Economizer Modes of Data Center Cooling Systems

Overview

The paper starts from a terminology problem: the industry has at least 15 to 17 distinct economizer modes, and their definitions are imprecise. The authors fold them into one classification, explain how each mode works, and compare them qualitatively on energy, water use, footprint, maintenance and risk. "Free cooling" stops being a label that can mean different things to a designer and a vendor, and each mode gets a profile that can be compared with the others.

The quantitative comparison rests on an hourly model of a 1 MW site in St. Louis running at 50% load (500 kW of IT). Some modes save more than 70% of annual cooling energy, which corresponds to more than a 15% reduction in annualized PUE. The best result comes from the air-to-air heat wheel (indirect) mode: about 381 MWh per year of cooling energy against a 2,761 MWh baseline, an 86% saving. The space taken by economizer equipment ranges from roughly 0.41 to 3.34 ft2 per kW depending on the mode.

The paper also explains what increases economizer hours. Raising the IT supply temperature and the chilled-water temperature adds hours; the recommended chilled-water supply is 10 to 15 C rather than the traditional 7.2 C. In a chilled-water plant, delivering 20 C supply air at 100% economizer requires outdoor wet-bulb to stay below about 2 C, and derating the chiller to 50% lifts that limit to about 7 C. Evaporative assist can lower supply air temperature by up to 19 C in suitable climates.

Limits and risks are stated plainly. A cooling tower consumes about 40 gpm per 1,000 tons of cooling capacity (151.4 L/min), so an energy gain has to be read together with water consumption. For modes that use outdoor air directly, air quality, loss of make-up water and interaction between fire suppression and fresh-air systems are the main threats to availability. The results are modelled for one climate and one site size; the ranking of modes may hold elsewhere, but the percentage savings will change.

Key points

1. Some economizer modes save more than 70% of annual cooling energy, corresponding to more than a 15% cut in annualized PUE.
2. At least 15 to 17 economizer-mode types exist, with imprecise industry definitions.
3. In the 1 MW site (500 kW IT, 50% load) the best mode, the air-to-air heat wheel, uses about 381 MWh/yr against a 2,761 MWh baseline (86% saving).
4. Evaporative assist can lower supply air temperature by up to 19 C (35 F) in suitable climates.
5. Cooling towers consume about 40 gpm per 1,000 tons of capacity (151.4 L/min).
6. For 20 C supply air at 100% economizer, outdoor wet-bulb must stay below about 2 C; with the chiller derated to 50%, about 7 C is enough.
7. Economizer equipment space ranges from about 0.41 to 3.34 ft2 per kW depending on mode.
8. Recommended chilled-water supply is 10 to 15 C (50 to 59 F) instead of the traditional 7.2 C (45 F).

Using it in design

• Compute economizer hours from hourly climate data (dry bulb and wet bulb) against the chosen supply setpoint and approach temperatures, and report partial and full economizer hours separately.
• For tower-based plants, show water use (about 0.043 L/min per kW of rejected heat, or roughly 2.6 L per kWh of heat at full tower duty) next to the energy result so the energy-versus-water trade-off is visible.
• Use 15 C as the default chilled-water supply temperature for new builds.
• Add a risk flag for direct fresh-air modes at sites with poor air quality or wildfire smoke, and review how fire suppression interacts with the fresh-air system.

Who should read it

Mechanical designers choosing a cooling plant type and an economizer strategy for a given climate, and the decision makers who fund it.

Related standards

• ASHRAE 90.1
• ASHRAE 90.4
• ASHRAE TC9.9 Thermal Guidelines
• The Green Grid PUE

Publication details

Authors

John Niemann; John Bean; Victor Avelar

Publisher

Schneider Electric

Year

2011

Document number

WP 132

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