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The Greening of IT
142 How Companies Can Make a Difference for the Environment

5. Deploying Supplemental Cooling

Supplemental cooling is a relatively new approach to data center cooling.
Introduced in 2002, this approach gained rapid acceptance as data center
managers seek solutions to help:

■ Overcome cooling capacity limitations of raised floor systems in high
heat density applications.
■ Increase cooling system efficiency and flexibility.

Raised-floor cooling proved an effective approach to data center environ-
mental management; however, as rack densities exceed 5 kW, and load diver-
sity across the room increases, supplemental cooling should be evaluated for
its impact on cooling system performance and efficiency.
At higher densities, equipment in the bottom of the rack might consume so
much cold air that remaining quantities of cold air are insufficient to cool equip-
ment at the top of the rack. The height of the raised floor creates a physical lim-
itation on the volume of air that can be distributed into the room, so adding
ptg
additional room air conditioners might not solve the problem.
Uptime Institute reports that equipment located in the top-third of a data
center rack fails twice as often as equipment in the bottom two-thirds of the
same rack. The organization also estimates that, for every increase of 18° F
above 70° F, long-term electronics reliability falls by 50 percent. The solu-
tion to rising rack densities and high room diversity proved to be a pumped
refrigerant cooling infrastructure that supports cooling modules placed
directly above or alongside high-density racks to supplement the air coming
up through the floor. This solution has a number of advantages, including
increased cooling system scalability, greater flexibility, and improved energy
efficiency.
Two factors contribute to improved energy efficiency: the location of the
cooling modules and the refrigerant used. Higher-density applications
require fluid-based cooling to effectively remove the high concentrations of
heat generated. From an efficiency perspective, refrigerant performs better
than water for high-density cooling. The R134 refrigerant used in the
Liebert XD system is pumped as a liquid but converts to gas when it reaches
the air. This phase change contributes to greater system efficiency. R134 is
approximately 700 percent more effective in moving heat than water, which
coincidentally is 700 percent more effective than air. It also ensures that
expensive IT equipment is not damaged in the event of a refrigerant leak.