2015). Free Cooling for Data Centers. STULZ. Release 1, September 2015. https://www.approvedbusiness.co.uk/storage/brochures/7909-stulz-white-paper-free-cooling-for-data-centers-v1.pdf
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