For Data Center Energy Efficiency, Hot-Aisle Beats Cold-Aisle Containment

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There’s little argument that hot-aisle and cold-aisle containment systems provide big energy savings in a data center, while eliminating the hot spots found in uncontained data centers.  But which strategy results in more savings?

To find out, here at APC Schneider Electric we did some analysis and found that, while both strategies offer energy savings, hot-aisle containment can provide 40% more savings. And while it can be difficult to retrofit an existing data center to support hot-aisle containment, making cold-aisle the only option, we can definitively say that hot-aisle containment should always be used for new data centers.

Either type of containment system requires a hot aisle/cold aisle layout, where a row of racks is positioned with the rack fronts facing the rack fronts of the adjacent row. This layout forms alternating hot and cold aisles, since hot air is expelled out the back of each row, making the rear the hot aisle and the front the cold aisle.

Hot vs. cold-aisle containment

A cold-aisle containment system (CACS) encloses the cold aisle, allowing the rest of the data center to become a large hot-air return while a hot-aisle containment system (HACS) encloses the hot aisle, collecting the hot exhaust air from IT equipment. The hot air may be ducted to a computer room air handler (CRAH) or large remote air conditioning unit using a large chimney located over the entire hot aisle.

The idea behind both methods is the same: to separate hot and cold air streams from one another. Doing so provides the following efficiencies:

  • Cooling systems can be set to a higher supply temperature, thereby saving energy and increasing cooling capacity, and still supply safe operating temperatures.
  • Elimination of hot spots. Containment allows cooling unit supply air to reach the front of IT equipment without mixing with hot air. When no mixing of hot and cold air occurs, the supply air temperature can be increased without risk of hot spots.
  • Increased economizer hours. When outdoor temperature is lower than indoor temperature, the cooling system compressors don’t need to work to reject heat to the outdoors, which saves energy.
  • Reduced humidification/dehumidification costs. By eliminating mixing between hot and cold air, the cooling system’s supply air temperatures can be increased, allowing the cooling system to operate above the dew point temperature. When supplying air above the dew point, no humidity is removed from the air. If no humidity is removed, adding humidity is not required, saving energy and water.
  • Better overall physical infrastructure utilization, which enables right-sizing of the cooling infrastructure and results in equipment running at higher efficiency.

HACS wins the energy audit

While both the CACS and HACS will save you money, our analysis shows that, at the same 75°F/24°C work environment, the HACS consumes 40% less cooling system energy than the CACS. The majority of these savings are attributed to the economizer hours. At that temperature, the CACS is unable to benefit from any economizer hours due to the low chilled water supply temperature.

From our analysis, it’s clear that under practical work environment temperature constraints and temperate climates, hot-aisle containment provides significantly more economizer hours and lower PUE compared to cold-aisle containment. This is true regardless of the cooling architecture or heat rejection method used.  

To learn more, download the white paper, “Implementing Hot and Cold Air Containment in Existing Data Centers.”

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