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The Merits of Open Bath Immersion Cooling of Datacom Equipment
Keywords: High Performance Computing, Immersion Cooling, System Level Savings
Data Center Inefficiency In December 2006, Congress passed Public Law 109-431 requiring the US EPA to study and report on the energy efficiency of datacenters in the US. The report1, released in August of 2007, estimated that power consumption attributable to data centers doubled between 2000 and 2006 and is expected to double again reaching 3% of the Nation's power consumption by 2011. Mindful of the economic and environmental implications, the EPA encouraged public and private sector research into new technologies. Even before the EPA report, the IT industry was proactively addressing the issue in conferences, trade organizations like the GreenGrid and Technical Committees (TC) like American Society of Heating Refrigeration and Air Conditioning Engineers (ASHRAE) TC 9.9. They concluded that legacy air cooling technology is a leading cause of inefficiency often consuming more than 50% of a data center's electricity. Indeed the path of heat transfer in a typical datacenter is convoluted (Figure 1) and the causes of inefficiency are well known to those skilled in this field (Figure 2). The Committee has since drafted standards to facilitate the propagation of more efficient liquid cooling technology. Emerging Technologies Hybrid air/liquid cooling technologies are emerging that address one or more of the aforementioned causes. Water-cooled forced air racks, for example, are being sold2. These reduce or eliminate mixing of warm and cool airstreams thus increasing overall efficiency and the amount of equipment that can be placed in a given amount of floor space. However, the efficiency gains achievable with these hybrid air/liquid technologies are limited by the required node and rack fan power and the need for near 20C chilled facility water Datacenters have been built that use no cooled water. Instead they duct outside air directly to the racks. Facilities such as these can achieve a Power Usage Effectiveness (PUE) of 1.10 so that 1.1W is drawn by the facility for every 1W delivered to the racks3. This number captures various inefficiencies in power conversion and some attributable to cooling but fails to capture the power consumption of fans within the racks which typically is 5% or more of the power they receive. Furthermore, facilities like these must be located in relatively cool climates; have limited ability to utilize the heat they output and; may need to have their computing load shifted elsewhere when ambient temperatures become too high. Cooling efficiency and the thermodynamic availability of the heat removed will increase dramatically if forced air is eliminated as a link between the outdoor air and the components on each chassis within a rack. However, implementation of most liquid cooling schemes be they single- or two-phase, direct- or indirect-contact is complicated by the requirement that each server or node within a rack be hot swappable. This implies a myriad of cold plates and couplings or clamshells and hermetic connectors each with its inherent costs and potential for leakage (Table I).
Steve A. Pignato, EMMD Key Account Manager / Heat Transfer Specialist
St. Paul, MN

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